2 @c This is part of the GNU Guile Reference Manual.
3 @c Copyright (C) 1996, 1997, 2000, 2001, 2002, 2003, 2004, 2006, 2007, 2008
4 @c Free Software Foundation, Inc.
5 @c See the file guile.texi for copying conditions.
8 @node Simple Data Types
9 @section Simple Generic Data Types
11 This chapter describes those of Guile's simple data types which are
12 primarily used for their role as items of generic data. By
13 @dfn{simple} we mean data types that are not primarily used as
14 containers to hold other data --- i.e.@: pairs, lists, vectors and so on.
15 For the documentation of such @dfn{compound} data types, see
16 @ref{Compound Data Types}.
18 @c One of the great strengths of Scheme is that there is no straightforward
19 @c distinction between ``data'' and ``functionality''. For example,
20 @c Guile's support for dynamic linking could be described:
24 @c either in a ``data-centric'' way, as the behaviour and properties of the
25 @c ``dynamically linked object'' data type, and the operations that may be
26 @c applied to instances of this type
29 @c or in a ``functionality-centric'' way, as the set of procedures that
30 @c constitute Guile's support for dynamic linking, in the context of the
34 @c The contents of this chapter are, therefore, a matter of judgment. By
35 @c @dfn{generic}, we mean to select those data types whose typical use as
36 @c @emph{data} in a wide variety of programming contexts is more important
37 @c than their use in the implementation of a particular piece of
38 @c @emph{functionality}. The last section of this chapter provides
39 @c references for all the data types that are documented not here but in a
40 @c ``functionality-centric'' way elsewhere in the manual.
43 * Booleans:: True/false values.
44 * Numbers:: Numerical data types.
45 * Characters:: Single characters.
46 * Character Sets:: Sets of characters.
47 * Strings:: Sequences of characters.
48 * Regular Expressions:: Pattern matching and substitution.
50 * Keywords:: Self-quoting, customizable display keywords.
51 * Other Types:: "Functionality-centric" data types.
59 The two boolean values are @code{#t} for true and @code{#f} for false.
61 Boolean values are returned by predicate procedures, such as the general
62 equality predicates @code{eq?}, @code{eqv?} and @code{equal?}
63 (@pxref{Equality}) and numerical and string comparison operators like
64 @code{string=?} (@pxref{String Comparison}) and @code{<=}
74 (equal? "house" "houses")
82 In test condition contexts like @code{if} and @code{cond} (@pxref{if
83 cond case}), where a group of subexpressions will be evaluated only if a
84 @var{condition} expression evaluates to ``true'', ``true'' means any
85 value at all except @code{#f}.
98 A result of this asymmetry is that typical Scheme source code more often
99 uses @code{#f} explicitly than @code{#t}: @code{#f} is necessary to
100 represent an @code{if} or @code{cond} false value, whereas @code{#t} is
101 not necessary to represent an @code{if} or @code{cond} true value.
103 It is important to note that @code{#f} is @strong{not} equivalent to any
104 other Scheme value. In particular, @code{#f} is not the same as the
105 number 0 (like in C and C++), and not the same as the ``empty list''
106 (like in some Lisp dialects).
108 In C, the two Scheme boolean values are available as the two constants
109 @code{SCM_BOOL_T} for @code{#t} and @code{SCM_BOOL_F} for @code{#f}.
110 Care must be taken with the false value @code{SCM_BOOL_F}: it is not
111 false when used in C conditionals. In order to test for it, use
112 @code{scm_is_false} or @code{scm_is_true}.
115 @deffn {Scheme Procedure} not x
116 @deffnx {C Function} scm_not (x)
117 Return @code{#t} if @var{x} is @code{#f}, else return @code{#f}.
121 @deffn {Scheme Procedure} boolean? obj
122 @deffnx {C Function} scm_boolean_p (obj)
123 Return @code{#t} if @var{obj} is either @code{#t} or @code{#f}, else
127 @deftypevr {C Macro} SCM SCM_BOOL_T
128 The @code{SCM} representation of the Scheme object @code{#t}.
131 @deftypevr {C Macro} SCM SCM_BOOL_F
132 The @code{SCM} representation of the Scheme object @code{#f}.
135 @deftypefn {C Function} int scm_is_true (SCM obj)
136 Return @code{0} if @var{obj} is @code{#f}, else return @code{1}.
139 @deftypefn {C Function} int scm_is_false (SCM obj)
140 Return @code{1} if @var{obj} is @code{#f}, else return @code{0}.
143 @deftypefn {C Function} int scm_is_bool (SCM obj)
144 Return @code{1} if @var{obj} is either @code{#t} or @code{#f}, else
148 @deftypefn {C Function} SCM scm_from_bool (int val)
149 Return @code{#f} if @var{val} is @code{0}, else return @code{#t}.
152 @deftypefn {C Function} int scm_to_bool (SCM val)
153 Return @code{1} if @var{val} is @code{SCM_BOOL_T}, return @code{0}
154 when @var{val} is @code{SCM_BOOL_F}, else signal a `wrong type' error.
156 You should probably use @code{scm_is_true} instead of this function
157 when you just want to test a @code{SCM} value for trueness.
161 @subsection Numerical data types
164 Guile supports a rich ``tower'' of numerical types --- integer,
165 rational, real and complex --- and provides an extensive set of
166 mathematical and scientific functions for operating on numerical
167 data. This section of the manual documents those types and functions.
169 You may also find it illuminating to read R5RS's presentation of numbers
170 in Scheme, which is particularly clear and accessible: see
171 @ref{Numbers,,,r5rs,R5RS}.
174 * Numerical Tower:: Scheme's numerical "tower".
175 * Integers:: Whole numbers.
176 * Reals and Rationals:: Real and rational numbers.
177 * Complex Numbers:: Complex numbers.
178 * Exactness:: Exactness and inexactness.
179 * Number Syntax:: Read syntax for numerical data.
180 * Integer Operations:: Operations on integer values.
181 * Comparison:: Comparison predicates.
182 * Conversion:: Converting numbers to and from strings.
183 * Complex:: Complex number operations.
184 * Arithmetic:: Arithmetic functions.
185 * Scientific:: Scientific functions.
186 * Primitive Numerics:: Primitive numeric functions.
187 * Bitwise Operations:: Logical AND, OR, NOT, and so on.
188 * Random:: Random number generation.
192 @node Numerical Tower
193 @subsubsection Scheme's Numerical ``Tower''
196 Scheme's numerical ``tower'' consists of the following categories of
201 Whole numbers, positive or negative; e.g.@: --5, 0, 18.
204 The set of numbers that can be expressed as @math{@var{p}/@var{q}}
205 where @var{p} and @var{q} are integers; e.g.@: @math{9/16} works, but
206 pi (an irrational number) doesn't. These include integers
210 The set of numbers that describes all possible positions along a
211 one-dimensional line. This includes rationals as well as irrational
214 @item complex numbers
215 The set of numbers that describes all possible positions in a two
216 dimensional space. This includes real as well as imaginary numbers
217 (@math{@var{a}+@var{b}i}, where @var{a} is the @dfn{real part},
218 @var{b} is the @dfn{imaginary part}, and @math{i} is the square root of
222 It is called a tower because each category ``sits on'' the one that
223 follows it, in the sense that every integer is also a rational, every
224 rational is also real, and every real number is also a complex number
225 (but with zero imaginary part).
227 In addition to the classification into integers, rationals, reals and
228 complex numbers, Scheme also distinguishes between whether a number is
229 represented exactly or not. For example, the result of
230 @m{2\sin(\pi/4),2*sin(pi/4)} is exactly @m{\sqrt{2},2^(1/2)}, but Guile
231 can represent neither @m{\pi/4,pi/4} nor @m{\sqrt{2},2^(1/2)} exactly.
232 Instead, it stores an inexact approximation, using the C type
235 Guile can represent exact rationals of any magnitude, inexact
236 rationals that fit into a C @code{double}, and inexact complex numbers
237 with @code{double} real and imaginary parts.
239 The @code{number?} predicate may be applied to any Scheme value to
240 discover whether the value is any of the supported numerical types.
242 @deffn {Scheme Procedure} number? obj
243 @deffnx {C Function} scm_number_p (obj)
244 Return @code{#t} if @var{obj} is any kind of number, else @code{#f}.
253 (number? "hello there!")
256 (define pi 3.141592654)
261 @deftypefn {C Function} int scm_is_number (SCM obj)
262 This is equivalent to @code{scm_is_true (scm_number_p (obj))}.
265 The next few subsections document each of Guile's numerical data types
269 @subsubsection Integers
271 @tpindex Integer numbers
275 Integers are whole numbers, that is numbers with no fractional part,
276 such as 2, 83, and @minus{}3789.
278 Integers in Guile can be arbitrarily big, as shown by the following
282 (define (factorial n)
283 (let loop ((n n) (product 1))
286 (loop (- n 1) (* product n)))))
292 @result{} 2432902008176640000
295 @result{} -119622220865480194561963161495657715064383733760000000000
298 Readers whose background is in programming languages where integers are
299 limited by the need to fit into just 4 or 8 bytes of memory may find
300 this surprising, or suspect that Guile's representation of integers is
301 inefficient. In fact, Guile achieves a near optimal balance of
302 convenience and efficiency by using the host computer's native
303 representation of integers where possible, and a more general
304 representation where the required number does not fit in the native
305 form. Conversion between these two representations is automatic and
306 completely invisible to the Scheme level programmer.
308 The infinities @samp{+inf.0} and @samp{-inf.0} are considered to be
309 inexact integers. They are explained in detail in the next section,
310 together with reals and rationals.
312 C has a host of different integer types, and Guile offers a host of
313 functions to convert between them and the @code{SCM} representation.
314 For example, a C @code{int} can be handled with @code{scm_to_int} and
315 @code{scm_from_int}. Guile also defines a few C integer types of its
316 own, to help with differences between systems.
318 C integer types that are not covered can be handled with the generic
319 @code{scm_to_signed_integer} and @code{scm_from_signed_integer} for
320 signed types, or with @code{scm_to_unsigned_integer} and
321 @code{scm_from_unsigned_integer} for unsigned types.
323 Scheme integers can be exact and inexact. For example, a number
324 written as @code{3.0} with an explicit decimal-point is inexact, but
325 it is also an integer. The functions @code{integer?} and
326 @code{scm_is_integer} report true for such a number, but the functions
327 @code{scm_is_signed_integer} and @code{scm_is_unsigned_integer} only
328 allow exact integers and thus report false. Likewise, the conversion
329 functions like @code{scm_to_signed_integer} only accept exact
332 The motivation for this behavior is that the inexactness of a number
333 should not be lost silently. If you want to allow inexact integers,
334 you can explicitly insert a call to @code{inexact->exact} or to its C
335 equivalent @code{scm_inexact_to_exact}. (Only inexact integers will
336 be converted by this call into exact integers; inexact non-integers
337 will become exact fractions.)
339 @deffn {Scheme Procedure} integer? x
340 @deffnx {C Function} scm_integer_p (x)
341 Return @code{#t} if @var{x} is an exact or inexact integer number, else
359 @deftypefn {C Function} int scm_is_integer (SCM x)
360 This is equivalent to @code{scm_is_true (scm_integer_p (x))}.
363 @defvr {C Type} scm_t_int8
364 @defvrx {C Type} scm_t_uint8
365 @defvrx {C Type} scm_t_int16
366 @defvrx {C Type} scm_t_uint16
367 @defvrx {C Type} scm_t_int32
368 @defvrx {C Type} scm_t_uint32
369 @defvrx {C Type} scm_t_int64
370 @defvrx {C Type} scm_t_uint64
371 @defvrx {C Type} scm_t_intmax
372 @defvrx {C Type} scm_t_uintmax
373 The C types are equivalent to the corresponding ISO C types but are
374 defined on all platforms, with the exception of @code{scm_t_int64} and
375 @code{scm_t_uint64}, which are only defined when a 64-bit type is
376 available. For example, @code{scm_t_int8} is equivalent to
379 You can regard these definitions as a stop-gap measure until all
380 platforms provide these types. If you know that all the platforms
381 that you are interested in already provide these types, it is better
382 to use them directly instead of the types provided by Guile.
385 @deftypefn {C Function} int scm_is_signed_integer (SCM x, scm_t_intmax min, scm_t_intmax max)
386 @deftypefnx {C Function} int scm_is_unsigned_integer (SCM x, scm_t_uintmax min, scm_t_uintmax max)
387 Return @code{1} when @var{x} represents an exact integer that is
388 between @var{min} and @var{max}, inclusive.
390 These functions can be used to check whether a @code{SCM} value will
391 fit into a given range, such as the range of a given C integer type.
392 If you just want to convert a @code{SCM} value to a given C integer
393 type, use one of the conversion functions directly.
396 @deftypefn {C Function} scm_t_intmax scm_to_signed_integer (SCM x, scm_t_intmax min, scm_t_intmax max)
397 @deftypefnx {C Function} scm_t_uintmax scm_to_unsigned_integer (SCM x, scm_t_uintmax min, scm_t_uintmax max)
398 When @var{x} represents an exact integer that is between @var{min} and
399 @var{max} inclusive, return that integer. Else signal an error,
400 either a `wrong-type' error when @var{x} is not an exact integer, or
401 an `out-of-range' error when it doesn't fit the given range.
404 @deftypefn {C Function} SCM scm_from_signed_integer (scm_t_intmax x)
405 @deftypefnx {C Function} SCM scm_from_unsigned_integer (scm_t_uintmax x)
406 Return the @code{SCM} value that represents the integer @var{x}. This
407 function will always succeed and will always return an exact number.
410 @deftypefn {C Function} char scm_to_char (SCM x)
411 @deftypefnx {C Function} {signed char} scm_to_schar (SCM x)
412 @deftypefnx {C Function} {unsigned char} scm_to_uchar (SCM x)
413 @deftypefnx {C Function} short scm_to_short (SCM x)
414 @deftypefnx {C Function} {unsigned short} scm_to_ushort (SCM x)
415 @deftypefnx {C Function} int scm_to_int (SCM x)
416 @deftypefnx {C Function} {unsigned int} scm_to_uint (SCM x)
417 @deftypefnx {C Function} long scm_to_long (SCM x)
418 @deftypefnx {C Function} {unsigned long} scm_to_ulong (SCM x)
419 @deftypefnx {C Function} {long long} scm_to_long_long (SCM x)
420 @deftypefnx {C Function} {unsigned long long} scm_to_ulong_long (SCM x)
421 @deftypefnx {C Function} size_t scm_to_size_t (SCM x)
422 @deftypefnx {C Function} ssize_t scm_to_ssize_t (SCM x)
423 @deftypefnx {C Function} scm_t_int8 scm_to_int8 (SCM x)
424 @deftypefnx {C Function} scm_t_uint8 scm_to_uint8 (SCM x)
425 @deftypefnx {C Function} scm_t_int16 scm_to_int16 (SCM x)
426 @deftypefnx {C Function} scm_t_uint16 scm_to_uint16 (SCM x)
427 @deftypefnx {C Function} scm_t_int32 scm_to_int32 (SCM x)
428 @deftypefnx {C Function} scm_t_uint32 scm_to_uint32 (SCM x)
429 @deftypefnx {C Function} scm_t_int64 scm_to_int64 (SCM x)
430 @deftypefnx {C Function} scm_t_uint64 scm_to_uint64 (SCM x)
431 @deftypefnx {C Function} scm_t_intmax scm_to_intmax (SCM x)
432 @deftypefnx {C Function} scm_t_uintmax scm_to_uintmax (SCM x)
433 When @var{x} represents an exact integer that fits into the indicated
434 C type, return that integer. Else signal an error, either a
435 `wrong-type' error when @var{x} is not an exact integer, or an
436 `out-of-range' error when it doesn't fit the given range.
438 The functions @code{scm_to_long_long}, @code{scm_to_ulong_long},
439 @code{scm_to_int64}, and @code{scm_to_uint64} are only available when
440 the corresponding types are.
443 @deftypefn {C Function} SCM scm_from_char (char x)
444 @deftypefnx {C Function} SCM scm_from_schar (signed char x)
445 @deftypefnx {C Function} SCM scm_from_uchar (unsigned char x)
446 @deftypefnx {C Function} SCM scm_from_short (short x)
447 @deftypefnx {C Function} SCM scm_from_ushort (unsigned short x)
448 @deftypefnx {C Function} SCM scm_from_int (int x)
449 @deftypefnx {C Function} SCM scm_from_uint (unsigned int x)
450 @deftypefnx {C Function} SCM scm_from_long (long x)
451 @deftypefnx {C Function} SCM scm_from_ulong (unsigned long x)
452 @deftypefnx {C Function} SCM scm_from_long_long (long long x)
453 @deftypefnx {C Function} SCM scm_from_ulong_long (unsigned long long x)
454 @deftypefnx {C Function} SCM scm_from_size_t (size_t x)
455 @deftypefnx {C Function} SCM scm_from_ssize_t (ssize_t x)
456 @deftypefnx {C Function} SCM scm_from_int8 (scm_t_int8 x)
457 @deftypefnx {C Function} SCM scm_from_uint8 (scm_t_uint8 x)
458 @deftypefnx {C Function} SCM scm_from_int16 (scm_t_int16 x)
459 @deftypefnx {C Function} SCM scm_from_uint16 (scm_t_uint16 x)
460 @deftypefnx {C Function} SCM scm_from_int32 (scm_t_int32 x)
461 @deftypefnx {C Function} SCM scm_from_uint32 (scm_t_uint32 x)
462 @deftypefnx {C Function} SCM scm_from_int64 (scm_t_int64 x)
463 @deftypefnx {C Function} SCM scm_from_uint64 (scm_t_uint64 x)
464 @deftypefnx {C Function} SCM scm_from_intmax (scm_t_intmax x)
465 @deftypefnx {C Function} SCM scm_from_uintmax (scm_t_uintmax x)
466 Return the @code{SCM} value that represents the integer @var{x}.
467 These functions will always succeed and will always return an exact
471 @deftypefn {C Function} void scm_to_mpz (SCM val, mpz_t rop)
472 Assign @var{val} to the multiple precision integer @var{rop}.
473 @var{val} must be an exact integer, otherwise an error will be
474 signalled. @var{rop} must have been initialized with @code{mpz_init}
475 before this function is called. When @var{rop} is no longer needed
476 the occupied space must be freed with @code{mpz_clear}.
477 @xref{Initializing Integers,,, gmp, GNU MP Manual}, for details.
480 @deftypefn {C Function} SCM scm_from_mpz (mpz_t val)
481 Return the @code{SCM} value that represents @var{val}.
484 @node Reals and Rationals
485 @subsubsection Real and Rational Numbers
486 @tpindex Real numbers
487 @tpindex Rational numbers
492 Mathematically, the real numbers are the set of numbers that describe
493 all possible points along a continuous, infinite, one-dimensional line.
494 The rational numbers are the set of all numbers that can be written as
495 fractions @var{p}/@var{q}, where @var{p} and @var{q} are integers.
496 All rational numbers are also real, but there are real numbers that
497 are not rational, for example @m{\sqrt2, the square root of 2}, and
500 Guile can represent both exact and inexact rational numbers, but it
501 can not represent irrational numbers. Exact rationals are represented
502 by storing the numerator and denominator as two exact integers.
503 Inexact rationals are stored as floating point numbers using the C
506 Exact rationals are written as a fraction of integers. There must be
507 no whitespace around the slash:
514 Even though the actual encoding of inexact rationals is in binary, it
515 may be helpful to think of it as a decimal number with a limited
516 number of significant figures and a decimal point somewhere, since
517 this corresponds to the standard notation for non-whole numbers. For
523 -5648394822220000000000.0
527 The limited precision of Guile's encoding means that any ``real'' number
528 in Guile can be written in a rational form, by multiplying and then dividing
529 by sufficient powers of 10 (or in fact, 2). For example,
530 @samp{-0.00000142857931198} is the same as @minus{}142857931198 divided by
531 100000000000000000. In Guile's current incarnation, therefore, the
532 @code{rational?} and @code{real?} predicates are equivalent.
535 Dividing by an exact zero leads to a error message, as one might
536 expect. However, dividing by an inexact zero does not produce an
537 error. Instead, the result of the division is either plus or minus
538 infinity, depending on the sign of the divided number.
540 The infinities are written @samp{+inf.0} and @samp{-inf.0},
541 respectivly. This syntax is also recognized by @code{read} as an
542 extension to the usual Scheme syntax.
544 Dividing zero by zero yields something that is not a number at all:
545 @samp{+nan.0}. This is the special `not a number' value.
547 On platforms that follow @acronym{IEEE} 754 for their floating point
548 arithmetic, the @samp{+inf.0}, @samp{-inf.0}, and @samp{+nan.0} values
549 are implemented using the corresponding @acronym{IEEE} 754 values.
550 They behave in arithmetic operations like @acronym{IEEE} 754 describes
551 it, i.e., @code{(= +nan.0 +nan.0)} @result{} @code{#f}.
553 The infinities are inexact integers and are considered to be both even
554 and odd. While @samp{+nan.0} is not @code{=} to itself, it is
555 @code{eqv?} to itself.
557 To test for the special values, use the functions @code{inf?} and
560 @deffn {Scheme Procedure} real? obj
561 @deffnx {C Function} scm_real_p (obj)
562 Return @code{#t} if @var{obj} is a real number, else @code{#f}. Note
563 that the sets of integer and rational values form subsets of the set
564 of real numbers, so the predicate will also be fulfilled if @var{obj}
565 is an integer number or a rational number.
568 @deffn {Scheme Procedure} rational? x
569 @deffnx {C Function} scm_rational_p (x)
570 Return @code{#t} if @var{x} is a rational number, @code{#f} otherwise.
571 Note that the set of integer values forms a subset of the set of
572 rational numbers, i. e. the predicate will also be fulfilled if
573 @var{x} is an integer number.
575 Since Guile can not represent irrational numbers, every number
576 satisfying @code{real?} also satisfies @code{rational?} in Guile.
579 @deffn {Scheme Procedure} rationalize x eps
580 @deffnx {C Function} scm_rationalize (x, eps)
581 Returns the @emph{simplest} rational number differing
582 from @var{x} by no more than @var{eps}.
584 As required by @acronym{R5RS}, @code{rationalize} only returns an
585 exact result when both its arguments are exact. Thus, you might need
586 to use @code{inexact->exact} on the arguments.
589 (rationalize (inexact->exact 1.2) 1/100)
595 @deffn {Scheme Procedure} inf? x
596 @deffnx {C Function} scm_inf_p (x)
597 Return @code{#t} if @var{x} is either @samp{+inf.0} or @samp{-inf.0},
601 @deffn {Scheme Procedure} nan? x
602 @deffnx {C Function} scm_nan_p (x)
603 Return @code{#t} if @var{x} is @samp{+nan.0}, @code{#f} otherwise.
606 @deffn {Scheme Procedure} nan
607 @deffnx {C Function} scm_nan ()
611 @deffn {Scheme Procedure} inf
612 @deffnx {C Function} scm_inf ()
616 @deffn {Scheme Procedure} numerator x
617 @deffnx {C Function} scm_numerator (x)
618 Return the numerator of the rational number @var{x}.
621 @deffn {Scheme Procedure} denominator x
622 @deffnx {C Function} scm_denominator (x)
623 Return the denominator of the rational number @var{x}.
626 @deftypefn {C Function} int scm_is_real (SCM val)
627 @deftypefnx {C Function} int scm_is_rational (SCM val)
628 Equivalent to @code{scm_is_true (scm_real_p (val))} and
629 @code{scm_is_true (scm_rational_p (val))}, respectively.
632 @deftypefn {C Function} double scm_to_double (SCM val)
633 Returns the number closest to @var{val} that is representable as a
634 @code{double}. Returns infinity for a @var{val} that is too large in
635 magnitude. The argument @var{val} must be a real number.
638 @deftypefn {C Function} SCM scm_from_double (double val)
639 Return the @code{SCM} value that representats @var{val}. The returned
640 value is inexact according to the predicate @code{inexact?}, but it
641 will be exactly equal to @var{val}.
644 @node Complex Numbers
645 @subsubsection Complex Numbers
646 @tpindex Complex numbers
650 Complex numbers are the set of numbers that describe all possible points
651 in a two-dimensional space. The two coordinates of a particular point
652 in this space are known as the @dfn{real} and @dfn{imaginary} parts of
653 the complex number that describes that point.
655 In Guile, complex numbers are written in rectangular form as the sum of
656 their real and imaginary parts, using the symbol @code{i} to indicate
671 Polar form can also be used, with an @samp{@@} between magnitude and
675 1@@3.141592 @result{} -1.0 (approx)
676 -1@@1.57079 @result{} 0.0-1.0i (approx)
679 Guile represents a complex number with a non-zero imaginary part as a
680 pair of inexact rationals, so the real and imaginary parts of a
681 complex number have the same properties of inexactness and limited
682 precision as single inexact rational numbers. Guile can not represent
683 exact complex numbers with non-zero imaginary parts.
685 @deffn {Scheme Procedure} complex? z
686 @deffnx {C Function} scm_complex_p (z)
687 Return @code{#t} if @var{x} is a complex number, @code{#f}
688 otherwise. Note that the sets of real, rational and integer
689 values form subsets of the set of complex numbers, i. e. the
690 predicate will also be fulfilled if @var{x} is a real,
691 rational or integer number.
694 @deftypefn {C Function} int scm_is_complex (SCM val)
695 Equivalent to @code{scm_is_true (scm_complex_p (val))}.
699 @subsubsection Exact and Inexact Numbers
700 @tpindex Exact numbers
701 @tpindex Inexact numbers
705 @rnindex exact->inexact
706 @rnindex inexact->exact
708 R5RS requires that a calculation involving inexact numbers always
709 produces an inexact result. To meet this requirement, Guile
710 distinguishes between an exact integer value such as @samp{5} and the
711 corresponding inexact real value which, to the limited precision
712 available, has no fractional part, and is printed as @samp{5.0}. Guile
713 will only convert the latter value to the former when forced to do so by
714 an invocation of the @code{inexact->exact} procedure.
716 @deffn {Scheme Procedure} exact? z
717 @deffnx {C Function} scm_exact_p (z)
718 Return @code{#t} if the number @var{z} is exact, @code{#f}
734 @deffn {Scheme Procedure} inexact? z
735 @deffnx {C Function} scm_inexact_p (z)
736 Return @code{#t} if the number @var{z} is inexact, @code{#f}
740 @deffn {Scheme Procedure} inexact->exact z
741 @deffnx {C Function} scm_inexact_to_exact (z)
742 Return an exact number that is numerically closest to @var{z}, when
743 there is one. For inexact rationals, Guile returns the exact rational
744 that is numerically equal to the inexact rational. Inexact complex
745 numbers with a non-zero imaginary part can not be made exact.
752 The following happens because 12/10 is not exactly representable as a
753 @code{double} (on most platforms). However, when reading a decimal
754 number that has been marked exact with the ``#e'' prefix, Guile is
755 able to represent it correctly.
759 @result{} 5404319552844595/4503599627370496
767 @c begin (texi-doc-string "guile" "exact->inexact")
768 @deffn {Scheme Procedure} exact->inexact z
769 @deffnx {C Function} scm_exact_to_inexact (z)
770 Convert the number @var{z} to its inexact representation.
775 @subsubsection Read Syntax for Numerical Data
777 The read syntax for integers is a string of digits, optionally
778 preceded by a minus or plus character, a code indicating the
779 base in which the integer is encoded, and a code indicating whether
780 the number is exact or inexact. The supported base codes are:
785 the integer is written in binary (base 2)
789 the integer is written in octal (base 8)
793 the integer is written in decimal (base 10)
797 the integer is written in hexadecimal (base 16)
800 If the base code is omitted, the integer is assumed to be decimal. The
801 following examples show how these base codes are used.
820 The codes for indicating exactness (which can, incidentally, be applied
821 to all numerical values) are:
830 the number is inexact.
833 If the exactness indicator is omitted, the number is exact unless it
834 contains a radix point. Since Guile can not represent exact complex
835 numbers, an error is signalled when asking for them.
845 ERROR: Wrong type argument
848 Guile also understands the syntax @samp{+inf.0} and @samp{-inf.0} for
849 plus and minus infinity, respectively. The value must be written
850 exactly as shown, that is, they always must have a sign and exactly
851 one zero digit after the decimal point. It also understands
852 @samp{+nan.0} and @samp{-nan.0} for the special `not-a-number' value.
853 The sign is ignored for `not-a-number' and the value is always printed
856 @node Integer Operations
857 @subsubsection Operations on Integer Values
866 @deffn {Scheme Procedure} odd? n
867 @deffnx {C Function} scm_odd_p (n)
868 Return @code{#t} if @var{n} is an odd number, @code{#f}
872 @deffn {Scheme Procedure} even? n
873 @deffnx {C Function} scm_even_p (n)
874 Return @code{#t} if @var{n} is an even number, @code{#f}
878 @c begin (texi-doc-string "guile" "quotient")
879 @c begin (texi-doc-string "guile" "remainder")
880 @deffn {Scheme Procedure} quotient n d
881 @deffnx {Scheme Procedure} remainder n d
882 @deffnx {C Function} scm_quotient (n, d)
883 @deffnx {C Function} scm_remainder (n, d)
884 Return the quotient or remainder from @var{n} divided by @var{d}. The
885 quotient is rounded towards zero, and the remainder will have the same
886 sign as @var{n}. In all cases quotient and remainder satisfy
887 @math{@var{n} = @var{q}*@var{d} + @var{r}}.
890 (remainder 13 4) @result{} 1
891 (remainder -13 4) @result{} -1
895 @c begin (texi-doc-string "guile" "modulo")
896 @deffn {Scheme Procedure} modulo n d
897 @deffnx {C Function} scm_modulo (n, d)
898 Return the remainder from @var{n} divided by @var{d}, with the same
902 (modulo 13 4) @result{} 1
903 (modulo -13 4) @result{} 3
904 (modulo 13 -4) @result{} -3
905 (modulo -13 -4) @result{} -1
909 @c begin (texi-doc-string "guile" "gcd")
910 @deffn {Scheme Procedure} gcd x@dots{}
911 @deffnx {C Function} scm_gcd (x, y)
912 Return the greatest common divisor of all arguments.
913 If called without arguments, 0 is returned.
915 The C function @code{scm_gcd} always takes two arguments, while the
916 Scheme function can take an arbitrary number.
919 @c begin (texi-doc-string "guile" "lcm")
920 @deffn {Scheme Procedure} lcm x@dots{}
921 @deffnx {C Function} scm_lcm (x, y)
922 Return the least common multiple of the arguments.
923 If called without arguments, 1 is returned.
925 The C function @code{scm_lcm} always takes two arguments, while the
926 Scheme function can take an arbitrary number.
929 @deffn {Scheme Procedure} modulo-expt n k m
930 @deffnx {C Function} scm_modulo_expt (n, k, m)
931 Return @var{n} raised to the integer exponent
932 @var{k}, modulo @var{m}.
941 @subsubsection Comparison Predicates
946 The C comparison functions below always takes two arguments, while the
947 Scheme functions can take an arbitrary number. Also keep in mind that
948 the C functions return one of the Scheme boolean values
949 @code{SCM_BOOL_T} or @code{SCM_BOOL_F} which are both true as far as C
950 is concerned. Thus, always write @code{scm_is_true (scm_num_eq_p (x,
951 y))} when testing the two Scheme numbers @code{x} and @code{y} for
952 equality, for example.
954 @c begin (texi-doc-string "guile" "=")
955 @deffn {Scheme Procedure} =
956 @deffnx {C Function} scm_num_eq_p (x, y)
957 Return @code{#t} if all parameters are numerically equal.
960 @c begin (texi-doc-string "guile" "<")
961 @deffn {Scheme Procedure} <
962 @deffnx {C Function} scm_less_p (x, y)
963 Return @code{#t} if the list of parameters is monotonically
967 @c begin (texi-doc-string "guile" ">")
968 @deffn {Scheme Procedure} >
969 @deffnx {C Function} scm_gr_p (x, y)
970 Return @code{#t} if the list of parameters is monotonically
974 @c begin (texi-doc-string "guile" "<=")
975 @deffn {Scheme Procedure} <=
976 @deffnx {C Function} scm_leq_p (x, y)
977 Return @code{#t} if the list of parameters is monotonically
981 @c begin (texi-doc-string "guile" ">=")
982 @deffn {Scheme Procedure} >=
983 @deffnx {C Function} scm_geq_p (x, y)
984 Return @code{#t} if the list of parameters is monotonically
988 @c begin (texi-doc-string "guile" "zero?")
989 @deffn {Scheme Procedure} zero? z
990 @deffnx {C Function} scm_zero_p (z)
991 Return @code{#t} if @var{z} is an exact or inexact number equal to
995 @c begin (texi-doc-string "guile" "positive?")
996 @deffn {Scheme Procedure} positive? x
997 @deffnx {C Function} scm_positive_p (x)
998 Return @code{#t} if @var{x} is an exact or inexact number greater than
1002 @c begin (texi-doc-string "guile" "negative?")
1003 @deffn {Scheme Procedure} negative? x
1004 @deffnx {C Function} scm_negative_p (x)
1005 Return @code{#t} if @var{x} is an exact or inexact number less than
1011 @subsubsection Converting Numbers To and From Strings
1012 @rnindex number->string
1013 @rnindex string->number
1015 @deffn {Scheme Procedure} number->string n [radix]
1016 @deffnx {C Function} scm_number_to_string (n, radix)
1017 Return a string holding the external representation of the
1018 number @var{n} in the given @var{radix}. If @var{n} is
1019 inexact, a radix of 10 will be used.
1022 @deffn {Scheme Procedure} string->number string [radix]
1023 @deffnx {C Function} scm_string_to_number (string, radix)
1024 Return a number of the maximally precise representation
1025 expressed by the given @var{string}. @var{radix} must be an
1026 exact integer, either 2, 8, 10, or 16. If supplied, @var{radix}
1027 is a default radix that may be overridden by an explicit radix
1028 prefix in @var{string} (e.g. "#o177"). If @var{radix} is not
1029 supplied, then the default radix is 10. If string is not a
1030 syntactically valid notation for a number, then
1031 @code{string->number} returns @code{#f}.
1034 @deftypefn {C Function} SCM scm_c_locale_stringn_to_number (const char *string, size_t len, unsigned radix)
1035 As per @code{string->number} above, but taking a C string, as pointer
1036 and length. The string characters should be in the current locale
1037 encoding (@code{locale} in the name refers only to that, there's no
1038 locale-dependent parsing).
1043 @subsubsection Complex Number Operations
1044 @rnindex make-rectangular
1051 @deffn {Scheme Procedure} make-rectangular real imaginary
1052 @deffnx {C Function} scm_make_rectangular (real, imaginary)
1053 Return a complex number constructed of the given @var{real} and
1054 @var{imaginary} parts.
1057 @deffn {Scheme Procedure} make-polar x y
1058 @deffnx {C Function} scm_make_polar (x, y)
1060 Return the complex number @var{x} * e^(i * @var{y}).
1063 @c begin (texi-doc-string "guile" "real-part")
1064 @deffn {Scheme Procedure} real-part z
1065 @deffnx {C Function} scm_real_part (z)
1066 Return the real part of the number @var{z}.
1069 @c begin (texi-doc-string "guile" "imag-part")
1070 @deffn {Scheme Procedure} imag-part z
1071 @deffnx {C Function} scm_imag_part (z)
1072 Return the imaginary part of the number @var{z}.
1075 @c begin (texi-doc-string "guile" "magnitude")
1076 @deffn {Scheme Procedure} magnitude z
1077 @deffnx {C Function} scm_magnitude (z)
1078 Return the magnitude of the number @var{z}. This is the same as
1079 @code{abs} for real arguments, but also allows complex numbers.
1082 @c begin (texi-doc-string "guile" "angle")
1083 @deffn {Scheme Procedure} angle z
1084 @deffnx {C Function} scm_angle (z)
1085 Return the angle of the complex number @var{z}.
1088 @deftypefn {C Function} SCM scm_c_make_rectangular (double re, double im)
1089 @deftypefnx {C Function} SCM scm_c_make_polar (double x, double y)
1090 Like @code{scm_make_rectangular} or @code{scm_make_polar},
1091 respectively, but these functions take @code{double}s as their
1095 @deftypefn {C Function} double scm_c_real_part (z)
1096 @deftypefnx {C Function} double scm_c_imag_part (z)
1097 Returns the real or imaginary part of @var{z} as a @code{double}.
1100 @deftypefn {C Function} double scm_c_magnitude (z)
1101 @deftypefnx {C Function} double scm_c_angle (z)
1102 Returns the magnitude or angle of @var{z} as a @code{double}.
1107 @subsubsection Arithmetic Functions
1122 The C arithmetic functions below always takes two arguments, while the
1123 Scheme functions can take an arbitrary number. When you need to
1124 invoke them with just one argument, for example to compute the
1125 equivalent od @code{(- x)}, pass @code{SCM_UNDEFINED} as the second
1126 one: @code{scm_difference (x, SCM_UNDEFINED)}.
1128 @c begin (texi-doc-string "guile" "+")
1129 @deffn {Scheme Procedure} + z1 @dots{}
1130 @deffnx {C Function} scm_sum (z1, z2)
1131 Return the sum of all parameter values. Return 0 if called without any
1135 @c begin (texi-doc-string "guile" "-")
1136 @deffn {Scheme Procedure} - z1 z2 @dots{}
1137 @deffnx {C Function} scm_difference (z1, z2)
1138 If called with one argument @var{z1}, -@var{z1} is returned. Otherwise
1139 the sum of all but the first argument are subtracted from the first
1143 @c begin (texi-doc-string "guile" "*")
1144 @deffn {Scheme Procedure} * z1 @dots{}
1145 @deffnx {C Function} scm_product (z1, z2)
1146 Return the product of all arguments. If called without arguments, 1 is
1150 @c begin (texi-doc-string "guile" "/")
1151 @deffn {Scheme Procedure} / z1 z2 @dots{}
1152 @deffnx {C Function} scm_divide (z1, z2)
1153 Divide the first argument by the product of the remaining arguments. If
1154 called with one argument @var{z1}, 1/@var{z1} is returned.
1157 @deffn {Scheme Procedure} 1+ z
1158 @deffnx {C Function} scm_oneplus (z)
1159 Return @math{@var{z} + 1}.
1162 @deffn {Scheme Procedure} 1- z
1163 @deffnx {C function} scm_oneminus (z)
1164 Return @math{@var{z} - 1}.
1167 @c begin (texi-doc-string "guile" "abs")
1168 @deffn {Scheme Procedure} abs x
1169 @deffnx {C Function} scm_abs (x)
1170 Return the absolute value of @var{x}.
1172 @var{x} must be a number with zero imaginary part. To calculate the
1173 magnitude of a complex number, use @code{magnitude} instead.
1176 @c begin (texi-doc-string "guile" "max")
1177 @deffn {Scheme Procedure} max x1 x2 @dots{}
1178 @deffnx {C Function} scm_max (x1, x2)
1179 Return the maximum of all parameter values.
1182 @c begin (texi-doc-string "guile" "min")
1183 @deffn {Scheme Procedure} min x1 x2 @dots{}
1184 @deffnx {C Function} scm_min (x1, x2)
1185 Return the minimum of all parameter values.
1188 @c begin (texi-doc-string "guile" "truncate")
1189 @deffn {Scheme Procedure} truncate x
1190 @deffnx {C Function} scm_truncate_number (x)
1191 Round the inexact number @var{x} towards zero.
1194 @c begin (texi-doc-string "guile" "round")
1195 @deffn {Scheme Procedure} round x
1196 @deffnx {C Function} scm_round_number (x)
1197 Round the inexact number @var{x} to the nearest integer. When exactly
1198 halfway between two integers, round to the even one.
1201 @c begin (texi-doc-string "guile" "floor")
1202 @deffn {Scheme Procedure} floor x
1203 @deffnx {C Function} scm_floor (x)
1204 Round the number @var{x} towards minus infinity.
1207 @c begin (texi-doc-string "guile" "ceiling")
1208 @deffn {Scheme Procedure} ceiling x
1209 @deffnx {C Function} scm_ceiling (x)
1210 Round the number @var{x} towards infinity.
1213 @deftypefn {C Function} double scm_c_truncate (double x)
1214 @deftypefnx {C Function} double scm_c_round (double x)
1215 Like @code{scm_truncate_number} or @code{scm_round_number},
1216 respectively, but these functions take and return @code{double}
1221 @subsubsection Scientific Functions
1223 The following procedures accept any kind of number as arguments,
1224 including complex numbers.
1227 @c begin (texi-doc-string "guile" "sqrt")
1228 @deffn {Scheme Procedure} sqrt z
1229 Return the square root of @var{z}. Of the two possible roots
1230 (positive and negative), the one with the a positive real part is
1231 returned, or if that's zero then a positive imaginary part. Thus,
1234 (sqrt 9.0) @result{} 3.0
1235 (sqrt -9.0) @result{} 0.0+3.0i
1236 (sqrt 1.0+1.0i) @result{} 1.09868411346781+0.455089860562227i
1237 (sqrt -1.0-1.0i) @result{} 0.455089860562227-1.09868411346781i
1242 @c begin (texi-doc-string "guile" "expt")
1243 @deffn {Scheme Procedure} expt z1 z2
1244 Return @var{z1} raised to the power of @var{z2}.
1248 @c begin (texi-doc-string "guile" "sin")
1249 @deffn {Scheme Procedure} sin z
1250 Return the sine of @var{z}.
1254 @c begin (texi-doc-string "guile" "cos")
1255 @deffn {Scheme Procedure} cos z
1256 Return the cosine of @var{z}.
1260 @c begin (texi-doc-string "guile" "tan")
1261 @deffn {Scheme Procedure} tan z
1262 Return the tangent of @var{z}.
1266 @c begin (texi-doc-string "guile" "asin")
1267 @deffn {Scheme Procedure} asin z
1268 Return the arcsine of @var{z}.
1272 @c begin (texi-doc-string "guile" "acos")
1273 @deffn {Scheme Procedure} acos z
1274 Return the arccosine of @var{z}.
1278 @c begin (texi-doc-string "guile" "atan")
1279 @deffn {Scheme Procedure} atan z
1280 @deffnx {Scheme Procedure} atan y x
1281 Return the arctangent of @var{z}, or of @math{@var{y}/@var{x}}.
1285 @c begin (texi-doc-string "guile" "exp")
1286 @deffn {Scheme Procedure} exp z
1287 Return e to the power of @var{z}, where e is the base of natural
1288 logarithms (2.71828@dots{}).
1292 @c begin (texi-doc-string "guile" "log")
1293 @deffn {Scheme Procedure} log z
1294 Return the natural logarithm of @var{z}.
1297 @c begin (texi-doc-string "guile" "log10")
1298 @deffn {Scheme Procedure} log10 z
1299 Return the base 10 logarithm of @var{z}.
1302 @c begin (texi-doc-string "guile" "sinh")
1303 @deffn {Scheme Procedure} sinh z
1304 Return the hyperbolic sine of @var{z}.
1307 @c begin (texi-doc-string "guile" "cosh")
1308 @deffn {Scheme Procedure} cosh z
1309 Return the hyperbolic cosine of @var{z}.
1312 @c begin (texi-doc-string "guile" "tanh")
1313 @deffn {Scheme Procedure} tanh z
1314 Return the hyperbolic tangent of @var{z}.
1317 @c begin (texi-doc-string "guile" "asinh")
1318 @deffn {Scheme Procedure} asinh z
1319 Return the hyperbolic arcsine of @var{z}.
1322 @c begin (texi-doc-string "guile" "acosh")
1323 @deffn {Scheme Procedure} acosh z
1324 Return the hyperbolic arccosine of @var{z}.
1327 @c begin (texi-doc-string "guile" "atanh")
1328 @deffn {Scheme Procedure} atanh z
1329 Return the hyperbolic arctangent of @var{z}.
1333 @node Primitive Numerics
1334 @subsubsection Primitive Numeric Functions
1336 Many of Guile's numeric procedures which accept any kind of numbers as
1337 arguments, including complex numbers, are implemented as Scheme
1338 procedures that use the following real number-based primitives. These
1339 primitives signal an error if they are called with complex arguments.
1341 @c begin (texi-doc-string "guile" "$abs")
1342 @deffn {Scheme Procedure} $abs x
1343 Return the absolute value of @var{x}.
1346 @c begin (texi-doc-string "guile" "$sqrt")
1347 @deffn {Scheme Procedure} $sqrt x
1348 Return the square root of @var{x}.
1351 @deffn {Scheme Procedure} $expt x y
1352 @deffnx {C Function} scm_sys_expt (x, y)
1353 Return @var{x} raised to the power of @var{y}. This
1354 procedure does not accept complex arguments.
1357 @c begin (texi-doc-string "guile" "$sin")
1358 @deffn {Scheme Procedure} $sin x
1359 Return the sine of @var{x}.
1362 @c begin (texi-doc-string "guile" "$cos")
1363 @deffn {Scheme Procedure} $cos x
1364 Return the cosine of @var{x}.
1367 @c begin (texi-doc-string "guile" "$tan")
1368 @deffn {Scheme Procedure} $tan x
1369 Return the tangent of @var{x}.
1372 @c begin (texi-doc-string "guile" "$asin")
1373 @deffn {Scheme Procedure} $asin x
1374 Return the arcsine of @var{x}.
1377 @c begin (texi-doc-string "guile" "$acos")
1378 @deffn {Scheme Procedure} $acos x
1379 Return the arccosine of @var{x}.
1382 @c begin (texi-doc-string "guile" "$atan")
1383 @deffn {Scheme Procedure} $atan x
1384 Return the arctangent of @var{x} in the range @minus{}@math{PI/2} to
1388 @deffn {Scheme Procedure} $atan2 x y
1389 @deffnx {C Function} scm_sys_atan2 (x, y)
1390 Return the arc tangent of the two arguments @var{x} and
1391 @var{y}. This is similar to calculating the arc tangent of
1392 @var{x} / @var{y}, except that the signs of both arguments
1393 are used to determine the quadrant of the result. This
1394 procedure does not accept complex arguments.
1397 @c begin (texi-doc-string "guile" "$exp")
1398 @deffn {Scheme Procedure} $exp x
1399 Return e to the power of @var{x}, where e is the base of natural
1400 logarithms (2.71828@dots{}).
1403 @c begin (texi-doc-string "guile" "$log")
1404 @deffn {Scheme Procedure} $log x
1405 Return the natural logarithm of @var{x}.
1408 @c begin (texi-doc-string "guile" "$sinh")
1409 @deffn {Scheme Procedure} $sinh x
1410 Return the hyperbolic sine of @var{x}.
1413 @c begin (texi-doc-string "guile" "$cosh")
1414 @deffn {Scheme Procedure} $cosh x
1415 Return the hyperbolic cosine of @var{x}.
1418 @c begin (texi-doc-string "guile" "$tanh")
1419 @deffn {Scheme Procedure} $tanh x
1420 Return the hyperbolic tangent of @var{x}.
1423 @c begin (texi-doc-string "guile" "$asinh")
1424 @deffn {Scheme Procedure} $asinh x
1425 Return the hyperbolic arcsine of @var{x}.
1428 @c begin (texi-doc-string "guile" "$acosh")
1429 @deffn {Scheme Procedure} $acosh x
1430 Return the hyperbolic arccosine of @var{x}.
1433 @c begin (texi-doc-string "guile" "$atanh")
1434 @deffn {Scheme Procedure} $atanh x
1435 Return the hyperbolic arctangent of @var{x}.
1438 C functions for the above are provided by the standard mathematics
1439 library. Naturally these expect and return @code{double} arguments
1440 (@pxref{Mathematics,,, libc, GNU C Library Reference Manual}).
1442 @multitable {xx} {Scheme Procedure} {C Function}
1443 @item @tab Scheme Procedure @tab C Function
1445 @item @tab @code{$abs} @tab @code{fabs}
1446 @item @tab @code{$sqrt} @tab @code{sqrt}
1447 @item @tab @code{$sin} @tab @code{sin}
1448 @item @tab @code{$cos} @tab @code{cos}
1449 @item @tab @code{$tan} @tab @code{tan}
1450 @item @tab @code{$asin} @tab @code{asin}
1451 @item @tab @code{$acos} @tab @code{acos}
1452 @item @tab @code{$atan} @tab @code{atan}
1453 @item @tab @code{$atan2} @tab @code{atan2}
1454 @item @tab @code{$exp} @tab @code{exp}
1455 @item @tab @code{$expt} @tab @code{pow}
1456 @item @tab @code{$log} @tab @code{log}
1457 @item @tab @code{$sinh} @tab @code{sinh}
1458 @item @tab @code{$cosh} @tab @code{cosh}
1459 @item @tab @code{$tanh} @tab @code{tanh}
1460 @item @tab @code{$asinh} @tab @code{asinh}
1461 @item @tab @code{$acosh} @tab @code{acosh}
1462 @item @tab @code{$atanh} @tab @code{atanh}
1465 @code{asinh}, @code{acosh} and @code{atanh} are C99 standard but might
1466 not be available on older systems. Guile provides the following
1467 equivalents (on all systems).
1469 @deftypefn {C Function} double scm_asinh (double x)
1470 @deftypefnx {C Function} double scm_acosh (double x)
1471 @deftypefnx {C Function} double scm_atanh (double x)
1472 Return the hyperbolic arcsine, arccosine or arctangent of @var{x}
1477 @node Bitwise Operations
1478 @subsubsection Bitwise Operations
1480 For the following bitwise functions, negative numbers are treated as
1481 infinite precision twos-complements. For instance @math{-6} is bits
1482 @math{@dots{}111010}, with infinitely many ones on the left. It can
1483 be seen that adding 6 (binary 110) to such a bit pattern gives all
1486 @deffn {Scheme Procedure} logand n1 n2 @dots{}
1487 @deffnx {C Function} scm_logand (n1, n2)
1488 Return the bitwise @sc{and} of the integer arguments.
1491 (logand) @result{} -1
1492 (logand 7) @result{} 7
1493 (logand #b111 #b011 #b001) @result{} 1
1497 @deffn {Scheme Procedure} logior n1 n2 @dots{}
1498 @deffnx {C Function} scm_logior (n1, n2)
1499 Return the bitwise @sc{or} of the integer arguments.
1502 (logior) @result{} 0
1503 (logior 7) @result{} 7
1504 (logior #b000 #b001 #b011) @result{} 3
1508 @deffn {Scheme Procedure} logxor n1 n2 @dots{}
1509 @deffnx {C Function} scm_loxor (n1, n2)
1510 Return the bitwise @sc{xor} of the integer arguments. A bit is
1511 set in the result if it is set in an odd number of arguments.
1514 (logxor) @result{} 0
1515 (logxor 7) @result{} 7
1516 (logxor #b000 #b001 #b011) @result{} 2
1517 (logxor #b000 #b001 #b011 #b011) @result{} 1
1521 @deffn {Scheme Procedure} lognot n
1522 @deffnx {C Function} scm_lognot (n)
1523 Return the integer which is the ones-complement of the integer
1524 argument, ie.@: each 0 bit is changed to 1 and each 1 bit to 0.
1527 (number->string (lognot #b10000000) 2)
1528 @result{} "-10000001"
1529 (number->string (lognot #b0) 2)
1534 @deffn {Scheme Procedure} logtest j k
1535 @deffnx {C Function} scm_logtest (j, k)
1536 Test whether @var{j} and @var{k} have any 1 bits in common. This is
1537 equivalent to @code{(not (zero? (logand j k)))}, but without actually
1538 calculating the @code{logand}, just testing for non-zero.
1541 (logtest #b0100 #b1011) @result{} #f
1542 (logtest #b0100 #b0111) @result{} #t
1546 @deffn {Scheme Procedure} logbit? index j
1547 @deffnx {C Function} scm_logbit_p (index, j)
1548 Test whether bit number @var{index} in @var{j} is set. @var{index}
1549 starts from 0 for the least significant bit.
1552 (logbit? 0 #b1101) @result{} #t
1553 (logbit? 1 #b1101) @result{} #f
1554 (logbit? 2 #b1101) @result{} #t
1555 (logbit? 3 #b1101) @result{} #t
1556 (logbit? 4 #b1101) @result{} #f
1560 @deffn {Scheme Procedure} ash n cnt
1561 @deffnx {C Function} scm_ash (n, cnt)
1562 Return @var{n} shifted left by @var{cnt} bits, or shifted right if
1563 @var{cnt} is negative. This is an ``arithmetic'' shift.
1565 This is effectively a multiplication by @m{2^{cnt}, 2^@var{cnt}}, and
1566 when @var{cnt} is negative it's a division, rounded towards negative
1567 infinity. (Note that this is not the same rounding as @code{quotient}
1570 With @var{n} viewed as an infinite precision twos complement,
1571 @code{ash} means a left shift introducing zero bits, or a right shift
1575 (number->string (ash #b1 3) 2) @result{} "1000"
1576 (number->string (ash #b1010 -1) 2) @result{} "101"
1578 ;; -23 is bits ...11101001, -6 is bits ...111010
1579 (ash -23 -2) @result{} -6
1583 @deffn {Scheme Procedure} logcount n
1584 @deffnx {C Function} scm_logcount (n)
1585 Return the number of bits in integer @var{n}. If @var{n} is
1586 positive, the 1-bits in its binary representation are counted.
1587 If negative, the 0-bits in its two's-complement binary
1588 representation are counted. If zero, 0 is returned.
1591 (logcount #b10101010)
1600 @deffn {Scheme Procedure} integer-length n
1601 @deffnx {C Function} scm_integer_length (n)
1602 Return the number of bits necessary to represent @var{n}.
1604 For positive @var{n} this is how many bits to the most significant one
1605 bit. For negative @var{n} it's how many bits to the most significant
1606 zero bit in twos complement form.
1609 (integer-length #b10101010) @result{} 8
1610 (integer-length #b1111) @result{} 4
1611 (integer-length 0) @result{} 0
1612 (integer-length -1) @result{} 0
1613 (integer-length -256) @result{} 8
1614 (integer-length -257) @result{} 9
1618 @deffn {Scheme Procedure} integer-expt n k
1619 @deffnx {C Function} scm_integer_expt (n, k)
1620 Return @var{n} raised to the power @var{k}. @var{k} must be an exact
1621 integer, @var{n} can be any number.
1623 Negative @var{k} is supported, and results in @m{1/n^|k|, 1/n^abs(k)}
1624 in the usual way. @math{@var{n}^0} is 1, as usual, and that includes
1628 (integer-expt 2 5) @result{} 32
1629 (integer-expt -3 3) @result{} -27
1630 (integer-expt 5 -3) @result{} 1/125
1631 (integer-expt 0 0) @result{} 1
1635 @deffn {Scheme Procedure} bit-extract n start end
1636 @deffnx {C Function} scm_bit_extract (n, start, end)
1637 Return the integer composed of the @var{start} (inclusive)
1638 through @var{end} (exclusive) bits of @var{n}. The
1639 @var{start}th bit becomes the 0-th bit in the result.
1642 (number->string (bit-extract #b1101101010 0 4) 2)
1644 (number->string (bit-extract #b1101101010 4 9) 2)
1651 @subsubsection Random Number Generation
1653 Pseudo-random numbers are generated from a random state object, which
1654 can be created with @code{seed->random-state}. The @var{state}
1655 parameter to the various functions below is optional, it defaults to
1656 the state object in the @code{*random-state*} variable.
1658 @deffn {Scheme Procedure} copy-random-state [state]
1659 @deffnx {C Function} scm_copy_random_state (state)
1660 Return a copy of the random state @var{state}.
1663 @deffn {Scheme Procedure} random n [state]
1664 @deffnx {C Function} scm_random (n, state)
1665 Return a number in [0, @var{n}).
1667 Accepts a positive integer or real n and returns a
1668 number of the same type between zero (inclusive) and
1669 @var{n} (exclusive). The values returned have a uniform
1673 @deffn {Scheme Procedure} random:exp [state]
1674 @deffnx {C Function} scm_random_exp (state)
1675 Return an inexact real in an exponential distribution with mean
1676 1. For an exponential distribution with mean @var{u} use @code{(*
1677 @var{u} (random:exp))}.
1680 @deffn {Scheme Procedure} random:hollow-sphere! vect [state]
1681 @deffnx {C Function} scm_random_hollow_sphere_x (vect, state)
1682 Fills @var{vect} with inexact real random numbers the sum of whose
1683 squares is equal to 1.0. Thinking of @var{vect} as coordinates in
1684 space of dimension @var{n} @math{=} @code{(vector-length @var{vect})},
1685 the coordinates are uniformly distributed over the surface of the unit
1689 @deffn {Scheme Procedure} random:normal [state]
1690 @deffnx {C Function} scm_random_normal (state)
1691 Return an inexact real in a normal distribution. The distribution
1692 used has mean 0 and standard deviation 1. For a normal distribution
1693 with mean @var{m} and standard deviation @var{d} use @code{(+ @var{m}
1694 (* @var{d} (random:normal)))}.
1697 @deffn {Scheme Procedure} random:normal-vector! vect [state]
1698 @deffnx {C Function} scm_random_normal_vector_x (vect, state)
1699 Fills @var{vect} with inexact real random numbers that are
1700 independent and standard normally distributed
1701 (i.e., with mean 0 and variance 1).
1704 @deffn {Scheme Procedure} random:solid-sphere! vect [state]
1705 @deffnx {C Function} scm_random_solid_sphere_x (vect, state)
1706 Fills @var{vect} with inexact real random numbers the sum of whose
1707 squares is less than 1.0. Thinking of @var{vect} as coordinates in
1708 space of dimension @var{n} @math{=} @code{(vector-length @var{vect})},
1709 the coordinates are uniformly distributed within the unit
1711 @c FIXME: What does this mean, particularly the n-sphere part?
1714 @deffn {Scheme Procedure} random:uniform [state]
1715 @deffnx {C Function} scm_random_uniform (state)
1716 Return a uniformly distributed inexact real random number in
1720 @deffn {Scheme Procedure} seed->random-state seed
1721 @deffnx {C Function} scm_seed_to_random_state (seed)
1722 Return a new random state using @var{seed}.
1725 @defvar *random-state*
1726 The global random state used by the above functions when the
1727 @var{state} parameter is not given.
1730 Note that the initial value of @code{*random-state*} is the same every
1731 time Guile starts up. Therefore, if you don't pass a @var{state}
1732 parameter to the above procedures, and you don't set
1733 @code{*random-state*} to @code{(seed->random-state your-seed)}, where
1734 @code{your-seed} is something that @emph{isn't} the same every time,
1735 you'll get the same sequence of ``random'' numbers on every run.
1737 For example, unless the relevant source code has changed, @code{(map
1738 random (cdr (iota 30)))}, if the first use of random numbers since
1739 Guile started up, will always give:
1742 (map random (cdr (iota 19)))
1744 (0 1 1 2 2 2 1 2 6 7 10 0 5 3 12 5 5 12)
1747 To use the time of day as the random seed, you can use code like this:
1750 (let ((time (gettimeofday)))
1751 (set! *random-state*
1752 (seed->random-state (+ (car time)
1757 And then (depending on the time of day, of course):
1760 (map random (cdr (iota 19)))
1762 (0 0 1 0 2 4 5 4 5 5 9 3 10 1 8 3 14 17)
1765 For security applications, such as password generation, you should use
1766 more bits of seed. Otherwise an open source password generator could
1767 be attacked by guessing the seed@dots{} but that's a subject for
1772 @subsection Characters
1775 In Scheme, a character literal is written as @code{#\@var{name}} where
1776 @var{name} is the name of the character that you want. Printable
1777 characters have their usual single character name; for example,
1778 @code{#\a} is a lower case @code{a}.
1780 Most of the ``control characters'' (those below codepoint 32) in the
1781 @acronym{ASCII} character set, as well as the space, may be referred
1782 to by longer names: for example, @code{#\tab}, @code{#\esc},
1783 @code{#\stx}, and so on. The following table describes the
1784 @acronym{ASCII} names for each character.
1786 @multitable @columnfractions .25 .25 .25 .25
1787 @item 0 = @code{#\nul}
1788 @tab 1 = @code{#\soh}
1789 @tab 2 = @code{#\stx}
1790 @tab 3 = @code{#\etx}
1791 @item 4 = @code{#\eot}
1792 @tab 5 = @code{#\enq}
1793 @tab 6 = @code{#\ack}
1794 @tab 7 = @code{#\bel}
1795 @item 8 = @code{#\bs}
1796 @tab 9 = @code{#\ht}
1797 @tab 10 = @code{#\nl}
1798 @tab 11 = @code{#\vt}
1799 @item 12 = @code{#\np}
1800 @tab 13 = @code{#\cr}
1801 @tab 14 = @code{#\so}
1802 @tab 15 = @code{#\si}
1803 @item 16 = @code{#\dle}
1804 @tab 17 = @code{#\dc1}
1805 @tab 18 = @code{#\dc2}
1806 @tab 19 = @code{#\dc3}
1807 @item 20 = @code{#\dc4}
1808 @tab 21 = @code{#\nak}
1809 @tab 22 = @code{#\syn}
1810 @tab 23 = @code{#\etb}
1811 @item 24 = @code{#\can}
1812 @tab 25 = @code{#\em}
1813 @tab 26 = @code{#\sub}
1814 @tab 27 = @code{#\esc}
1815 @item 28 = @code{#\fs}
1816 @tab 29 = @code{#\gs}
1817 @tab 30 = @code{#\rs}
1818 @tab 31 = @code{#\us}
1819 @item 32 = @code{#\sp}
1822 The ``delete'' character (octal 177) may be referred to with the name
1825 Several characters have more than one name:
1827 @multitable {@code{#\backspace}} {Original}
1828 @item Alias @tab Original
1829 @item @code{#\space} @tab @code{#\sp}
1830 @item @code{#\newline} @tab @code{#\nl}
1831 @item @code{#\tab} @tab @code{#\ht}
1832 @item @code{#\backspace} @tab @code{#\bs}
1833 @item @code{#\return} @tab @code{#\cr}
1834 @item @code{#\page} @tab @code{#\np}
1835 @item @code{#\null} @tab @code{#\nul}
1839 @deffn {Scheme Procedure} char? x
1840 @deffnx {C Function} scm_char_p (x)
1841 Return @code{#t} iff @var{x} is a character, else @code{#f}.
1845 @deffn {Scheme Procedure} char=? x y
1846 Return @code{#t} iff @var{x} is the same character as @var{y}, else @code{#f}.
1850 @deffn {Scheme Procedure} char<? x y
1851 Return @code{#t} iff @var{x} is less than @var{y} in the @acronym{ASCII} sequence,
1856 @deffn {Scheme Procedure} char<=? x y
1857 Return @code{#t} iff @var{x} is less than or equal to @var{y} in the
1858 @acronym{ASCII} sequence, else @code{#f}.
1862 @deffn {Scheme Procedure} char>? x y
1863 Return @code{#t} iff @var{x} is greater than @var{y} in the @acronym{ASCII}
1864 sequence, else @code{#f}.
1868 @deffn {Scheme Procedure} char>=? x y
1869 Return @code{#t} iff @var{x} is greater than or equal to @var{y} in the
1870 @acronym{ASCII} sequence, else @code{#f}.
1874 @deffn {Scheme Procedure} char-ci=? x y
1875 Return @code{#t} iff @var{x} is the same character as @var{y} ignoring
1876 case, else @code{#f}.
1880 @deffn {Scheme Procedure} char-ci<? x y
1881 Return @code{#t} iff @var{x} is less than @var{y} in the @acronym{ASCII} sequence
1882 ignoring case, else @code{#f}.
1886 @deffn {Scheme Procedure} char-ci<=? x y
1887 Return @code{#t} iff @var{x} is less than or equal to @var{y} in the
1888 @acronym{ASCII} sequence ignoring case, else @code{#f}.
1892 @deffn {Scheme Procedure} char-ci>? x y
1893 Return @code{#t} iff @var{x} is greater than @var{y} in the @acronym{ASCII}
1894 sequence ignoring case, else @code{#f}.
1898 @deffn {Scheme Procedure} char-ci>=? x y
1899 Return @code{#t} iff @var{x} is greater than or equal to @var{y} in the
1900 @acronym{ASCII} sequence ignoring case, else @code{#f}.
1903 @rnindex char-alphabetic?
1904 @deffn {Scheme Procedure} char-alphabetic? chr
1905 @deffnx {C Function} scm_char_alphabetic_p (chr)
1906 Return @code{#t} iff @var{chr} is alphabetic, else @code{#f}.
1909 @rnindex char-numeric?
1910 @deffn {Scheme Procedure} char-numeric? chr
1911 @deffnx {C Function} scm_char_numeric_p (chr)
1912 Return @code{#t} iff @var{chr} is numeric, else @code{#f}.
1915 @rnindex char-whitespace?
1916 @deffn {Scheme Procedure} char-whitespace? chr
1917 @deffnx {C Function} scm_char_whitespace_p (chr)
1918 Return @code{#t} iff @var{chr} is whitespace, else @code{#f}.
1921 @rnindex char-upper-case?
1922 @deffn {Scheme Procedure} char-upper-case? chr
1923 @deffnx {C Function} scm_char_upper_case_p (chr)
1924 Return @code{#t} iff @var{chr} is uppercase, else @code{#f}.
1927 @rnindex char-lower-case?
1928 @deffn {Scheme Procedure} char-lower-case? chr
1929 @deffnx {C Function} scm_char_lower_case_p (chr)
1930 Return @code{#t} iff @var{chr} is lowercase, else @code{#f}.
1933 @deffn {Scheme Procedure} char-is-both? chr
1934 @deffnx {C Function} scm_char_is_both_p (chr)
1935 Return @code{#t} iff @var{chr} is either uppercase or lowercase, else
1939 @rnindex char->integer
1940 @deffn {Scheme Procedure} char->integer chr
1941 @deffnx {C Function} scm_char_to_integer (chr)
1942 Return the number corresponding to ordinal position of @var{chr} in the
1943 @acronym{ASCII} sequence.
1946 @rnindex integer->char
1947 @deffn {Scheme Procedure} integer->char n
1948 @deffnx {C Function} scm_integer_to_char (n)
1949 Return the character at position @var{n} in the @acronym{ASCII} sequence.
1952 @rnindex char-upcase
1953 @deffn {Scheme Procedure} char-upcase chr
1954 @deffnx {C Function} scm_char_upcase (chr)
1955 Return the uppercase character version of @var{chr}.
1958 @rnindex char-downcase
1959 @deffn {Scheme Procedure} char-downcase chr
1960 @deffnx {C Function} scm_char_downcase (chr)
1961 Return the lowercase character version of @var{chr}.
1964 @node Character Sets
1965 @subsection Character Sets
1967 The features described in this section correspond directly to SRFI-14.
1969 The data type @dfn{charset} implements sets of characters
1970 (@pxref{Characters}). Because the internal representation of
1971 character sets is not visible to the user, a lot of procedures for
1972 handling them are provided.
1974 Character sets can be created, extended, tested for the membership of a
1975 characters and be compared to other character sets.
1977 The Guile implementation of character sets currently deals only with
1978 8-bit characters. In the future, when Guile gets support for
1979 international character sets, this will change, but the functions
1980 provided here will always then be able to efficiently cope with very
1981 large character sets.
1984 * Character Set Predicates/Comparison::
1985 * Iterating Over Character Sets:: Enumerate charset elements.
1986 * Creating Character Sets:: Making new charsets.
1987 * Querying Character Sets:: Test charsets for membership etc.
1988 * Character-Set Algebra:: Calculating new charsets.
1989 * Standard Character Sets:: Variables containing predefined charsets.
1992 @node Character Set Predicates/Comparison
1993 @subsubsection Character Set Predicates/Comparison
1995 Use these procedures for testing whether an object is a character set,
1996 or whether several character sets are equal or subsets of each other.
1997 @code{char-set-hash} can be used for calculating a hash value, maybe for
1998 usage in fast lookup procedures.
2000 @deffn {Scheme Procedure} char-set? obj
2001 @deffnx {C Function} scm_char_set_p (obj)
2002 Return @code{#t} if @var{obj} is a character set, @code{#f}
2006 @deffn {Scheme Procedure} char-set= . char_sets
2007 @deffnx {C Function} scm_char_set_eq (char_sets)
2008 Return @code{#t} if all given character sets are equal.
2011 @deffn {Scheme Procedure} char-set<= . char_sets
2012 @deffnx {C Function} scm_char_set_leq (char_sets)
2013 Return @code{#t} if every character set @var{cs}i is a subset
2014 of character set @var{cs}i+1.
2017 @deffn {Scheme Procedure} char-set-hash cs [bound]
2018 @deffnx {C Function} scm_char_set_hash (cs, bound)
2019 Compute a hash value for the character set @var{cs}. If
2020 @var{bound} is given and non-zero, it restricts the
2021 returned value to the range 0 @dots{} @var{bound - 1}.
2024 @c ===================================================================
2026 @node Iterating Over Character Sets
2027 @subsubsection Iterating Over Character Sets
2029 Character set cursors are a means for iterating over the members of a
2030 character sets. After creating a character set cursor with
2031 @code{char-set-cursor}, a cursor can be dereferenced with
2032 @code{char-set-ref}, advanced to the next member with
2033 @code{char-set-cursor-next}. Whether a cursor has passed past the last
2034 element of the set can be checked with @code{end-of-char-set?}.
2036 Additionally, mapping and (un-)folding procedures for character sets are
2039 @deffn {Scheme Procedure} char-set-cursor cs
2040 @deffnx {C Function} scm_char_set_cursor (cs)
2041 Return a cursor into the character set @var{cs}.
2044 @deffn {Scheme Procedure} char-set-ref cs cursor
2045 @deffnx {C Function} scm_char_set_ref (cs, cursor)
2046 Return the character at the current cursor position
2047 @var{cursor} in the character set @var{cs}. It is an error to
2048 pass a cursor for which @code{end-of-char-set?} returns true.
2051 @deffn {Scheme Procedure} char-set-cursor-next cs cursor
2052 @deffnx {C Function} scm_char_set_cursor_next (cs, cursor)
2053 Advance the character set cursor @var{cursor} to the next
2054 character in the character set @var{cs}. It is an error if the
2055 cursor given satisfies @code{end-of-char-set?}.
2058 @deffn {Scheme Procedure} end-of-char-set? cursor
2059 @deffnx {C Function} scm_end_of_char_set_p (cursor)
2060 Return @code{#t} if @var{cursor} has reached the end of a
2061 character set, @code{#f} otherwise.
2064 @deffn {Scheme Procedure} char-set-fold kons knil cs
2065 @deffnx {C Function} scm_char_set_fold (kons, knil, cs)
2066 Fold the procedure @var{kons} over the character set @var{cs},
2067 initializing it with @var{knil}.
2070 @deffn {Scheme Procedure} char-set-unfold p f g seed [base_cs]
2071 @deffnx {C Function} scm_char_set_unfold (p, f, g, seed, base_cs)
2072 This is a fundamental constructor for character sets.
2074 @item @var{g} is used to generate a series of ``seed'' values
2075 from the initial seed: @var{seed}, (@var{g} @var{seed}),
2076 (@var{g}^2 @var{seed}), (@var{g}^3 @var{seed}), @dots{}
2077 @item @var{p} tells us when to stop -- when it returns true
2078 when applied to one of the seed values.
2079 @item @var{f} maps each seed value to a character. These
2080 characters are added to the base character set @var{base_cs} to
2081 form the result; @var{base_cs} defaults to the empty set.
2085 @deffn {Scheme Procedure} char-set-unfold! p f g seed base_cs
2086 @deffnx {C Function} scm_char_set_unfold_x (p, f, g, seed, base_cs)
2087 This is a fundamental constructor for character sets.
2089 @item @var{g} is used to generate a series of ``seed'' values
2090 from the initial seed: @var{seed}, (@var{g} @var{seed}),
2091 (@var{g}^2 @var{seed}), (@var{g}^3 @var{seed}), @dots{}
2092 @item @var{p} tells us when to stop -- when it returns true
2093 when applied to one of the seed values.
2094 @item @var{f} maps each seed value to a character. These
2095 characters are added to the base character set @var{base_cs} to
2096 form the result; @var{base_cs} defaults to the empty set.
2100 @deffn {Scheme Procedure} char-set-for-each proc cs
2101 @deffnx {C Function} scm_char_set_for_each (proc, cs)
2102 Apply @var{proc} to every character in the character set
2103 @var{cs}. The return value is not specified.
2106 @deffn {Scheme Procedure} char-set-map proc cs
2107 @deffnx {C Function} scm_char_set_map (proc, cs)
2108 Map the procedure @var{proc} over every character in @var{cs}.
2109 @var{proc} must be a character -> character procedure.
2112 @c ===================================================================
2114 @node Creating Character Sets
2115 @subsubsection Creating Character Sets
2117 New character sets are produced with these procedures.
2119 @deffn {Scheme Procedure} char-set-copy cs
2120 @deffnx {C Function} scm_char_set_copy (cs)
2121 Return a newly allocated character set containing all
2122 characters in @var{cs}.
2125 @deffn {Scheme Procedure} char-set . rest
2126 @deffnx {C Function} scm_char_set (rest)
2127 Return a character set containing all given characters.
2130 @deffn {Scheme Procedure} list->char-set list [base_cs]
2131 @deffnx {C Function} scm_list_to_char_set (list, base_cs)
2132 Convert the character list @var{list} to a character set. If
2133 the character set @var{base_cs} is given, the character in this
2134 set are also included in the result.
2137 @deffn {Scheme Procedure} list->char-set! list base_cs
2138 @deffnx {C Function} scm_list_to_char_set_x (list, base_cs)
2139 Convert the character list @var{list} to a character set. The
2140 characters are added to @var{base_cs} and @var{base_cs} is
2144 @deffn {Scheme Procedure} string->char-set str [base_cs]
2145 @deffnx {C Function} scm_string_to_char_set (str, base_cs)
2146 Convert the string @var{str} to a character set. If the
2147 character set @var{base_cs} is given, the characters in this
2148 set are also included in the result.
2151 @deffn {Scheme Procedure} string->char-set! str base_cs
2152 @deffnx {C Function} scm_string_to_char_set_x (str, base_cs)
2153 Convert the string @var{str} to a character set. The
2154 characters from the string are added to @var{base_cs}, and
2155 @var{base_cs} is returned.
2158 @deffn {Scheme Procedure} char-set-filter pred cs [base_cs]
2159 @deffnx {C Function} scm_char_set_filter (pred, cs, base_cs)
2160 Return a character set containing every character from @var{cs}
2161 so that it satisfies @var{pred}. If provided, the characters
2162 from @var{base_cs} are added to the result.
2165 @deffn {Scheme Procedure} char-set-filter! pred cs base_cs
2166 @deffnx {C Function} scm_char_set_filter_x (pred, cs, base_cs)
2167 Return a character set containing every character from @var{cs}
2168 so that it satisfies @var{pred}. The characters are added to
2169 @var{base_cs} and @var{base_cs} is returned.
2172 @deffn {Scheme Procedure} ucs-range->char-set lower upper [error [base_cs]]
2173 @deffnx {C Function} scm_ucs_range_to_char_set (lower, upper, error, base_cs)
2174 Return a character set containing all characters whose
2175 character codes lie in the half-open range
2176 [@var{lower},@var{upper}).
2178 If @var{error} is a true value, an error is signalled if the
2179 specified range contains characters which are not contained in
2180 the implemented character range. If @var{error} is @code{#f},
2181 these characters are silently left out of the resultung
2184 The characters in @var{base_cs} are added to the result, if
2188 @deffn {Scheme Procedure} ucs-range->char-set! lower upper error base_cs
2189 @deffnx {C Function} scm_ucs_range_to_char_set_x (lower, upper, error, base_cs)
2190 Return a character set containing all characters whose
2191 character codes lie in the half-open range
2192 [@var{lower},@var{upper}).
2194 If @var{error} is a true value, an error is signalled if the
2195 specified range contains characters which are not contained in
2196 the implemented character range. If @var{error} is @code{#f},
2197 these characters are silently left out of the resultung
2200 The characters are added to @var{base_cs} and @var{base_cs} is
2204 @deffn {Scheme Procedure} ->char-set x
2205 @deffnx {C Function} scm_to_char_set (x)
2206 Coerces x into a char-set. @var{x} may be a string, character or char-set. A string is converted to the set of its constituent characters; a character is converted to a singleton set; a char-set is returned as-is.
2209 @c ===================================================================
2211 @node Querying Character Sets
2212 @subsubsection Querying Character Sets
2214 Access the elements and other information of a character set with these
2217 @deffn {Scheme Procedure} char-set-size cs
2218 @deffnx {C Function} scm_char_set_size (cs)
2219 Return the number of elements in character set @var{cs}.
2222 @deffn {Scheme Procedure} char-set-count pred cs
2223 @deffnx {C Function} scm_char_set_count (pred, cs)
2224 Return the number of the elements int the character set
2225 @var{cs} which satisfy the predicate @var{pred}.
2228 @deffn {Scheme Procedure} char-set->list cs
2229 @deffnx {C Function} scm_char_set_to_list (cs)
2230 Return a list containing the elements of the character set
2234 @deffn {Scheme Procedure} char-set->string cs
2235 @deffnx {C Function} scm_char_set_to_string (cs)
2236 Return a string containing the elements of the character set
2237 @var{cs}. The order in which the characters are placed in the
2238 string is not defined.
2241 @deffn {Scheme Procedure} char-set-contains? cs ch
2242 @deffnx {C Function} scm_char_set_contains_p (cs, ch)
2243 Return @code{#t} iff the character @var{ch} is contained in the
2244 character set @var{cs}.
2247 @deffn {Scheme Procedure} char-set-every pred cs
2248 @deffnx {C Function} scm_char_set_every (pred, cs)
2249 Return a true value if every character in the character set
2250 @var{cs} satisfies the predicate @var{pred}.
2253 @deffn {Scheme Procedure} char-set-any pred cs
2254 @deffnx {C Function} scm_char_set_any (pred, cs)
2255 Return a true value if any character in the character set
2256 @var{cs} satisfies the predicate @var{pred}.
2259 @c ===================================================================
2261 @node Character-Set Algebra
2262 @subsubsection Character-Set Algebra
2264 Character sets can be manipulated with the common set algebra operation,
2265 such as union, complement, intersection etc. All of these procedures
2266 provide side-effecting variants, which modify their character set
2269 @deffn {Scheme Procedure} char-set-adjoin cs . rest
2270 @deffnx {C Function} scm_char_set_adjoin (cs, rest)
2271 Add all character arguments to the first argument, which must
2275 @deffn {Scheme Procedure} char-set-delete cs . rest
2276 @deffnx {C Function} scm_char_set_delete (cs, rest)
2277 Delete all character arguments from the first argument, which
2278 must be a character set.
2281 @deffn {Scheme Procedure} char-set-adjoin! cs . rest
2282 @deffnx {C Function} scm_char_set_adjoin_x (cs, rest)
2283 Add all character arguments to the first argument, which must
2287 @deffn {Scheme Procedure} char-set-delete! cs . rest
2288 @deffnx {C Function} scm_char_set_delete_x (cs, rest)
2289 Delete all character arguments from the first argument, which
2290 must be a character set.
2293 @deffn {Scheme Procedure} char-set-complement cs
2294 @deffnx {C Function} scm_char_set_complement (cs)
2295 Return the complement of the character set @var{cs}.
2298 @deffn {Scheme Procedure} char-set-union . rest
2299 @deffnx {C Function} scm_char_set_union (rest)
2300 Return the union of all argument character sets.
2303 @deffn {Scheme Procedure} char-set-intersection . rest
2304 @deffnx {C Function} scm_char_set_intersection (rest)
2305 Return the intersection of all argument character sets.
2308 @deffn {Scheme Procedure} char-set-difference cs1 . rest
2309 @deffnx {C Function} scm_char_set_difference (cs1, rest)
2310 Return the difference of all argument character sets.
2313 @deffn {Scheme Procedure} char-set-xor . rest
2314 @deffnx {C Function} scm_char_set_xor (rest)
2315 Return the exclusive-or of all argument character sets.
2318 @deffn {Scheme Procedure} char-set-diff+intersection cs1 . rest
2319 @deffnx {C Function} scm_char_set_diff_plus_intersection (cs1, rest)
2320 Return the difference and the intersection of all argument
2324 @deffn {Scheme Procedure} char-set-complement! cs
2325 @deffnx {C Function} scm_char_set_complement_x (cs)
2326 Return the complement of the character set @var{cs}.
2329 @deffn {Scheme Procedure} char-set-union! cs1 . rest
2330 @deffnx {C Function} scm_char_set_union_x (cs1, rest)
2331 Return the union of all argument character sets.
2334 @deffn {Scheme Procedure} char-set-intersection! cs1 . rest
2335 @deffnx {C Function} scm_char_set_intersection_x (cs1, rest)
2336 Return the intersection of all argument character sets.
2339 @deffn {Scheme Procedure} char-set-difference! cs1 . rest
2340 @deffnx {C Function} scm_char_set_difference_x (cs1, rest)
2341 Return the difference of all argument character sets.
2344 @deffn {Scheme Procedure} char-set-xor! cs1 . rest
2345 @deffnx {C Function} scm_char_set_xor_x (cs1, rest)
2346 Return the exclusive-or of all argument character sets.
2349 @deffn {Scheme Procedure} char-set-diff+intersection! cs1 cs2 . rest
2350 @deffnx {C Function} scm_char_set_diff_plus_intersection_x (cs1, cs2, rest)
2351 Return the difference and the intersection of all argument
2355 @c ===================================================================
2357 @node Standard Character Sets
2358 @subsubsection Standard Character Sets
2360 In order to make the use of the character set data type and procedures
2361 useful, several predefined character set variables exist.
2367 Currently, the contents of these character sets are recomputed upon a
2368 successful @code{setlocale} call (@pxref{Locales}) in order to reflect
2369 the characters available in the current locale's codeset. For
2370 instance, @code{char-set:letter} contains 52 characters under an ASCII
2371 locale (e.g., the default @code{C} locale) and 117 characters under an
2372 ISO-8859-1 (``Latin-1'') locale.
2374 @defvr {Scheme Variable} char-set:lower-case
2375 @defvrx {C Variable} scm_char_set_lower_case
2376 All lower-case characters.
2379 @defvr {Scheme Variable} char-set:upper-case
2380 @defvrx {C Variable} scm_char_set_upper_case
2381 All upper-case characters.
2384 @defvr {Scheme Variable} char-set:title-case
2385 @defvrx {C Variable} scm_char_set_title_case
2386 This is empty, because ASCII has no titlecase characters.
2389 @defvr {Scheme Variable} char-set:letter
2390 @defvrx {C Variable} scm_char_set_letter
2391 All letters, e.g. the union of @code{char-set:lower-case} and
2392 @code{char-set:upper-case}.
2395 @defvr {Scheme Variable} char-set:digit
2396 @defvrx {C Variable} scm_char_set_digit
2400 @defvr {Scheme Variable} char-set:letter+digit
2401 @defvrx {C Variable} scm_char_set_letter_and_digit
2402 The union of @code{char-set:letter} and @code{char-set:digit}.
2405 @defvr {Scheme Variable} char-set:graphic
2406 @defvrx {C Variable} scm_char_set_graphic
2407 All characters which would put ink on the paper.
2410 @defvr {Scheme Variable} char-set:printing
2411 @defvrx {C Variable} scm_char_set_printing
2412 The union of @code{char-set:graphic} and @code{char-set:whitespace}.
2415 @defvr {Scheme Variable} char-set:whitespace
2416 @defvrx {C Variable} scm_char_set_whitespace
2417 All whitespace characters.
2420 @defvr {Scheme Variable} char-set:blank
2421 @defvrx {C Variable} scm_char_set_blank
2422 All horizontal whitespace characters, that is @code{#\space} and
2426 @defvr {Scheme Variable} char-set:iso-control
2427 @defvrx {C Variable} scm_char_set_iso_control
2428 The ISO control characters with the codes 0--31 and 127.
2431 @defvr {Scheme Variable} char-set:punctuation
2432 @defvrx {C Variable} scm_char_set_punctuation
2433 The characters @code{!"#%&'()*,-./:;?@@[\\]_@{@}}
2436 @defvr {Scheme Variable} char-set:symbol
2437 @defvrx {C Variable} scm_char_set_symbol
2438 The characters @code{$+<=>^`|~}.
2441 @defvr {Scheme Variable} char-set:hex-digit
2442 @defvrx {C Variable} scm_char_set_hex_digit
2443 The hexadecimal digits @code{0123456789abcdefABCDEF}.
2446 @defvr {Scheme Variable} char-set:ascii
2447 @defvrx {C Variable} scm_char_set_ascii
2448 All ASCII characters.
2451 @defvr {Scheme Variable} char-set:empty
2452 @defvrx {C Variable} scm_char_set_empty
2453 The empty character set.
2456 @defvr {Scheme Variable} char-set:full
2457 @defvrx {C Variable} scm_char_set_full
2458 This character set contains all possible characters.
2465 Strings are fixed-length sequences of characters. They can be created
2466 by calling constructor procedures, but they can also literally get
2467 entered at the @acronym{REPL} or in Scheme source files.
2469 @c Guile provides a rich set of string processing procedures, because text
2470 @c handling is very important when Guile is used as a scripting language.
2472 Strings always carry the information about how many characters they are
2473 composed of with them, so there is no special end-of-string character,
2474 like in C. That means that Scheme strings can contain any character,
2475 even the @samp{#\nul} character @samp{\0}.
2477 To use strings efficiently, you need to know a bit about how Guile
2478 implements them. In Guile, a string consists of two parts, a head and
2479 the actual memory where the characters are stored. When a string (or
2480 a substring of it) is copied, only a new head gets created, the memory
2481 is usually not copied. The two heads start out pointing to the same
2484 When one of these two strings is modified, as with @code{string-set!},
2485 their common memory does get copied so that each string has its own
2486 memory and modifying one does not accidently modify the other as well.
2487 Thus, Guile's strings are `copy on write'; the actual copying of their
2488 memory is delayed until one string is written to.
2490 This implementation makes functions like @code{substring} very
2491 efficient in the common case that no modifications are done to the
2494 If you do know that your strings are getting modified right away, you
2495 can use @code{substring/copy} instead of @code{substring}. This
2496 function performs the copy immediately at the time of creation. This
2497 is more efficient, especially in a multi-threaded program. Also,
2498 @code{substring/copy} can avoid the problem that a short substring
2499 holds on to the memory of a very large original string that could
2500 otherwise be recycled.
2502 If you want to avoid the copy altogether, so that modifications of one
2503 string show up in the other, you can use @code{substring/shared}. The
2504 strings created by this procedure are called @dfn{mutation sharing
2505 substrings} since the substring and the original string share
2506 modifications to each other.
2508 If you want to prevent modifications, use @code{substring/read-only}.
2510 Guile provides all procedures of SRFI-13 and a few more.
2513 * String Syntax:: Read syntax for strings.
2514 * String Predicates:: Testing strings for certain properties.
2515 * String Constructors:: Creating new string objects.
2516 * List/String Conversion:: Converting from/to lists of characters.
2517 * String Selection:: Select portions from strings.
2518 * String Modification:: Modify parts or whole strings.
2519 * String Comparison:: Lexicographic ordering predicates.
2520 * String Searching:: Searching in strings.
2521 * Alphabetic Case Mapping:: Convert the alphabetic case of strings.
2522 * Reversing and Appending Strings:: Appending strings to form a new string.
2523 * Mapping Folding and Unfolding:: Iterating over strings.
2524 * Miscellaneous String Operations:: Replicating, insertion, parsing, ...
2525 * Conversion to/from C::
2529 @subsubsection String Read Syntax
2531 @c In the following @code is used to get a good font in TeX etc, but
2532 @c is omitted for Info format, so as not to risk any confusion over
2533 @c whether surrounding ` ' quotes are part of the escape or are
2534 @c special in a string (they're not).
2536 The read syntax for strings is an arbitrarily long sequence of
2537 characters enclosed in double quotes (@nicode{"}).
2539 Backslash is an escape character and can be used to insert the
2540 following special characters. @nicode{\"} and @nicode{\\} are R5RS
2541 standard, the rest are Guile extensions, notice they follow C string
2546 Backslash character.
2549 Double quote character (an unescaped @nicode{"} is otherwise the end
2553 NUL character (ASCII 0).
2556 Bell character (ASCII 7).
2559 Formfeed character (ASCII 12).
2562 Newline character (ASCII 10).
2565 Carriage return character (ASCII 13).
2568 Tab character (ASCII 9).
2571 Vertical tab character (ASCII 11).
2574 Character code given by two hexadecimal digits. For example
2575 @nicode{\x7f} for an ASCII DEL (127).
2579 The following are examples of string literals:
2589 @node String Predicates
2590 @subsubsection String Predicates
2592 The following procedures can be used to check whether a given string
2593 fulfills some specified property.
2596 @deffn {Scheme Procedure} string? obj
2597 @deffnx {C Function} scm_string_p (obj)
2598 Return @code{#t} if @var{obj} is a string, else @code{#f}.
2601 @deftypefn {C Function} int scm_is_string (SCM obj)
2602 Returns @code{1} if @var{obj} is a string, @code{0} otherwise.
2605 @deffn {Scheme Procedure} string-null? str
2606 @deffnx {C Function} scm_string_null_p (str)
2607 Return @code{#t} if @var{str}'s length is zero, and
2608 @code{#f} otherwise.
2610 (string-null? "") @result{} #t
2612 (string-null? y) @result{} #f
2616 @deffn {Scheme Procedure} string-any char_pred s [start [end]]
2617 @deffnx {C Function} scm_string_any (char_pred, s, start, end)
2618 Check if @var{char_pred} is true for any character in string @var{s}.
2620 @var{char_pred} can be a character to check for any equal to that, or
2621 a character set (@pxref{Character Sets}) to check for any in that set,
2622 or a predicate procedure to call.
2624 For a procedure, calls @code{(@var{char_pred} c)} are made
2625 successively on the characters from @var{start} to @var{end}. If
2626 @var{char_pred} returns true (ie.@: non-@code{#f}), @code{string-any}
2627 stops and that return value is the return from @code{string-any}. The
2628 call on the last character (ie.@: at @math{@var{end}-1}), if that
2629 point is reached, is a tail call.
2631 If there are no characters in @var{s} (ie.@: @var{start} equals
2632 @var{end}) then the return is @code{#f}.
2635 @deffn {Scheme Procedure} string-every char_pred s [start [end]]
2636 @deffnx {C Function} scm_string_every (char_pred, s, start, end)
2637 Check if @var{char_pred} is true for every character in string
2640 @var{char_pred} can be a character to check for every character equal
2641 to that, or a character set (@pxref{Character Sets}) to check for
2642 every character being in that set, or a predicate procedure to call.
2644 For a procedure, calls @code{(@var{char_pred} c)} are made
2645 successively on the characters from @var{start} to @var{end}. If
2646 @var{char_pred} returns @code{#f}, @code{string-every} stops and
2647 returns @code{#f}. The call on the last character (ie.@: at
2648 @math{@var{end}-1}), if that point is reached, is a tail call and the
2649 return from that call is the return from @code{string-every}.
2651 If there are no characters in @var{s} (ie.@: @var{start} equals
2652 @var{end}) then the return is @code{#t}.
2655 @node String Constructors
2656 @subsubsection String Constructors
2658 The string constructor procedures create new string objects, possibly
2659 initializing them with some specified character data. See also
2660 @xref{String Selection}, for ways to create strings from existing
2663 @c FIXME::martin: list->string belongs into `List/String Conversion'
2665 @deffn {Scheme Procedure} string char@dots{}
2667 Return a newly allocated string made from the given character
2671 (string #\x #\y #\z) @result{} "xyz"
2672 (string) @result{} ""
2676 @deffn {Scheme Procedure} list->string lst
2677 @deffnx {C Function} scm_string (lst)
2678 @rnindex list->string
2679 Return a newly allocated string made from a list of characters.
2682 (list->string '(#\a #\b #\c)) @result{} "abc"
2686 @deffn {Scheme Procedure} reverse-list->string lst
2687 @deffnx {C Function} scm_reverse_list_to_string (lst)
2688 Return a newly allocated string made from a list of characters, in
2692 (reverse-list->string '(#\a #\B #\c)) @result{} "cBa"
2696 @rnindex make-string
2697 @deffn {Scheme Procedure} make-string k [chr]
2698 @deffnx {C Function} scm_make_string (k, chr)
2699 Return a newly allocated string of
2700 length @var{k}. If @var{chr} is given, then all elements of
2701 the string are initialized to @var{chr}, otherwise the contents
2702 of the @var{string} are unspecified.
2705 @deftypefn {C Function} SCM scm_c_make_string (size_t len, SCM chr)
2706 Like @code{scm_make_string}, but expects the length as a
2710 @deffn {Scheme Procedure} string-tabulate proc len
2711 @deffnx {C Function} scm_string_tabulate (proc, len)
2712 @var{proc} is an integer->char procedure. Construct a string
2713 of size @var{len} by applying @var{proc} to each index to
2714 produce the corresponding string element. The order in which
2715 @var{proc} is applied to the indices is not specified.
2718 @deffn {Scheme Procedure} string-join ls [delimiter [grammar]]
2719 @deffnx {C Function} scm_string_join (ls, delimiter, grammar)
2720 Append the string in the string list @var{ls}, using the string
2721 @var{delim} as a delimiter between the elements of @var{ls}.
2722 @var{grammar} is a symbol which specifies how the delimiter is
2723 placed between the strings, and defaults to the symbol
2728 Insert the separator between list elements. An empty string
2729 will produce an empty list.
2731 Like @code{infix}, but will raise an error if given the empty
2734 Insert the separator after every list element.
2736 Insert the separator before each list element.
2740 @node List/String Conversion
2741 @subsubsection List/String conversion
2743 When processing strings, it is often convenient to first convert them
2744 into a list representation by using the procedure @code{string->list},
2745 work with the resulting list, and then convert it back into a string.
2746 These procedures are useful for similar tasks.
2748 @rnindex string->list
2749 @deffn {Scheme Procedure} string->list str [start [end]]
2750 @deffnx {C Function} scm_substring_to_list (str, start, end)
2751 @deffnx {C Function} scm_string_to_list (str)
2752 Convert the string @var{str} into a list of characters.
2755 @deffn {Scheme Procedure} string-split str chr
2756 @deffnx {C Function} scm_string_split (str, chr)
2757 Split the string @var{str} into the a list of the substrings delimited
2758 by appearances of the character @var{chr}. Note that an empty substring
2759 between separator characters will result in an empty string in the
2763 (string-split "root:x:0:0:root:/root:/bin/bash" #\:)
2765 ("root" "x" "0" "0" "root" "/root" "/bin/bash")
2767 (string-split "::" #\:)
2771 (string-split "" #\:)
2778 @node String Selection
2779 @subsubsection String Selection
2781 Portions of strings can be extracted by these procedures.
2782 @code{string-ref} delivers individual characters whereas
2783 @code{substring} can be used to extract substrings from longer strings.
2785 @rnindex string-length
2786 @deffn {Scheme Procedure} string-length string
2787 @deffnx {C Function} scm_string_length (string)
2788 Return the number of characters in @var{string}.
2791 @deftypefn {C Function} size_t scm_c_string_length (SCM str)
2792 Return the number of characters in @var{str} as a @code{size_t}.
2796 @deffn {Scheme Procedure} string-ref str k
2797 @deffnx {C Function} scm_string_ref (str, k)
2798 Return character @var{k} of @var{str} using zero-origin
2799 indexing. @var{k} must be a valid index of @var{str}.
2802 @deftypefn {C Function} SCM scm_c_string_ref (SCM str, size_t k)
2803 Return character @var{k} of @var{str} using zero-origin
2804 indexing. @var{k} must be a valid index of @var{str}.
2807 @rnindex string-copy
2808 @deffn {Scheme Procedure} string-copy str [start [end]]
2809 @deffnx {C Function} scm_substring_copy (str, start, end)
2810 @deffnx {C Function} scm_string_copy (str)
2811 Return a copy of the given string @var{str}.
2813 The returned string shares storage with @var{str} initially, but it is
2814 copied as soon as one of the two strings is modified.
2818 @deffn {Scheme Procedure} substring str start [end]
2819 @deffnx {C Function} scm_substring (str, start, end)
2820 Return a new string formed from the characters
2821 of @var{str} beginning with index @var{start} (inclusive) and
2822 ending with index @var{end} (exclusive).
2823 @var{str} must be a string, @var{start} and @var{end} must be
2824 exact integers satisfying:
2826 0 <= @var{start} <= @var{end} <= @code{(string-length @var{str})}.
2828 The returned string shares storage with @var{str} initially, but it is
2829 copied as soon as one of the two strings is modified.
2832 @deffn {Scheme Procedure} substring/shared str start [end]
2833 @deffnx {C Function} scm_substring_shared (str, start, end)
2834 Like @code{substring}, but the strings continue to share their storage
2835 even if they are modified. Thus, modifications to @var{str} show up
2836 in the new string, and vice versa.
2839 @deffn {Scheme Procedure} substring/copy str start [end]
2840 @deffnx {C Function} scm_substring_copy (str, start, end)
2841 Like @code{substring}, but the storage for the new string is copied
2845 @deffn {Scheme Procedure} substring/read-only str start [end]
2846 @deffnx {C Function} scm_substring_read_only (str, start, end)
2847 Like @code{substring}, but the resulting string can not be modified.
2850 @deftypefn {C Function} SCM scm_c_substring (SCM str, size_t start, size_t end)
2851 @deftypefnx {C Function} SCM scm_c_substring_shared (SCM str, size_t start, size_t end)
2852 @deftypefnx {C Function} SCM scm_c_substring_copy (SCM str, size_t start, size_t end)
2853 @deftypefnx {C Function} SCM scm_c_substring_read_only (SCM str, size_t start, size_t end)
2854 Like @code{scm_substring}, etc. but the bounds are given as a @code{size_t}.
2857 @deffn {Scheme Procedure} string-take s n
2858 @deffnx {C Function} scm_string_take (s, n)
2859 Return the @var{n} first characters of @var{s}.
2862 @deffn {Scheme Procedure} string-drop s n
2863 @deffnx {C Function} scm_string_drop (s, n)
2864 Return all but the first @var{n} characters of @var{s}.
2867 @deffn {Scheme Procedure} string-take-right s n
2868 @deffnx {C Function} scm_string_take_right (s, n)
2869 Return the @var{n} last characters of @var{s}.
2872 @deffn {Scheme Procedure} string-drop-right s n
2873 @deffnx {C Function} scm_string_drop_right (s, n)
2874 Return all but the last @var{n} characters of @var{s}.
2877 @deffn {Scheme Procedure} string-pad s len [chr [start [end]]]
2878 @deffnx {Scheme Procedure} string-pad-right s len [chr [start [end]]]
2879 @deffnx {C Function} scm_string_pad (s, len, chr, start, end)
2880 @deffnx {C Function} scm_string_pad_right (s, len, chr, start, end)
2881 Take characters @var{start} to @var{end} from the string @var{s} and
2882 either pad with @var{char} or truncate them to give @var{len}
2885 @code{string-pad} pads or truncates on the left, so for example
2888 (string-pad "x" 3) @result{} " x"
2889 (string-pad "abcde" 3) @result{} "cde"
2892 @code{string-pad-right} pads or truncates on the right, so for example
2895 (string-pad-right "x" 3) @result{} "x "
2896 (string-pad-right "abcde" 3) @result{} "abc"
2900 @deffn {Scheme Procedure} string-trim s [char_pred [start [end]]]
2901 @deffnx {Scheme Procedure} string-trim-right s [char_pred [start [end]]]
2902 @deffnx {Scheme Procedure} string-trim-both s [char_pred [start [end]]]
2903 @deffnx {C Function} scm_string_trim (s, char_pred, start, end)
2904 @deffnx {C Function} scm_string_trim_right (s, char_pred, start, end)
2905 @deffnx {C Function} scm_string_trim_both (s, char_pred, start, end)
2906 Trim occurrances of @var{char_pred} from the ends of @var{s}.
2908 @code{string-trim} trims @var{char_pred} characters from the left
2909 (start) of the string, @code{string-trim-right} trims them from the
2910 right (end) of the string, @code{string-trim-both} trims from both
2913 @var{char_pred} can be a character, a character set, or a predicate
2914 procedure to call on each character. If @var{char_pred} is not given
2915 the default is whitespace as per @code{char-set:whitespace}
2916 (@pxref{Standard Character Sets}).
2919 (string-trim " x ") @result{} "x "
2920 (string-trim-right "banana" #\a) @result{} "banan"
2921 (string-trim-both ".,xy:;" char-set:punctuation)
2923 (string-trim-both "xyzzy" (lambda (c)
2930 @node String Modification
2931 @subsubsection String Modification
2933 These procedures are for modifying strings in-place. This means that the
2934 result of the operation is not a new string; instead, the original string's
2935 memory representation is modified.
2937 @rnindex string-set!
2938 @deffn {Scheme Procedure} string-set! str k chr
2939 @deffnx {C Function} scm_string_set_x (str, k, chr)
2940 Store @var{chr} in element @var{k} of @var{str} and return
2941 an unspecified value. @var{k} must be a valid index of
2945 @deftypefn {C Function} void scm_c_string_set_x (SCM str, size_t k, SCM chr)
2946 Like @code{scm_string_set_x}, but the index is given as a @code{size_t}.
2949 @rnindex string-fill!
2950 @deffn {Scheme Procedure} string-fill! str chr [start [end]]
2951 @deffnx {C Function} scm_substring_fill_x (str, chr, start, end)
2952 @deffnx {C Function} scm_string_fill_x (str, chr)
2953 Stores @var{chr} in every element of the given @var{str} and
2954 returns an unspecified value.
2957 @deffn {Scheme Procedure} substring-fill! str start end fill
2958 @deffnx {C Function} scm_substring_fill_x (str, start, end, fill)
2959 Change every character in @var{str} between @var{start} and
2960 @var{end} to @var{fill}.
2963 (define y "abcdefg")
2964 (substring-fill! y 1 3 #\r)
2970 @deffn {Scheme Procedure} substring-move! str1 start1 end1 str2 start2
2971 @deffnx {C Function} scm_substring_move_x (str1, start1, end1, str2, start2)
2972 Copy the substring of @var{str1} bounded by @var{start1} and @var{end1}
2973 into @var{str2} beginning at position @var{start2}.
2974 @var{str1} and @var{str2} can be the same string.
2977 @deffn {Scheme Procedure} string-copy! target tstart s [start [end]]
2978 @deffnx {C Function} scm_string_copy_x (target, tstart, s, start, end)
2979 Copy the sequence of characters from index range [@var{start},
2980 @var{end}) in string @var{s} to string @var{target}, beginning
2981 at index @var{tstart}. The characters are copied left-to-right
2982 or right-to-left as needed -- the copy is guaranteed to work,
2983 even if @var{target} and @var{s} are the same string. It is an
2984 error if the copy operation runs off the end of the target
2989 @node String Comparison
2990 @subsubsection String Comparison
2992 The procedures in this section are similar to the character ordering
2993 predicates (@pxref{Characters}), but are defined on character sequences.
2995 The first set is specified in R5RS and has names that end in @code{?}.
2996 The second set is specified in SRFI-13 and the names have no ending
2997 @code{?}. The predicates ending in @code{-ci} ignore the character case
2998 when comparing strings.
3001 @deffn {Scheme Procedure} string=? s1 s2
3002 Lexicographic equality predicate; return @code{#t} if the two
3003 strings are the same length and contain the same characters in
3004 the same positions, otherwise return @code{#f}.
3006 The procedure @code{string-ci=?} treats upper and lower case
3007 letters as though they were the same character, but
3008 @code{string=?} treats upper and lower case as distinct
3013 @deffn {Scheme Procedure} string<? s1 s2
3014 Lexicographic ordering predicate; return @code{#t} if @var{s1}
3015 is lexicographically less than @var{s2}.
3019 @deffn {Scheme Procedure} string<=? s1 s2
3020 Lexicographic ordering predicate; return @code{#t} if @var{s1}
3021 is lexicographically less than or equal to @var{s2}.
3025 @deffn {Scheme Procedure} string>? s1 s2
3026 Lexicographic ordering predicate; return @code{#t} if @var{s1}
3027 is lexicographically greater than @var{s2}.
3031 @deffn {Scheme Procedure} string>=? s1 s2
3032 Lexicographic ordering predicate; return @code{#t} if @var{s1}
3033 is lexicographically greater than or equal to @var{s2}.
3036 @rnindex string-ci=?
3037 @deffn {Scheme Procedure} string-ci=? s1 s2
3038 Case-insensitive string equality predicate; return @code{#t} if
3039 the two strings are the same length and their component
3040 characters match (ignoring case) at each position; otherwise
3044 @rnindex string-ci<?
3045 @deffn {Scheme Procedure} string-ci<? s1 s2
3046 Case insensitive lexicographic ordering predicate; return
3047 @code{#t} if @var{s1} is lexicographically less than @var{s2}
3052 @deffn {Scheme Procedure} string-ci<=? s1 s2
3053 Case insensitive lexicographic ordering predicate; return
3054 @code{#t} if @var{s1} is lexicographically less than or equal
3055 to @var{s2} regardless of case.
3058 @rnindex string-ci>?
3059 @deffn {Scheme Procedure} string-ci>? s1 s2
3060 Case insensitive lexicographic ordering predicate; return
3061 @code{#t} if @var{s1} is lexicographically greater than
3062 @var{s2} regardless of case.
3065 @rnindex string-ci>=?
3066 @deffn {Scheme Procedure} string-ci>=? s1 s2
3067 Case insensitive lexicographic ordering predicate; return
3068 @code{#t} if @var{s1} is lexicographically greater than or
3069 equal to @var{s2} regardless of case.
3072 @deffn {Scheme Procedure} string-compare s1 s2 proc_lt proc_eq proc_gt [start1 [end1 [start2 [end2]]]]
3073 @deffnx {C Function} scm_string_compare (s1, s2, proc_lt, proc_eq, proc_gt, start1, end1, start2, end2)
3074 Apply @var{proc_lt}, @var{proc_eq}, @var{proc_gt} to the
3075 mismatch index, depending upon whether @var{s1} is less than,
3076 equal to, or greater than @var{s2}. The mismatch index is the
3077 largest index @var{i} such that for every 0 <= @var{j} <
3078 @var{i}, @var{s1}[@var{j}] = @var{s2}[@var{j}] -- that is,
3079 @var{i} is the first position that does not match.
3082 @deffn {Scheme Procedure} string-compare-ci s1 s2 proc_lt proc_eq proc_gt [start1 [end1 [start2 [end2]]]]
3083 @deffnx {C Function} scm_string_compare_ci (s1, s2, proc_lt, proc_eq, proc_gt, start1, end1, start2, end2)
3084 Apply @var{proc_lt}, @var{proc_eq}, @var{proc_gt} to the
3085 mismatch index, depending upon whether @var{s1} is less than,
3086 equal to, or greater than @var{s2}. The mismatch index is the
3087 largest index @var{i} such that for every 0 <= @var{j} <
3088 @var{i}, @var{s1}[@var{j}] = @var{s2}[@var{j}] -- that is,
3089 @var{i} is the first position that does not match. The
3090 character comparison is done case-insensitively.
3093 @deffn {Scheme Procedure} string= s1 s2 [start1 [end1 [start2 [end2]]]]
3094 @deffnx {C Function} scm_string_eq (s1, s2, start1, end1, start2, end2)
3095 Return @code{#f} if @var{s1} and @var{s2} are not equal, a true
3099 @deffn {Scheme Procedure} string<> s1 s2 [start1 [end1 [start2 [end2]]]]
3100 @deffnx {C Function} scm_string_neq (s1, s2, start1, end1, start2, end2)
3101 Return @code{#f} if @var{s1} and @var{s2} are equal, a true
3105 @deffn {Scheme Procedure} string< s1 s2 [start1 [end1 [start2 [end2]]]]
3106 @deffnx {C Function} scm_string_lt (s1, s2, start1, end1, start2, end2)
3107 Return @code{#f} if @var{s1} is greater or equal to @var{s2}, a
3108 true value otherwise.
3111 @deffn {Scheme Procedure} string> s1 s2 [start1 [end1 [start2 [end2]]]]
3112 @deffnx {C Function} scm_string_gt (s1, s2, start1, end1, start2, end2)
3113 Return @code{#f} if @var{s1} is less or equal to @var{s2}, a
3114 true value otherwise.
3117 @deffn {Scheme Procedure} string<= s1 s2 [start1 [end1 [start2 [end2]]]]
3118 @deffnx {C Function} scm_string_le (s1, s2, start1, end1, start2, end2)
3119 Return @code{#f} if @var{s1} is greater to @var{s2}, a true
3123 @deffn {Scheme Procedure} string>= s1 s2 [start1 [end1 [start2 [end2]]]]
3124 @deffnx {C Function} scm_string_ge (s1, s2, start1, end1, start2, end2)
3125 Return @code{#f} if @var{s1} is less to @var{s2}, a true value
3129 @deffn {Scheme Procedure} string-ci= s1 s2 [start1 [end1 [start2 [end2]]]]
3130 @deffnx {C Function} scm_string_ci_eq (s1, s2, start1, end1, start2, end2)
3131 Return @code{#f} if @var{s1} and @var{s2} are not equal, a true
3132 value otherwise. The character comparison is done
3136 @deffn {Scheme Procedure} string-ci<> s1 s2 [start1 [end1 [start2 [end2]]]]
3137 @deffnx {C Function} scm_string_ci_neq (s1, s2, start1, end1, start2, end2)
3138 Return @code{#f} if @var{s1} and @var{s2} are equal, a true
3139 value otherwise. The character comparison is done
3143 @deffn {Scheme Procedure} string-ci< s1 s2 [start1 [end1 [start2 [end2]]]]
3144 @deffnx {C Function} scm_string_ci_lt (s1, s2, start1, end1, start2, end2)
3145 Return @code{#f} if @var{s1} is greater or equal to @var{s2}, a
3146 true value otherwise. The character comparison is done
3150 @deffn {Scheme Procedure} string-ci> s1 s2 [start1 [end1 [start2 [end2]]]]
3151 @deffnx {C Function} scm_string_ci_gt (s1, s2, start1, end1, start2, end2)
3152 Return @code{#f} if @var{s1} is less or equal to @var{s2}, a
3153 true value otherwise. The character comparison is done
3157 @deffn {Scheme Procedure} string-ci<= s1 s2 [start1 [end1 [start2 [end2]]]]
3158 @deffnx {C Function} scm_string_ci_le (s1, s2, start1, end1, start2, end2)
3159 Return @code{#f} if @var{s1} is greater to @var{s2}, a true
3160 value otherwise. The character comparison is done
3164 @deffn {Scheme Procedure} string-ci>= s1 s2 [start1 [end1 [start2 [end2]]]]
3165 @deffnx {C Function} scm_string_ci_ge (s1, s2, start1, end1, start2, end2)
3166 Return @code{#f} if @var{s1} is less to @var{s2}, a true value
3167 otherwise. The character comparison is done
3171 @deffn {Scheme Procedure} string-hash s [bound [start [end]]]
3172 @deffnx {C Function} scm_substring_hash (s, bound, start, end)
3173 Compute a hash value for @var{S}. the optional argument @var{bound} is a non-negative exact integer specifying the range of the hash function. A positive value restricts the return value to the range [0,bound).
3176 @deffn {Scheme Procedure} string-hash-ci s [bound [start [end]]]
3177 @deffnx {C Function} scm_substring_hash_ci (s, bound, start, end)
3178 Compute a hash value for @var{S}. the optional argument @var{bound} is a non-negative exact integer specifying the range of the hash function. A positive value restricts the return value to the range [0,bound).
3181 @node String Searching
3182 @subsubsection String Searching
3184 @deffn {Scheme Procedure} string-index s char_pred [start [end]]
3185 @deffnx {C Function} scm_string_index (s, char_pred, start, end)
3186 Search through the string @var{s} from left to right, returning
3187 the index of the first occurence of a character which
3191 equals @var{char_pred}, if it is character,
3194 satisifies the predicate @var{char_pred}, if it is a procedure,
3197 is in the set @var{char_pred}, if it is a character set.
3201 @deffn {Scheme Procedure} string-rindex s char_pred [start [end]]
3202 @deffnx {C Function} scm_string_rindex (s, char_pred, start, end)
3203 Search through the string @var{s} from right to left, returning
3204 the index of the last occurence of a character which
3208 equals @var{char_pred}, if it is character,
3211 satisifies the predicate @var{char_pred}, if it is a procedure,
3214 is in the set if @var{char_pred} is a character set.
3218 @deffn {Scheme Procedure} string-prefix-length s1 s2 [start1 [end1 [start2 [end2]]]]
3219 @deffnx {C Function} scm_string_prefix_length (s1, s2, start1, end1, start2, end2)
3220 Return the length of the longest common prefix of the two
3224 @deffn {Scheme Procedure} string-prefix-length-ci s1 s2 [start1 [end1 [start2 [end2]]]]
3225 @deffnx {C Function} scm_string_prefix_length_ci (s1, s2, start1, end1, start2, end2)
3226 Return the length of the longest common prefix of the two
3227 strings, ignoring character case.
3230 @deffn {Scheme Procedure} string-suffix-length s1 s2 [start1 [end1 [start2 [end2]]]]
3231 @deffnx {C Function} scm_string_suffix_length (s1, s2, start1, end1, start2, end2)
3232 Return the length of the longest common suffix of the two
3236 @deffn {Scheme Procedure} string-suffix-length-ci s1 s2 [start1 [end1 [start2 [end2]]]]
3237 @deffnx {C Function} scm_string_suffix_length_ci (s1, s2, start1, end1, start2, end2)
3238 Return the length of the longest common suffix of the two
3239 strings, ignoring character case.
3242 @deffn {Scheme Procedure} string-prefix? s1 s2 [start1 [end1 [start2 [end2]]]]
3243 @deffnx {C Function} scm_string_prefix_p (s1, s2, start1, end1, start2, end2)
3244 Is @var{s1} a prefix of @var{s2}?
3247 @deffn {Scheme Procedure} string-prefix-ci? s1 s2 [start1 [end1 [start2 [end2]]]]
3248 @deffnx {C Function} scm_string_prefix_ci_p (s1, s2, start1, end1, start2, end2)
3249 Is @var{s1} a prefix of @var{s2}, ignoring character case?
3252 @deffn {Scheme Procedure} string-suffix? s1 s2 [start1 [end1 [start2 [end2]]]]
3253 @deffnx {C Function} scm_string_suffix_p (s1, s2, start1, end1, start2, end2)
3254 Is @var{s1} a suffix of @var{s2}?
3257 @deffn {Scheme Procedure} string-suffix-ci? s1 s2 [start1 [end1 [start2 [end2]]]]
3258 @deffnx {C Function} scm_string_suffix_ci_p (s1, s2, start1, end1, start2, end2)
3259 Is @var{s1} a suffix of @var{s2}, ignoring character case?
3262 @deffn {Scheme Procedure} string-index-right s char_pred [start [end]]
3263 @deffnx {C Function} scm_string_index_right (s, char_pred, start, end)
3264 Search through the string @var{s} from right to left, returning
3265 the index of the last occurence of a character which
3269 equals @var{char_pred}, if it is character,
3272 satisifies the predicate @var{char_pred}, if it is a procedure,
3275 is in the set if @var{char_pred} is a character set.
3279 @deffn {Scheme Procedure} string-skip s char_pred [start [end]]
3280 @deffnx {C Function} scm_string_skip (s, char_pred, start, end)
3281 Search through the string @var{s} from left to right, returning
3282 the index of the first occurence of a character which
3286 does not equal @var{char_pred}, if it is character,
3289 does not satisify the predicate @var{char_pred}, if it is a
3293 is not in the set if @var{char_pred} is a character set.
3297 @deffn {Scheme Procedure} string-skip-right s char_pred [start [end]]
3298 @deffnx {C Function} scm_string_skip_right (s, char_pred, start, end)
3299 Search through the string @var{s} from right to left, returning
3300 the index of the last occurence of a character which
3304 does not equal @var{char_pred}, if it is character,
3307 does not satisfy the predicate @var{char_pred}, if it is a
3311 is not in the set if @var{char_pred} is a character set.
3315 @deffn {Scheme Procedure} string-count s char_pred [start [end]]
3316 @deffnx {C Function} scm_string_count (s, char_pred, start, end)
3317 Return the count of the number of characters in the string
3322 equals @var{char_pred}, if it is character,
3325 satisifies the predicate @var{char_pred}, if it is a procedure.
3328 is in the set @var{char_pred}, if it is a character set.
3332 @deffn {Scheme Procedure} string-contains s1 s2 [start1 [end1 [start2 [end2]]]]
3333 @deffnx {C Function} scm_string_contains (s1, s2, start1, end1, start2, end2)
3334 Does string @var{s1} contain string @var{s2}? Return the index
3335 in @var{s1} where @var{s2} occurs as a substring, or false.
3336 The optional start/end indices restrict the operation to the
3337 indicated substrings.
3340 @deffn {Scheme Procedure} string-contains-ci s1 s2 [start1 [end1 [start2 [end2]]]]
3341 @deffnx {C Function} scm_string_contains_ci (s1, s2, start1, end1, start2, end2)
3342 Does string @var{s1} contain string @var{s2}? Return the index
3343 in @var{s1} where @var{s2} occurs as a substring, or false.
3344 The optional start/end indices restrict the operation to the
3345 indicated substrings. Character comparison is done
3349 @node Alphabetic Case Mapping
3350 @subsubsection Alphabetic Case Mapping
3352 These are procedures for mapping strings to their upper- or lower-case
3353 equivalents, respectively, or for capitalizing strings.
3355 @deffn {Scheme Procedure} string-upcase str [start [end]]
3356 @deffnx {C Function} scm_substring_upcase (str, start, end)
3357 @deffnx {C Function} scm_string_upcase (str)
3358 Upcase every character in @code{str}.
3361 @deffn {Scheme Procedure} string-upcase! str [start [end]]
3362 @deffnx {C Function} scm_substring_upcase_x (str, start, end)
3363 @deffnx {C Function} scm_string_upcase_x (str)
3364 Destructively upcase every character in @code{str}.
3374 @deffn {Scheme Procedure} string-downcase str [start [end]]
3375 @deffnx {C Function} scm_substring_downcase (str, start, end)
3376 @deffnx {C Function} scm_string_downcase (str)
3377 Downcase every character in @var{str}.
3380 @deffn {Scheme Procedure} string-downcase! str [start [end]]
3381 @deffnx {C Function} scm_substring_downcase_x (str, start, end)
3382 @deffnx {C Function} scm_string_downcase_x (str)
3383 Destructively downcase every character in @var{str}.
3388 (string-downcase! y)
3395 @deffn {Scheme Procedure} string-capitalize str
3396 @deffnx {C Function} scm_string_capitalize (str)
3397 Return a freshly allocated string with the characters in
3398 @var{str}, where the first character of every word is
3402 @deffn {Scheme Procedure} string-capitalize! str
3403 @deffnx {C Function} scm_string_capitalize_x (str)
3404 Upcase the first character of every word in @var{str}
3405 destructively and return @var{str}.
3408 y @result{} "hello world"
3409 (string-capitalize! y) @result{} "Hello World"
3410 y @result{} "Hello World"
3414 @deffn {Scheme Procedure} string-titlecase str [start [end]]
3415 @deffnx {C Function} scm_string_titlecase (str, start, end)
3416 Titlecase every first character in a word in @var{str}.
3419 @deffn {Scheme Procedure} string-titlecase! str [start [end]]
3420 @deffnx {C Function} scm_string_titlecase_x (str, start, end)
3421 Destructively titlecase every first character in a word in
3425 @node Reversing and Appending Strings
3426 @subsubsection Reversing and Appending Strings
3428 @deffn {Scheme Procedure} string-reverse str [start [end]]
3429 @deffnx {C Function} scm_string_reverse (str, start, end)
3430 Reverse the string @var{str}. The optional arguments
3431 @var{start} and @var{end} delimit the region of @var{str} to
3435 @deffn {Scheme Procedure} string-reverse! str [start [end]]
3436 @deffnx {C Function} scm_string_reverse_x (str, start, end)
3437 Reverse the string @var{str} in-place. The optional arguments
3438 @var{start} and @var{end} delimit the region of @var{str} to
3439 operate on. The return value is unspecified.
3442 @rnindex string-append
3443 @deffn {Scheme Procedure} string-append . args
3444 @deffnx {C Function} scm_string_append (args)
3445 Return a newly allocated string whose characters form the
3446 concatenation of the given strings, @var{args}.
3450 (string-append h "world"))
3451 @result{} "hello world"
3455 @deffn {Scheme Procedure} string-append/shared . ls
3456 @deffnx {C Function} scm_string_append_shared (ls)
3457 Like @code{string-append}, but the result may share memory
3458 with the argument strings.
3461 @deffn {Scheme Procedure} string-concatenate ls
3462 @deffnx {C Function} scm_string_concatenate (ls)
3463 Append the elements of @var{ls} (which must be strings)
3464 together into a single string. Guaranteed to return a freshly
3468 @deffn {Scheme Procedure} string-concatenate-reverse ls [final_string [end]]
3469 @deffnx {C Function} scm_string_concatenate_reverse (ls, final_string, end)
3470 Without optional arguments, this procedure is equivalent to
3473 (string-concatenate (reverse ls))
3476 If the optional argument @var{final_string} is specified, it is
3477 consed onto the beginning to @var{ls} before performing the
3478 list-reverse and string-concatenate operations. If @var{end}
3479 is given, only the characters of @var{final_string} up to index
3482 Guaranteed to return a freshly allocated string.
3485 @deffn {Scheme Procedure} string-concatenate/shared ls
3486 @deffnx {C Function} scm_string_concatenate_shared (ls)
3487 Like @code{string-concatenate}, but the result may share memory
3488 with the strings in the list @var{ls}.
3491 @deffn {Scheme Procedure} string-concatenate-reverse/shared ls [final_string [end]]
3492 @deffnx {C Function} scm_string_concatenate_reverse_shared (ls, final_string, end)
3493 Like @code{string-concatenate-reverse}, but the result may
3494 share memory with the the strings in the @var{ls} arguments.
3497 @node Mapping Folding and Unfolding
3498 @subsubsection Mapping, Folding, and Unfolding
3500 @deffn {Scheme Procedure} string-map proc s [start [end]]
3501 @deffnx {C Function} scm_string_map (proc, s, start, end)
3502 @var{proc} is a char->char procedure, it is mapped over
3503 @var{s}. The order in which the procedure is applied to the
3504 string elements is not specified.
3507 @deffn {Scheme Procedure} string-map! proc s [start [end]]
3508 @deffnx {C Function} scm_string_map_x (proc, s, start, end)
3509 @var{proc} is a char->char procedure, it is mapped over
3510 @var{s}. The order in which the procedure is applied to the
3511 string elements is not specified. The string @var{s} is
3512 modified in-place, the return value is not specified.
3515 @deffn {Scheme Procedure} string-for-each proc s [start [end]]
3516 @deffnx {C Function} scm_string_for_each (proc, s, start, end)
3517 @var{proc} is mapped over @var{s} in left-to-right order. The
3518 return value is not specified.
3521 @deffn {Scheme Procedure} string-for-each-index proc s [start [end]]
3522 @deffnx {C Function} scm_string_for_each_index (proc, s, start, end)
3523 Call @code{(@var{proc} i)} for each index i in @var{s}, from left to
3526 For example, to change characters to alternately upper and lower case,
3529 (define str (string-copy "studly"))
3530 (string-for-each-index (lambda (i)
3532 ((if (even? i) char-upcase char-downcase)
3533 (string-ref str i))))
3535 str @result{} "StUdLy"
3539 @deffn {Scheme Procedure} string-fold kons knil s [start [end]]
3540 @deffnx {C Function} scm_string_fold (kons, knil, s, start, end)
3541 Fold @var{kons} over the characters of @var{s}, with @var{knil}
3542 as the terminating element, from left to right. @var{kons}
3543 must expect two arguments: The actual character and the last
3544 result of @var{kons}' application.
3547 @deffn {Scheme Procedure} string-fold-right kons knil s [start [end]]
3548 @deffnx {C Function} scm_string_fold_right (kons, knil, s, start, end)
3549 Fold @var{kons} over the characters of @var{s}, with @var{knil}
3550 as the terminating element, from right to left. @var{kons}
3551 must expect two arguments: The actual character and the last
3552 result of @var{kons}' application.
3555 @deffn {Scheme Procedure} string-unfold p f g seed [base [make_final]]
3556 @deffnx {C Function} scm_string_unfold (p, f, g, seed, base, make_final)
3558 @item @var{g} is used to generate a series of @emph{seed}
3559 values from the initial @var{seed}: @var{seed}, (@var{g}
3560 @var{seed}), (@var{g}^2 @var{seed}), (@var{g}^3 @var{seed}),
3562 @item @var{p} tells us when to stop -- when it returns true
3563 when applied to one of these seed values.
3564 @item @var{f} maps each seed value to the corresponding
3565 character in the result string. These chars are assembled
3566 into the string in a left-to-right order.
3567 @item @var{base} is the optional initial/leftmost portion
3568 of the constructed string; it default to the empty
3570 @item @var{make_final} is applied to the terminal seed
3571 value (on which @var{p} returns true) to produce
3572 the final/rightmost portion of the constructed string.
3573 The default is nothing extra.
3577 @deffn {Scheme Procedure} string-unfold-right p f g seed [base [make_final]]
3578 @deffnx {C Function} scm_string_unfold_right (p, f, g, seed, base, make_final)
3580 @item @var{g} is used to generate a series of @emph{seed}
3581 values from the initial @var{seed}: @var{seed}, (@var{g}
3582 @var{seed}), (@var{g}^2 @var{seed}), (@var{g}^3 @var{seed}),
3584 @item @var{p} tells us when to stop -- when it returns true
3585 when applied to one of these seed values.
3586 @item @var{f} maps each seed value to the corresponding
3587 character in the result string. These chars are assembled
3588 into the string in a right-to-left order.
3589 @item @var{base} is the optional initial/rightmost portion
3590 of the constructed string; it default to the empty
3592 @item @var{make_final} is applied to the terminal seed
3593 value (on which @var{p} returns true) to produce
3594 the final/leftmost portion of the constructed string.
3595 It defaults to @code{(lambda (x) )}.
3599 @node Miscellaneous String Operations
3600 @subsubsection Miscellaneous String Operations
3602 @deffn {Scheme Procedure} xsubstring s from [to [start [end]]]
3603 @deffnx {C Function} scm_xsubstring (s, from, to, start, end)
3604 This is the @emph{extended substring} procedure that implements
3605 replicated copying of a substring of some string.
3607 @var{s} is a string, @var{start} and @var{end} are optional
3608 arguments that demarcate a substring of @var{s}, defaulting to
3609 0 and the length of @var{s}. Replicate this substring up and
3610 down index space, in both the positive and negative directions.
3611 @code{xsubstring} returns the substring of this string
3612 beginning at index @var{from}, and ending at @var{to}, which
3613 defaults to @var{from} + (@var{end} - @var{start}).
3616 @deffn {Scheme Procedure} string-xcopy! target tstart s sfrom [sto [start [end]]]
3617 @deffnx {C Function} scm_string_xcopy_x (target, tstart, s, sfrom, sto, start, end)
3618 Exactly the same as @code{xsubstring}, but the extracted text
3619 is written into the string @var{target} starting at index
3620 @var{tstart}. The operation is not defined if @code{(eq?
3621 @var{target} @var{s})} or these arguments share storage -- you
3622 cannot copy a string on top of itself.
3625 @deffn {Scheme Procedure} string-replace s1 s2 [start1 [end1 [start2 [end2]]]]
3626 @deffnx {C Function} scm_string_replace (s1, s2, start1, end1, start2, end2)
3627 Return the string @var{s1}, but with the characters
3628 @var{start1} @dots{} @var{end1} replaced by the characters
3629 @var{start2} @dots{} @var{end2} from @var{s2}.
3632 @deffn {Scheme Procedure} string-tokenize s [token_set [start [end]]]
3633 @deffnx {C Function} scm_string_tokenize (s, token_set, start, end)
3634 Split the string @var{s} into a list of substrings, where each
3635 substring is a maximal non-empty contiguous sequence of
3636 characters from the character set @var{token_set}, which
3637 defaults to @code{char-set:graphic}.
3638 If @var{start} or @var{end} indices are provided, they restrict
3639 @code{string-tokenize} to operating on the indicated substring
3643 @deffn {Scheme Procedure} string-filter s char_pred [start [end]]
3644 @deffnx {C Function} scm_string_filter (s, char_pred, start, end)
3645 Filter the string @var{s}, retaining only those characters which
3646 satisfy @var{char_pred}.
3648 If @var{char_pred} is a procedure, it is applied to each character as
3649 a predicate, if it is a character, it is tested for equality and if it
3650 is a character set, it is tested for membership.
3653 @deffn {Scheme Procedure} string-delete s char_pred [start [end]]
3654 @deffnx {C Function} scm_string_delete (s, char_pred, start, end)
3655 Delete characters satisfying @var{char_pred} from @var{s}.
3657 If @var{char_pred} is a procedure, it is applied to each character as
3658 a predicate, if it is a character, it is tested for equality and if it
3659 is a character set, it is tested for membership.
3662 @node Conversion to/from C
3663 @subsubsection Conversion to/from C
3665 When creating a Scheme string from a C string or when converting a
3666 Scheme string to a C string, the concept of character encoding becomes
3669 In C, a string is just a sequence of bytes, and the character encoding
3670 describes the relation between these bytes and the actual characters
3671 that make up the string. For Scheme strings, character encoding is
3672 not an issue (most of the time), since in Scheme you never get to see
3673 the bytes, only the characters.
3675 Well, ideally, anyway. Right now, Guile simply equates Scheme
3676 characters and bytes, ignoring the possibility of multi-byte encodings
3677 completely. This will change in the future, where Guile will use
3678 Unicode codepoints as its characters and UTF-8 or some other encoding
3679 as its internal encoding. When you exclusively use the functions
3680 listed in this section, you are `future-proof'.
3682 Converting a Scheme string to a C string will often allocate fresh
3683 memory to hold the result. You must take care that this memory is
3684 properly freed eventually. In many cases, this can be achieved by
3685 using @code{scm_dynwind_free} inside an appropriate dynwind context,
3686 @xref{Dynamic Wind}.
3688 @deftypefn {C Function} SCM scm_from_locale_string (const char *str)
3689 @deftypefnx {C Function} SCM scm_from_locale_stringn (const char *str, size_t len)
3690 Creates a new Scheme string that has the same contents as @var{str}
3691 when interpreted in the current locale character encoding.
3693 For @code{scm_from_locale_string}, @var{str} must be null-terminated.
3695 For @code{scm_from_locale_stringn}, @var{len} specifies the length of
3696 @var{str} in bytes, and @var{str} does not need to be null-terminated.
3697 If @var{len} is @code{(size_t)-1}, then @var{str} does need to be
3698 null-terminated and the real length will be found with @code{strlen}.
3701 @deftypefn {C Function} SCM scm_take_locale_string (char *str)
3702 @deftypefnx {C Function} SCM scm_take_locale_stringn (char *str, size_t len)
3703 Like @code{scm_from_locale_string} and @code{scm_from_locale_stringn},
3704 respectively, but also frees @var{str} with @code{free} eventually.
3705 Thus, you can use this function when you would free @var{str} anyway
3706 immediately after creating the Scheme string. In certain cases, Guile
3707 can then use @var{str} directly as its internal representation.
3710 @deftypefn {C Function} {char *} scm_to_locale_string (SCM str)
3711 @deftypefnx {C Function} {char *} scm_to_locale_stringn (SCM str, size_t *lenp)
3712 Returns a C string in the current locale encoding with the same
3713 contents as @var{str}. The C string must be freed with @code{free}
3714 eventually, maybe by using @code{scm_dynwind_free}, @xref{Dynamic
3717 For @code{scm_to_locale_string}, the returned string is
3718 null-terminated and an error is signalled when @var{str} contains
3719 @code{#\nul} characters.
3721 For @code{scm_to_locale_stringn} and @var{lenp} not @code{NULL},
3722 @var{str} might contain @code{#\nul} characters and the length of the
3723 returned string in bytes is stored in @code{*@var{lenp}}. The
3724 returned string will not be null-terminated in this case. If
3725 @var{lenp} is @code{NULL}, @code{scm_to_locale_stringn} behaves like
3726 @code{scm_to_locale_string}.
3729 @deftypefn {C Function} size_t scm_to_locale_stringbuf (SCM str, char *buf, size_t max_len)
3730 Puts @var{str} as a C string in the current locale encoding into the
3731 memory pointed to by @var{buf}. The buffer at @var{buf} has room for
3732 @var{max_len} bytes and @code{scm_to_local_stringbuf} will never store
3733 more than that. No terminating @code{'\0'} will be stored.
3735 The return value of @code{scm_to_locale_stringbuf} is the number of
3736 bytes that are needed for all of @var{str}, regardless of whether
3737 @var{buf} was large enough to hold them. Thus, when the return value
3738 is larger than @var{max_len}, only @var{max_len} bytes have been
3739 stored and you probably need to try again with a larger buffer.
3742 @node Regular Expressions
3743 @subsection Regular Expressions
3744 @tpindex Regular expressions
3746 @cindex regular expressions
3748 @cindex emacs regexp
3750 A @dfn{regular expression} (or @dfn{regexp}) is a pattern that
3751 describes a whole class of strings. A full description of regular
3752 expressions and their syntax is beyond the scope of this manual;
3753 an introduction can be found in the Emacs manual (@pxref{Regexps,
3754 , Syntax of Regular Expressions, emacs, The GNU Emacs Manual}), or
3755 in many general Unix reference books.
3757 If your system does not include a POSIX regular expression library,
3758 and you have not linked Guile with a third-party regexp library such
3759 as Rx, these functions will not be available. You can tell whether
3760 your Guile installation includes regular expression support by
3761 checking whether @code{(provided? 'regex)} returns true.
3763 The following regexp and string matching features are provided by the
3764 @code{(ice-9 regex)} module. Before using the described functions,
3765 you should load this module by executing @code{(use-modules (ice-9
3769 * Regexp Functions:: Functions that create and match regexps.
3770 * Match Structures:: Finding what was matched by a regexp.
3771 * Backslash Escapes:: Removing the special meaning of regexp
3776 @node Regexp Functions
3777 @subsubsection Regexp Functions
3779 By default, Guile supports POSIX extended regular expressions.
3780 That means that the characters @samp{(}, @samp{)}, @samp{+} and
3781 @samp{?} are special, and must be escaped if you wish to match the
3784 This regular expression interface was modeled after that
3785 implemented by SCSH, the Scheme Shell. It is intended to be
3786 upwardly compatible with SCSH regular expressions.
3788 Zero bytes (@code{#\nul}) cannot be used in regex patterns or input
3789 strings, since the underlying C functions treat that as the end of
3790 string. If there's a zero byte an error is thrown.
3792 Patterns and input strings are treated as being in the locale
3793 character set if @code{setlocale} has been called (@pxref{Locales}),
3794 and in a multibyte locale this includes treating multi-byte sequences
3795 as a single character. (Guile strings are currently merely bytes,
3796 though this may change in the future, @xref{Conversion to/from C}.)
3798 @deffn {Scheme Procedure} string-match pattern str [start]
3799 Compile the string @var{pattern} into a regular expression and compare
3800 it with @var{str}. The optional numeric argument @var{start} specifies
3801 the position of @var{str} at which to begin matching.
3803 @code{string-match} returns a @dfn{match structure} which
3804 describes what, if anything, was matched by the regular
3805 expression. @xref{Match Structures}. If @var{str} does not match
3806 @var{pattern} at all, @code{string-match} returns @code{#f}.
3809 Two examples of a match follow. In the first example, the pattern
3810 matches the four digits in the match string. In the second, the pattern
3814 (string-match "[0-9][0-9][0-9][0-9]" "blah2002")
3815 @result{} #("blah2002" (4 . 8))
3817 (string-match "[A-Za-z]" "123456")
3821 Each time @code{string-match} is called, it must compile its
3822 @var{pattern} argument into a regular expression structure. This
3823 operation is expensive, which makes @code{string-match} inefficient if
3824 the same regular expression is used several times (for example, in a
3825 loop). For better performance, you can compile a regular expression in
3826 advance and then match strings against the compiled regexp.
3828 @deffn {Scheme Procedure} make-regexp pat flag@dots{}
3829 @deffnx {C Function} scm_make_regexp (pat, flaglst)
3830 Compile the regular expression described by @var{pat}, and
3831 return the compiled regexp structure. If @var{pat} does not
3832 describe a legal regular expression, @code{make-regexp} throws
3833 a @code{regular-expression-syntax} error.
3835 The @var{flag} arguments change the behavior of the compiled
3836 regular expression. The following values may be supplied:
3838 @defvar regexp/icase
3839 Consider uppercase and lowercase letters to be the same when
3843 @defvar regexp/newline
3844 If a newline appears in the target string, then permit the
3845 @samp{^} and @samp{$} operators to match immediately after or
3846 immediately before the newline, respectively. Also, the
3847 @samp{.} and @samp{[^...]} operators will never match a newline
3848 character. The intent of this flag is to treat the target
3849 string as a buffer containing many lines of text, and the
3850 regular expression as a pattern that may match a single one of
3854 @defvar regexp/basic
3855 Compile a basic (``obsolete'') regexp instead of the extended
3856 (``modern'') regexps that are the default. Basic regexps do
3857 not consider @samp{|}, @samp{+} or @samp{?} to be special
3858 characters, and require the @samp{@{...@}} and @samp{(...)}
3859 metacharacters to be backslash-escaped (@pxref{Backslash
3860 Escapes}). There are several other differences between basic
3861 and extended regular expressions, but these are the most
3865 @defvar regexp/extended
3866 Compile an extended regular expression rather than a basic
3867 regexp. This is the default behavior; this flag will not
3868 usually be needed. If a call to @code{make-regexp} includes
3869 both @code{regexp/basic} and @code{regexp/extended} flags, the
3870 one which comes last will override the earlier one.
3874 @deffn {Scheme Procedure} regexp-exec rx str [start [flags]]
3875 @deffnx {C Function} scm_regexp_exec (rx, str, start, flags)
3876 Match the compiled regular expression @var{rx} against
3877 @code{str}. If the optional integer @var{start} argument is
3878 provided, begin matching from that position in the string.
3879 Return a match structure describing the results of the match,
3880 or @code{#f} if no match could be found.
3882 The @var{flags} argument changes the matching behavior. The following
3883 flag values may be supplied, use @code{logior} (@pxref{Bitwise
3884 Operations}) to combine them,
3886 @defvar regexp/notbol
3887 Consider that the @var{start} offset into @var{str} is not the
3888 beginning of a line and should not match operator @samp{^}.
3890 If @var{rx} was created with the @code{regexp/newline} option above,
3891 @samp{^} will still match after a newline in @var{str}.
3894 @defvar regexp/noteol
3895 Consider that the end of @var{str} is not the end of a line and should
3896 not match operator @samp{$}.
3898 If @var{rx} was created with the @code{regexp/newline} option above,
3899 @samp{$} will still match before a newline in @var{str}.
3904 ;; Regexp to match uppercase letters
3905 (define r (make-regexp "[A-Z]*"))
3907 ;; Regexp to match letters, ignoring case
3908 (define ri (make-regexp "[A-Z]*" regexp/icase))
3910 ;; Search for bob using regexp r
3911 (match:substring (regexp-exec r "bob"))
3912 @result{} "" ; no match
3914 ;; Search for bob using regexp ri
3915 (match:substring (regexp-exec ri "Bob"))
3916 @result{} "Bob" ; matched case insensitive
3919 @deffn {Scheme Procedure} regexp? obj
3920 @deffnx {C Function} scm_regexp_p (obj)
3921 Return @code{#t} if @var{obj} is a compiled regular expression,
3922 or @code{#f} otherwise.
3926 @deffn {Scheme Procedure} list-matches regexp str [flags]
3927 Return a list of match structures which are the non-overlapping
3928 matches of @var{regexp} in @var{str}. @var{regexp} can be either a
3929 pattern string or a compiled regexp. The @var{flags} argument is as
3930 per @code{regexp-exec} above.
3933 (map match:substring (list-matches "[a-z]+" "abc 42 def 78"))
3934 @result{} ("abc" "def")
3938 @deffn {Scheme Procedure} fold-matches regexp str init proc [flags]
3939 Apply @var{proc} to the non-overlapping matches of @var{regexp} in
3940 @var{str}, to build a result. @var{regexp} can be either a pattern
3941 string or a compiled regexp. The @var{flags} argument is as per
3942 @code{regexp-exec} above.
3944 @var{proc} is called as @code{(@var{proc} match prev)} where
3945 @var{match} is a match structure and @var{prev} is the previous return
3946 from @var{proc}. For the first call @var{prev} is the given
3947 @var{init} parameter. @code{fold-matches} returns the final value
3950 For example to count matches,
3953 (fold-matches "[a-z][0-9]" "abc x1 def y2" 0
3954 (lambda (match count)
3961 Regular expressions are commonly used to find patterns in one string
3962 and replace them with the contents of another string. The following
3963 functions are convenient ways to do this.
3965 @c begin (scm-doc-string "regex.scm" "regexp-substitute")
3966 @deffn {Scheme Procedure} regexp-substitute port match [item@dots{}]
3967 Write to @var{port} selected parts of the match structure @var{match}.
3968 Or if @var{port} is @code{#f} then form a string from those parts and
3971 Each @var{item} specifies a part to be written, and may be one of the
3976 A string. String arguments are written out verbatim.
3979 An integer. The submatch with that number is written
3980 (@code{match:substring}). Zero is the entire match.
3983 The symbol @samp{pre}. The portion of the matched string preceding
3984 the regexp match is written (@code{match:prefix}).
3987 The symbol @samp{post}. The portion of the matched string following
3988 the regexp match is written (@code{match:suffix}).
3991 For example, changing a match and retaining the text before and after,
3994 (regexp-substitute #f (string-match "[0-9]+" "number 25 is good")
3996 @result{} "number 37 is good"
3999 Or matching a @sc{yyyymmdd} format date such as @samp{20020828} and
4000 re-ordering and hyphenating the fields.
4003 (define date-regex "([0-9][0-9][0-9][0-9])([0-9][0-9])([0-9][0-9])")
4004 (define s "Date 20020429 12am.")
4005 (regexp-substitute #f (string-match date-regex s)
4006 'pre 2 "-" 3 "-" 1 'post " (" 0 ")")
4007 @result{} "Date 04-29-2002 12am. (20020429)"
4012 @c begin (scm-doc-string "regex.scm" "regexp-substitute")
4013 @deffn {Scheme Procedure} regexp-substitute/global port regexp target [item@dots{}]
4014 @cindex search and replace
4015 Write to @var{port} selected parts of matches of @var{regexp} in
4016 @var{target}. If @var{port} is @code{#f} then form a string from
4017 those parts and return that. @var{regexp} can be a string or a
4020 This is similar to @code{regexp-substitute}, but allows global
4021 substitutions on @var{target}. Each @var{item} behaves as per
4022 @code{regexp-substitute}, with the following differences,
4026 A function. Called as @code{(@var{item} match)} with the match
4027 structure for the @var{regexp} match, it should return a string to be
4028 written to @var{port}.
4031 The symbol @samp{post}. This doesn't output anything, but instead
4032 causes @code{regexp-substitute/global} to recurse on the unmatched
4033 portion of @var{target}.
4035 This @emph{must} be supplied to perform a global search and replace on
4036 @var{target}; without it @code{regexp-substitute/global} returns after
4037 a single match and output.
4040 For example, to collapse runs of tabs and spaces to a single hyphen
4044 (regexp-substitute/global #f "[ \t]+" "this is the text"
4046 @result{} "this-is-the-text"
4049 Or using a function to reverse the letters in each word,
4052 (regexp-substitute/global #f "[a-z]+" "to do and not-do"
4053 'pre (lambda (m) (string-reverse (match:substring m))) 'post)
4054 @result{} "ot od dna ton-od"
4057 Without the @code{post} symbol, just one regexp match is made. For
4058 example the following is the date example from
4059 @code{regexp-substitute} above, without the need for the separate
4060 @code{string-match} call.
4063 (define date-regex "([0-9][0-9][0-9][0-9])([0-9][0-9])([0-9][0-9])")
4064 (define s "Date 20020429 12am.")
4065 (regexp-substitute/global #f date-regex s
4066 'pre 2 "-" 3 "-" 1 'post " (" 0 ")")
4068 @result{} "Date 04-29-2002 12am. (20020429)"
4073 @node Match Structures
4074 @subsubsection Match Structures
4076 @cindex match structures
4078 A @dfn{match structure} is the object returned by @code{string-match} and
4079 @code{regexp-exec}. It describes which portion of a string, if any,
4080 matched the given regular expression. Match structures include: a
4081 reference to the string that was checked for matches; the starting and
4082 ending positions of the regexp match; and, if the regexp included any
4083 parenthesized subexpressions, the starting and ending positions of each
4086 In each of the regexp match functions described below, the @code{match}
4087 argument must be a match structure returned by a previous call to
4088 @code{string-match} or @code{regexp-exec}. Most of these functions
4089 return some information about the original target string that was
4090 matched against a regular expression; we will call that string
4091 @var{target} for easy reference.
4093 @c begin (scm-doc-string "regex.scm" "regexp-match?")
4094 @deffn {Scheme Procedure} regexp-match? obj
4095 Return @code{#t} if @var{obj} is a match structure returned by a
4096 previous call to @code{regexp-exec}, or @code{#f} otherwise.
4099 @c begin (scm-doc-string "regex.scm" "match:substring")
4100 @deffn {Scheme Procedure} match:substring match [n]
4101 Return the portion of @var{target} matched by subexpression number
4102 @var{n}. Submatch 0 (the default) represents the entire regexp match.
4103 If the regular expression as a whole matched, but the subexpression
4104 number @var{n} did not match, return @code{#f}.
4108 (define s (string-match "[0-9][0-9][0-9][0-9]" "blah2002foo"))
4112 ;; match starting at offset 6 in the string
4114 (string-match "[0-9][0-9][0-9][0-9]" "blah987654" 6))
4118 @c begin (scm-doc-string "regex.scm" "match:start")
4119 @deffn {Scheme Procedure} match:start match [n]
4120 Return the starting position of submatch number @var{n}.
4123 In the following example, the result is 4, since the match starts at
4127 (define s (string-match "[0-9][0-9][0-9][0-9]" "blah2002foo"))
4132 @c begin (scm-doc-string "regex.scm" "match:end")
4133 @deffn {Scheme Procedure} match:end match [n]
4134 Return the ending position of submatch number @var{n}.
4137 In the following example, the result is 8, since the match runs between
4138 characters 4 and 8 (i.e. the ``2002'').
4141 (define s (string-match "[0-9][0-9][0-9][0-9]" "blah2002foo"))
4146 @c begin (scm-doc-string "regex.scm" "match:prefix")
4147 @deffn {Scheme Procedure} match:prefix match
4148 Return the unmatched portion of @var{target} preceding the regexp match.
4151 (define s (string-match "[0-9][0-9][0-9][0-9]" "blah2002foo"))
4157 @c begin (scm-doc-string "regex.scm" "match:suffix")
4158 @deffn {Scheme Procedure} match:suffix match
4159 Return the unmatched portion of @var{target} following the regexp match.
4163 (define s (string-match "[0-9][0-9][0-9][0-9]" "blah2002foo"))
4168 @c begin (scm-doc-string "regex.scm" "match:count")
4169 @deffn {Scheme Procedure} match:count match
4170 Return the number of parenthesized subexpressions from @var{match}.
4171 Note that the entire regular expression match itself counts as a
4172 subexpression, and failed submatches are included in the count.
4175 @c begin (scm-doc-string "regex.scm" "match:string")
4176 @deffn {Scheme Procedure} match:string match
4177 Return the original @var{target} string.
4181 (define s (string-match "[0-9][0-9][0-9][0-9]" "blah2002foo"))
4183 @result{} "blah2002foo"
4187 @node Backslash Escapes
4188 @subsubsection Backslash Escapes
4190 Sometimes you will want a regexp to match characters like @samp{*} or
4191 @samp{$} exactly. For example, to check whether a particular string
4192 represents a menu entry from an Info node, it would be useful to match
4193 it against a regexp like @samp{^* [^:]*::}. However, this won't work;
4194 because the asterisk is a metacharacter, it won't match the @samp{*} at
4195 the beginning of the string. In this case, we want to make the first
4198 You can do this by preceding the metacharacter with a backslash
4199 character @samp{\}. (This is also called @dfn{quoting} the
4200 metacharacter, and is known as a @dfn{backslash escape}.) When Guile
4201 sees a backslash in a regular expression, it considers the following
4202 glyph to be an ordinary character, no matter what special meaning it
4203 would ordinarily have. Therefore, we can make the above example work by
4204 changing the regexp to @samp{^\* [^:]*::}. The @samp{\*} sequence tells
4205 the regular expression engine to match only a single asterisk in the
4208 Since the backslash is itself a metacharacter, you may force a regexp to
4209 match a backslash in the target string by preceding the backslash with
4210 itself. For example, to find variable references in a @TeX{} program,
4211 you might want to find occurrences of the string @samp{\let\} followed
4212 by any number of alphabetic characters. The regular expression
4213 @samp{\\let\\[A-Za-z]*} would do this: the double backslashes in the
4214 regexp each match a single backslash in the target string.
4216 @c begin (scm-doc-string "regex.scm" "regexp-quote")
4217 @deffn {Scheme Procedure} regexp-quote str
4218 Quote each special character found in @var{str} with a backslash, and
4219 return the resulting string.
4222 @strong{Very important:} Using backslash escapes in Guile source code
4223 (as in Emacs Lisp or C) can be tricky, because the backslash character
4224 has special meaning for the Guile reader. For example, if Guile
4225 encounters the character sequence @samp{\n} in the middle of a string
4226 while processing Scheme code, it replaces those characters with a
4227 newline character. Similarly, the character sequence @samp{\t} is
4228 replaced by a horizontal tab. Several of these @dfn{escape sequences}
4229 are processed by the Guile reader before your code is executed.
4230 Unrecognized escape sequences are ignored: if the characters @samp{\*}
4231 appear in a string, they will be translated to the single character
4234 This translation is obviously undesirable for regular expressions, since
4235 we want to be able to include backslashes in a string in order to
4236 escape regexp metacharacters. Therefore, to make sure that a backslash
4237 is preserved in a string in your Guile program, you must use @emph{two}
4238 consecutive backslashes:
4241 (define Info-menu-entry-pattern (make-regexp "^\\* [^:]*"))
4244 The string in this example is preprocessed by the Guile reader before
4245 any code is executed. The resulting argument to @code{make-regexp} is
4246 the string @samp{^\* [^:]*}, which is what we really want.
4248 This also means that in order to write a regular expression that matches
4249 a single backslash character, the regular expression string in the
4250 source code must include @emph{four} backslashes. Each consecutive pair
4251 of backslashes gets translated by the Guile reader to a single
4252 backslash, and the resulting double-backslash is interpreted by the
4253 regexp engine as matching a single backslash character. Hence:
4256 (define tex-variable-pattern (make-regexp "\\\\let\\\\=[A-Za-z]*"))
4259 The reason for the unwieldiness of this syntax is historical. Both
4260 regular expression pattern matchers and Unix string processing systems
4261 have traditionally used backslashes with the special meanings
4262 described above. The POSIX regular expression specification and ANSI C
4263 standard both require these semantics. Attempting to abandon either
4264 convention would cause other kinds of compatibility problems, possibly
4265 more severe ones. Therefore, without extending the Scheme reader to
4266 support strings with different quoting conventions (an ungainly and
4267 confusing extension when implemented in other languages), we must adhere
4268 to this cumbersome escape syntax.
4275 Symbols in Scheme are widely used in three ways: as items of discrete
4276 data, as lookup keys for alists and hash tables, and to denote variable
4279 A @dfn{symbol} is similar to a string in that it is defined by a
4280 sequence of characters. The sequence of characters is known as the
4281 symbol's @dfn{name}. In the usual case --- that is, where the symbol's
4282 name doesn't include any characters that could be confused with other
4283 elements of Scheme syntax --- a symbol is written in a Scheme program by
4284 writing the sequence of characters that make up the name, @emph{without}
4285 any quotation marks or other special syntax. For example, the symbol
4286 whose name is ``multiply-by-2'' is written, simply:
4292 Notice how this differs from a @emph{string} with contents
4293 ``multiply-by-2'', which is written with double quotation marks, like
4300 Looking beyond how they are written, symbols are different from strings
4301 in two important respects.
4303 The first important difference is uniqueness. If the same-looking
4304 string is read twice from two different places in a program, the result
4305 is two @emph{different} string objects whose contents just happen to be
4306 the same. If, on the other hand, the same-looking symbol is read twice
4307 from two different places in a program, the result is the @emph{same}
4308 symbol object both times.
4310 Given two read symbols, you can use @code{eq?} to test whether they are
4311 the same (that is, have the same name). @code{eq?} is the most
4312 efficient comparison operator in Scheme, and comparing two symbols like
4313 this is as fast as comparing, for example, two numbers. Given two
4314 strings, on the other hand, you must use @code{equal?} or
4315 @code{string=?}, which are much slower comparison operators, to
4316 determine whether the strings have the same contents.
4319 (define sym1 (quote hello))
4320 (define sym2 (quote hello))
4321 (eq? sym1 sym2) @result{} #t
4323 (define str1 "hello")
4324 (define str2 "hello")
4325 (eq? str1 str2) @result{} #f
4326 (equal? str1 str2) @result{} #t
4329 The second important difference is that symbols, unlike strings, are not
4330 self-evaluating. This is why we need the @code{(quote @dots{})}s in the
4331 example above: @code{(quote hello)} evaluates to the symbol named
4332 "hello" itself, whereas an unquoted @code{hello} is @emph{read} as the
4333 symbol named "hello" and evaluated as a variable reference @dots{} about
4334 which more below (@pxref{Symbol Variables}).
4337 * Symbol Data:: Symbols as discrete data.
4338 * Symbol Keys:: Symbols as lookup keys.
4339 * Symbol Variables:: Symbols as denoting variables.
4340 * Symbol Primitives:: Operations related to symbols.
4341 * Symbol Props:: Function slots and property lists.
4342 * Symbol Read Syntax:: Extended read syntax for symbols.
4343 * Symbol Uninterned:: Uninterned symbols.
4348 @subsubsection Symbols as Discrete Data
4350 Numbers and symbols are similar to the extent that they both lend
4351 themselves to @code{eq?} comparison. But symbols are more descriptive
4352 than numbers, because a symbol's name can be used directly to describe
4353 the concept for which that symbol stands.
4355 For example, imagine that you need to represent some colours in a
4356 computer program. Using numbers, you would have to choose arbitrarily
4357 some mapping between numbers and colours, and then take care to use that
4358 mapping consistently:
4361 ;; 1=red, 2=green, 3=purple
4363 (if (eq? (colour-of car) 1)
4368 You can make the mapping more explicit and the code more readable by
4376 (if (eq? (colour-of car) red)
4381 But the simplest and clearest approach is not to use numbers at all, but
4382 symbols whose names specify the colours that they refer to:
4385 (if (eq? (colour-of car) 'red)
4389 The descriptive advantages of symbols over numbers increase as the set
4390 of concepts that you want to describe grows. Suppose that a car object
4391 can have other properties as well, such as whether it has or uses:
4395 automatic or manual transmission
4397 leaded or unleaded fuel
4399 power steering (or not).
4403 Then a car's combined property set could be naturally represented and
4404 manipulated as a list of symbols:
4407 (properties-of car1)
4409 (red manual unleaded power-steering)
4411 (if (memq 'power-steering (properties-of car1))
4412 (display "Unfit people can drive this car.\n")
4413 (display "You'll need strong arms to drive this car!\n"))
4415 Unfit people can drive this car.
4418 Remember, the fundamental property of symbols that we are relying on
4419 here is that an occurrence of @code{'red} in one part of a program is an
4420 @emph{indistinguishable} symbol from an occurrence of @code{'red} in
4421 another part of a program; this means that symbols can usefully be
4422 compared using @code{eq?}. At the same time, symbols have naturally
4423 descriptive names. This combination of efficiency and descriptive power
4424 makes them ideal for use as discrete data.
4428 @subsubsection Symbols as Lookup Keys
4430 Given their efficiency and descriptive power, it is natural to use
4431 symbols as the keys in an association list or hash table.
4433 To illustrate this, consider a more structured representation of the car
4434 properties example from the preceding subsection. Rather than
4435 mixing all the properties up together in a flat list, we could use an
4436 association list like this:
4439 (define car1-properties '((colour . red)
4440 (transmission . manual)
4442 (steering . power-assisted)))
4445 Notice how this structure is more explicit and extensible than the flat
4446 list. For example it makes clear that @code{manual} refers to the
4447 transmission rather than, say, the windows or the locking of the car.
4448 It also allows further properties to use the same symbols among their
4449 possible values without becoming ambiguous:
4452 (define car1-properties '((colour . red)
4453 (transmission . manual)
4455 (steering . power-assisted)
4457 (locking . manual)))
4460 With a representation like this, it is easy to use the efficient
4461 @code{assq-XXX} family of procedures (@pxref{Association Lists}) to
4462 extract or change individual pieces of information:
4465 (assq-ref car1-properties 'fuel) @result{} unleaded
4466 (assq-ref car1-properties 'transmission) @result{} manual
4468 (assq-set! car1-properties 'seat-colour 'black)
4471 (transmission . manual)
4473 (steering . power-assisted)
4474 (seat-colour . black)
4475 (locking . manual)))
4478 Hash tables also have keys, and exactly the same arguments apply to the
4479 use of symbols in hash tables as in association lists. The hash value
4480 that Guile uses to decide where to add a symbol-keyed entry to a hash
4481 table can be obtained by calling the @code{symbol-hash} procedure:
4483 @deffn {Scheme Procedure} symbol-hash symbol
4484 @deffnx {C Function} scm_symbol_hash (symbol)
4485 Return a hash value for @var{symbol}.
4488 See @ref{Hash Tables} for information about hash tables in general, and
4489 for why you might choose to use a hash table rather than an association
4493 @node Symbol Variables
4494 @subsubsection Symbols as Denoting Variables
4496 When an unquoted symbol in a Scheme program is evaluated, it is
4497 interpreted as a variable reference, and the result of the evaluation is
4498 the appropriate variable's value.
4500 For example, when the expression @code{(string-length "abcd")} is read
4501 and evaluated, the sequence of characters @code{string-length} is read
4502 as the symbol whose name is "string-length". This symbol is associated
4503 with a variable whose value is the procedure that implements string
4504 length calculation. Therefore evaluation of the @code{string-length}
4505 symbol results in that procedure.
4507 The details of the connection between an unquoted symbol and the
4508 variable to which it refers are explained elsewhere. See @ref{Binding
4509 Constructs}, for how associations between symbols and variables are
4510 created, and @ref{Modules}, for how those associations are affected by
4511 Guile's module system.
4514 @node Symbol Primitives
4515 @subsubsection Operations Related to Symbols
4517 Given any Scheme value, you can determine whether it is a symbol using
4518 the @code{symbol?} primitive:
4521 @deffn {Scheme Procedure} symbol? obj
4522 @deffnx {C Function} scm_symbol_p (obj)
4523 Return @code{#t} if @var{obj} is a symbol, otherwise return
4527 @deftypefn {C Function} int scm_is_symbol (SCM val)
4528 Equivalent to @code{scm_is_true (scm_symbol_p (val))}.
4531 Once you know that you have a symbol, you can obtain its name as a
4532 string by calling @code{symbol->string}. Note that Guile differs by
4533 default from R5RS on the details of @code{symbol->string} as regards
4536 @rnindex symbol->string
4537 @deffn {Scheme Procedure} symbol->string s
4538 @deffnx {C Function} scm_symbol_to_string (s)
4539 Return the name of symbol @var{s} as a string. By default, Guile reads
4540 symbols case-sensitively, so the string returned will have the same case
4541 variation as the sequence of characters that caused @var{s} to be
4544 If Guile is set to read symbols case-insensitively (as specified by
4545 R5RS), and @var{s} comes into being as part of a literal expression
4546 (@pxref{Literal expressions,,,r5rs, The Revised^5 Report on Scheme}) or
4547 by a call to the @code{read} or @code{string-ci->symbol} procedures,
4548 Guile converts any alphabetic characters in the symbol's name to
4549 lower case before creating the symbol object, so the string returned
4550 here will be in lower case.
4552 If @var{s} was created by @code{string->symbol}, the case of characters
4553 in the string returned will be the same as that in the string that was
4554 passed to @code{string->symbol}, regardless of Guile's case-sensitivity
4555 setting at the time @var{s} was created.
4557 It is an error to apply mutation procedures like @code{string-set!} to
4558 strings returned by this procedure.
4561 Most symbols are created by writing them literally in code. However it
4562 is also possible to create symbols programmatically using the following
4563 @code{string->symbol} and @code{string-ci->symbol} procedures:
4565 @rnindex string->symbol
4566 @deffn {Scheme Procedure} string->symbol string
4567 @deffnx {C Function} scm_string_to_symbol (string)
4568 Return the symbol whose name is @var{string}. This procedure can create
4569 symbols with names containing special characters or letters in the
4570 non-standard case, but it is usually a bad idea to create such symbols
4571 because in some implementations of Scheme they cannot be read as
4575 @deffn {Scheme Procedure} string-ci->symbol str
4576 @deffnx {C Function} scm_string_ci_to_symbol (str)
4577 Return the symbol whose name is @var{str}. If Guile is currently
4578 reading symbols case-insensitively, @var{str} is converted to lowercase
4579 before the returned symbol is looked up or created.
4582 The following examples illustrate Guile's detailed behaviour as regards
4583 the case-sensitivity of symbols:
4586 (read-enable 'case-insensitive) ; R5RS compliant behaviour
4588 (symbol->string 'flying-fish) @result{} "flying-fish"
4589 (symbol->string 'Martin) @result{} "martin"
4591 (string->symbol "Malvina")) @result{} "Malvina"
4593 (eq? 'mISSISSIppi 'mississippi) @result{} #t
4594 (string->symbol "mISSISSIppi") @result{} mISSISSIppi
4595 (eq? 'bitBlt (string->symbol "bitBlt")) @result{} #f
4597 (string->symbol (symbol->string 'LolliPop))) @result{} #t
4598 (string=? "K. Harper, M.D."
4600 (string->symbol "K. Harper, M.D."))) @result{} #t
4602 (read-disable 'case-insensitive) ; Guile default behaviour
4604 (symbol->string 'flying-fish) @result{} "flying-fish"
4605 (symbol->string 'Martin) @result{} "Martin"
4607 (string->symbol "Malvina")) @result{} "Malvina"
4609 (eq? 'mISSISSIppi 'mississippi) @result{} #f
4610 (string->symbol "mISSISSIppi") @result{} mISSISSIppi
4611 (eq? 'bitBlt (string->symbol "bitBlt")) @result{} #t
4613 (string->symbol (symbol->string 'LolliPop))) @result{} #t
4614 (string=? "K. Harper, M.D."
4616 (string->symbol "K. Harper, M.D."))) @result{} #t
4619 From C, there are lower level functions that construct a Scheme symbol
4620 from a C string in the current locale encoding.
4622 When you want to do more from C, you should convert between symbols
4623 and strings using @code{scm_symbol_to_string} and
4624 @code{scm_string_to_symbol} and work with the strings.
4626 @deffn {C Function} scm_from_locale_symbol (const char *name)
4627 @deffnx {C Function} scm_from_locale_symboln (const char *name, size_t len)
4628 Construct and return a Scheme symbol whose name is specified by
4629 @var{name}. For @code{scm_from_locale_symbol}, @var{name} must be null
4630 terminated; for @code{scm_from_locale_symboln} the length of @var{name} is
4631 specified explicitly by @var{len}.
4634 @deftypefn {C Function} SCM scm_take_locale_symbol (char *str)
4635 @deftypefnx {C Function} SCM scm_take_locale_symboln (char *str, size_t len)
4636 Like @code{scm_from_locale_symbol} and @code{scm_from_locale_symboln},
4637 respectively, but also frees @var{str} with @code{free} eventually.
4638 Thus, you can use this function when you would free @var{str} anyway
4639 immediately after creating the Scheme string. In certain cases, Guile
4640 can then use @var{str} directly as its internal representation.
4643 The size of a symbol can also be obtained from C:
4645 @deftypefn {C Function} size_t scm_c_symbol_length (SCM sym)
4646 Return the number of characters in @var{sym}.
4649 Finally, some applications, especially those that generate new Scheme
4650 code dynamically, need to generate symbols for use in the generated
4651 code. The @code{gensym} primitive meets this need:
4653 @deffn {Scheme Procedure} gensym [prefix]
4654 @deffnx {C Function} scm_gensym (prefix)
4655 Create a new symbol with a name constructed from a prefix and a counter
4656 value. The string @var{prefix} can be specified as an optional
4657 argument. Default prefix is @samp{@w{ g}}. The counter is increased by 1
4658 at each call. There is no provision for resetting the counter.
4661 The symbols generated by @code{gensym} are @emph{likely} to be unique,
4662 since their names begin with a space and it is only otherwise possible
4663 to generate such symbols if a programmer goes out of their way to do
4664 so. Uniqueness can be guaranteed by instead using uninterned symbols
4665 (@pxref{Symbol Uninterned}), though they can't be usefully written out
4670 @subsubsection Function Slots and Property Lists
4672 In traditional Lisp dialects, symbols are often understood as having
4673 three kinds of value at once:
4677 a @dfn{variable} value, which is used when the symbol appears in
4678 code in a variable reference context
4681 a @dfn{function} value, which is used when the symbol appears in
4682 code in a function name position (i.e. as the first element in an
4686 a @dfn{property list} value, which is used when the symbol is given as
4687 the first argument to Lisp's @code{put} or @code{get} functions.
4690 Although Scheme (as one of its simplifications with respect to Lisp)
4691 does away with the distinction between variable and function namespaces,
4692 Guile currently retains some elements of the traditional structure in
4693 case they turn out to be useful when implementing translators for other
4694 languages, in particular Emacs Lisp.
4696 Specifically, Guile symbols have two extra slots. for a symbol's
4697 property list, and for its ``function value.'' The following procedures
4698 are provided to access these slots.
4700 @deffn {Scheme Procedure} symbol-fref symbol
4701 @deffnx {C Function} scm_symbol_fref (symbol)
4702 Return the contents of @var{symbol}'s @dfn{function slot}.
4705 @deffn {Scheme Procedure} symbol-fset! symbol value
4706 @deffnx {C Function} scm_symbol_fset_x (symbol, value)
4707 Set the contents of @var{symbol}'s function slot to @var{value}.
4710 @deffn {Scheme Procedure} symbol-pref symbol
4711 @deffnx {C Function} scm_symbol_pref (symbol)
4712 Return the @dfn{property list} currently associated with @var{symbol}.
4715 @deffn {Scheme Procedure} symbol-pset! symbol value
4716 @deffnx {C Function} scm_symbol_pset_x (symbol, value)
4717 Set @var{symbol}'s property list to @var{value}.
4720 @deffn {Scheme Procedure} symbol-property sym prop
4721 From @var{sym}'s property list, return the value for property
4722 @var{prop}. The assumption is that @var{sym}'s property list is an
4723 association list whose keys are distinguished from each other using
4724 @code{equal?}; @var{prop} should be one of the keys in that list. If
4725 the property list has no entry for @var{prop}, @code{symbol-property}
4729 @deffn {Scheme Procedure} set-symbol-property! sym prop val
4730 In @var{sym}'s property list, set the value for property @var{prop} to
4731 @var{val}, or add a new entry for @var{prop}, with value @var{val}, if
4732 none already exists. For the structure of the property list, see
4733 @code{symbol-property}.
4736 @deffn {Scheme Procedure} symbol-property-remove! sym prop
4737 From @var{sym}'s property list, remove the entry for property
4738 @var{prop}, if there is one. For the structure of the property list,
4739 see @code{symbol-property}.
4742 Support for these extra slots may be removed in a future release, and it
4743 is probably better to avoid using them. For a more modern and Schemely
4744 approach to properties, see @ref{Object Properties}.
4747 @node Symbol Read Syntax
4748 @subsubsection Extended Read Syntax for Symbols
4750 The read syntax for a symbol is a sequence of letters, digits, and
4751 @dfn{extended alphabetic characters}, beginning with a character that
4752 cannot begin a number. In addition, the special cases of @code{+},
4753 @code{-}, and @code{...} are read as symbols even though numbers can
4754 begin with @code{+}, @code{-} or @code{.}.
4756 Extended alphabetic characters may be used within identifiers as if
4757 they were letters. The set of extended alphabetic characters is:
4760 ! $ % & * + - . / : < = > ? @@ ^ _ ~
4763 In addition to the standard read syntax defined above (which is taken
4764 from R5RS (@pxref{Formal syntax,,,r5rs,The Revised^5 Report on
4765 Scheme})), Guile provides an extended symbol read syntax that allows the
4766 inclusion of unusual characters such as space characters, newlines and
4767 parentheses. If (for whatever reason) you need to write a symbol
4768 containing characters not mentioned above, you can do so as follows.
4772 Begin the symbol with the characters @code{#@{},
4775 write the characters of the symbol and
4778 finish the symbol with the characters @code{@}#}.
4781 Here are a few examples of this form of read syntax. The first symbol
4782 needs to use extended syntax because it contains a space character, the
4783 second because it contains a line break, and the last because it looks
4795 Although Guile provides this extended read syntax for symbols,
4796 widespread usage of it is discouraged because it is not portable and not
4800 @node Symbol Uninterned
4801 @subsubsection Uninterned Symbols
4803 What makes symbols useful is that they are automatically kept unique.
4804 There are no two symbols that are distinct objects but have the same
4805 name. But of course, there is no rule without exception. In addition
4806 to the normal symbols that have been discussed up to now, you can also
4807 create special @dfn{uninterned} symbols that behave slightly
4810 To understand what is different about them and why they might be useful,
4811 we look at how normal symbols are actually kept unique.
4813 Whenever Guile wants to find the symbol with a specific name, for
4814 example during @code{read} or when executing @code{string->symbol}, it
4815 first looks into a table of all existing symbols to find out whether a
4816 symbol with the given name already exists. When this is the case, Guile
4817 just returns that symbol. When not, a new symbol with the name is
4818 created and entered into the table so that it can be found later.
4820 Sometimes you might want to create a symbol that is guaranteed `fresh',
4821 i.e. a symbol that did not exist previously. You might also want to
4822 somehow guarantee that no one else will ever unintentionally stumble
4823 across your symbol in the future. These properties of a symbol are
4824 often needed when generating code during macro expansion. When
4825 introducing new temporary variables, you want to guarantee that they
4826 don't conflict with variables in other people's code.
4828 The simplest way to arrange for this is to create a new symbol but
4829 not enter it into the global table of all symbols. That way, no one
4830 will ever get access to your symbol by chance. Symbols that are not in
4831 the table are called @dfn{uninterned}. Of course, symbols that
4832 @emph{are} in the table are called @dfn{interned}.
4834 You create new uninterned symbols with the function @code{make-symbol}.
4835 You can test whether a symbol is interned or not with
4836 @code{symbol-interned?}.
4838 Uninterned symbols break the rule that the name of a symbol uniquely
4839 identifies the symbol object. Because of this, they can not be written
4840 out and read back in like interned symbols. Currently, Guile has no
4841 support for reading uninterned symbols. Note that the function
4842 @code{gensym} does not return uninterned symbols for this reason.
4844 @deffn {Scheme Procedure} make-symbol name
4845 @deffnx {C Function} scm_make_symbol (name)
4846 Return a new uninterned symbol with the name @var{name}. The returned
4847 symbol is guaranteed to be unique and future calls to
4848 @code{string->symbol} will not return it.
4851 @deffn {Scheme Procedure} symbol-interned? symbol
4852 @deffnx {C Function} scm_symbol_interned_p (symbol)
4853 Return @code{#t} if @var{symbol} is interned, otherwise return
4860 (define foo-1 (string->symbol "foo"))
4861 (define foo-2 (string->symbol "foo"))
4862 (define foo-3 (make-symbol "foo"))
4863 (define foo-4 (make-symbol "foo"))
4867 ; Two interned symbols with the same name are the same object,
4871 ; but a call to make-symbol with the same name returns a
4876 ; A call to make-symbol always returns a new object, even for
4880 @result{} #<uninterned-symbol foo 8085290>
4881 ; Uninterned symbols print differently from interned symbols,
4885 ; but they are still symbols,
4887 (symbol-interned? foo-3)
4889 ; just not interned.
4894 @subsection Keywords
4897 Keywords are self-evaluating objects with a convenient read syntax that
4898 makes them easy to type.
4900 Guile's keyword support conforms to R5RS, and adds a (switchable) read
4901 syntax extension to permit keywords to begin with @code{:} as well as
4902 @code{#:}, or to end with @code{:}.
4905 * Why Use Keywords?:: Motivation for keyword usage.
4906 * Coding With Keywords:: How to use keywords.
4907 * Keyword Read Syntax:: Read syntax for keywords.
4908 * Keyword Procedures:: Procedures for dealing with keywords.
4911 @node Why Use Keywords?
4912 @subsubsection Why Use Keywords?
4914 Keywords are useful in contexts where a program or procedure wants to be
4915 able to accept a large number of optional arguments without making its
4916 interface unmanageable.
4918 To illustrate this, consider a hypothetical @code{make-window}
4919 procedure, which creates a new window on the screen for drawing into
4920 using some graphical toolkit. There are many parameters that the caller
4921 might like to specify, but which could also be sensibly defaulted, for
4926 color depth -- Default: the color depth for the screen
4929 background color -- Default: white
4932 width -- Default: 600
4935 height -- Default: 400
4938 If @code{make-window} did not use keywords, the caller would have to
4939 pass in a value for each possible argument, remembering the correct
4940 argument order and using a special value to indicate the default value
4944 (make-window 'default ;; Color depth
4945 'default ;; Background color
4948 @dots{}) ;; More make-window arguments
4951 With keywords, on the other hand, defaulted arguments are omitted, and
4952 non-default arguments are clearly tagged by the appropriate keyword. As
4953 a result, the invocation becomes much clearer:
4956 (make-window #:width 800 #:height 100)
4959 On the other hand, for a simpler procedure with few arguments, the use
4960 of keywords would be a hindrance rather than a help. The primitive
4961 procedure @code{cons}, for example, would not be improved if it had to
4965 (cons #:car x #:cdr y)
4968 So the decision whether to use keywords or not is purely pragmatic: use
4969 them if they will clarify the procedure invocation at point of call.
4971 @node Coding With Keywords
4972 @subsubsection Coding With Keywords
4974 If a procedure wants to support keywords, it should take a rest argument
4975 and then use whatever means is convenient to extract keywords and their
4976 corresponding arguments from the contents of that rest argument.
4978 The following example illustrates the principle: the code for
4979 @code{make-window} uses a helper procedure called
4980 @code{get-keyword-value} to extract individual keyword arguments from
4984 (define (get-keyword-value args keyword default)
4985 (let ((kv (memq keyword args)))
4986 (if (and kv (>= (length kv) 2))
4990 (define (make-window . args)
4991 (let ((depth (get-keyword-value args #:depth screen-depth))
4992 (bg (get-keyword-value args #:bg "white"))
4993 (width (get-keyword-value args #:width 800))
4994 (height (get-keyword-value args #:height 100))
4999 But you don't need to write @code{get-keyword-value}. The @code{(ice-9
5000 optargs)} module provides a set of powerful macros that you can use to
5001 implement keyword-supporting procedures like this:
5004 (use-modules (ice-9 optargs))
5006 (define (make-window . args)
5007 (let-keywords args #f ((depth screen-depth)
5015 Or, even more economically, like this:
5018 (use-modules (ice-9 optargs))
5020 (define* (make-window #:key (depth screen-depth)
5027 For further details on @code{let-keywords}, @code{define*} and other
5028 facilities provided by the @code{(ice-9 optargs)} module, see
5029 @ref{Optional Arguments}.
5032 @node Keyword Read Syntax
5033 @subsubsection Keyword Read Syntax
5035 Guile, by default, only recognizes a keyword syntax that is compatible
5036 with R5RS. A token of the form @code{#:NAME}, where @code{NAME} has the
5037 same syntax as a Scheme symbol (@pxref{Symbol Read Syntax}), is the
5038 external representation of the keyword named @code{NAME}. Keyword
5039 objects print using this syntax as well, so values containing keyword
5040 objects can be read back into Guile. When used in an expression,
5041 keywords are self-quoting objects.
5043 If the @code{keyword} read option is set to @code{'prefix}, Guile also
5044 recognizes the alternative read syntax @code{:NAME}. Otherwise, tokens
5045 of the form @code{:NAME} are read as symbols, as required by R5RS.
5047 @cindex SRFI-88 keyword syntax
5049 If the @code{keyword} read option is set to @code{'postfix}, Guile
5050 recognizes the SRFI-88 read syntax @code{NAME:} (@pxref{SRFI-88}).
5051 Otherwise, tokens of this form are read as symbols.
5053 To enable and disable the alternative non-R5RS keyword syntax, you use
5054 the @code{read-set!} procedure documented in @ref{User level options
5055 interfaces} and @ref{Reader options}. Note that the @code{prefix} and
5056 @code{postfix} syntax are mutually exclusive.
5059 (read-set! keywords 'prefix)
5069 (read-set! keywords 'postfix)
5079 (read-set! keywords #f)
5087 ERROR: In expression :type:
5088 ERROR: Unbound variable: :type
5089 ABORT: (unbound-variable)
5092 @node Keyword Procedures
5093 @subsubsection Keyword Procedures
5095 @deffn {Scheme Procedure} keyword? obj
5096 @deffnx {C Function} scm_keyword_p (obj)
5097 Return @code{#t} if the argument @var{obj} is a keyword, else
5101 @deffn {Scheme Procedure} keyword->symbol keyword
5102 @deffnx {C Function} scm_keyword_to_symbol (keyword)
5103 Return the symbol with the same name as @var{keyword}.
5106 @deffn {Scheme Procedure} symbol->keyword symbol
5107 @deffnx {C Function} scm_symbol_to_keyword (symbol)
5108 Return the keyword with the same name as @var{symbol}.
5111 @deftypefn {C Function} int scm_is_keyword (SCM obj)
5112 Equivalent to @code{scm_is_true (scm_keyword_p (@var{obj}))}.
5115 @deftypefn {C Function} SCM scm_from_locale_keyword (const char *str)
5116 @deftypefnx {C Function} SCM scm_from_locale_keywordn (const char *str, size_t len)
5117 Equivalent to @code{scm_symbol_to_keyword (scm_from_locale_symbol
5118 (@var{str}))} and @code{scm_symbol_to_keyword (scm_from_locale_symboln
5119 (@var{str}, @var{len}))}, respectively.
5123 @subsection ``Functionality-Centric'' Data Types
5125 Procedures and macros are documented in their own chapter: see
5126 @ref{Procedures and Macros}.
5128 Variable objects are documented as part of the description of Guile's
5129 module system: see @ref{Variables}.
5131 Asyncs, dynamic roots and fluids are described in the chapter on
5132 scheduling: see @ref{Scheduling}.
5134 Hooks are documented in the chapter on general utility functions: see
5137 Ports are described in the chapter on I/O: see @ref{Input and Output}.
5141 @c TeX-master: "guile.texi"