Issue 3954: Report bad return values in user-readable form

An error message like

  markup-def.ly:11:20: error: music function cannot return
  (#<procedure line-markup (layout props args)> (Test))

is not really user-readable.  At the price of some possible ambiguity,
using a LilyPond syntax form like

  markup-def.ly:11:20: error: music function cannot return \markup \line { Test}

is quite preferable.

diff --git a/scm/ly-syntax-constructors.scm b/scm/ly-syntax-constructors.scm
index 2712fc21f3c331228192f563285b8b34fc246777..73e871686e3ee079f488ec646b0d550d870206ba 100644 (file)
--- a/scm/ly-syntax-constructors.scm
+++ b/scm/ly-syntax-constructors.scm
@@ -64,7 +64,8 @@
           (if good
               (ly:parser-error parser
                                (format #f (_ "~a function cannot return ~a")
-                                       (type-name pred) m)
+                                       (type-name pred)
+                                       (value->lily-string m parser))
                                loc))
           (and (pair? (car sig)) (cdar sig))))))