all: fix the interface of yyexpected_tokens

The user gives yyexpected_tokens a limit: the max number of tokens she
wants to hear about.  That's because an error message that reports a
bazillion of possible tokens is useless.

In that case yyexpected_tokens returned 0, so the user would not know
if there are too many expected tokens or none (yes, that's possible).

There are several ways to tell the user in which situation she's in:

- return some E2MANY, a negative value.  Then it makes the pattern

    int argsize = yypcontext_expected_tokens (ctx, arg, ARGS_MAX);
    if (argsize < 0)
      return argsize;

  no longer valid, as for E2MANY (i) the user must generate the error
  message anyway, and (ii) she should not return E2MANY

- return ARGS_MAX + 1.  Then it makes it dangerous for the user, as
  she has to iterate update `min (ARGS_MAX, argsize)`.

Returning 0 is definitely simpler and safer for the user, as it tells
her "this is not an error, just generate your message without a list
of expecting tokens".  So let's still return 0, but set arg[0] to the
empty token when the list is really empty.

* data/skeletons/glr.c, data/skeletons/lalr1.cc, data/skeletons/lalr1.java
* data/skeletons/yacc.c (yyexpected_tokens): Put the empty symbol
first if there are no possible tokens at all.
* examples/c/bistromathic/parse.y: Demonstrate how to use that.

2 files changed, 7 insertions, 4 deletions

diff --git a/examples/c/bistromathic/bistromathic.test b/examples/c/bistromathic/bistromathic.test
index 3bb83b77..495146bd 100755
--- a/examples/c/bistromathic/bistromathic.test
+++ b/examples/c/bistromathic/bistromathic.test
@@ -96,7 +96,7 @@ cat >input <<EOF
 EOF
 run 0 '> *
 > ''
-err: 1.1: syntax error: expected end of file or - or ( or exit or number or function or variable before *'
+err: 1.1: syntax error: expected end of file or - or ( or exit or number or function etc., before *'
 
 cat >input <<EOF
 1 + 2 * * 3
diff --git a/examples/c/bistromathic/parse.y b/examples/c/bistromathic/parse.y
index 38fedb46..c2ddb124 100644
--- a/examples/c/bistromathic/parse.y
+++ b/examples/c/bistromathic/parse.y
@@ -333,7 +333,7 @@ error_format_string (int argc)
     case 5: return _("%@: syntax error: expected %0e or %1e or %2e or %3e before %u");
     case 6: return _("%@: syntax error: expected %0e or %1e or %2e or %3e or %4e before %u");
     case 7: return _("%@: syntax error: expected %0e or %1e or %2e or %3e or %4e or %5e before %u");
-    case 8: return _("%@: syntax error: expected %0e or %1e or %2e or %3e or %4e or %5e or %6e before %u");
+    case 8: return _("%@: syntax error: expected %0e or %1e or %2e or %3e or %4e or %5e etc., before %u");
     }
 }
 
@@ -341,12 +341,15 @@ error_format_string (int argc)
 int
 yyreport_syntax_error (const yypcontext_t *ctx)
 {
-  enum { ARGS_MAX = 7 };
+  enum { ARGS_MAX = 6 };
   yysymbol_kind_t arg[ARGS_MAX];
   int argsize = yypcontext_expected_tokens (ctx, arg, ARGS_MAX);
   if (argsize < 0)
     return argsize;
-  const char *format = error_format_string (1 + argsize);
+  const int too_many_expected_tokens = argsize == 0 && arg[0] != YYSYMBOL_YYEMPTY;
+  if (too_many_expected_tokens)
+    argsize = ARGS_MAX;
+  const char *format = error_format_string (1 + argsize + too_many_expected_tokens);
 
   while (*format)
     // %@: location.