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grammar: introduce %empty

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Provide a means to explicitly denote empty right-hand sides of rules:
instead of

  exp:  { ... }

allow

  exp: %empty { ... }

Make sure that %empty is properly used.

With help from Joel E. Denny and Gabriel Rassoul.
http://lists.gnu.org/archive/html/bison-patches/2013-01/msg00142.html

* src/reader.h, src/reader.c (grammar_current_rule_empty_set): New.
* src/parse-gram.y (%empty): New token.
Use it.
* src/scan-gram.l (%empty): Scan it.
* src/reader.c (grammar_rule_check): Check that %empty is properly used.
* tests/actions.at (Invalid uses of %empty, Valid uses of %empty): New.
This commit is contained in:
Akim Demaille
2013-01-29 14:19:40 +01:00
parent 9e4917b2a1
commit ae2b48f5c0
8 changed files with 98 additions and 0 deletions
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17
src/reader.c
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@@ -333,6 +333,12 @@ grammar_rule_check (const symbol_list *r)
}
}
/* Check that %empty => empty rule. */
if (r->percent_empty_loc.start.file
&& r->next && r->next->content.sym)
complain (&r->percent_empty_loc, complaint,
_("%%empty on non-empty rule"));
/* See comments in grammar_current_rule_prec_set for how POSIX
mandates this complaint. It's only for identifiers, so skip
it for char literals and strings, which are always tokens. */
@@ -434,6 +440,17 @@ grammar_current_rule_prec_set (symbol *precsym, location loc)
current_rule->ruleprec = precsym;
}
/* Set %empty for the current rule. */
void
grammar_current_rule_empty_set (location loc)
{
if (current_rule->percent_empty_loc.start.file)
complain (&loc, complaint, _("only one %s allowed per rule"), "%empty");
else
current_rule->percent_empty_loc = loc;
}
/* Attach dynamic precedence DPREC to the current rule. */
void