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doc: clarify the purpose of symbol_type constructors

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Reported by Frank Heckenbach.
http://lists.gnu.org/archive/html/bug-bison/2019-02/msg00006.html

* doc/bison.texi (Complete Symbols): Here.
This commit is contained in:
Akim Demaille
2019-02-04 12:26:35 +01:00
parent 9f26e6d6b3
commit aa21c457f2
1 changed files with 13 additions and 7 deletions
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doc/bison.texi
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@@ -11732,15 +11732,21 @@ symbol_type (int token, const int&, const location_type&);
symbol_type (int token, const location_type&);
@end example
@noindent
which should be used in a Flex-scanner as follows.
Correct matching between token types and value types is checked via
@code{assert}; for instance, @samp{symbol_type (ID, 42)} would abort. Named
constructors are preferable (see below), as they offer better type safety
(for instance @samp{make_ID (42)} would not even compile), but symbol_type
constructors may help when token types are discovered at run-time, e.g.,
@example
%%
[a-z]+ return yy::parser::symbol_type (TOK_IDENTIFIER, yytext, loc);
[0-9]+ return yy::parser::symbol_type (TOK_INTEGER, text_to_int (yytext), loc);
":" return yy::parser::symbol_type (':', loc);
<<EOF>> return yy::parser::symbol_type (0, loc);
@group
[a-z]+ @{
if (auto i = lookup_keyword (yytext))
return yy::parser::symbol_type (i, loc);
else
return yy::parser::make_ID (yytext, loc);
@}
@end group
@end example
@sp 1