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doc: document YYEOF, YYUNDEF and YYerror

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* doc/bison.texi (Special Tokens): New.
* examples/c/bistromathic/parse.y: Formatting changes.
This commit is contained in:
Akim Demaille
2020-04-29 08:23:55 +02:00
parent 547545a795
commit 99efa35369
4 changed files with 72 additions and 29 deletions
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doc/bison.texi
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@@ -314,6 +314,7 @@ Parser C-Language Interface
The Lexical Analyzer Function @code{yylex}
* Calling Convention:: How @code{yyparse} calls @code{yylex}.
* Special Tokens:: Signaling end-of-file and errors to the parser.
* Tokens from Literals:: Finding token kinds from string aliases.
* Token Values:: How @code{yylex} must return the semantic value
of the token it has read.
@@ -7115,6 +7116,7 @@ that need it. @xref{Invocation}.
@menu
* Calling Convention:: How @code{yyparse} calls @code{yylex}.
* Special Tokens:: Signaling end-of-file and errors to the parser.
* Tokens from Literals:: Finding token kinds from string aliases.
* Token Values:: How @code{yylex} must return the semantic value
of the token it has read.
@@ -7168,6 +7170,49 @@ This interface has been designed so that the output from the @code{lex}
utility can be used without change as the definition of @code{yylex}.
@node Special Tokens
@subsection Special Tokens
In addition to the user defined tokens, Bison generates a few special tokens
that @code{yylex} may return.
The @code{YYEOF} token denotes the end of file, and signals to the parser
that there is nothing left afterwards. @xref{Calling Convention}, for an
example.
Returning @code{YYUNDEF} tells the parser that some lexical error was found.
It will emit an error message about an ``invalid token'', and enter
error-recovery (@pxref{Error Recovery}). Returning an unknown token kind
results in the exact same behavior.
Returning @code{YYerror} requires the parser to enter error-recovery
@emph{without} emitting an error message. This way the lexical analyzer can
produce an accurate error messages about the invalid input (something the
parser cannot do), and yet benefit from the error-recovery features of the
parser.
@example
int
yylex (void)
@{
@dots{}
switch (c)
@{
@dots{}
case '0': case '1': case '2': case '3': case '4':
case '5': case '6': case '7': case '8': case '9':
@dots{}
return TOK_NUM;
@dots{}
case EOF:
return YYEOF;
default:
yyerror ("syntax error: invalid character: %c", c);
return YYerror;
@}
@}
@end example
@node Tokens from Literals
@subsection Finding Tokens by String Literals