When yyparse detects a syntax error, it calls the error reporting
function yyerror to print an error message (usually but not always
"parse error"). It is up to the programmer to supply yyerror
(see section Parser C-Language Interface), so here is the definition we will use:
#include <stdio.h>
yyerror (s) /* Called by yyparse on error */
char *s;
{
printf ("%s\n", s);
}
After yyerror returns, the Bison parser may recover from the error
and continue parsing if the grammar contains a suitable error rule
(see section Error Recovery). Otherwise, yyparse returns nonzero. We
have not written any error rules in this example, so any invalid input will
cause the calculator program to exit. This is not clean behavior for a
real calculator, but it is adequate in the first example.
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