slv@seg.npl.co.uk (S Voogd) (01/10/91)
Dear comp.compiler-readers,
While working with YACC, we came across a POSSIBLE BUG
in the precedence mechanism of YACC.
You can give a token a precedence and associativity by
declaring this in the declaration-part of a YACC-file
According to the manual you can also change the precedence
and associativity of a token in the grammar-rules by
using the '%prec'-mechanism.
But it seems to us that this mechanism doesn't work!!!
We enclose a simple desk calculator and reverse the usual
precedence of '+' with '*' using the '%prec'-mechanism.
It seems that the associativity of tokens can be changed.(?)
But their precedence levels are now the same.
Are we doing something wrong?
Is this a BUG on YACC?
If it is a bug, is there a way of changing the prec. and
ass. of tokens ? (We really need it!)
Please mail us with comments, results etc. or discuss this on the net.
Simon Voogd Graeme Parkin
e-mail : slv@seg.npl.co.uk gip@seg.npl.co.uk
Tel : +44 81 943 7009 +44 81 943 7006
National Physical Laboratory (UK)
--------Test Results-----------------------------------------
normal precedences :
4+7*2 = 4+(7*2) = 18
2*7+4 = (2*7)+4 = 18
reversed precedences :
4+7*2 = (4+7)*2 = 22
2*7+4 = 2*(7+4) = 22
BUT the enclosed calculator gives the following results :
4+7*2 = 22 [ = (4+7)*2 ]
2*7+4 = 18 [ = (2*7)+4 ] (surely should be 22 !!!!!)
--------Simple Desk Calculator-------------------------------
%{
#include <stdio.h>
#define YYSTYPE double
%}
%token NUMBER
%left PLUS
%left MUL
%%
lines : lines expr '\n'
{printf("%g\n", $2); }
| lines '\n'
|
| error '\n'
{ yyerror("reenter last line:");
yyerrok; }
;
expr : expr PLUS expr %prec MUL
{ $$ = $1 + $3; }
| expr MUL expr %prec PLUS
{ $$ = $1 * $3; }
| NUMBER
;
%%
yylex()
{ int c;
while((c = getchar()) == ' ') {}
if(isdigit(c))
{ yylval = c - '0';
return(NUMBER);
}
if(c == '+')
{ yylval = PLUS;
return(PLUS);
}
if(c == '*')
{ yylval = MUL;
return(MUL);
}
if(c == EOF)
{ yylval = 0;
return(EOF);
}
return(c);
}
yywrap(){};
yyerror(s) char *s;{
fprintf(stderr,"%s\n",s);
};
main(){
return(yyparse() );
}
-------------------------------------------------------------
[This does appear to confuse yacc's precedence mechanism -- the two
productions are indeed treated as of equal precedence. The standard unary
minus precedence does work, I haven't narrowed the bug further. -John]
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