Re: Useless rule?

[Paul Eggert]

> From: "Jon Watte" <address@hidden>
> Date: Fri, 10 Jan 2003 23:46:45 -0800
> 
> pieces.y:232.3-233.50: warning: useless rule: edge_height_list: '{' INTEGER
> ',' INTEGER '}' edge_height_list ',' '{' INTEGER ',' INTEGER '}'

It does seem to me that Bison's error message is less than useful.

The problem is in this rule:

        edge_height_list:
          '{' INTEGER ',' INTEGER '}'
          edge_height_list ',' '{' INTEGER ',' INTEGER '}'
          ;

You forgot a '|' before the 3rd line.  This rule is useless because
it cannot possibly generate a finite string.  Bison is within its rights
to call it a useless rule, but its diagnostic is pretty cryptic.

On the same example, Yacc generates the slightly-more-useful message
"nonterminal edge_height_list never derives any token string".  This
is still too cryptic, but it's better.