Re: Useless nonterminals - what and why?

[Matthew Palmer]

On 21 Nov 2000, Akim Demaille wrote:

> >>>>> "Matthew" == Matthew Palmer <address@hidden> writes:
> 
> Matthew> OK, hacking on somebody else's bison file, and I have gotten
> Matthew> this message. Not real sure exactly what it means, beyond a
> Matthew> definition somebody gave that "A useless nonterminal is one
> Matthew> which doesn't produce any strings" or similar.
> 
> That's a pretty theoretical definition, which in essence means this
> symbol is not reachable starting from the start symbol.  Typically,
> it's not used in any rule.
> 
> But Bison is usually quite good at error message: what exactly did it
> say? 

That, and that only.  "2 useless nonterminals and 3 unused rules".  The
parse.output listed which rules and nonterminals it was, but nothing more.

> Your snippet has several flaws, but most probably due to the
> fact that you cut some other relevant parts.  In particular I can't
> see any useless nonterminal here.

I thought I'd provided the flow of control, perhaps not.  What fixed it,
AFAICT, was placing a required colon and port number after the hostname, as
in

http_proxy:     host COLON_TOK PORTNUM_TOK

Although at some stage I think I added the single-element list thingy in
there, too, which may have fixed it.  Basically I was just moving things
around trying to fix it, and stumblerd across the solution.  I don't know
what exactly fixed it, because I didn't keep perfect records of everything I
did, but it now produces a valid parser, with no errors, and which produces
the required behaviour.


-- 
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#include <disclaimer.h>
Matthew Palmer
address@hidden