Re: Useless nonterminals - what and why?

[Matthew Palmer]

On Sat, 18 Nov 2000, Hans Aberg wrote:

> At 01.11 +1100 0-11-19, Matthew Palmer wrote:
> >OK, hacking on somebody else's bison file, and I have gotten this message.
> >Not real sure exactly what it means, beyond a definition somebody gave that
> >"A useless nonterminal is one which doesn't produce any strings" or similar.
> 
> I can just make a quick guess until you get a reply from the real
> experts... :-)
> 
> >http_proxy_list: http_proxy_list http_proxy { ... }
> 
> But it seems me if you just have such a rule and nothing else, you have
> nothing to start up the pattern <http_proxy_list>: Think of it as a loop
> which needs some kind of initialization. It could be the empty string
>   http_proxy_list: http_proxy_list http_proxy { ... }
>                   |                           { ... }
> or the well known <foo_bar> variable
>     http_proxy_list: http_proxy_list http_proxy { ... }
>                   | foo_bar                     { ... }

Interesting.  Even though I'll get a hit with http_proxy, I still need the
empty list?

I seem to have fixed it now, anyway.  Made it a requirement to have a comma
between entries, as in

http_proxy_list: http_proxy_list COMMA_TOK http_proxy

and then make the opt_port required, as in

http_proxy:     host COLON_TOK PORTNUM_TOK

and it compiles and parses fine.

I think it's because there would be circumstances where it couldn't work out
when it was actually parsing a http_proxy entry or something else, and the
poor dear might get confused... <g>


-- 
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Matthew Palmer
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