Re: Useless nonterminals - what and why?

[Matthew Palmer]

On Tue, 21 Nov 2000, Hans Aberg wrote:

> At 09.57 +1100 0-11-21, Matthew Palmer wrote:
> >> Think of the rule as written (as when debugging is turned on)
> >>   http_proxy_list <- http_proxy_list http_proxy
> ...
> >> there is not any other rule for finding a http_proxy_list, so you can
> >> never find the RHS http_proxy_list.
> >
> >Aha, is that how it works?  Funny, the bison manual never mentioned anything
> >of that sort, and it recommended using this sort of recursion to prevent
> >huge stack blowouts.
> 
> Which quote in the Bison manual do you have in your mind?

Section 3.4 on Recursive Rules, where it states "you should always use left
recursion because it can parse a sequence of any number of elements with
bounded stack space".  I neglected to study the examples properly, where
they define a list as being " element | list element".  Oops.

> -- There is not anything wrong with the relation itself, but it needs an
> additional startup relation.

It shall be done in future.

> >> Note that you forgot terminate the rules with ";" in the original post.
> >> Have you done it now?
> >
> >I'm not sure (haven't got the code with me ATM), but I don't think any of
> >the rules are terminated by a semi-colon, and it works just fine.  I may be
> >wrong on that, as I didn't take a huge pile of notice.
> 
> It looks wrong to me -- perhaps Bison substitutes an empty relation, making
> your code work or something.

Checked the code - no semicolons.  As to making the thing work, on further
inspection it would seem that I now have a simple http_proxy as an
alternative http_proxy_list.  I did a lot of things to make it work, and
that must have been one of them.

Certainly I've learned my lesson now about it all, and understand grammar
definition a lot better.  Thanks for your patience.


-- 
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#include <disclaimer.h>
Matthew Palmer
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