[m-rev.] Re: for review: parsing_utils improvements

[Ian MacLarty]

On Tue, Sep 29, 2009 at 3:36 PM, Ralph Becket <rafe at csse.unimelb.edu.au> wrote:
> Ian MacLarty, Tuesday, 29 September 2009:
>> > I guess what I'm looking for here is a demonstration that your changes
>> > make it noticably easier to handle errors than with the existing
>> > interface.
>>
>> Consider this partial sparql (http://www.w3.org/TR/rdf-sparql-query/)
>> parser that I wrote.  I've attached a copy of the old version which
>> uses a result type to keep track of errors (sparql.m).  I've also
>> attached a reworked version (sparql.m.new) using the new error handling.
>> It is considerably shorter and would have been much less of a pain to
>> write.
>>
>> If you can tell me a better way to implement the error handling using
>> the old system, then please let me know.
>
> I'm trying to work out why throwing an exception at the point you
> identify an error won't do the trick for you.  sparql.m.new only has
> two calls to fail_with_message.
>
> Ah, now I see: if you want to parse A | B | C and A reports an error,
> you want to try B and C to see if you get a parse before reporting an
> error.  This makes sense if your grammar isn't LR(k).
>
> Okay, I think I'm happier with your scheme now.  However, it would be
> better to have two mutvars, one for the offset and one for the
> maybe(error), to avoid the need for an allocation every time you call
> record_progress.
>

Okay.

> Having said that, aren't grammars like this fairly rare?  I mean,
> can the mutable stuff be separated out just for the parsers that need
> it?

I don't know. What did you have in mind?

What about the idea of adding a record_progress flag to the src type
so that if you're not using parse/4 you only pay the price of checking
the flag when the record_progress predicate is called?

Ian.

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