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

[Ian MacLarty]

On Tue, Sep 29, 2009 at 3:21 PM, Ian MacLarty
<maclarty at csse.unimelb.edu.au> wrote:
> On Tue, Sep 29, 2009 at 2:30 PM, Ralph Becket <rafe at csse.unimelb.edu.au> wrote:
>> Hi Ian,
>>
>> Paul forwarded your diff to me (I have an over eager spam filter...).  I
>> have some Real Objections!
>>
>> (1) You call record_progress all over the place.  Why not just in, say,
>> the next_char predicate?  The code is pretty ugly with all those
>> impure scopes.
>>
>
> Because, as I understand it, not all the predicates in the interface
> go through next_char.
>
>> (2) Every call to record_progress makes an allocation and updates
>> your error reference.  This is a Bad Thing for performance.
>>
>
> Yes, but from my point of view I'm more interesting in writing parsers
> quickly than making them go fast.
>
> What if I add a second mutable that was updated by record_progress
> that only contained the current offset.  This would avoid an
> allocation and a dereference everytime record_progress was called.
> Then in (the relatively rarely called) fail_with_message I could
> record the message and offset in an error_info mutable as I currently
> do.  It would also update the offset in the second mutable.  Then when
> it comes time to decide what offset to return in the parse predicate I
> can compare the offset in the error_info and the new mutable to see
> what to return.
>

Another compromise could be to set a flag in the src that indicates
whether the mutable should be updated at all.  If you ran the parser
outside the parse/4 predicate then the mutable would simply never be
updated.

Ian.

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