[mercury-users] Libraries and DCG convenience...

[Henk Vandecasteele]

Dear,

I noticed that problem as well with some mercury-librarys.
It would be nice if the libraries were designed such that
people, where convenient, could use DCG.

Henk

Luke Evans wrote:
> 
> Thanks for the full reply (enjoyed the Maslow quote!).
> 
> I take the point about DCG's being designed for parsing (and I remember
> doing the standard banal exercises with them many years ago now for my
> AI/Prolog course).  However, they ARE pressed into service for IO in Mercury
> - so it's natural to think (as one learning the language), that they're
> considered a 'respectable' tool for other similar tasks.  If this isn't the
> case, then it seems to me that Mercury should have the appropriate
> 'respectable tool' - fit for purpose.
> 
> I've read the abstract and intro from your document, and I can't help but
> agree with the opening points.  I'll read the rest over lunch today.
> 
> As a relative newcomer to the Mercury language (and certainly to creating
> 'real' programs which do things I would normally program in Java), my
> comments are observational.  The particular code that brought it all to a
> head creates 9 graph nodes and 10 links between them to initialise my (not
> very big) graph.  Of course (just as you imply in your document), I end up
> with 36 variables which I don't really care about but I have to 'manage' (by
> typing in and 'renumbering' if I edit the lines).  IMHO this is exactly as
> if I'd coded all my IO long-hand (non-DCG), yet we don't do this - we're
> advised to use DCG's in this case.
> 
> In the absence of the 'respectable tool' for this, I AM going to add wrapper
> predicates which move the 'GraphIn', 'GraphOut' variables to the end of the
> argument list, and then employ DCG's to make my list of Graph deltas much
> more approachable (and more easily modified).
> 
> Now, where did I put that hammer...
> 
> Lwe
> 
> 
> -----Original Message-----
> From: Peter Schachte [mailto:schachte at cs.mu.OZ.AU]
> Sent: 22 June 1999 06:59
> To: mercury-users at cs.mu.OZ.AU
> Subject: Re: [mercury-users] Libraries and DCG convenience...
> 
> On Mon, Jun 21, 1999 at 07:12:56PM +0100, Luke Evans wrote:
> > There's clearly a convention with predicates (visible in the standard
> > libraries) where predicate arguments are presented as:
> > <ADT state before> [arguments describing the delta] <ADT state after>
> 
> [snip]
> 
> > Threading the graph variables through all this is painful and seems (IMHO)
> > to be exactly what DCG's were designed for.
> 
> Actually, it's not.  DCGs were designed for natural language parsing.
> Pressing them into use for this sort of thing does them a real
> disservice, I believe, as it makes them seem awkward.  They are
> actually quite comfortable for parsing.
> 
>         If you only have a hammer, you tend to see every problem
>         as a nail.
>                 -- Maslow
> 
> The problems with using DCGs for hiding argument threads include:
> 
>      o  they can only hide one thread (one pair of arguments)
>      o  they only hide the last two arguments
>      o  the need to insert braces to avoid adding a thread to some
>         goals is easily forgotten
>      o  unit clauses don't get the thread added; you have to use a
>         different syntax for unit clauses
>      o  in Mercury, you have a pred declaration for a predicate with
>         arity n, but the clauses have arity n-2
> 
> I still think that the DCG idea of introducing a special notation in
> which the sequence of goals is significant is the right one, because
> sequence is a comfortable way to present threading (just as it's good
> for presenting complex conditionals as if-then-elses).  But most of
> the other design details of DCGs aren't right for this purpose.  My
> proposal is put forth in
> 
>         http://www.cs.mu.oz.au/~schachte/papers/iclp97.ps.gz
> 
> but it won't do you much good, because my implementation is for
> Prolog.  I do believe it could be adapted to Mercury without much
> trouble, but that hasn't been done yet.  So everyone keeps hammering
> those wood screws, and wishing they weren't threaded (sorry for the
> egregious pun :-).
> 
> --
> Peter Schachte                     Things are only impossible until they're
> mailto:schachte at cs.mu.OZ.AU        not. -- Jean-Luc Picard, Star Trek: The
> http://www.cs.mu.oz.au/~schachte/  Next Generation
> PGP: finger schachte at 128.250.37.3
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