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

[Peter Schachte]

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  
--------------------------------------------------------------------------
mercury-users mailing list
post:  mercury-users at cs.mu.oz.au
administrative address: owner-mercury-users at cs.mu.oz.au
unsubscribe: Address: mercury-users-request at cs.mu.oz.au Message: unsubscribe
subscribe:   Address: mercury-users-request at cs.mu.oz.au Message: subscribe
--------------------------------------------------------------------------