Re: Extended DCG?

I never bother with phrase/3, and just invoke my predicate xy(Rag, List, []).  So maybe I could do xy(Rag, (List1, List2), ([],[])) and design xy to work?  It's not clear to me how to pull this all together.
 
Alan Baljeu


________________________________
 From: "Abdallah, Samer" <[email protected]>
To: Alan Baljeu <[email protected]> 
Cc: Prolog <[email protected]> 
Sent: Wednesday, February 12, 2014 1:48:07 PM
Subject: Re: [SWIPL] Extended DCG?
 

Hi all,
I would just like to use this opportunity to request again that phrase/3 be 
allowed to work with arbitrary types in the last two positions, not just
lists, or an alternative, say gphrase/3  (for generalised phrase), or
rundcg/3 which works exactly like phrase/3 but allows arbitrary types.
Or even better, put phrase/3 back how it was and introduce a type
checking list_phrase/3 or safe_phrase/3 for people who want that.

It used to be the case that an extended DCG for writing a transducer
as  Alan would like, was easy to write using ordinary DCG notation.
The type of the threaded argument would be a pair of lists (L,R)
instead of just one list. In my DCG library I have some DCG rules for
working with this conveniently. For example, operators (\<)//1 and (\>)//1 
apply a DCG phrase to the left or right member of the pair:

:- op(800, \<, fy).
:- op(800, \>, fy).

\<(P, (L1,R), (L2,R)) :- phrase(P,L1,L2). 
\>(P, (L,R1), (L,R2)) :- phrase(P,R1,R2)

Then you can do things like \< integer(X) to read an integer from
from the left sequence and \> [int(X)] to write a term to the right, and so on.
Alan's scenario is easily and efficiently dealt with.

Forcing us to use lists for the arguments to phrase/3 forces an ugly
encoding of the pair in a singleton list, e.g. as [(L,R)], and, if the list
is [X] and we already have a bunch of predicates that want to work
on X,  forces an unnecessary layer of meta-predicates to get through 
the wrapping.

For the moment, I have deleted the two offending lines from boot/dcg.pl
in my local installations, but of course, this is not good for the portability
of my code!

cheers,
Samer


(

On 11 Feb 2014, at 16:51, Alan Baljeu <[email protected]>
wrote:
> I have a scenario where I'm mapping a list to a second list, but it's not 1 to 1.  For this, it would be nice to use an extended DCG, that supports two independent lists.  Does such a thing exist?
>  
> Alan Baljeu
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