Re: term_expansion/2 with DCG clauses

Hi Jan,

Sorry to somehow hijack the thread but...

On 13/08/2013, at 15:12, Jan Wielemaker <[email protected]> wrote:

> It is still not very clear what you want.  If you want to
> produce DCG's from term-expansion, this is just fine.  DCG
> expansion is done automatically if term-expansion produces
> (Head --> Body) terms.

... the above behavior come as a surprise to me. A simple example:

$ swipl
...
?- assertz(term_expansion(a(X), (X --> []))).
true.

?- expand_term(a(z), T).
T = (z(_G2260, _G2260):-true).

I would expect T = (z-->[]). GNU Prolog, XSB, YAP, Logtalk all produce T = (z-->[]). The Quintus Prolog 3.5 manual states (pp 1315) that:

"expand_term/2 calls term_expansion/2 first; if it succeeds, the standard grammar rule expansion is not tried."

Is SWI-Prolog the only one deviating from the original Quintus definition?

Cheers,

Paulo


> Maybe your use of =.. explains some of the misconceptions.
> Just write
> 
> user:term_expansion((abnf(Atom) :- _), DCGS) :-
> 	...
> 
> And you create your DCG simply using (Head --> Body).  Note
> the additional (), wich you need to put these beasts in a
> normal Prolog argument.
> 
> Of course, this is really ugly.  Since we have quasy quotations,
> you can make your source
> 
> {|abnf||
> repeat         =  1*DIGIT / (*DIGIT "*" *DIGIT)
> |}.
> 
> See http://www.swi-prolog.org/pldoc/man?section=quasiquotations
> 
> This would be cool to have.  Be aware it is not always simple.
> Some DCGs are left-recursive and will loop.  Others are for various
> reasons very poorly structured for naive translation info DCGs.  There
> are ways to fix this automatically ...
> 
> 	Cheers --- Jan
> 
> 
> 
> On 08/13/2013 03:49 PM, Wouter Beek wrote:
>> Hi Feliks,
>> 
>> My specific use case is that I want to generate DCGs based on ABNF (a
>> variant of BNF). Most RFC standards define their syntax in ABNF and
>> having them converted into DCGs seems convenient.
>> 
>> Here is my use of term_expansion/2. The conversion from ABNF to DCGs
>> is done in rule//1.
>> ~~~{.pl}
>> system:term_expansion(ABNF, DCGs):-
>>   ABNF =.. [:-,abnf(Atom),_],
>>   atom_codes(Atom, Codes),
>>   once(phrase(rule(DCGs), Codes)),
>> abnf('repeat         =  1*DIGIT / (*DIGIT "*" *DIGIT)'):- true.
>> ~~~
>> 
>> ---
>> Cheers!,
>> Wouter.
>> 
>> E-mail: [email protected]
>> WWW: www.wouterbeek.com
>> Tel.: 0647674624
>> 
>> 
>> On Tue, Aug 13, 2013 at 3:15 PM, Feliks Kluzniak
>> <[email protected]> wrote:
>>> Hi,
>>> 
>>> I find your question hard to understand.  Could you please give an example of what it is that you are after?
>>> 
>>> Thanks,
>>> -- Feliks
>>> 
>>> On Aug 13, 2013, at 14:02, Wouter Beek <[email protected]> wrote:
>>> 
>>>> Hi all,
>>>> 
>>>> What would be the recommended way for replacing regular Prolog
>>>> predicates with DCG rules? I would be nice if I could use the
>>>> syntactic sugar / `-->`-notation in formulating the replacing rules.
>>>> 
>>>> I was thinking of using term_expansion/2, but `ResultTerm =..
>>>> [-->,Head,Body]` would not be correct since `-->` is not the name of
>>>> the predicate.
>>>> 
>>>> ---
>>>> Cheers!,
>>>> Wouter.
>>>> 
>>>> E-mail: [email protected]
>>>> WWW: www.wouterbeek.com
>>>> Tel.: 0647674624
>>>> _______________________________________________
>>>> SWI-Prolog mailing list
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>>> 
>>> 
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> 
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Paulo Moura
Logtalk developer

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