Re: Ann: SWI-Prolog 7.1.0

On 24/11/2013 03:49, Jan Wielemaker wrote:
> On 11/22/2013 10:48 PM, Abramo Bagnara wrote:
>> ?- A1 = a(), functor(A1, a, 0), functor(A2, a, 0), writeln(A1-A2), A1 = A2.
>> a()-a
>> false.
>>
>> This behavior is indeed very surprising for me... Is it deliberate?
>
> Yes.  I tried changing functor/3 (and =../2), but the compatibility
> consequences are too involved and often hard to debug.  Therefore I left
> functor/3 and =../2 alone and added compound_name_arity/3 and
> compound_name_arguments/3.  You need these if you want to do

There appear to be four ways a() could be handled by functor/3 and the like:

(1)  As above.  functor(a(), a, 0) and functor(a, a, 0) succeed, and 
functor(A, a, 0) succeeds once with A = a.

(2)  As (1), but functor(A, a, 0) succeeds twice with A = a or A = a().

(3)  functor(a(), F, N) fails.

(4)  functor(a(), F, N) raises a domain_error.

I don't think I like (1) at all.  Currently working code that copies 
(and modifies) terms will now silently change a() into a in many places.

(2) introduces non-determinism where we've not had it before, and will 
be painful to implement efficiently.  (Or at least, I will find it 
difficult to implement efficiently.)  I think it would be a poor choice.

(3) will probably cause unnecessary surprise, and I don't like it either.

(4) is probably the safest.  Any code it breaks will likely be broken by 
the new a() terms anyway, and the breakage will at least be obvious.

> term manipulation that is robust against a().  a() mainly plays
> a role in user-defined functions on maps and special purpose
> notation for DSLs.  This implies you mostly need to be aware
> of this in some parts of the Prolog infrastructure (e.g,
> goal expansion) and code that manages DSL expressions that use
> this.
>
> In my view, this is a notation that can make the representation
> of some DSLs much more understandable and you don't need to use
> it in other contexts.  That said, I like
>
> 	maplist(length(), ListOfLists, Lengths)
>
> more than
>
> 	maplist(length, ListOfLists, Lengths)

I'm afraid that I don't see any use for it sufficient for such a large 
change to the Prolog data model, and it will complicate a lot of my 
code, but if we're to have it, could we try out (4) please?


     Jeff Schultz