[mercury-users] Higher-order unification question

[doug.auclair at logicaltypes.com]

Dear Ralph, hello.

>> 	  Rnds = (pred(P::(pred(in, in, out) is det), A::in, B::out,
>> 		       RNG0::in, RNG::out) is det :-
>> 	          P(X, RNG0, RNG),
>> 		  B = [X|A]),
>> the call:
>> 	  svunfold(counter(10), Rnds(random_int), [], Ans, J0, _),

you wrote:
>Rnds has arity 5, svunfold expects a predicate of arity 4.

Rnds may have arity 5, but I'm trying to produce a predicate
with arity 4 by binding the first argument and currying the 
remaining arguments.  I see now, by rereading section 8.1
again (that would be a third reading), that this doesn't work
for higher order terms but it does work for ground pred type
terms.  So when Rnds becomes rnds/5:

:- pred rnds(pred(T, RNG, RNG), list(T), list(T), RNG, RNG) <= rng(RNG).
:- mode rnds(pred(out, in, out) is det, in, out, in, out) is det.
rnds(P, A, [X|A], !RNG) :- P(X, !RNG).

then the above code (s/Rnds/rnds/) now works.

Problem solved.  My misunderstanding.  However, it does seem a bit
odd that higher-order currying is allowed, but not for ground higher
order terms, which require (an extra) forwarding lambda term, what 
is the need to make this a language specification requirement?

Would it be possible to allow currying of higher order terms and
builtins (I found my (+)(1) function didn't work when I was mapping
a list(int))?  I think allowing this would make higher order programming
much easier to use (and to teach and to learn) in Mercury.

Sincerely,
Doug Auclair

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