[mercury-users] Higher-order unification question

[Ralph Becket]

doug.auclair at logicaltypes.com, Monday, 26 February 2007:
> Hi, I receive an error when I pass in an applied higher order term.
> What am I doing wrong?
> 
> The goal's type:
> 
> :- pred svunfold(counter, pred(T, T, SV, SV), T, T, SV, SV).
> :- mode svunfold(in, pred(in, out, in, out) is det, in, out, in, out)
>         is det.
> 
> The argument def:
> 
> 	  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, _),

Rnds has arity 5, svunfold expects a predicate of arity 4.

> (where random_int/3 is from Julian's typeclassed RNG protocol).
> 
> The exact compile error message:
> 
> test_series.m:026: In clause for predicate `main'/2:
> test_series.m:026:   in argument 2 of call to predicate `svunfold'/6:
> test_series.m:026:   in unification of argument
> test_series.m:026:   and term `Rnds(V_55)':
> test_series.m:026:   type error in argument(s) of higher-order term (with arity
> test_series.m:026:   1).
> test_series.m:026:   Functor (Rnds) has type `pred(pred(T, V_7, V_6),
> test_series.m:026:   (list.list(T)), (list.list(T)), V_7, V_6)',
> test_series.m:026:   expected type was `((func V_10) = V_11)'.
> ** Error making `Mercury/cs/test_series.c'.
> make: *** [test_series] Error 1
> 
> Why is it looking for (func T0) = T1 as the type? It should be looking for

I have no idea.  Have you given us enough information?
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