[mercury-users] problem with existentially quantified higher order argument

[Andrea Bini]

hi all,
I have a problem using a discriminated union with a data constructor
containing a higher order type with a type variable existentially quatified.
I tried solving the problem by myself reading the guide but I didn't
succeed. The code is the following:

:- pred get_int_range(int::in, int::in, list(int)::in, list(int)::out) is
det.
get_int_range(A,B,Lin,Lout) :-
  ( if
      A = B
    then
      Lout = [B|Lin]
    else
      B2 = B - 1,
      get_int_range(A,B2,[B|Lin],Lout)
  ).

:- type gen ---> some [T] gen(pred(list(T))).
:- inst gen1 == bound(gen(pred(out) is det)).

:- pred try_gen(gen::in(gen1)) is det.
try_gen(gen(Gen)) :-
  call(Gen,_Dom).

main(!IO) :-
  My_Int_Gen = get_int_range(1, 5, []),
  try_gen('new gen'(My_Int_Gen)).


compiling it as it is I get the following compiler error:

Uncaught Mercury exception:
Software Error: modes.m: Unexpected: modecheck_set_var_inst: unify_inst
failed
Stack dump not available in this grade.

removing the try_gen invocation in the main predicate I get this cryptic, at
least for me, error:

prova_errore.m:026: In clause for `try_gen(in((prova_errore.gen1)))':
prova_errore.m:026:   in argument 1 (i.e. the predicate term) of
higher-order
prova_errore.m:026:   predicate call:
prova_errore.m:026:   mode error: variable `Gen' has instantiatedness
`free',
prova_errore.m:026:   expecting higher-order pred inst (of arity 1).

I think the problem is in the inst gen1 that is not correct but I cannot
understand why. If I use universally quantification, using the same inst,
all works correctly and I also have no problem if I use existential
quantification on a type variable that is not in an higher-order type (e.g.
if gen was declared like this ":- type gen ---> some [T] gen(list(T)).").

can you help me?

thanks a lot!
best regards!

Andrea Bini

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