%---------------------------------------------------------------------------%
% vim: ts=4 sw=4 et ft=mercury
%---------------------------------------------------------------------------%

:- module any_free_unify.

:- interface.
:- import_module io.

:- pred main(io::di, io::uo) is det.

:- implementation.

:- import_module bool.
:- import_module list.
:- import_module std_util.

main(!IO) :-
    test_any_free_unify([], Result1),
    io.print(Result1, !IO),
    io.nl(!IO).

:- solver type foo
    where
        representation is int,
        ground         is ground,
        any            is ground.

:- pred init_foo(foo::out(any)) is det.
:- pragma promise_pure(init_foo/1).

init_foo(X) :-
    impure X = 'representation to any foo/0'(42).

:- pred test_any_free_unify(list(foo), bool).
:- mode test_any_free_unify(in(list_skel(any)), out) is det.

% In the unification in the condition of the if-then-else, the variable
% List has an inst which includes `any' components. Normally, we can't
% check whether `any' has become further instantiated over a goal so we
% do not allow it in a negated context. However, in this case, the `any'
% component is unified with `free' so we know that it cannot have become
% further instantiated. Therefore we should allow the unification
% in the condition.

test_any_free_unify(List, Result) :-
    promise_pure
        ( if List = [_ | _] then
            Result = no
        else
            Result = yes
        ).