[mercury-users] existential type question

[Peter Ross]

I don't understand why the program below doesn't work.  Can anyone help?

% mmc -e arg2.m
arg2.m:022: In clause for predicate `arg2:p/2':
arg2.m:022:   in unification of variable `MaybeArg'
arg2.m:022:   and term `arg(Arg)':
arg2.m:022:   type error in argument(s) of functor `arg/1'.
arg2.m:022:   Argument 1 (Arg) has type `(some [T] T)',
arg2.m:022:   expected type was `(some [U] U)'.
For more information, try recompiling with `-E'.

:- module arg2.

:- interface.

:- type t
    ---> f
    ;    g.

:- type maybe_arg
    ---> some [U] arg(U)
    ;    no.

:- pred p(t::in, maybe_arg::out) is cc_multi.

:- implementation.

:- import_module std_util.

p(I, MaybeArg) :-
    q(I, Univ),
    Arg = univ_value(Univ),
    MaybeArg = arg(Arg).

:- pred q(t::in, univ::out) is cc_multi.

q(f, univ(1)).
q(f, univ(2)).
q(g, univ(3)).

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