[m-users.] Existential types question

Hi

The following fails to compile:


:- type t ---> some [X] (t(X) => class(X)).

:- typeclass class(X) where [
pred tuwas(X::in, X::out) is det
].

:- instance class(int) where [
tuwas(I, I+1)
].


:- pred nochwas(t::in, t::out) is det.

nochwas(T, U) :-
    T = t(X),
    tuwas(X,Y),
    U = t(Y).   % <-- line 28


main(!IO) :-
    nochwas('new t'(1), t(Y)),
    io.print(Y, !IO).


I get this error message:

test1.m:028: In clause for predicate `nochwas'/2:
test1.m:028:   in unification of variable `U'
test1.m:028:   and term `t(Y)':
test1.m:028:   type error in argument of functor `t'/1.
test1.m:028:   Argument 1 (Y) has type
test1.m:028:     some [X_1] (X_1),
test1.m:028:   expected type was
test1.m:028:     some [X_3] (X_3).


I don't understand this message...


TIA,
Volker

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