[mercury-users] Mode polymorphism

Peter Hawkins peter at hawkins.emu.id.au
Fri Jul 29 10:48:41 AEST 2005


Why doesn't this work?

:- module test2.
:- interface.
:- import_module io.

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

:- implementation.

:- import_module list.

% Version of map with mode polymorphism
:- func map_poly(func(T1) = T2, list(T1)) = list(T2).
:- mode map_poly((func(in(I)) = (out(O)) is det), in(list(I))) = 
    is det.
map_poly(_Func, []) = [].
map_poly(Func, [X|Xs]) = [Y|Ys] :-
    Y = apply(Func, X),
    Ys = map_poly(Func, Xs).

:- func get_list = (list(list(int))::out(list(non_empty_list))) is det.
get_list = [[1],[2]].

:- func do_something(list(int)::in(non_empty_list)) =
    (list(int)::out(non_empty_list)) is det.
do_something(X) = X.

:- pred print_non_empty_list(list(list(int))::in(list(non_empty_list)),
    io.state::di, io.state::uo) is det.
print_non_empty_list([], !IO).
print_non_empty_list([X|Xs], !IO) :-
    io.print(X, !IO),
    print_non_empty_list(Xs, !IO).

main(!IO) :-
    X = get_list,
    Y = map(do_something, X),
    print_non_empty_list(Y, !IO),

I get this error message:
$ mmc --make test2
Making Mercury/int3s/test2.int3
Making Mercury/cs/test2.c
test2.m:038: In clause for `main(di, uo)':
test2.m:038:   in argument 1 of call to predicate 
test2.m:038:   mode error: variable `Y' has instantiatedness `ground',
test2.m:038:   expected instantiatedness was `bound((list.[]) ; 
list.'[|]'(bound(list.'[|]'(ground, ground)), ...))'.
For more information, try recompiling with `-E'.
** Error making `Mercury/cs/test2.c'.


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