[mercury-users] Mode polymorphism

[Peter Hawkins]

Mark Brown wrote:

>You get this error message because Y has inst `ground' rather than
>`non_empty_list' ;-).  You forgot to call map_poly instead of map.
>  
>
Oops. Let's try again:
:- import_module io.

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

:- implementation.

:- import_module list.
:- import_module int.
:- import_module exception.

% 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))) = 
out(list(O))
    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]].

:- 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),
    io.nl(!IO),
    print_non_empty_list(Xs, !IO).

:- func get_elem(list(T)::in(list(I)), int::in) = (T::out(I)) is det.
get_elem(L, X) = Out :-
    (if X = 0, L = [A | _] then
        Out = A
     else if X > 0, L = [_ | As] then
        Out = get_elem(As, X-1)
     else
        throw("bad")
    ).

main(!IO) :-
    A = get_list,
    B = [0,1],
    Z = map_poly(get_elem(A), B),
    print_non_empty_list(Z, !IO),
    true.


I now get this error:
$ mmc --make -E test2
Making Mercury/cs/test2.c
test2.m:043: In clause for `main(di, uo)':
test2.m:043:   mode error in conjunction. The next 4 error messages
test2.m:043:   indicate possible causes of this error.
test2.m:046: In clause for `main(di, uo)':
test2.m:046:   in argument 1 of call to function `test2.map_poly/2':
test2.m:046:   mode error: variable `V_14' has instantiatedness `free',
test2.m:046:   expected instantiatedness was `(func(((V_1 =< ground) >> 
(V_1 =< ground))) = (free >> (V_2 =< ground)) is det)'.
test2.m:046: In clause for `main(di, uo)':
test2.m:046:   mode error: argument 2 became too instantiated.
test2.m:046:   Final instantiatedness of `V_21' was 
`bound(list.'[|]'(ground, ground))',
test2.m:046:   expected final instantiatedness was `(V_2 =< ground)'.
test2.m:047: In clause for `main(di, uo)':
test2.m:047:   in argument 1 of call to predicate 
`test2.print_non_empty_list/3':
test2.m:047:   mode error: variable `Z' has instantiatedness `free',
test2.m:047:   expected instantiatedness was `bound((list.[]) ; 
list.'[|]'(bound(list.'[|]'(ground, ground)), ...))'.


=)
Peter
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