[mercury-users] Cann't understand compiler error

[stormie at hotmail.it]

Can anyone help me to explain the compilation errors?
I am trying to combine elements two lists of type1 to get a list of type2.

Thanks in advance...
-

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
stormie at stormie-laptop:~/logic/dvl_mercury/testmod$ mmc  --make exp1
Making Mercury/int3s/exp1.int3
Making Mercury/ints/exp1.int
Making Mercury/cs/exp1.c
exp1.m:040: In `combine'(in, in, out):
exp1.m:040:   error: determinism declaration not satisfied.
exp1.m:040:   Declared `det', inferred `semidet'.
exp1.m:041:   In argument 2 of clause head:
exp1.m:041:   unification of `HeadVar__2' and `list.[]' can fail.
exp1.m:042:   In argument 2 of clause head:
exp1.m:042:   unification of `HeadVar__2' and `list.[B | Bs]' can fail.


%%%%%%%%%% Start of Mercury Code
:- module exp1.

:- interface.

:- import_module io.

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

:- implementation.

:- import_module list,string.

:- type type1      ---> t(string).

:- type type2     --->  c(type1,type1).

main(!IO) :-
    io.write_string('Halo!! !! \n', !IO),
    X = [t('1'),t('2'),t('3'),t('4')],
    Y = [t('a'),t('b'),t('c'),t('d')],
    combine(X,Y,Out),
    displayt2(Out,!IO).


:- pred displayt1(list(type1)::in,io::di, io::uo) is det.
displayt1([],!IO).
displayt1([X|Xs],!IO):-
    displayt1(Xs,!IO),
    io.write(X,!IO).


:- pred displayt2(list(type2)::in,io::di, io::uo) is det.
displayt2([],!IO).
displayt2([X|Xs],!IO):-
    displayt2(Xs,!IO),
    io.write(X,!IO).


:- pred combine( list(type1)::in, list(type1)::in, list(type2)::out) is det.
combine( [], [] ,G) :- G = [].
combine( [A|As],[B|Bs], G):-
    combine(As, Bs, G1),
    append([c(A,B)],G1,G).


%%%%%%%%%%%%%%%%%%%%%%% End of Mercury Code


--
I am able to work out similar logic in prolog, 
e.g. with following using constraints
combine( [], [] ,G) :- G = [].
combine( [A|As],[B|Bs], G):-
    combine(As,Bs, G1),
    append([c(A,B)],G1,G)

applying goal of 
?- combine([x,y,z],[1,2,3],G).
G = [c(x, 1), c(y, 2), c(z, 3)] ;

No

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