[mercury-users] Type class usage question

[peter at hawkins.emu.id.au]

Hi...

I'm trying to implement something using mercury but I'm hoping someone
here can give me a better idea how to do it.

I'm trying to have code that manipulates two different kinds of
constraints. Each kind has a corresponding constraint store that deals
strictly with that type of constraint. The constraint store will be
reasonably different for each kind, so simple polymorphism doesn't cut
it, I think.


:- typeclass constraint(T) where [
    func constraint_to_thing(T) = thing
].

:- typeclass constraint_store(T) where [
    % Add a constraint into the store
    pred add_constraint(C, T, T) <= constraint(C),
    mode add_constraint(in, in, out) is det,

    % Return the number of constraints in the store
    func num_constraints(T) = int
].

Then I might have:

:- type int_constraint ---> (a == b) ; (a > b).
:- instance constraint(int_constraint) where [
    pred(constraint_to_thing/3) is int_constraint_to_thing
]. 

:- type int_constraint_store ---> c_store(
        constraints :: list(int_constraint),
        foo :: int,
        bar :: string
    ).
:- instance constraint_store(int_constraint_store) where [
    pred(add_constraint/3) is int_constraint_store_add,
    num_constraints(T) = size(T)
].

:- pred int_constraint_store_add(int_constraint, int_constraint_store,
            int_constraint_store).
:- mode int_constraint_store_add(in, in, out) is det.
int_constraint_store_add(C, S0, S) :-
    L = S0 ^ constraints,
    insert(C, L, L0),
    S = (S0 ^ constraints := L).

and I might have another implementation too.

Unfortunately this doesn't compile, since the type of the first argument
int_constraint_store_add should be (by the typeclass definition)
some [C] C
not:
int_constraint

Now, what I really want is to have the constraint type as a parameter of
the constraint store type class. Unfortunately I can't do that since
then that constraint type has to be an argument to every predicate that
applies to constraint store type class (including for example
num_constraints, which obviously doesn't actually care).

How do I express what I want in Mercury?

=)
Peter
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