[mercury-users] Superclasses and "solutions"

[Kerry Raymond]

I am trying to represent some O-O models in Mercury and am having some
difficulties finding the right Mercury approach to inheritance. I thought
that typeclasses would be the right approach, but am encountering
compilationerrors rejecting what I am doing. I would welcome some expert advice
on what the problem is.

To make life simpler, I've found the smallest program to illustrate the
problem. I have two types, "a" and "b", and a common abstract supertype
of them "super". Each of "a" and "b" implement a concrete method "f_a"
and "f_b" which are linked to the abstract method "f_x" of super. Everything
goes haywire when I attempt to call "solutions".

:- module try.
:- import_module std_util.

:- type a ---> aa.
:- type b ---> bb.

:- pred f_a(a).

f_a(aa).

:- pred f_b(b).

f_b(bb).

:- typeclass super(T) where [
        pred f_x(T),
        mode f_x(out) is multi
].

:- instance super(a) where [ pred(f_x/1) is f_a ].

:- instance super(b) where [ pred(f_x/1) is f_b ].

:- pred foo(int).
:- mode foo(in) is det.

foo(_) :- solutions((pred(X::out) is nondet:- f_x(X)),_List).


The compiler does not like this call to solutions. It says:

try.m:025: In predicate `try:foo/1':
try.m:025:   type error: unsatisfied typeclass constraint(s):
try.m:025:   try:super(T).
try.m:025: In predicate `try:foo/1':
try.m:025:   warning: unresolved polymorphism.
try.m:025:   The variables with unbound types were:
try.m:025:       X :: T
try.m:025:       V_6 :: T
try.m:025:       V_5 :: (pred T)
try.m:025:       _List :: (list:list(T))
try.m:025:   The unbound type variable(s) will be implicitly
try.m:025:   bound to the builtin type `void'.



===========================================================================
Dr Kerry Raymond, Distinguished Research Fellow           kerry at dstc.edu.au
CRC for Enterprise Distributed Systems Technology       Ph: +61 7 3365 4310
University of Queensland 4072 Australia                Fax: +61 7 3365 4311
===================================================== www.dstc.edu.au/kerry





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