[m-rev.] for review: use typeclass constraints to help resolve overloading

[Peter Ross]

On Wed, Jan 16, 2002 at 07:16:28AM +0100, Peter Ross wrote:
> On Wed, Jan 16, 2002 at 03:15:36PM +1100, Peter Ross wrote:
> > +:- pred instance_decl_exists(module_info::in, (type)::in,
> > +		{int, class_constraint}::in) is semidet.
> > +
> > +instance_decl_exists(ModuleInfo, ArgType, {N, constraint(Name, Types)}) :-
> > +	module_info_instances(ModuleInfo, Instances),
> > +	map__lookup(Instances, class_id(Name, list__length(Types)),
> > +			InstanceDefns),
> > +	list__filter((pred(InstanceDefn::in) is semidet :-
> > +			InstanceDefn = hlds_instance_defn(_, _, _, _,
> > +					InstanceTypes, _, _, _, _),
> > +			list__index1_det(InstanceTypes, N, InstanceType),
> > +			type_list_subsumes([ArgType], [InstanceType], _)
> > +		), InstanceDefns, Compatible, _Incompatible),
> > +
> > +		% There must be at least one instance declaration which
> > +		% metions the ArgType.
> > +	Compatible = [_ | _].
> 
> The arguments of type_list_subsumes are the wrong way around.

Actually you have to check both ways: as sometimes the instance type is
more concrete then the arg type and vice versa.

(
    type_list_subsumes([InstanceType], [ArgType], _)
;
    type_list_subsumes([ArgType], [InstanceType], _)
)
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