[mercury-users] Higher-order types using typeclasses

[Julien Fischer]

On Fri, 16 Mar 2007, Mark Brown wrote:

> On 16-Mar-2007, Ondrej Bojar <bojar at csse.unimelb.edu.au> wrote:
>> Hi,
>>
>> I would like to abstract over container implementation when I am already
>> abstracting over the element type.
>>
>> If successful, I would be able to implement:
>>
>> :- pred my_sample_predicate(ContainerOfX::in, X::out, ...)
>>        <= (container(ContainerOfX, X)) is ...
>>
>> The container typeclass defines required operations on the container:
>>
>> :- typeclass container(Cont, Elem) <= (Cont->Elem) where [
>>         pred get_a_member(Elem::out, Cont::in, Cont::out) is nondet,
>>         func count(Cont) = int
>>    ].
>>
>> And I would instantiate the typeclass for various containers:
>>
>> :- instance container(bag(X), X) where [
>>      pred(get_a_member/3) is bag_nondet_member,
>>      func(count/1) is bag_count
>>    ].
>>
>> However, the intended instantiation is not possible:
>>
>> t_drop_fr_reins.m:492: Error: types in instance declarations must be
>> functors wi
>> th distinct variables as arguments: container(bag(_1), _1).
>>
>> Are there any fundamental reasons for the restriction?
>
> No.  The main requirement is that any proof that a sequence of types
> is in a typeclass must be unique -- there must not be more than one way
> to construct the dictionary that satisfies a given typeclass constraint.
>
> The current restrictions on typeclass instances are sufficient to ensure
> this uniqueness, but they aren't necessary.  In theory it is possible to
> allow variables as instance arguments, and to allow variables to occur
> more than once in the arguments, as long as no two instances overlap
> (that is, no two sequences of instance arguments for a typeclass
> can be unified).
>
> When Mercury's typeclass system was implemented this wasn't understood
> so well, and as a result the restrictions are built fairly deeply into
> the implementation.

Out of curiosity where are these restrictions deeply built into the
implementation?  The type checker? RTTI?

Julien.
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