RULES and type classes

[Andreas Schropp]

> //
> In particular, it would be nice to be able to specialise based on the
> instances, as we do for [a] --> [Int], e.g.
>
>     RULES sum = sumInt :: [Int] -> Int
>
> is fine in the current system. So I could imagine some nice
> specialisations based on say, the good old Ord:
>
>     RULES nub = nubOrd :: (Eq a, Ord a) => [a] -> [a]
>
> which might use a Map, say.
>
> I don't know how costly this instance matching would be.
> -- Don
In principle I think you should be able to get constraint-specialized implementations right now, using Oleg's type-class overloaded (aka 2-polymorphic) functions (http://okmij.org/ftp/Haskell/poly2.txt). Some prototyping, not tested:

    import Poly2
    {- -- from Poly2: (these can also be expressed as
    `TypeCls'-Membership-instance -> feasable with deriving machinery?)
    type Fractionals = Float :*: Double :*: HNil
    type Nums = Int :*: Integer :*: AllOf Fractionals :*: HNil
    type Ords = Bool :*: Char :*: AllOf Nums :*: HNil
    type Eqs = AllOf (TypeCl OpenEqs) :*: AllOfBut Ords Fractionals :*:
    HNil -}

    data NubLabel

    -- Ord-specialized instance, guarded by Membership-Predicate
    instance GFN Z NubLabel (Member Ords)
    instance (Ord a) => Apply (GFnA Z NubLabel) [a] [a] where -- we get
    here if Guard succeded
        apply _ = Map.tolist . Map.fromListWith const

    -- default instance
    instance TypeCast pred Otherwise => GFN n NubLabel pred
    instance (Eq a) => Apply (GFnA n NubLabel) [a] [a] where
        apply _ = List.nub

    nub_opt xs = apply (GFn NubLabel) xs

But it probably needs some deriving-infrastructure for the `TypeCls' member-predicate to make it feasible.


BTW: rewriting rules being turing-complete, I'm wondering if theres some "freaky hackery" to be discovered, or yet going on? :>