Long time no Haskell....simple typeclass question

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Long time no Haskell....simple typeclass question

Nicholls, Mark-2
This is the basic problem...

I have a data type, that has some functions that require typeclass constraints (this example doesn't...but that's not the point).

I have a typeclass, and I want to declare my datatype inhabits that type class....

> data Func a b = Func (a->b)

> exeFunc :: (Ord a) => Func a b -> a -> b
> exeFunc (Func g) = g

> class IFunc f where
>   exe :: f a b -> a -> b

> instance IFunc Func where
>   exe = exeFunc

Follow your nose and obviously

"No instance for (Ord a) arising from a use of 'exeFunc'"

But how do I constain "a" in the instance declaration, when "a" doesn't even appear

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Long time no Haskell....simple typeclass question

Nicholls, Mark-2
I can obviously follow Haskell's advice and go...

> class IFunc f where
>   exe :: (Ord a) => f a b -> a -> b

But that's not really what I want to say...that's constraining the class to make it clear that the type inhabits it...not constraining the instance.


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Long time no Haskell....simple typeclass question

Nicholls, Mark-2
Actually I can fix it by following the Haskell advice and reading http://blog.omega-prime.co.uk/?p=127 and turning on some extensions...

Basically....
You DO follow Haskell's advice by amending the class declaration with a class content on the exe function, but you can let the instance declaration declare the constraint by....
turning the constraint into a type (ConstraintKinds)
You then use type families  (TypeFamilies) to calculate the constraint in the instance declaration.
 
> {-# LANGUAGE TypeFamilies, ConstraintKinds #-}

> import GHC.Prim

> data Func a b = Func (a->b)

> exeFunc :: (Ord a) => Func a b -> a -> b
> exeFunc (Func g) = g

> class IFunc f where
>   type ExeConstraint f a b :: Constraint
>   exe :: (ExeConstraint f a b) => f a b -> a -> b

> instance IFunc Func where
>   type ExeConstraint Func a b = Ord a
>   exe = exeFunc

As opposed to the instance declaration for the function type, which has no constraint

> instance IFunc (->) where
>   type ExeConstraint (->) a b = ()
>   exe f = f


In some sense it still feels clumsy....with my F bounded OO type head on...which I'm told I must ignore....in something like C# this would be something like...

    interface IOrd {}

    interface IFunc<A,B>
    {
        B Exe(A a);
    }

    class Fun<A,B> : IFunc<A,B>
        where A : IOrd
    {
        readonly Func<A,B> f;

        public Fun(Func<A,B> f) { this.f = f; }

        public B Exe(A a)
        {
            return this.f(a);
        }
    }

i.e. here the IFunc declaration is closed...and blissfully unaware of any horrible constraints that need to be applied in order for the compiler to make the data type inhabit the type.

If I want to make my typeclass definitions as closed in Haskell, it would seem that I would always have to declare them with indexed types as constraints?
 
Or am I missing something? (I expect there is a reason, and I'm aware I'm comparing apple Haskell type(classe)s with orange C# types).

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