Re: [Haskell-cafe] Instances of constrained datatypes

One way to do roughly what you want is to pass the dictionary yourself:

data EqDict a = EqDict { leq :: a -> a -> Bool }

data EqList a = EqList (EqDict a) [a]

test :: EqList a -> EqList a -> Bool test (EqList dict (a0:as)) (EqList _ (b0:bs)) = (leq dict) a0 b0

In this way the definition of equality on elements of type 'a' is passed with the list type, so it can be used wherever the list type is used, without requiring extra constraints. Keean. Keean Schupke wrote:

I think it is more a problem of imlpementation than one of what is desirable. A Constrained data type:

data (Eq v) => EqList v = EqList [v]

The problem is how to get the dictionary for the class Eq to the application site:

f :: EqList v -> EqList v f (EqList (u0:us)) (EqList (v0:vs)) | v0 == u0 = ...

Which of course does not work... the constraint needs to be in the function type signature:

f :: Eq v => EqList v -> EqList v

Things are worse though, as even functions that use no methods of Eq will require the constraint.

The constraint on the data type does not stop you construction EqLists from non Eq members... of course this gets detected the moment you try and use it in a constrained function.

In other words using the constraint in the data type does nothing... you may as well just do:

f :: Eq v => [v] -> [v]

Infact I believe it was decided to remove the feature from Haskell98 entirely, but there was apparently some use for the 'syntax' although with a different effect.

Keean.

Cale Gibbard wrote:

...

I don't believe you can, but it would be nice. There are certain types, such as Set, where it's not really possible to just remove the constraint from the data declaration, and yet it would be nice if sets could be instances of Monad and Functor. Currently, to be an instance of Functor or Monad, your type has to be a functor defined on the whole category of types.

Could this issue be fixed somehow? Constrained instances would make various typeclass-based libraries more applicable. What would it break to allow instances where the types of functions defined by the typeclass are further restricted? I suppose that checking that types are correct becomes more difficult and non-local, because functions which are defined using the typeclass won't already have that constraint for obvious reasons. Still, the constraint is in the instance, which must be around when the functions actually get applied. There are probably bad interactions with the module system, but I'm not certain.

People must have talked about this before... was a consensus reached that I'm not aware of?

- Cale

On Apr 6, 2005 2:10 AM, Arjun Guha wrote:

...

This is a contrived example, but contains the essence of what I'd like to do. Suppose I have this datatype:

...

data (Eq v) => EqList v = EqList [v]

I'd like to make it an instance of Functor. However, fmap takes an arbitrary function of type a -> b. I need an Eq constraint on a and b. Is there any way to do this without creating my own `EqFunctor' class with explicitly-kinded quantification:

...

class (Eq a) => EqFunctor (f :: * -> *) a where eqfmap:: (Eq b) => (a -> b) -> f a -> f b

Thanks.

-Arjun

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