[Haskell-cafe] higher kind in parametric type

[Frank Staals]

Olaf Klinke <olf at aatal-apotheke.de> writes:

> Dear cafe,
>
> I am writing a library for parsing higher-kinded data, based on the czipwith package [1]. I ran into the following problem.
>
> I have a type class
> class Config p f | f -> p where
>     f :: (* -> *) -> *
> is the higher-kinded data and
>     p :: * -> *
> is the associated parser type.
>
> I want to add a class member that does not mention f in its type, e.g. 
>
> heading :: p String
>
> Naturally, this would lead to ambiguity checks to fail, as the usage of heading does not tell which Config instance to use. My usual workaround would be to wrap `heading` in a phantom type, e.g.
>     data Heading f p = Heading (p String)
> and give `heading` the type Heading f p. However, ghc-8.0.2 complains about f not being a type:
>
>     • Expecting one more argument to ‘f’
>       Expected a type, but ‘f’ has kind ‘(* -> *) -> *’
>     • In the first argument of ‘Heading’, namely ‘f’
>       In the type signature:
>         heading :: Heading f p
>       In the class declaration for ‘Config’
>
> Is there a restriction of the kinds that can be used in parametric types?
>
> Cheers,
> Olaf
>
> [1] http://hackage.haskell.org/package/czipwith

I think you can just a kind signature to 'f' in your Heading
datatype. I.e. the following seems to compile/typecheck here:


{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE FunctionalDependencies #-}
{-# LANGUAGE KindSignatures #-}
{-# LANGUAGE PolyKinds #-}

newtype Heading (f :: k) (p :: * -> *) = Heading (p String)


class Config (p :: * -> *) (f :: (* -> *) -> *) | f -> p where
  foo :: Heading f p -> String




-- 

- Frank