Re: [Haskell-cafe] higher kind in parametric type

11 Jul 2018


      Olaf Klinke  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

Re: [Haskell-cafe] higher kind in parametric type

Frank Staals