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

On 07/11/2018 03:15 PM, Olaf Klinke wrote:

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Am 11.07.2018 um 16:31 schrieb Frank Staals :

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

You solved it, Frank, thanks a lot!

Actually, it seems that this line makes all the difference: {-# LANGUAGE PolyKinds #-}

In ghc-7.4.2 my code compiles with the pragma, and yields the above error message without. Usually the compiler is kind enough to suggest adding the language extension when encountering an error. This time it didn't. Maybe this is a case for the ghc developers?

Hello, There is also a solution without the PolyKinds extension: newtype Heading (f :: (* -> *) -> *) (p :: * -> *) = ... I think the error message could be improved here but should probably not recommend the PolyKinds extension because it has a few surprising behaviors (IIRC it can break existing code, though I don't have an example off-hand). A safer alternative is to encourage kind annotations on higher-kinded types, that also mirrors the existing practice of giving signatures to toplevel functions. Li-yao