gistfile1.txt

[1 of 1] Compiling Main             ( polykinds.hs, polykinds.o )

polykinds.hs:14:24: error:
    • Expected kind ‘k’, but ‘Void’ has kind ‘*’
    • In the first argument of ‘Test’, namely ‘Void’
      In the type signature:
        test :: Test Void b => b -> Proxy a
      In the class declaration for ‘Test’

polykinds.hs

{-# LANGUAGE PolyKinds #-}
{-# LANGUAGE DefaultSignatures #-}
{-# LANGUAGE FlexibleContexts #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE MultiParamTypeClasses #-}

import Data.Proxy

data Void
data Void1 a

class Test (a :: k) b where
  test :: b -> Proxy a
  default test :: Test Void b => b -> Proxy a
  test = undefined

instance Test Void1 b
instance Test Void b