Unable to resolve instance for polykinded superclass constraint on associated-type-family.
The following program doesn't compile:
{-# LANGUAGE FlexibleContexts #-}
{-# LANGUAGE PolyKinds #-}
{-# LANGUAGE TypeFamilies #-}
module Test where
class Back t
class Back (FrontBack t) => Front t where
type FrontBack t :: k
instance Back Bool
instance Front Int where
type FrontBack Int = Bool
with the error message:
• No instance for (Back (FrontBack Int))
arising from the superclasses of an instance declaration
• In the instance declaration for ‘Front Int’The example successfully compiles if the kind annotation on FrontBack is removed.