Starring.hs

{-# LANGUAGE FlexibleContexts #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE TypeInType #-}
{-# LANGUAGE TypeSynonymInstances #-}

import Data.Kind (Type)
import GHC.TypeLits (Nat)

class Starring ka
instance Starring Type
instance Starring kb => Starring (ka -> kb)

a :: Starring Type => ()
a = ()

b :: Starring (Type -> Type) => ()
b = ()

c :: Starring Nat => ()
c = ()

d :: Starring (Nat -> Type) => ()
d = ()

e :: Starring (Type -> Nat) => ()
e = ()


-- GHCi:
--
-- > a
-- ()
-- > b
-- ()
-- > c
-- <interactive>:25:1: error:
--     • No instance for (Starring Nat) arising from a use of ‘c’
--     • When instantiating ‘it’, initially inferred to have
--       this overly-general type:
--         Starring Nat => ()
--       NB: This instantiation can be caused by the monomorphism restriction.
-- > d
-- ()
-- > e
-- <interactive>:27:1: error:
--     • No instance for (Starring Nat) arising from a use of ‘e’
--     • When instantiating ‘it’, initially inferred to have
--       this overly-general type:
--         Starring (Type -> Nat) => ()
--       NB: This instantiation can be caused by the monomorphism restriction.