StarMaker.hs

{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE TypeInType #-}

import Data.Kind
import GHC.Exts (Constraint)
import GHC.TypeLits (Nat)

type family StarMaker k :: Constraint where
  StarMaker Type = ()
  StarMaker (_ -> x) = StarMaker x

a :: StarMaker Type => ()
a = ()

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

c :: StarMaker Nat => ()
c = ()

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

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

-- Example GHCI interaction
--
-- @
-- > a
-- ()
--
-- > b
-- ()
--
-- > c
-- <interactive>:36:1: error:
--     • Could not deduce: StarMaker Nat arising from a use of ‘c’
--     • When instantiating ‘it’, initially inferred to have
--       this overly-general type:
--         StarMaker Nat => ()
--       NB: This instantiation can be caused by the monomorphism restriction.
-- > d
-- ()
--
-- > e
-- <interactive>:38:1: error:
--     • Could not deduce: StarMaker Nat arising from a use of ‘e’
--     • When instantiating ‘it’, initially inferred to have
--       this overly-general type:
--         StarMaker (Type -> Nat) => ()
--       NB: This instantiation can be caused by the monomorphism restriction.
-- @