ConstraintKinds

gistfile1.txt

{-# LANGUAGE AllowAmbiguousTypes, ConstraintKinds, FlexibleInstances, KindSignatures, RankNTypes, TypeApplications #-}
module Lib3 where

import ClassyPrelude
import Data.Kind (Constraint)

type MyConstraint a = (Show a, Monoid a)

-- The error goes away if I replace 'MyConstraint with Show' on L10,18, but I want to be able to express multiple constraints on L18.
fooF :: forall a. MyConstraint a => a -> String
fooF = const ""

foo :: forall (c :: * -> Constraint) a1 a2 b. (c a1, c a2) => (forall x. c x => x -> b) -> (a1, a2) -> [b]
foo f (x1, x2) = [f x1, f x2]

foo' :: (Int, Bool) -> [String]
-- "The type synonym ‘MyConstraint’ should have 1 argument, but has been given none"
foo' = foo @MyConstraint fooF

solution1 -- Requires UndecidableInstances

{-# LANGUAGE AllowAmbiguousTypes, ConstraintKinds, FlexibleInstances, KindSignatures, RankNTypes, TypeApplications #-}
{-# LANGUAGE UndecidableInstances #-}
module Lib3 where

import ClassyPrelude
import Data.Kind (Constraint)

class MyConstraint a where
  fooF :: a -> String

instance (Show a, Enum a) => MyConstraint a where
  fooF = const ""


foo :: forall (c :: * -> Constraint) a1 a2 b. (c a1, c a2) => (forall x. c x => x -> b) -> (a1, a2) -> [b]
foo f (x1, x2) = [f x1, f x2]

foo' :: (Int, Bool) -> [String]
foo' = foo @MyConstraint fooF