The Constraint kind

MonadSet.hs

-- https://jeltsch.wordpress.com/2013/02/14/the-constraint-kind/
{-# LANGUAGE ConstraintKinds, TypeFamilies #-}

import Prelude hiding (Monad (..))
import Data.Set
import GHC.Exts (Constraint)

setReturn :: el -> Set el
setReturn = singleton

setBind :: (Ord el, Ord el') =>
           Set el -> (el -> Set el') -> Set el'
setBind set1 gen2 = unions (Prelude.map gen2 (toList set1))

class Monad mon where
    type Object mon val :: Constraint
    type Object mon val = ()

    return :: Object mon val =>
              val -> mon val

    (>>=)  :: (Object mon val, Object mon val') =>
              mon val -> (val -> mon val') -> mon val'

instance Monad Set where
    type Object Set el = Ord el

    return = setReturn

    (>>=) = setBind