smoosh.hs

{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE TypeFamilies #-}

module Smoosh
  ( Smoosh (..)
  )
where

import Control.Monad (join)

type family Smooshed a where
  Smooshed (f (f a)) = Smooshed (f a)
  Smooshed a = a

class Smoosh a where
  smoosh :: a -> Smooshed a

instance {-# OVERLAPPABLE #-} a ~ Smooshed a => Smoosh a where
  smoosh = id

instance (Monad m, Smoosh (m a)) => Smoosh (m (m a)) where
  smoosh = smoosh . join