https://twitter.com/keizo_bookman/status/634688932916035584 これ?

Flatten.hs

{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE FlexibleInstances, FlexibleContexts #-}
module Flatten (flatten) where

import Control.Monad

class Flatten m x a where
  flatten :: x -> m a
instance Monad m => Flatten m (m a) a where
  flatten = id
instance (Monad m, Flatten m (m x) a) => Flatten m (m (m x)) a where
  flatten = flatten . join

flattenList1 :: [Int]
flattenList1 = flatten [[1::Int]]

flattenList2 :: [Int]
flattenList2 = flatten [[[2::Int]]]

flattenMaybe1 :: Maybe String
flattenMaybe1 = flatten (Just (Just ""))

flattenMaybe2 :: Maybe String
flattenMaybe2 = flatten (Just (Just (Just "")))