readerMonad.hs

instance Monad ((->) r) where
  return = const
  {-Note:
    (>>=) :: (r -> a) -> (a -> (r -> b)) -> (r -> b)
    f :: (r -> a)
    g :: (a -> (r -> b))
  -}
  f >>= g = \r -> g (f r) r

-- using >>= and lambda functions
mean' = (fromIntegral . length) >>= \l -> sum >>= \s -> return $ s / l

-- translation using do notation
mean = do
  l <- (fromIntegral . length)
  s <- sum
  return $ s / l

-- these are equivalent
ans = mean  [1, 2, 3, 9, 2] -- 3.4
ans'= mean' [1, 2, 3, 9, 2] -- 3.4