mapm_example.hs

mapM' :: Monad m => (a -> m b) -> [a] -> m [b]
mapM' _ [] = return []
mapM' f (x : xs) = do
  y <- f x
  ys <- mapM' f xs
  return (y : ys)

mapM'' :: Monad m => (a -> m b) -> [a] -> m [b]
mapM'' _ [] = return []
mapM'' f (x : xs) =
  f x >>= \y ->
  mapM'' f xs >>= \ys ->
  return (y : ys)

ex1 = mapM' (\a -> [a, a]) [1, 0]
-- [[1,0],[1,0],[1,0],[1,0]]
ex2 = mapM' (\x -> Just x) [1, 0]
-- Just [1, 0]
ex3 = mapM' (\x -> if x > 0 then Just x else Nothing) [10, -1, -2, 5, 6]
-- Nothing