〈Haskell Tutorial（31）do 區塊與 <- 綁定〉的習題解答

ForM.hs

sequence' :: Monad m => [m a] -> m [a]
sequence' [] = return []
sequence' (x:xs) = do
    value <- x
    values <- sequence' xs
    return (value : values)
 
forM' :: Monad m => [a] -> (a -> m b) -> m [b]
forM' xs f = sequence' $ map f xs