寻找mapAccumLM

Question

我正在寻找一种在IO monad中使用mapAccumL的方法 - 类似于mapM的模拟，即使用这种类型的签名：

mapAccumLM :: (Monad m) => (a -> b -> m(a, c)) -> a -> [b] -> m(a, [c])

这么简单吗？

Answer 1

这基本上是mapM上的StateT a IO：

mapAccumLM f a xs = runStateT (mapM (StateT . f) xs) a

Answer 2

与mapAccumL非常相似：

mapAccumLM :: Monad m => (a -> b -> m(a, c)) -> a -> [b] -> m(a, [c])
mapAccumLM _ a [] = return (a, [])
mapAccumLM f a (x:xs) = do
  (a', c) <- f a x
  (a'', cs) <- mapAccumLM f a' xs
  return (a'', c:cs)