Haskell 堆栈实现未按预期工作

Question

本人自学Haskell，自己实现了一个栈如下：

pop :: [a] -> [a]
pop [] = error "Cannot pop from an empty list!"
pop xs = init xs

push :: a -> [a] -> [a]
push = (:)

peek :: [a] -> a
peek [] = error "Cannot peek from an empty list!"
peek (x:xs) = last (x:xs)


isEmpty :: [Int] -> Bool
isEmpty [] = True
isEmpty xs = False

test :: [Int] -> Either String [Int]
test = fmap reverse . foldM step []
    where step xs x | even x = Right (push x xs)
                    | odd x = do
                        var <- Right (pop xs)
                        if isEmpty var then Left "Successful"
                        else (if not (isEmpty var) then return xs else Left "Unsuccessful")

压栈是没有问题的，但是这个实现不能按预期的那样出栈，例如：

Input: [0,2,3,1]
Output: Right [0,2]
Expected: Left "Successful"

我认为问题出在 test 函数的 do 部分，但我正在努力使其工作。提前致谢！

Answer 1

在评论的帮助下，test 函数现在可以使用以下实现工作：

test :: [Int] -> Either String [Int]
test = fmap reverse . foldM step []
    where step xs x | even x = Right (push x xs)
                    | odd x = if isEmpty (pop xs) then Left "Successful"
                        else (if not (isEmpty (pop xs)) then return (pop xs) else Left "Unsuccessful")

Haskell 堆栈实现未按预期工作

Haskell Stack Implementation not working as expected

stack

haskell