Question

import Control.Monad.State

type Stack = [Integer]

pop :: State Stack Integer
pop = state $ \(x:xs) -> (x, xs)

push :: Integer -> State Stack ()
push x = state $ \xs -> ((), (x:xs))

main :: IO()
main = print $ runState `enter code here` [1,2,3,4]

使用＆＃34; pop＆gt;＆gt; =（\ s1 - ＆gt; pop＆gt;＆gt; =（\ s2 - ＆gt; push enter code here）＆＃34; 我应该在这写什么？

Answer 1

编译器可以figure out the type to fill in a blank。如果我在代码

中添加了一个洞_

add = pop >>= \s1 -> pop >>= \s2 -> push _

编译器告诉我它应该有类型Integer。

   Found hole `_' with type: Integer

你可以把Integer放在那里，这是从堆栈中弹出的值的总和？

Answer 2

你是说这个？

popAndSum :: Int -> [Int] -> [Int]
popAndSum elem list = fst xs ++ [sum']
        where
            xs = splitAt ((length list) - elem) list
            sum' = foldl (+) 0 $ snd xs

popAndSum 2 [1,2,3,4,5] = [1,2,3,9]

popAndSum 3 [1,2,3,4,5] = [1,2,12]

我怎样才能从堆栈中弹出两个元素然后将它们作为值的总和推回？

2 个答案: