Question

问：榆树0.15不会让我（尾巴＆gt;＆gt;头）xs。如何以优雅的方式解决这个问题？

module Fibonacci where

import List exposing ((::), head, map2, reverse, tail)

fibonacci : Int -> List Int
fibonacci n =
    let fibonacci' n acc =
        if n <= 2
            then acc
            else fibonacci' (n-1) ((head acc + (tail >> head) acc) :: acc)
    in
        fibonacci' n [1,1] |> reverse

自：

head : List a -> Maybe a
tail : List a -> Maybe (List a)

我不知道如何转换Maybe (List a) to be List a

那么，在没有定义新函数的情况下，有没有简单的方法来组合这两个函数？

Answer 1

从技术上讲，你想要的是在Maybe monad 中的 Kleisli composition：

import List exposing (head, tail) import Graphics.Element exposing (show) (>=>) : (a -> Maybe b) -> (b -> Maybe c) -> (a -> Maybe c) f >=> g = \a -> case f a of Nothing -> Nothing Just b -> g b -- or: f >=> g = f >> (\x -> Maybe.andThen x g) main = show ((tail >=> head) [1,2,3,4,5])

请注意(>=>)的类型与(>>)的类型非常相似，但所有返回类型都包含在Maybe中。您可以通过这种方式链接任意Maybe - 返回函数（例如tail >=> tail >=> tail >=> head）。

您无法将Maybe a安全地转换为a，因为您没有选择，只有在给定Nothing时崩溃。但可以将Maybe (Maybe a)值展平为Maybe a：

flattenMaybe : Maybe (Maybe a) -> Maybe a flattenMaybe mma = case mma of Just (Just a) -> Just a _ -> Nothing

撰写尾部和头部

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