是否有一种无条件的方式将条件检查转换为输入的Maybe类型？ [英] Is there a point-free way to convert a conditional check into a Maybe type of the input?

问题描述

我只是在通过haskell中的一些简单练习，想知道是否有一种将if-then-else语句转换为 Maybe 类型的无点方法：如果条件为false，则返回 Nothing ;如果条件为真，则返回 Just 输入。

总之，给出一些：

  maybeIf ::（a  - > Bool ） - > a  - >也许一个
 maybeIf cond a =如果cond a然后一个其他没有
  

关于 a 的免费实现？我也一直在看更具体的版本， a - >也许是一个，并且感觉在 Control.Arrow 中可能有答案。但是，由于 Maybe 是一种数据类型，而if-else语句控制数据流，所以我不确定是否有干净的方式来执行它。

解决方案

您可以从 Data.Foldable find $ c $>然后它很简单：

$ p $ import Data.Foldable（find）

maybeIf cond =找到cond。只需


函数 find 并不复杂，你可以非常容易地定义它，你可以用 Maybe 的方式来定义它，但它与你自己实现 maybeIf ，所以你可能得不到多少，这取决于你为什么要这样做。

I am just working through some simple exercises in haskell and was wondering if there was a point-free way of converting an if-then-else statement into a Maybe type: Nothing being returned if the condition is false, and Just the input if the condition is true.

In short, given some:

maybeIf :: (a -> Bool) -> a -> Maybe a
maybeIf cond a = if cond a then Just a else Nothing

Is there an implementation that is point-free with respect to a? I've also been looking at a more concrete version, a -> Maybe a, and feel like there may be an answer somewhere in Control.Arrow. However, since Maybe is a data type and if-else statements control data flow, I'm unsure if there is a clean way of doing it.

解决方案

You can import find from Data.Foldable and then it's quite simply:

import Data.Foldable(find)

maybeIf cond = find cond . Just

The function find is not complicated so you could quite easily define it yourself less generically, in terms of Maybe, but it isn't actually so different from your own implementation of maybeIf so you might not gain much, depending on why you wanted to do it.

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