在Haskell中总结Maybe Int列表的习惯用法 [英] Idiomatic way to sum a list of Maybe Int in haskell

问题描述

有没有更实用的方法来实现以下目标？我觉得我错过了一种摆脱拉姆达的方式，但无法找到将其转换为无点的方法。也许还有另一种非应用方式，那就更简单了？

  import Data.Maybe 
 import Control （只有0）[只有3，只有4] 
（\ xy  - >纯（+）* x * y） - 只有7 
 
 foldl（\ xy  - >纯（+）* x * y）（只是0）[只有3，只有4，没有] 
  -  Nothing 
  

解决方案

从Control.Monad使用 sequence ：

 > fmap sum $ sequence [Just 3，Just 4] 
只需7 
> fmap sum $ sequence [Just 3，Just 4，Nothing] 
 Nothing 
  

- 免费表格：



  sumMaybe :: Num a => [也许a]  - >也许
 sumMaybe = fmap总和。序列
  




Is there a more idiomatic way to implement the following? I feel like I'm missing a way to get rid of the lambda, but couldn't figure out a way to convert it to point-free. Maybe there is another non-applicative way as well that is more straight forward?

import Data.Maybe
import Control.Applicative

foldl (\x y -> pure (+) <*> x <*> y) (Just 0) [Just 3, Just 4]
-- Just 7

foldl (\x y -> pure (+) <*> x <*> y) (Just 0) [Just 3, Just 4, Nothing]
-- Nothing

解决方案

I'd just use sequence from Control.Monad:

> fmap sum $ sequence [Just 3, Just 4]
Just 7
> fmap sum $ sequence [Just 3, Just 4, Nothing]
Nothing

For the point-free form:

sumMaybe :: Num a => [Maybe a] -> Maybe a
sumMaybe = fmap sum . sequence

这篇关于在Haskell中总结Maybe Int列表的习惯用法的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！