简化嵌套可能模式匹配 [英] Simplifying nested Maybe pattern matching

问题描述

在我的代码中有以下结构：

  f :: Maybe A  - > X 
 f a = a 
的情况a无 - > x 
（Just b） - > 
的情况b没有 - > y 
（Just c） - >案例c的
无 - > z 
（Just d） - > d 
  

我没有看到一种明显的方法来简化它，而不是使用嵌套的也许函数，这不会使整个事情看起来好多了。有没有什么聪明的，但仍然可以理解的技巧可以让这个构造更加优雅？解析方案

UPDATED 2

Monad或者适合您

  import Data.Maybe（也许）
 
 maybeE :: e  - >也许是 - >或者e 
 maybeE e =也许（左e）右
 
 f ::可能（可能（也许是d）） - > ed ed 
fa = maybeE xa 
>> = maybeE y 
>> = maybeE z 
  

UPDATED 3

如果我们不想要 type，我们可以重写函数：

$ p $ import Data.Either（）

或者'= id id

f :: Maybe（Maybe（Maybe d）） - > d
fa ='$ maybeE xa
>> = maybeE y
>> = maybeE z


I have the following construct in my code:

f :: Maybe A -> X
f a = case a of
  Nothing -> x
  (Just b) -> case b of
    Nothing -> y
    (Just c) -> case c of
      Nothing -> z
      (Just d) -> d

I'm not seeing an obvious way to simplify this instead of using nested maybe functions, which wouldn't make the whole thing look much better. Are there any clever - but still understandable - tricks that would help make this construct more "elegant"?

解决方案

UPDATED 2

Monad Either is for you

import Data.Maybe (maybe)

maybeE :: e -> Maybe a -> Either e a
maybeE e = maybe (Left e) Right

f :: Maybe (Maybe (Maybe d)) -> Either e d
f a  =   maybeE x a 
     >>= maybeE y
     >>= maybeE z

UPDATED 3

If we want to have not Either type, we could rewrite function:

import Data.Either(either)

either' = either id id

f :: Maybe (Maybe (Maybe d)) -> d
f a = either' $ maybeE x a 
            >>= maybeE y
            >>= maybeE z

这篇关于简化嵌套可能模式匹配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！