在Haskell中更改函数的返回类型？ [英] Changing the return type of a function in Haskell?

问题描述

考虑例如 比较在 Data.Ord 中。它返回一个 Ordering 。但我只关心这两项是否平等。因此，我需要将生成的 Ordering 转换为 Bool 。

  isEqualOn fxy = if fxy == EQ then True else False 
  

（或 isEqualOn fxy =比较fxy == EQ >

是否有更具构图性的方法可以做到这一点（比较适合比较或以无意义的方式写入）而不必写出所有内容？理想情况下，我正在寻找一些适用于n元函数的东西。

- 更新 -

<正如答案/注释所示，上面的具体示例（ isEqualOn ）可以使用上的标准来实现，在（==）上作为使用。然而，我的问题是关于改变函数返回类型的一般技巧，例如在评论中的SEC方法中以组合/无意义的方式来改变函数返回类型。 解决方案您在 isEqualOn 的定义将与<== c $ c> on（==）相同：

  \> import Data.Function（on）
 \> ：t（on（==））
（on（==））:: Eq a1 => （a  - > a1） - > a  - > a  - > Bool 
  

例如以绝对值进行比较：

  \> let isEqualOn = on（==）
 \> isEqualOn abs 2（-2）
 True 
  

Is there is a succinct way to change the return type of a function?

Consider for example comparing in Data.Ord. It returns an Ordering. Yet I am only interested in whether the two items are equal or not. So I need to convert the resulting Ordering to a Bool.

A straight forward way I can think of is:

isEqualOn f x y = if comparing f x y==EQ then True else False 

(or isEqualOn f x y = comparing f x y==EQ as here as pointed out in the comments).

Is there a more compositional way to do this (sort of adapting comparing or writing it in a pointless manner) without having to write out everything? Ideally, I am looking for something that works on n-ary functions as well.

-- Update --

As suggested by the answers/comments, the specific example above (isEqualOn) can be implemented using the standard on function as on (==). However, my question is about the general technique to change function return types in a compositional/pointless manner as exemplified by the SEC approach in the comments.

解决方案

Your definition of isEqualOn would be the same as on (==):

\> import Data.Function (on)
\> :t (on (==))
(on (==)) :: Eq a1 => (a -> a1) -> a -> a -> Bool

for example to compare on absolute value:

\> let isEqualOn = on (==)
\> isEqualOn abs 2 (-2)
True

这篇关于在Haskell中更改函数的返回类型？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！