Haskell Prelude中的'const'有什么意义？ [英] What's the point of 'const' in the Haskell Prelude?

问题描述

通过Haskell序曲，我看到一个函数 const ：

  const x _ = x 
  

我似乎无法找到任何与此功能相关的内容。

有什么意义？任何人都可以举一个例子说明这个函数可以用在哪里？

解决方案

当你不愿意传递给高阶函数不需要所有的灵活性。例如，一元序列运算符>>> 可以用一元绑定运算符定义为

  x>>> y = x>> = const y 
  

它比使用lambda

  x>>> y = x>> = \ _  - > y 
  

并且您甚至可以使用它无点

 （>>）=（。const）。 （> =）
  

虽然我不特别推荐在这种情况下。 p>

Looking through the Haskell Prelude, I see a function const:

const x _ = x

I can't seem to find anything relevant regarding this function.

What's the point? Can anyone give an example of where this function might be used?

解决方案

It's useful for passing to higher-order functions when you don't need all their flexibility. For example, the monadic sequence operator >> can be defined in terms of the monadic bind operator as

x >> y = x >>= const y

It's somewhat neater than using a lambda

x >> y = x >>= \_ -> y

and you can even use it point-free

(>>) = (. const) . (>>=)

although I don't particularly recommend that in this case.

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