解释 Contramap | Ordering

本文介绍了解释 Contramap的处理方法，对大家解决问题具有一定的参考价值，需要的朋友们下面随着小编来一起学习吧！

问题描述

有人可以向我解释 contramap 吗?这个实现会是什么样子?好的使用示例应该是什么样的?

Can someone explain contramap to me? What would this implementation look like? What would good examples of usage look like?

// contravariant functor
trait Contravariant[F[_]] {
  def contramap[A, B](f: B => A): F[A] => F[B]
}

来源:http://tmorris.net/posts/functors-and-things-using-scala/index.html

推荐答案

如果看下面的Ordering.on 标准库方法:

If you look at the following Ordering.on method of the standard library:

def on[U](f: U => T): Ordering[U]

您将看到 on 将 Ordering[T] 转换为 Ordering[U] 同时从 获取函数U 到 T.所以方法 on 见证了一个事实，Ordering 可以被看作是一个 Contravariant 函子:

You'll see that on transforms an Ordering[T] into an Ordering[U] while taking a function from U to T. So the method on witnesses the fact that Ordering can be seen as a Contravariant functor with:

def contramap[A, B](f: B => A) = (fa: Ordering[A]) => fa.on(f)

我还看到了 Tony 的博文，它帮助我终于理解了这个来自retronym 的三年历史answer我的问题之一.

I also saw the blog post from Tony and it helped me finally makes sense of this three year old answer from retronym to one of my question.

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