为什么没有<<在Haskell标准库中？ [英] Why is there no << in the Haskell standard library?

问题描述

Monad 类定义了一个>> 方法，该方法对两个monadic动作进行排序：

 >> :: Monad m => m a  - > m b  - > mb 
  

绑定操作符>> = 具有翻转参数等价物， = << ;一元函数组合（'鱼'）运算符> => 和 <= 。但似乎没有<< （在Hoogling的几分钟后）。这是为什么？

编辑：我知道这不是什么大不了的事。我只是喜欢某些代码行与左指向的操作符的方式。 x < - doSomething =<< doSomethingElse 看起来更好，箭头的方式都是相同的，比 x < - doSomethingElse>> = doSomething 。

解决方案

据我所知，没有很好的理由。请注意，您的 Monad 也应该是 Applicative 的实例，因此您可以使用< ; * 和 *> 来代替您的排序工具。

The Monad class defines a >> method, which sequences two monadic actions:

>> :: Monad m => m a -> m b -> m b

The binding operator >>= has a flipped-argument equivalent, =<<; as do the monadic function composition ('fish') operators >=> and <=<. There doesn't seem to be a <<, though (after a few minutes of Hoogling). Why is this?

Edit: I know it's not a big deal. I just like the way certain lines of code look with the left-pointing operators. x <- doSomething =<< doSomethingElse just looks nicer, with the arrows all going the same way, than x <- doSomethingElse >>= doSomething.

解决方案

To the best of my knowledge there is no good reason. Note, that your Monad should also be an instance of Applicative, so you can use <* and *> instead as your sequencing tools.

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