如何从monadic动作中提取价值 [英] How to extract value from monadic action

问题描述

是否有内置函数，其签名 ::（Monad m）=> m a - > a ？

Is there a built-in function with signature :: (Monad m) => m a -> a ?

Hoogle告诉我们没有这样的功能。

Hoogle tells that there is no such function.

你能解释一下为什么吗？

Can you explain why?

推荐答案

函数：

A monad only supplies two functions:

return :: Monad m => a -> m a
(>>=) :: Monad m => m a -> (a -> m b) -> m b

这两个都返回类型为 ma ，因此无法以任何方式组合这些函数来获取类型为 Monad m =>的函数。 m a - >一个。要做到这一点，你需要的不仅仅是这两个函数，所以你需要知道更多关于 m 的知识，而不是它的单子。

Both of these return something of type m a, so there is no way to combine these in any way to get a function of type Monad m => m a -> a. To do that, you'll need more than these two functions, so you need to know more about m than that it's a monad.

例如， Identity monad具有 runIdentity :: Identity a - >一个和几个monad具有相似的功能，但是没有办法一般地提供它。事实上，无法从monad中逃离对像 IO 这样的单子来说至关重要。

For example, the Identity monad has runIdentity :: Identity a -> a, and several monads have similar functions, but there is no way to provide it generically. In fact, the inability to "escape" from the monad is essential for monads like IO.

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