“状态"的数据构造函数在哪里? [英] Where is the data constructor for 'State'?

问题描述

在阅读了有关Haskell状态单子的一些教程之后，我想亲自尝试一下.我阅读的教程声称Control.Monad.State提供以下定义:

After reading a couple of tutorials on Haskell state monads I wanted to try them out myself. The tutorials I read claim that the Control.Monad.State provide the following definition:

newtype State s a = State { runState :: s -> (a,s) }  

但是，我似乎很难找到State数据构造函数:

However, I seem to be having trouble find the State data constructor:

Prelude> import Control.Monad.State
Prelude Control.Monad.State> :t State

<interactive>:1:1:
    Not in scope: data constructor `State'
    Perhaps you meant `StateT' (imported from Control.Monad.State)

我还尝试了Hoogle搜索State，但是没有找到任何具有预期类型的​​数据构造函数.

I also tried a Hoogle search for State but did not find any data constructors with the expected type.

State构造函数在哪里?它曾经存在吗?还是我只是在错误的地方看?本质上，我想知道创建状态monad需要做什么.

Where did the State constructor go? Did it ever exist? Or am I just looking in the wrong place? Essentially I would like to know what I need to do to create a state monad.

推荐答案

它不再存在.不幸的是，这使得网络上许多Haskell资源过时了.

It doesn't exist any more. Unfortunately, this makes many Haskell resources on the web about it outdated.

要创建值，只需使用 state 函数:

To create a value, you can just use the state function:

state :: (s -> (a, s)) -> State s a

runState曾经是State的字段，现在本身只是一个普通函数，但其​​工作方式与以前相同.

runState, which used to be a field of State, is now just a normal function itself, but it works in the same way as before.

State已根据StateT monad转换器进行了重写:

State has been rewritten in terms of the StateT monad transformer:

type State s = StateT s Identity

StateT本身具有构造函数StateT，其功能与旧的State构造函数非常相似:

StateT itself has a constructor StateT that functions very similarly to the old State constructor:

newtype StateT s m a = StateT { runStateT :: s -> m (a, s) }

唯一的区别是，还有一个额外的参数m.这只是一个插槽，您可以在其中添加任何其他monad，然后StateT可以使用状态处理功能进行扩展.自然地，要恢复State的旧功能，只需将m设置为Identity，这不会做任何事情.

The only difference is that there is an extra parameter m. This is just a slot where you can add in any other monad, which StateT then extends with state-handling capabilities. Naturally, to regain the old functionality of State, you just have to set m to Identity, which doesn't do anything.

newtype Identity a = Identity { runIdentity :: a }

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