如何使用Monad实现全局计数器？ [英] How to implement a global counter using Monad?

问题描述

我需要一个全局计数器，从0,1,2,3，.....
开始我明白这个不纯的代码应该分开执行......我刚刚开始了解Monad，但不知道如何使用Monad来实现这个全局计数器？这可能是理解它是否可能的非常有用的例子。

解释方案

状态monad给你状态，但只在monad中。它不会在重复调用函数时持续存在。

如果您想要真正的全局可变状态，您可能需要执行以下操作：

  import Data.IORef 
 
 type Counter = Int  - > IO Int 
 $ b $ makeCounter :: IO Counter 
 makeCounter = do 
r < -  newIORef 0 
 return（\i  - > do modifyIORef r（+ i） 
 readIORef r）
 
 testCounter :: Counter  - > IO（）
 testCounter counter = do 
b < -  counter 1 
c < -  counter 1 
d < -  counter 1 
 print [b，c，d] 
 
 main = do 
 counter < -  makeCounter 
 testCounter counter 
 testCounter counter 
  

这里'makeCounter'创建一个全局的可变变量，它保持它的状态跨越调用并破坏纯度。例如，在主函数中，对'testCounter'的两个相同的调用给出不同的结果。

 >主
 [1,2,3] 
 [4,5,6] 
  

I need a global counter, starting from 0, 1, 2, 3, ..... I kind of understand that this "impure" code should be separately implemented... I am just starting to understand Monad, but have no idea how to implement this global counter using Monad? This could be very useful example for understanding if it is possible

解决方案

State monad gives you state but only inside the monad. It is not persistent across repeated invocations of the function.

If you want truly global, mutable state you might want to do something like:

  import Data.IORef

  type Counter = Int -> IO Int

  makeCounter :: IO Counter
  makeCounter = do
      r <- newIORef 0
      return (\i -> do modifyIORef r (+i)
                       readIORef r)

  testCounter :: Counter -> IO ()
  testCounter counter = do
      b <- counter 1
      c <- counter 1
      d <- counter 1
      print [b,c,d]

  main = do
      counter <- makeCounter
      testCounter counter
      testCounter counter

Here 'makeCounter' creates a global, mutable variable that is keeps its state across invocations and destroys purity. For example, in the main function two identical calls to 'testCounter' gives different results.

> main
[1,2,3]
[4,5,6]

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