State Monad:如何在Haskell中“打印"中间值 [英] State monad: How to `print` intermediate value in Haskell

问题描述

我是Haskell的新手，我有以下代码

I'm new in haskell and I have following code

module StateTest where

import Control.Monad.State.Lazy

tick :: State Int Int
tick = do n <- get
          put (n+1)
          return n

plusOne :: Int -> Int
plusOne = execState tick

main = print $ plusOne 1

我想在put (n+1)之后打印状态值并继续这样计算

And I want to print state value after put (n+1) and continue computation like this

tick = do n <- get
          put (n+1)
          print
          return n

遵循此规则的整个代码看起来如何?

How whole code will look following to this?

推荐答案

如果要在状态计算中运行IO操作，则可以更改tick的类型以返回StateT Int IO Int并使用liftIO.然后，您可以使用execStateT:

If you want to run IO actions within a state computation you can change the type of tick to return a StateT Int IO Int and use liftIO. Then you can run it using execStateT:

import Control.Monad.State.Lazy
import Control.Monad.IO.Class (liftIO)

tick :: StateT Int IO Int
tick = do n <- get
          put (n+1)
          liftIO $ print (n+1)
          return n

plusOne :: Int -> IO Int
plusOne = execStateT tick

main = plusOne 1 >> pure ()

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