State monad:如何在Haskell中"打印"中间值

Question

我是haskell的新手,我有以下代码

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

我想在之后打印状态值put (n+1)并继续这样的计算

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

整个代码如何看待这个？

Answer 1

如果要在状态计算中运行IO操作,可以更改tick返回a StateT Int IO Int和use 的类型liftIO.然后你可以运行它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 ()