haskell - 状态单子(monad) : How to `print` intermediate value in Haskell

Question

我是 haskell 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 put (n+1) 之后打印状态值并像这样继续计算

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

接下来的整个代码会是什么样子？

Answer 1

如果您想在状态计算中运行 IO 操作，您可以更改 tick 的类型。返回 StateT Int IO Int并使用liftIO 。然后你可以使用 execStateT 运行它:

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 ()