如何编写状态 Monad 的实例

问题描述

我对 haskell 中的 monads 非常陌生，我正在尝试使用 monads ny 创建一些实例来发展我的知识，但我真的很困惑这个我遇到了一些错误并且已经在我这里工作了一段时间我仍然不确定任何帮助和解释是否值得赞赏这是我到目前为止所拥有的任何想法我哪里出错了？

newtype ST b = S (Int -> (b,Int))
runState :: ST b  -> Int -> (b,Int)
runState (S b) st = b st 

instance Monad ST where 
return :: b -> ST b
return x = S (\st -> (x,st))       the new state with a b 

(>>=) :: ST b -> (b -> ST c) -> ST c
c >>= c' = S (\st1 ->
                let (b,st2) = runState c st1
                    (c,st3) = runState (c' b) st2
                in  (c,st3))

解决方法

您可能还需要提供 Applicative 和 Functor 的实现：

import Control.Applicative
import Control.Monad (liftM,ap)

newtype ST b = S (Int -> (b,Int))
runState :: ST b  -> Int -> (b,Int)
runState (S b) st = b st 

instance Monad ST where 
  -- return :: b -> ST b
  return x = S (\st -> (x,st))      -- takes in the current state and returns the new state with a b 
  
  -- (>>=) :: ST b -> (b -> ST c) -> ST c
  c >>= c' = S (\st1 ->
                let (b,st2) = runState c st1
                    (c'',st3) = runState (c' b) st2
                in  (c'',st3))


instance Applicative ST where
  pure = return
  (<*>) = ap


instance Functor ST where
  fmap = liftM

我发现here。