Question

使用mtl，派生MonadState似乎让make状态操作正常解除了：


:set -XGeneralizedNewtypeDeriving
import Control.Applicative (Applicative)
import Control.Monad.Trans ( MonadIO, liftIO )
import Control.Monad.State (MonadState, evalStateT, modify, StateT, gets)

data State = State { int:: Int }
newtype St a = St { unSt :: StateT State IO a } deriving (Functor, Applicative, Monad, MonadIO, MonadState State)

let testLift = gets int >> return () :: St ()

对于变形金刚，没有MonadState


:set -XGeneralizedNewtypeDeriving
import Control.Monad.Trans.State.Strict (runStateT, evalStateT, modify, StateT, gets)
import Control.Monad.IO.Class ( MonadIO, liftIO )
import Control.Applicative (Applicative)


data State = State { int:: Int }
newtype St a = St { unSt :: StateT State IO a } deriving (Functor, Applicative, Monad, MonadIO)


let testLift = gets int >> return () :: St ()

Couldn't match type `StateT State m0' with `St'
Expected type: St Int
  Actual type: StateT State m0 Int
In the return type of a call of `gets'
In the first argument of `(>>)', namely `gets int'
In the expression: gets int >> return () :: St ()

我应该做些什么才能让变形金刚工作？

Answer 1

你应该使用mtl来完成这项工作。 mtl和transformers并非真正的竞争对手 - mtl depends on变形金刚！它的存在是为了添加使用函数依赖关系的类MonadState，因此它们不必是核心（Haskell-98）类型定义的一部分。还有一个使用类型系列的包（monads-tf），它也与transformers兼容（但实际上，你不应该使用它）。

也许我应该问：你为什么“从mtl转换为变形金刚”？

Answer 2

MonadState实例位于包monads-tf或monads-fd中，具体取决于您是使用类型函数还是函数依赖项。我尝试使用monads-tf（因为这是我更喜欢的包），GHC无法为MonadState派生St实例，所以你必须自己编写一个。{1}}。

{-# OPTIONS_GHC -package transformers -package monads-tf -hide-package mtl #-}
{-# LANGUAGE GeneralizedNewtypeDeriving, TypeFamilies #-}

import Control.Monad.Trans.State.Strict (runStateT, evalStateT, modify, StateT)
import Control.Monad.State.Class  -- MonadState class
import Control.Monad.IO.Class ( MonadIO, liftIO )
import Control.Applicative (Applicative)

data State = State { int:: Int }
newtype St a = St { unSt :: StateT State IO a } deriving (Functor, Applicative, Monad, MonadIO)

instance MonadState St where
  type StateType St = State
  get = St get
  put x = St (put x)

StateT newtype：从mtl切换到变换器

2 个答案: