mtl :从StateT切换到transformers

Question

使用mtl，派生MonadState似乎可以正确地提升make state操作：

: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 ()

对于转换器，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 ()

我应该怎么做才能让它与transformers一起工作呢？

Answer 1

你应该使用mtl来实现这一点。mtl和transformers并不是真正的竞争对手-- mtl depends on转换器！它的存在是为了添加像MonadState这样使用函数依赖的类，这样它们就不必成为核心(Haskell-98)类型定义的一部分。还有一个使用类型族的包(monads-tf)，它也与transformers兼容(但实际上，您不应该使用它)。

也许我应该问:为什么你要“从mtl切换到transformers"？

Answer 2

MonadState实例位于包monads-tf或monads-fd中，具体取决于您使用的是类型函数还是函数依赖项。我用monads-tf试过了(因为那是我喜欢的包)，GHC不能为St派生一个MonadState实例，所以你必须自己写一个。

{-# 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)