Haskell Monad变压器堆栈和类型签名

我正试图创建一个Monad Transformers堆栈,但在获取函数的正确类型签名时遇到了问题。(我对哈斯克尔还很陌生)

堆栈结合了多个状态转换器,因为我有多个状态需要跟踪(其中两个状态可以进行元组化,但我将在一秒钟内完成),还有一个用于日志记录的writert。

以下是我目前为止的情况:

module Pass1 where
import Control.Monad.Identity
import Control.Monad.State
import Control.Monad.Writer
import Data.Maybe
import qualified Data.Map as Map
import Types

data Msg = Error String
         | Warning String

type Pass1 a = WriterT [Msg] (StateT Int (StateT [Line] (StateT [Address] Identity))) a


runPass1 addrs instrs msgs = runIdentity (runStateT (runStateT (runStateT (runWriterT msgs) 1) instrs) addrs)


--popLine :: (MonadState s m) => m (Maybe s)
--popLine :: (Monad m) => StateT [Line] m (Maybe Line)
popLine :: (MonadState s m) => m (Maybe Line)
popLine = do
        ls <- get
        case ls of
          x:xs -> do
                    put xs
                    return $ Just x
          []   -> return Nothing


incLineNum :: (Num s, MonadState s m) => m ()
incLineNum = do
               ln <- get
               put $ ln + 1

curLineNum :: (MonadState s m) => m s
curLineNum = do
               ln <- get
               return ln

evalr = do l <- popLine
           --incLineNum
           return l

我想要 popLine 把…弄得一团糟 [Line] 状态与 xLineNum 影响的函数 Int 状态。 evalr 是要传递给 runPass1 .

每当我加载代码时,都会遇到以下各种错误:

Pass1.hs:23:14:
    No instance for (MonadState [t] m)
      arising from a use of `get' at Pass1.hs:23:14-16
    Possible fix: add an instance declaration for (MonadState [t] m)
    In a stmt of a 'do' expression: ls <- get
    In the expression:
        do ls <- get
           case ls of {
             x : xs -> do ...
             [] -> return Nothing }
    In the definition of `popLine':
        popLine = do ls <- get
                     case ls of {
                       x : xs -> ...
                       [] -> return Nothing }


Pass1.hs:22:0:
    Couldn't match expected type `s' against inferred type `[Line]'
      `s' is a rigid type variable bound by                        
          the type signature for `popLine' at Pass1.hs:21:23        
    When using functional dependencies to combine                  
      MonadState [Line] m,                                         
        arising from a use of `get' at Pass1.hs:23:14-16            
      MonadState s m,                                              
        arising from the type signature for `popLine'              
                     at Pass1.hs:(22,0)-(28,31)                     
    When generalising the type(s) for `popLine'         




Pass1.hs:23:14:
    Could not deduce (MonadState [Line] m)
      from the context (MonadState s m)   
      arising from a use of `get' at Pass1.hs:23:14-16
    Possible fix:                                    
      add (MonadState [Line] m) to the context of    
        the type signature for `popLine'             
      or add an instance declaration for (MonadState [Line] m)
    In a stmt of a 'do' expression: ls <- get
    In the expression:
        do ls <- get
           case ls of {
             x : xs -> do ...
             [] -> return Nothing }
    In the definition of `popLine':
        popLine = do ls <- get
                     case ls of {
                       x : xs -> ...
                       [] -> return Nothing }

所有签名似乎都不正确,但popline是第一个函数,因此它是唯一一个立即导致错误的函数。

我尝试在类型签名中添加它的建议(例如: popLine :: (MonadState [Line] m) => ... 但它的错误是这样的:

Pass1.hs:21:0:
    Non type-variable argument in the constraint: MonadState [Line] m
    (Use -XFlexibleContexts to permit this)                          
    In the type signature for `popLine':                             
      popLine :: (MonadState [Line] m) => m (Maybe Line)

每当我尝试做一些不是类型变量的事情时,我似乎总是收到这个消息。看起来像 (MonadState s m) 好的,但如果我用 [a] 而不是 s 它的错误与上述类似。(最初[line]和int是在一个单一状态下进行元组化的,但是我得到了这个错误,所以我认为我会尝试将它们置于不同的状态)。

GHC 6.10.4,库本图

那么,有人能告诉我发生了什么,并给我一个解释/展示正确的类型签名吗,或者有人知道关于这方面的一个很好的参考资料吗(到目前为止唯一有帮助的是“Monad Transformers step by step”,但它只使用一个aux state函数和一个statet)?

非常感谢。

编辑
下面是结合JFT和Edward建议的编译代码:

{-# LANGUAGE GeneralizedNewtypeDeriving #-} -- needed for: deriving (Functor,Monad)
{-# LANGUAGE MultiParamTypeClasses #-}      -- needed for: MonadState instance
{-# LANGUAGE FlexibleContexts #-}           -- needed for: (MonadState PassState m) => ...

module Pass1 where
import Control.Monad.State
import Control.Monad.Writer
import Data.Maybe
import Types

type Lines     = [Line]
type Addresses = [Address]
type LineNum   = Int
type Messages  = [Msg]
data Msg = Error String
         | Warning String

data PassState = PassState { passLineNum :: LineNum
                           , passLines :: Lines
                           , passAddresses :: Addresses
                           }

newtype Pass1 a = Pass1 { unPass1 :: WriterT Messages (State PassState) a
                        }
                        deriving (Functor,Monad)

instance MonadState PassState Pass1 where
        get   = Pass1 . lift $ get
        put s = Pass1 . lift $ put s



runPass1 :: PassState -> Pass1 a -> ((a, Messages), PassState)
runPass1 state = flip runState state .
                 runWriterT          .
                 unPass1


curLineNum :: (MonadState PassState m) => m LineNum
curLineNum = do
               state <- get
               return $ passLineNum state


nextLine :: (MonadState PassState m) => m (Maybe Line)
nextLine = do
             state <- get
             let c = passLineNum state
             let l = passLines state
             case l of
               x:xs -> do
                         put state { passLines = xs, passLineNum = (c+1) }
                         return $ Just x
               _ -> return Nothing



evalr :: Pass1 (Maybe Line,LineNum)
evalr = do
          l <- nextLine
          c <- curLineNum
          --tell $ Warning "hello"
          return (l,c)

我结合 incLineNum 和 弹出线 进入之内 nextLine 我仍然需要让作者单子部分工作,但我想我知道从这里到哪里去。谢谢,伙计们。