【发布时间】：2011-01-06 04:15:36
【问题描述】：
我正在尝试创建一堆 monad 转换器，但无法为我的函数获取正确的类型签名。 （我对 Haskell 还是很陌生）
堆栈组合了多个 StateT 转换器，因为我需要跟踪多个状态（其中两个可以被元组化，但我会在一秒钟内完成）和一个用于日志记录的 WriterT。
这是我目前所拥有的：
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
我希望popLine 与[Line] 状态和xLineNum 函数混淆以影响Int 状态。 evalr 是将传递给 runPass1 的计算。
每当我加载代码时，我都会遇到以下错误：
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 }
似乎没有一个签名是正确的，但 popLine 是第一个函数，所以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建议的内容（例如：popLine :: (MonadState [Line] m) => ...，但随后出现如下错误：
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)
每当我尝试做不是类型变量的事情时，我似乎总是会收到此消息。它似乎喜欢(MonadState s m) ok 和其他错误，但是当我用[a] 而不是s 尝试它时，它会出现类似于上面的错误。 （最初 [Line] 和 Int 被元组化为一个状态，但我遇到了这个错误，所以我想我会尝试将它们置于不同的状态）。
GHC 6.10.4，库本图
那么，任何人都可以告诉我发生了什么并给出解释/显示正确的类型签名，或者有没有人知道关于这些东西的一个很好的参考（到目前为止唯一有帮助的是“Monad Transformers Step by Step”，但只使用一个辅助状态函数和一个StateT）？
非常感谢。
编辑
这是包含 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)
我将 incLineNum 和 popLine 合并到 nextLine 我仍然需要让 Writer monad 部分工作，但我想我知道从这里去哪里。谢谢各位。
【问题讨论】：
标签： haskell types monad-transformers state-monad