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）
该堆栈结合了多个 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。
这是我到目前为止所拥有的：
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
我想popLine弄乱[Line]状态和xLineNum影响Int状态的功能。evalr是将传递给 的计算runPass1。
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 }
没有一个签名似乎是正确的，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所建议的内容（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)
每当我尝试做一些不是类型变量的事情时，我似乎总是会收到这条消息。它似乎喜欢(MonadState s m)在其他东西上 ok 和 error，但是当我用 a[a]而不是sit 尝试它时，错误类似于上面。（最初 [Line] 和 Int 在一个状态下进行元组处理，但我收到了这个错误，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，谁能告诉我发生了什么并给出解释/向我展示正确的类型签名，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）？
提前谢谢了。
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)
我结合incLineNum并popLine进入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。多谢你们。