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我正在爲我參加的課程編寫一個小型編譯器。所以我開始寫這個monad變換器來處理類型檢查，但是得到了一個非常神祕的類型錯誤。涉及函數依賴的東西，我不太清楚。 從程序的小摘錄，可以重現錯誤：這個類型檢查錯誤是什麼意思？
import Control.Monad.RWS.Lazy
import qualified Data.Map as M
import Control.Applicative
--Placeholders for other data types.
data TypeError = TypeError
data Ident = Ident String
data Type = Type
type Typer a = RWS FunctionTable [TypeError] IdentTable a
type FunctionTable = M.Map Ident Type
type IdentTable = [M.Map Ident Type]
emptyIdents :: IdentTable
emptyIdents = []
getIdent :: Ident -> Typer (Maybe Type)
getIdent id = getIdent' id <$> get
getIdent' :: Ident -> IdentTable -> Maybe Type
getIdent' _ [] = Nothing
getIdent' id (x:xs) =
case M.lookup id x of
Just t -> Just t
Nothing -> getIdent' id xs
putIdent :: Ident -> Type -> Typer()
putIdent id ty = modify $ \xs -> case xs of
[] -> [M.singleton id ty]
(x:xs) -> (M.insert id ty x) : xs
scopeEnter :: Typer()
scopeEnter = modify $ \ids -> emptyIdents : ids
scopeExit :: Typer()
scopeExit = modify tail
和實際的消息：
[1 of 1] Compiling Main (ErrorExample.hs, interpreted)
ErrorExample.hs:30:22:
Couldn't match type `M.Map Ident Type' with `Type'
When using functional dependencies to combine
MonadState
[[M.Map Ident Type]]
(RWST
(M.Map Ident Type)
[TypeError]
[M.Map Ident Type]
transformers-0.2.2.0:Data.Functor.Identity.Identity),
arising from a use of `modify' at ErrorExample.hs:35:18-23
MonadState
[M.Map Ident Type]
(RWST
(M.Map Ident Type)
[TypeError]
[M.Map Ident Type]
transformers-0.2.2.0:Data.Functor.Identity.Identity),
arising from a use of `modify' at ErrorExample.hs:30:22-27
In the expression: modify
In the expression:
modify
$ \ xs
-> case xs of {
[] -> [...]
(x : xs) -> (M.insert id ty x) : xs }
ErrorExample.hs:30:22:
Couldn't match type `[]' with `M.Map Ident'
When using functional dependencies to combine
MonadState
[[M.Map Ident Type]]
(RWST
(M.Map Ident Type)
[TypeError]
[M.Map Ident Type]
transformers-0.2.2.0:Data.Functor.Identity.Identity),
arising from a use of `modify' at ErrorExample.hs:35:18-23
MonadState
[M.Map Ident Type]
(RWST
(M.Map Ident Type)
[TypeError]
[M.Map Ident Type]
transformers-0.2.2.0:Data.Functor.Identity.Identity),
arising from a use of `modify' at ErrorExample.hs:30:22-27
In the expression: modify
In the expression:
modify
$ \ xs
-> case xs of {
[] -> [...]
(x : xs) -> (M.insert id ty x) : xs }
ErrorExample.hs:35:18:
Couldn't match type `Type' with `M.Map Ident Type'
When using functional dependencies to combine
MonadState s (RWST r w s m),
arising from the dependency `m -> s'
in the instance declaration in `Control.Monad.State.Class'
MonadState
[[M.Map Ident Type]]
(RWST
(M.Map Ident Type)
[TypeError]
[M.Map Ident Type]
transformers-0.2.2.0:Data.Functor.Identity.Identity),
arising from a use of `modify' at ErrorExample.hs:35:18-23
In the expression: modify
In the expression: modify $ \ ids -> emptyIdents : ids
ErrorExample.hs:35:18:
Couldn't match type `M.Map Ident' with `[]'
When using functional dependencies to combine
MonadState s (RWST r w s m),
arising from the dependency `m -> s'
in the instance declaration in `Control.Monad.State.Class'
MonadState
[[M.Map Ident Type]]
(RWST
(M.Map Ident Type)
[TypeError]
[M.Map Ident Type]
transformers-0.2.2.0:Data.Functor.Identity.Identity),
arising from a use of `modify' at ErrorExample.hs:35:18-23
In the expression: modify
In the expression: modify $ \ ids -> emptyIdents : ids
Failed, modules loaded: none.
Leaving GHCi.