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現在、受講しているコース用の小さなコンパイラを作成しています。そこで、型チェックを処理するためにこのモナド変換子を書き始めましたが、非常に不可解な型エラーが発生しました。私が本当によく知らない機能的な依存関係を含む何か。エラーを再現できるプログラムからの小さな抜粋:

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

そして実際のメッセージ：

[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.
4

1 に答える 1

4

これは非常に悪いエラーメッセージです。原因は、Stateのタイプの不一致が原因です。コードが[M.MapIdentType]と[[M.MapIdentType]]を混在させようとしています。

emptyIdents呼び出しをscopeEnterに手動でインライン化すると、次のようになります。

scopeEnter :: Typer ()
scopeEnter = modify $ \ids -> [] : ids

...これはあまり意味がありません[M.Map Ident Type]。タイプチェックを行うバージョンと比較してください。

scopeEnter :: Typer ()
scopeEnter = modify $ \ids -> M.empty : ids
于 2012-04-05T21:41:22.967 に答える