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我对收到的这个编译器错误消息感到困惑。函数 addAgent 和 withAgent 具有相似的类型签名和相似的实现，所以我不明白为什么 addAgent 编译但 withAgent 没有。预先感谢您的任何帮助！

{-# LANGUAGE TypeFamilies, FlexibleContexts #-}

import Control.Monad.IO.Class (liftIO)
import Control.Monad.State (StateT, execStateT, gets, modify)

class AgentDatabase d where
  type Elem d
  addAgent :: Elem d -> StateT d IO ()
  withAgent :: 
     (Elem d -> StateT d IO (Elem d)) -> String -> StateT d IO ()

data SimpleUniverse d = SimpleUniverse {
    agentDB :: d
    -- plus other fields
  }

-- I want the methods of class AgentDatabase to "penetrate" through
-- the outer wrapper of SimpleUniverse and operate on the agentDB field.
instance (AgentDatabase d) => 
    AgentDatabase (SimpleUniverse d) where
  type Elem (SimpleUniverse d) = Elem d

  -- When addAgent is invoked on a SimpleUniverse, apply it to the
  -- agentDB field inside.
  addAgent a = do
    db <- gets agentDB
    db' <- liftIO $ execStateT (addAgent a) db
    modify (\u -> u { agentDB=db' } )

  -- When withAgent is invoked on a SimpleUniverse, apply it to the
  -- agentDB field inside.
  withAgent program name = do
    db <- gets agentDB
    db' <- liftIO $ execStateT (withAgent program name) db -- line 33
    modify (\u -> u { agentDB=db' } )

我得到的错误信息是......

amy3.hs:33:11:
    Couldn't match type `d' with `SimpleUniverse d'
      `d' is a rigid type variable bound by
          the instance declaration at amy3.hs:19:25
    When using functional dependencies to combine
      Control.Monad.State.Class.MonadState s (StateT s m),
        arising from the dependency `m -> s'
        in the instance declaration in `Control.Monad.State.Class'
      Control.Monad.State.Class.MonadState
        (SimpleUniverse (SimpleUniverse d)) (StateT (SimpleUniverse d) IO),
        arising from a use of `gets' at amy3.hs:33:11-14
    In a stmt of a 'do' block: db <- gets agentDB
    In the expression:
      do { db <- gets agentDB;
           db' <- liftIO $ execStateT (withAgent program name) db;
           modify (\ u -> u {agentDB = db'}) }
Failed, modules loaded: none.
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1 回答 1

2

program有类型

Elem (SimpleUniverse d) -> StateT (SimpleUniverse  d) IO (Elem (SimpleUniverse d))

这确实简化了，但是

Elem d -> StateT (SimpleUniverse d) IO (Elem d)

并且内部withAgent采用类型的程序

Elem d -> StateT d IO (Elem d)

要解决此问题，此功能将有助于：

stateMap :: Monad m => (s -> t) -> (t -> s) -> StateT s m a -> StateT t m a
stateMap f g (StateT h) = StateT $ liftM (fmap f) . h . g
于 2012-11-28T16:14:54.110 回答