2

鉴于我有以下 DSL（使用 Free Monad）及其解释器：

data MyDslF next =
    GetThingById Int (Thing -> next)
  | Log Text next

type MyDslT = FT MyDslF

runMyDsl :: (MonadLogger m, MonadIO m, MonadCatch m) => MyDslT m a -> m a
runMyDsl = iterT run
  where
    run :: (MonadLogger m, MonadIO m, MonadCatch m) => MyDslF (m a) -> m a
    run (Log message continue)      = Logger.log message >> continue
    run (GetThingById id' continue) = SomeApi.getThingById id' >>= continue

我想在内部更改解释器以使用 MonadState ，以便如果 aThing已经为给定检索到Id，则没有第二次调用SomeApi

假设我已经知道如何使用getand编写记忆版本put，但我遇到的问题是在MonadState内部运行runMyDsl。我在想解决方案看起来类似于：

type ThingMap = Map Int Thing

runMyDsl :: (MonadLogger m, MonadIO m, MonadCatch m) => MyDslT m a -> m a
runMyDsl = flip evalStateT mempty . iterT run
  where
    run :: (MonadLogger m, MonadIO m, MonadCatch m, MonadState ThingMap m) => MyDslF (m a) -> m a
    run ..

但是类型不对齐，因为run返回(.. , MonadState ThingMap m) => m a和evalStateT期望StateT ThingMap m a。

4

1 回答 1

2

使用iterTM代替iterT：

runMyDsl :: (MonadLogger m, MonadIO m, MonadCatch m) => MyDslT m a -> m a
runMyDsl dsl = evalStateT (iterTM run dsl) Map.empty
  where
  run (Log message continue)      = logger message >> continue
  run (GetThingById id' continue) = do
    m <- get 
    case Map.lookup id' m of
      Nothing -> do 
         thing <- getThingById id' 
         put (Map.insert id' thing m)
         continue thing
      Just thing -> continue thing

等效地，iterT如果您首先提高MyDsl m a到MyDsl (StateT Int m) ausing ，则可以使用hoistFT lift，如下所示：

runMyDsl :: (MonadLogger m, MonadIO m, MonadCatch m) => MyDslT m a -> m a
runMyDsl dsl = evalStateT (iterT run (hoistFT lift dsl)) Map.empty

这使得dsl它MyDsl (StateT Int m) a实际上不涉及任何状态更新，但run确实涉及状态转换。

于 2016-07-15T11:31:44.353 回答