haskell - 如何使用可扩展效果和IO？

Question

我正在尝试重写 article 中的示例到新版本的包extensible-effects 1.11.0.0，但当我尝试使用 IO monad 的 lift 时，我得到“无法推断...”:(

ghc 7.10.1

{-# LANGUAGE FlexibleContexts, ScopedTypeVariables, TypeOperators #-}
{-# LANGUAGE DeriveFunctor, DeriveDataTypeable #-}
module Main where

import Data.Typeable
import Control.Eff
import Control.Eff.Lift

data Log v = Log String v deriving (Functor, Typeable)

log' :: Member Log r => String -> Eff r ()
log' txt = send . inj $ Log txt ()

verboseAddition :: Member Log r => Eff r Int
verboseAddition = do
   log' "I'm starting with 1..."
   x <- return 1

   log' "and I'm adding 2..."
   y <- return 2

   let r = x + y

   log' $ "Looks like the result is " ++ show r
   return r

runLogger :: Eff (Log :> r) a -> Eff r ([String],a)
runLogger = loop
   where 
     prefixLogWith txt (l,v) = (txt:l, v)
     loop = freeMap
       (\x -> return ([], x))
       (\u -> handleRelay u loop
              $ \(Log txt next) -> fmap (prefixLogWith txt) (loop next))

runIOLogger :: SetMember Lift (Lift IO) r => Eff (Log :> r) a -> Eff r a
runIOLogger = loop
   where
     loop = freeMap
       return
       (\u -> handleRelay u loop 
              $ \(Log txt next) -> 
                  lift (putStrLn txt)  >>  --  Could not deduce ...    :(
                  loop next)

main:: IO ()
main = --    print $ run $ runLogger verboseAddition -- ok
       runLift (runIOLogger verboseAddition) >>= print 

Could not deduce (extensible-effects-1.11.0.0:Data.OpenUnion.Internal.Base.M emberUImpl
                    extensible-effects-1.11.0.0:Data.OpenUnion.Internal.Open Union2.OU2
                    Lift
                    (Lift IO)
                    r)
  arising from a use of `lift'
from the context (SetMember Lift (Lift IO) r)
  bound by the type signature for
             runIOLogger :: SetMember Lift (Lift IO) r =>
                            Eff (Log :> r) a -> Eff r a
  at src\Main.hs:37:16-72
In the first argument of `(>>)', namely `lift (putStrLn txt)'
In the expression: lift (putStrLn txt) >> loop next
In the second argument of `($)', namely
  `\ (Log txt next) -> lift (putStrLn txt) >> loop next'

Answer 1

这有效:

runIOLogger :: SetMember Lift (Lift IO) r => Eff (Log :> r) a -> Eff r a
runIOLogger = freeMap
  return
  (\u -> handleRelay u runIOLogger $ \(Log txt next) -> do
           lift $ putStrLn txt
           runIOLogger next)

还有这个:

runIOLogger :: SetMember Lift (Lift IO) r => Eff (Log :> r) a -> Eff r a
runIOLogger = loop
   where
     loop :: SetMember Lift (Lift IO) r => Eff (Log :> r) a -> Eff r a
     loop = freeMap
       return
       (\u -> handleRelay u loop 
              $ \(Log txt next) -> 
                  lift (putStrLn txt)  >>  --  Could not deduce ...    :(
                  loop next)

GHC can't infer递归绑定(bind)的多态类型，相反，它首先假设递归绑定(bind)变量是单态的来推断类型，然后概括结果类型。这意味着只有当我们使用递归绑定(bind)的不同实例时，类型推断才会失败。例如，这无法检查:

id' a = fst (a, id' (a, a))

但这很好:

id' :: a -> a
id' a = fst (a, id' (a, a))

我很确定这里存在这样的问题，尽管我还没有努力找出推理到底在哪里停止了。