如何使用镜头和可扩展效果？

Question

我想使用lens和extensible-effects来做一个简单的例子 错误消息说类型是不明确的，因为带有参数HasObj x的类型类，而且GHC无法理解pos来自哪里，我猜。 makeClassy对于使用公共数据函数定义抽象数据类型非常有用，因此在我的实际项目中需要它。

我的问题是：我怎样才能让它发挥作用？ 或者，有没有办法从数据类型定义镜头并在Eff.State中使用它们？

{-# LANGUAGE TemplateHaskell, TypeOperators, DeriveDataTypeable #-}
import Control.Eff
import Control.Eff.Lift
import Control.Eff.State.Strict
import Control.Lens hiding (use, (%=), (.=))
import Control.Monad
import Data.Typeable

-- define data types
data Obj = Obj { _pos :: Int } deriving (Show)
data Person = Person {
  _objPerson :: Obj,
  _greeting :: String
  } deriving (Show, Typeable)

-- make typeclasses and lenses
makeClassy ''Obj
makeLenses ''Person

instance HasObj Person where
  obj = objPerson

-- define lens operators in Eff
use k = fmap (^.k) get
l %= f = modify (l %~ f)
l .= f = modify (l .~ f)

-- update :: StateT Person IO ()
update :: Eff (State Person :> Lift IO :> ()) ()
update = do
  lift . putStrLn =<< use greeting
  pos %= (+10)
  p <- use pos
  when (p >= 100) $ do
    greeting .= "at 100, yeah!"

main = do
  let p = Person (Obj 0) "hi!"
  print $ p

  p' <- runLift $ execState p update
  print $ p'

错误讯息：

LensEE.hs:28:3:
    No instance for (HasObj s0) arising from a use of ‘pos’
    The type variable ‘s0’ is ambiguous
    Note: there are several potential instances:
      instance HasObj Obj -- Defined at LensEE.hs:15:1
      instance HasObj Person -- Defined at LensEE.hs:18:10
    In the first argument of ‘(%=)’, namely ‘pos’
    In a stmt of a 'do' block: pos %= (+ 10)
    In the expression:
      do { lift . putStrLn =<< use greeting;
           pos %= (+ 10);
           p <- use pos;
           when (p >= 100) $ do { greeting .= "at 100, yeah!" } }

LensEE.hs:28:7:
    Overlapping instances for Member
                                (State s0) (State Person :> (Lift IO :> ()))
      arising from a use of ‘%=’
    Matching instances:
      instance [overlap ok] Member t r => Member t (t' :> r)
        -- Defined in ‘Data.OpenUnion1’
      instance [overlap ok] Member t (t :> r)
        -- Defined in ‘Data.OpenUnion1’
    (The choice depends on the instantiation of ‘s0’
     To pick the first instance above, use IncoherentInstances
     when compiling the other instance declarations)
    In a stmt of a 'do' block: pos %= (+ 10)
    In the expression:
      do { lift . putStrLn =<< use greeting;
           pos %= (+ 10);
           p <- use pos;
           when (p >= 100) $ do { greeting .= "at 100, yeah!" } }
    In an equation for ‘update’:
        update
          = do { lift . putStrLn =<< use greeting;
                 pos %= (+ 10);
                 p <- use pos;
                 .... }

LensEE.hs:29:8:
    Overlapping instances for Member
                                (State a0) (State Person :> (Lift IO :> ()))
      arising from a use of ‘use’
    Matching instances:
      instance [overlap ok] Member t r => Member t (t' :> r)
        -- Defined in ‘Data.OpenUnion1’
      instance [overlap ok] Member t (t :> r)
        -- Defined in ‘Data.OpenUnion1’
    (The choice depends on the instantiation of ‘a0’
     To pick the first instance above, use IncoherentInstances
     when compiling the other instance declarations)
    In a stmt of a 'do' block: p <- use pos
    In the expression:
      do { lift . putStrLn =<< use greeting;
           pos %= (+ 10);
           p <- use pos;
           when (p >= 100) $ do { greeting .= "at 100, yeah!" } }
    In an equation for ‘update’:
        update
          = do { lift . putStrLn =<< use greeting;
                 pos %= (+ 10);
                 p <- use pos;
                 .... }

LensEE.hs:29:12:
    No instance for (HasObj a0) arising from a use of ‘pos’
    The type variable ‘a0’ is ambiguous
    Note: there are several potential instances:
      instance HasObj Obj -- Defined at LensEE.hs:15:1
      instance HasObj Person -- Defined at LensEE.hs:18:10
    In the first argument of ‘use’, namely ‘pos’
    In a stmt of a 'do' block: p <- use pos
    In the expression:
      do { lift . putStrLn =<< use greeting;
           pos %= (+ 10);
           p <- use pos;
           when (p >= 100) $ do { greeting .= "at 100, yeah!" } }