Eff.StateをMonadStateのインスタンスにすればLensが使える

gistfile1.hs

{-# LANGUAGE FlexibleContexts, ScopedTypeVariables #-}
{-# LANGUAGE FlexibleInstances, MultiParamTypeClasses, UndecidableInstances #-}
{-# LANGUAGE DeriveDataTypeable #-}
{-# LANGUAGE TemplateHaskell #-}

module Main where
import Data.Typeable
import Control.Eff
import Control.Eff.State.Lazy as EST
import Control.Monad.State.Class (MonadState(..))
import Control.Lens

instance (Typeable x, Member (State x) r) => MonadState x (Eff r) where
  get = EST.get 
  put = EST.put 

----

data Foo = Foo
  { _hoge :: Int
  , _piyo :: String
  , _fuga :: Bool
  } deriving (Show, Typeable)

makeLenses ''Foo

proc :: (Member (State Foo) r) => Eff r ()
proc = do
  hoge *= 2
  p <- use $ piyo
  piyo .= "[" ++ p ++ "]"
  fuga .= True 

----

main = do
  let foo = Foo
              { _hoge = 100
              , _piyo = "Hello"
              , _fuga = False
              }
  print . run $ execState foo proc