小编Iru*_*ruT的帖子

This code is compiled fine:

data None = None { _f :: Int }
type Simpl = Env

type Env = Int

However, I got an error with this code:

{-# LANGUAGE TemplateHaskell #-}
import Control.Lens

data None = None { _f :: Int }

type Simpl = Env

makeLenses ''None

type Env = Int

Error:

Not in scope: type constructor or class `Env'

I just added a single line makeLenses ''None between type declarations.
This means TemplateHaskell code could …

我想使用镜头和可扩展效果来做一个简单的例子.
错误消息说类型是不明确的,因为带有参数的类型类HasObj x和GHC无法理解pos从哪里来,我猜.
makeClassy使用通用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 …