Haskell、Control.Lens：无法将类型“[Language.Haskell.TH.Syntax.Dec]”与“Language.Haskell.TH.Syntax.Exp”匹配

Question

我想学习如何使用 Control.Lens 包。我尝试使用 an official tutorial.

首先，我没有Control.Lens.Tutorial包。重现示例很重要吗？

我启动了 GHCi 并加载了模板Haskell 扩展：

Prelude> :set -XTemplateHaskell

然后我尝试重现教程的第一步：

Prelude> import Control.Lens hiding (element)
Prelude Control.Lens> import Control.Lens.TH
Prelude Control.Lens Control.Lens.TH> 
Prelude Control.Lens Control.Lens.TH> data Point = Point { _x :: Double, _y :: Double } deriving (Show)
Prelude Control.Lens Control.Lens.TH> data Atom = Atom { _element :: String, _point :: Point } deriving (Show)
Prelude Control.Lens Control.Lens.TH> $(makeLenses ''Point)

出现错误：

<interactive>:7:3: error:
    • Couldn't match type ‘[Language.Haskell.TH.Syntax.Dec]’
                     with ‘Language.Haskell.TH.Syntax.Exp’
      Expected type: Language.Haskell.TH.Lib.Internal.ExpQ
        Actual type: Language.Haskell.TH.Lib.Internal.DecsQ
    • In the expression: makeLenses ''Point
      In the untyped splice: $(makeLenses ''Point)

GHC 8.6.4，镜头 4.19.2 从 Cabal 安装。

教程是不是有点过时了？如果是，我在哪里可以阅读 Haskell 中关于镜头的实用介绍，其中包含简单的使用示例和尽可能少的范畴论？

Answer 1


  :{
  data Point = Point { _x :: Double, _y :: Double } deriving (Show)
  data Atom = Atom { _element :: String, _point :: Point } deriving (Show)
  makeLenses ''Point
  :}

Haskell、Control.Lens：无法将类型“[Language.Haskell.TH.Syntax.Dec]”与“Language.Haskell.TH.Syntax.Exp”匹配

Haskell, Control.Lens: Couldn't match type ‘[Language.Haskell.TH.Syntax.Dec]’ with ‘Language.Haskell.TH.Syntax.Exp’

haskell

template-haskell

haskell-lens