haskell - 无模板 haskell 的多态透镜

Question

我正在尝试为多种类型创建多态镜头减速(没有模板 haskell)。

module Sample where
import Control.Lens
data A = A {_value:: Int}
data B = B {_value:: Int}
data C = C {_value:: String}
value = lens _value (\i x -> i{_value=x}) -- <<< ERROR

但我收到以下错误:

Ambiguous occurrence ‘_value’
It could refer to either the field ‘_value’,
                         defined at library/Sample.hs:5:13
                      or the field ‘_value’, defined at 
library/Sample.hs:4:13
                      or the field ‘_value’, defined at 
library/Sample.hs:3:13
  |
6 | value = lens _value (\i x -> i{_value=x}) -- <<< ERROR
  |              ^^^^^^

因此，目标是让值(value)镜头适用于所有三种类型 A、B 和 C。有没有办法实现这一目标？谢谢。

Answer 1

无需 TH 即可通过 generic-lens 导出镜头。您可以将通用场镜头专门用于特定场，并为其指定如下名称。

{-# LANGUAGE DataKinds #-}
{-# LANGUAGE DeriveGeneric #-}
{-# LANGUAGE DuplicateRecordFields #-}
{-# LANGUAGE FlexibleContexts #-}
{-# LANGUAGE TypeApplications #-}

import GHC.Generics (Generic)
import Control.Lens (Lens, (^.))
import Data.Generics.Product (HasField(field))

data A = A { _value :: Int } deriving Generic
data B = B { _value :: Int } deriving Generic
data C = C { _value :: String } deriving Generic

value :: HasField "_value" s t a b => Lens s t a b
value = field @"_value"

main :: IO ()
main = print (A 0 ^. value, B 0 ^. value, C "0" ^. value)