haskell - 重载标签 : No instance for IsLabel on record data type

Question

在下面的代码中，

{-# LANGUAGE OverloadedLabels #-}
module Foo where

data R = R { x :: Int }

g :: Int
g = #x (R { x = 1 })

我希望这会进行类型检查，但我得到了:

foo.hs:7:5: error:
    • No instance for (GHC.OverloadedLabels.IsLabel "x" (R -> Int))
        arising from the overloaded label ‘#x’
        (maybe you haven't applied a function to enough arguments?)
    • In the expression: #x
      In the expression: #x (R {x = 1})
      In an equation for ‘g’: g = #x (R {x = 1})

鉴于 Overloaded Record Fields建议，我希望有一个 IsLabel "x" (R -> Int) 的内置实例.仍然是这种情况还是实现偏离了提案？

Answer 1

没有IsLabel OverloadedLabels 的实例在(至少是当前的)基础中(参见讨论 here)。
您可能想要使用一些定义孤立实例的库，例如 generic-lens .
当然，您可以自己定义它:

{-# LANGUAGE TypeApplications #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE ScopedTypeVariables #-}

import GHC.Records
import GHC.OverloadedLabels

instance HasField x r a => IsLabel x (r -> a) where
  fromLabel = getField @x