OverloadedLabels: No instance for IsLabel on record data type

Question

In the code below,

{-# LANGUAGE OverloadedLabels #-}
module Foo where

data R = R { x :: Int }

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

I expected this to type-check, but instead I get:

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})

Given the Overloaded Record Fields proposal, I expected there to be a built-in instance of IsLabel "x" (R -> Int). Is this still the case or does the implementation deviate from the proposal?

Answer 1

在（至少当前）基础中没有IsLabel实例OverloadedLabels（参见此处的讨论）。您可能想要使用一些定义孤立实例的库，例如generic-lens。当然，你可以自己定义：

{-# 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