Overloaded records in Haskell.

OverloadedRecords.hs

{-# LANGUAGE DataKinds #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE OverloadedLabels #-}
{-# LANGUAGE TemplateHaskell #-}
{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE FlexibleContexts #-}
{-# LANGUAGE UndecidableInstances #-}
{-# LANGUAGE MagicHash #-}
{-# LANGUAGE ScopedTypeVariables #-}

import qualified Control.Lens as Lens
import qualified Data.Default.Class as Default
import qualified Data.OverloadedRecords as OverloadedRecords
import Data.OverloadedRecords
import Data.Proxy
import GHC.Prim(Proxy#(..),proxy#)
import qualified Data.OverloadedRecords.TH as OverloadedRecords
import GHC.TypeLits

data B = B { bX :: Char, bZ :: Char } deriving (Show)

$(OverloadedRecords.overloadedRecord Default.def ''B)

data C = C { cX :: Char, cpart :: B } deriving (Show)

$(OverloadedRecords.overloadedRecord Default.def ''C)

instance {-# OVERLAPPABLE #-} HasField l B a => HasField l C a where
    getField _ z =  getField  (proxy# :: Proxy# l) (cpart z)

main :: IO ()
main = do
    let b = B '@' '#'
    print (OverloadedRecords.get #x b) -- '@'
    print (b Lens.^. #z) -- '#'
    print (OverloadedRecords.set' #x '$' b) -- B {bX = '$', bZ = '#'}
    print (b Lens.& #z Lens..~ '!') -- B {bX = '@', bZ = '!'}

    let c = C 'X' b 
    print (OverloadedRecords.get #x c) -- 'X' -- the field of the same name in the component is overshadowed
    print (OverloadedRecords.get #z c) -- '#'