AnonRecords.hs

{-# LANGUAGE DataKinds, FlexibleInstances, FunctionalDependencies,
             KindSignatures, MagicHash, MultiParamTypeClasses #-}

import GHC.TypeLits
import GHC.Exts

class HasField (x :: Symbol) r a | x r -> a where
  getField :: Proxy# x -> r -> a

data Record2 (n1 :: Symbol) v1 (n2 :: Symbol) v2 =
  Record2 v1 v2

instance HasField n1 (Record2 n1 v1 n2 v2) v1 where
  getField _ (Record2 x _) = x

instance HasField n2 (Record2 n1 v1 n2 v2) v2 where
  getField _ (Record2 _ y) = y

z1 = getField (proxy# :: Proxy# "bar") (Record2 True 42 :: Record2 "foo" Bool "bar" Int)
z2 = getField (proxy# :: Proxy# "foo") (Record2 True False :: Record2 "foo" Bool "bar" Bool)

-- z3 = getField (proxy# :: Proxy# "foo") (Record2 True False :: Record2 "foo" Bool "foo" Bool)
--
-- Uncommenting this line gives:
--     Overlapping instances for HasField "foo" (Record2 "foo" Bool "foo" Bool) Bool