{-# LANGUAGE DataKinds, OverloadedLabels, TypeFamilies #-}
module Main where

import Data.Proxy
import GHC.TypeLits
import GHC.OverloadedLabels

data Ei (l1 :: Symbol) a (l2 :: Symbol) b
  = Lef a | Rig b

newtype WasLabel s = WasLabel String

instance (KnownSymbol s, s ~ s') => IsLabel s (WasLabel s') where
  fromLabel = WasLabel (symbolVal (Proxy @s))

was :: forall s a b l1 l2. (KnownSymbol s, KnownSymbol l1, KnownSymbol l2)
    => WasLabel s -> Ei l1 a l2 b -> Bool
was (WasLabel s) (Lef _) = s == symbolVal (Proxy @l1)
was (WasLabel s) (Rig _) = s == symbolVal (Proxy @l2)

main :: IO ()
main = do
  let x = Lef 10 :: Ei "foo" Int "bar" Char
      y = Rig 'b' :: Ei "foo" Int "bar" Char
  print (was #foo x)
  print (was #bar x)
  print (was #foo y)
  print (was #bar y)