Instances from GADTs

{-# LANGUAGE GADTs #-}
{-# LANGUAGE RankNTypes #-}

data T k where
  A :: Eq k => T k

g :: T k -> (Eq k => a) -> a
g A = id

f :: T k -> k -> k -> Bool
f tt x y = g tt (x == y)

main :: IO ()
main = do
    print $ f (A :: T Int) 1 2
    print $ f (A :: T Bool) True False