{-# LANGUAGE TypeFamilies #-}

class A a where
    type Id a

    check :: a -> Bool

class B a where

data PhantomData a = PhantomData

data Test s t = Test {
    s :: s,
    phantom :: PhantomData t
}

class C a where
    data State a
    bar :: State a -> a -> a -> a

data X = X1 | X2
    deriving Show

instance C X where
    data State X = forall s t . (A s, B t) => XState (Test s t)
    bar (XState st) a b = if check (s st) then a else b

instance A Int where
    type Id Int = ()

    check 0 = True
    check _ = False

instance A String where
    type Id String = ()

    check "" = True
    check _ = False

instance B ()

data Y = Y1 | Y2
    deriving Show

instance C Y where
    data State Y = YState
    bar YState a b = a

main = do
    let t1 = Test (0 :: Int) (PhantomData :: PhantomData ())
    print $ bar (XState t1) X1 X2

    let t2 = Test ("Hi" :: String) (PhantomData :: PhantomData ())
    print $ bar (XState t2) X1 X2

    print $ bar YState Y1 Y2