{-# LANGUAGE TypeFamilies #-}

data F1 = F1
data F2 = F2
data F3 = F3

data G = G1 | G2 | G3 deriving (Show)

type family Convert a :: *  where  
  Convert F1 = G
  Convert F2 = G
  Convert F3 = G

class Convertable a where
  convert :: a -> G

instance Convertable F1 where
  convert F1 = G1

instance Convertable F2 where
  convert F2 = G2

instance Convertable F3 where
  convert F3 = G3