Annotated final

{-# LANGUAGE NoMonomorphismRestriction, TypeSynonymInstances, FlexibleInstances, MultiParamTypeClasses #-}

class ExpSYM repr where
    lit :: Int -> repr
    neg :: repr -> repr
    add :: repr -> repr -> repr

class Ann repr e where
  a :: e -> repr -> repr

instance ExpSYM Int where
    lit n = n
    neg e = - e
    add e1 e2 = e1 + e2

instance Ann Int e where
    a e = id
    
instance ExpSYM String where
    lit n = show n
    neg e = "(-" ++ e ++ ")"
    add e1 e2 = "(" ++ e1 ++ " + " ++ e2 ++ ")"

instance Show a => Ann String a where
    a e s = "@" ++ show e ++ " " ++ s

data P = P Int Int
instance Show P where show (P x y) = "(" ++ show x ++ "," ++ show y ++ ")"

tf1 = add (lit 8) (neg (add (lit 1) (lit 2)))
tf2 = a (P 1 1) $ add (a (P 2 1) $ lit 8) (a (P 3 1) $ neg (a (P 4 1) $ add (a (P 5 1) $ lit 1) (a (P 6 1) $ lit 2)))