{-# LANGUAGE FlexibleInstances #-}

unOp :: Read a => Show b => (a -> b) -> String -> String
unOp f x = show . f $ read x

binOp :: Read a => Read b => Show c => (a -> b -> c) -> String -> String -> String
binOp f x y = show $ read x `f` read y

instance Num String where
  (+) = binOp (+)
  (*) = binOp (*)
  abs = unOp abs
  signum = unOp signum
  fromInteger = show
  negate = unOp negate

main = print $ 2 + 2 == "4"