Capture.hs

data Exp = Lit Integer
         | Add Exp Exp
         | Sub Exp Exp
         | Mul Exp Exp
         | Neg Exp
         | Abs Exp
         | Sgn Exp
         | Var Var
         deriving Show

type Var = String

instance Num Exp where
    (+) = Add
    (-) = Sub
    (*) = Mul
    negate = Neg
    abs = Abs
    signum = Sgn
    fromInteger = Lit

-- subtract (Var "x") (Var "y") :: Exp
-- => Sub (Var "y") (Var "x")