data Expr = Value Integer
          | Signum Expr
          | Abs Expr
          | Add Expr Expr
          | Mul Expr Expr
            deriving (Show, Eq)

instance Num Expr where
    a + b = Add a b
    a * b = Mul a b
    signum = Signum
    abs = Abs
    fromInteger = Value


f1 x y = x * y
f2 x y = x + y

-- f1 1 2 :: Expr  =>  Mul (Value 1) (Value 2)
-- f1 1 2 :: Expr  =>  Add (Value 1) (Value 2)


f3 x y = a * a where
    a = x + y

-- f3 1 2 :: Expr  =>  Mul (Add (Value 1) (Value 2)) (Add (Value 1) (Value 2))
-- How to recover the sharing?
-- http://zwizwa.be/-/meta/20110114-130928