dt1.hs

data Alg = Num' Int 
         | Add Alg Alg 
         | Sub Alg Alg  
         | Mul Alg Alg  
     deriving (Show)

consume :: Alg -> Int
consume (Num' x)  = x
consume (Add a b) = (consume a) + (consume b)
consume (Sub a b) = (consume a) - (consume b)
consume (Mul a b) = (consume a) * (consume b)

-- ((7 + 5) * 3) - 10
main = putStrLn $ show $ consume (Sub (Mul (Add (Num' 7) (Num' 5)) (Num' 3)) (Num' 10))