module Calc where
    import ExprT
    import Parser
    eval::ExprT -> Integer
    eval (Lit a) = a
    eval (Add exp1 exp2) = eval exp1 + eval exp2
    eval (Mul exp1 exp2) = eval exp1 * eval exp2

    evalStr :: String -> Maybe Integer
    evalStr str =  case (parseExp Lit Add Mul str) of
                     Nothing -> Nothing
                     Just exp -> Just (eval exp)

    class Expr a where
            lit :: Integer -> a
            add, mul :: a -> a -> a

    instance Expr ExprT where
            lit a  = Lit a
            mul a b = Mul a b
            add a b = Add a b

    instance Expr Integer where
            lit a = a
            mul a b = a * b
            add a b = a + b


    newtype MinMax = MinMax Integer deriving (Eq, Show)

    instance Expr MinMax where
            lit a = MinMax a
            mul (MinMax a) (MinMax b) = if (a < b) then MinMax a else MinMax b
            add (MinMax a) (MinMax b) = if (a > b) then MinMax a else MinMax b

    newtype Mod7 = Mod7 Integer deriving (Eq, Show)

    instance Expr Mod7 where
            lit a = Mod7 (a `mod` 7)
            mul (Mod7 a) (Mod7 b) = Mod7 ((a*b) `mod` 7)
            add (Mod7 a) (Mod7 b) = Mod7 ((a+b) `mod` 7)