-- data MyList a = Cons a (MyList a) -- | Nil -- Cons (1, Cons (2, Nil)) -- f (1, f(2, base)) -- foldList :: (a -> b -> b) -> b -> MyList a -> b -- foldList _ base (Nil n) = base n data Exp = EInt Int | EAdd Exp Exp | EMin Exp Exp | EMul Exp Exp deriving (Show, Eq) -- EInt 5 => rInt 5 -- EAdd (EInt 5) (EInt 4) => rAdd (rInt 5) (rInt 4) foldE :: (Int -> a) -> (a -> a -> a) -> (a -> a -> a) -> (a -> a -> a) -> Exp -> a foldE rInt _ _ _ (EInt n) = rInt n foldE a rAdd b c (EAdd e1 e2) = rAdd (foldE a rAdd b c e1) (foldE a rAdd b c e2) foldE a b rMin c (EMin e1 e2) = rMin (foldE a b rMin c e1) (foldE a b rMin c e2) foldE a b c rMul (EMul e1 e2) = rMul (foldE a b c rMul e1) (foldE a b c rMul e2) eval :: Exp -> Int eval e = foldE id (-) (+) (*) e -- EInt 5 => rInt 5 => 5 -- (1 - 2) + 3 * (4 - 5) = -24 -- (1 + 2) - 3 * (4 + 5) testvalue = EMin (EAdd (EInt 1) (EInt 2)) (EMul (EInt 3) (EAdd (EInt 4) (EInt 5))) main = print $ eval testvalue