#!/usr/bin/env stack
-- stack --resolver=lts-12.6 script

module Main (main) where

data Expr =
    Val Int
    | Add Expr Expr
    | Sub Expr Expr
    | Mul Expr Expr
    | Div Expr Expr

eval :: Expr -> Maybe Int
eval (Val x) = pure x
eval (Add ex ey) = (+) <$> eval ex <*> eval ey
eval (Sub ex ey) = (-) <$> eval ex <*> eval ey
eval (Mul ex ey) = (*) <$> eval ex <*> eval ey
eval (Div ex ey) = do
    y <- eval ey
    if y == 0
        then Nothing
        else do
            x <- eval ex
            pure (x `div` y)

main :: IO ()
main = do
    -- Divide by 2: displays "Just 7"
    print $ eval (Div (Mul (Val 2) (Add (Val 3) (Val 4))) (Val 2))

    -- Divide by 0: displays "Nothing"
    print $ eval (Div (Mul (Val 2) (Add (Val 3) (Val 4))) (Val 0))