Write You a Haskell Stephen Diehl

data Expr
= Add Expr Expr
| Mul Expr Expr
| Sub Expr Expr
| Lit Int
deriving Show
eval :: Expr -> Int
eval ex = case ex of
Add a b -> eval a + eval b
Mul a b -> eval a * eval b
Sub a b -> eval a - eval b
Lit n -> n
int :: Parser Expr
int = do
n (a -> a -> a) -> Parser (a -> a -> a)
infixOp x f = reserved x >> return f
addop :: Parser (Expr -> Expr -> Expr)
addop = (infixOp ”+” Add) (infixOp ”-” Sub)
mulop :: Parser (Expr -> Expr -> Expr)
mulop = infixOp ”*” Mul
run :: String -> Expr
run = runParser expr
main :: IO ()
main = forever $ do
putStr ”> ”
a