addop, mulop :: Parser (Expr -> Expr -> Expr)
addop = (infixOp "+" Add) <|> (infixOp "-" Sub)
mulop = infixOp "*" Mul

int :: Parser Expr
int = do
  n <- number
  return (Lit n)

expr :: Parser Expr
expr = term `chainl1` addop

term :: Parser Expr
term = factor `chainl1` mulop

factor :: Parser Expr
factor = int <|> parens expr

run :: String -> Expr
run = runParser expr

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

evExpr :: String -> Maybe Integer
evExpr [] = Nothing
evExpr str = Just (eval (run str))

• Couldn't match type ‘Int’ with ‘Integer’

  Expected type: Maybe Integer
    Actual type: Maybe Int
• In the expression: Just (eval (run str))
  In an equation for ‘evExpr’: evExpr str = Just (eval (run str))
   |
37 | evExpr str = Just (eval (run str))
   |              ^^^^^^^^^^^^^^^^^^^^^

• Couldn't match type ‘SourceName
                       -> [Char] -> Either ParseError Expr’
                 with ‘Expr’
  Expected type: String -> Expr
    Actual type: () -> SourceName -> [Char] -> Either ParseError Expr
• Probable cause: ‘runParser’ is applied to too few arguments
  In the expression: runParser expr
  In an equation for ‘run’: run = runParser expr
   |
26 | run = runParser expr
   |       ^^^^^^^^^^^^^^