data Expr = I Int
        | B Bool           -- boolean constants
        | Add Expr Expr
        | Mul Expr Expr
        | Eq  Expr Expr    -- equality test

eval :: Expr -> Maybe (Either Int Bool)
-- Your implementation here.

data Expr = I Int         -- integer constants
          | B Bool        -- boolean constants
          | Add Expr Expr -- add two expressions
          | Mul Expr Expr -- multiply two expressions
          | Eq  Expr Expr -- equality test
          deriving (Show)

eval :: Expr -> Maybe (Either Int Bool)
eval (I n) = Just $ Left n
eval (B b) = Just $ Right b
eval (Add e1 e2) = ...
eval (Mul e1 e2) = ...
eval (Eq e1 e2) = ...