Union types

• On the programming side, a ∨ b is a union type

• It could contain a value of type a or a value of type b

• (Plus a tag telling you which one it is)

• In Haskell this is written Either a b

• There are two injectors for constructing union values:

        -- x :: a
        -- y :: b
        Left x  :: Either a b
        Right y :: Either a b