Either Disjoint or Union Types
Many languages support union types, and it is high time Scala did. Union types are coming in upcoming version of Scala  Dotty. Union types (
) are already being compared with Either
and Option
(disjoint unions).
In some ways, Either
and Option
may be expressed as union types.
// Option[String]
String  null
// Either[Int, String]
Int  String
Similar to disjoint union types, you can pattern match over Union types. However, the differences outshine the similarities.
Disjoint union types like Either
and Option
…
 constraint the universe of types to be unique 
Left
andRight
,Some
andNone
. There is only oneLeft
orSome
.  can be parameterized 
Either[L, R]
,Option[T]
 Defined in the standard library. Not language syntax.
On the other hand, Union Types (
) …

Not parameterized. Types are specific.

The types involved don’t have to be necessarily unique.
String  Int  String  Int
The above definition is valid although the universe of types is just
String
andInt
. 
Language syntax
There is one difference that stands out to me, in fact of disjoint union types. Either
and Option
are monads and so they give the niceties of map
, flatMap
and all those of a container. Can’t do that with Union types.
Update: Per phazer99’s reddit comment, union types could be augmented with extension methods to get the
map
andflatMap
. That’s interesting although I feel it does not make it truly monadic. For instance, what is the unit value? Will the monad laws hold good? Happy to stand corrected.
However, Union types give you edge over one thing. In fact, the thing I don’t like about Either
or Option
; or JVM in general. Disjoint union types are allocated on the heap; every instance. Union types are compile time construct and do not require extra allocation.
As you can see, grass is greener on either side. Both disjoint union and Union types have their place and are here to stay. You gotta choose the right one for the job; Either
disjoint types 
Union Types!