Unions

Unions define a type that must be exactly one of several possible variants. There are two types of unions:

• Union expressions
• Named unions

Union expressions

Unnamed unions, or union expressions, can be declared by combining the variants using the | operator.

alias Breed = Beagle | GermanShepherd | GoldenRetriever;

In this example, Breed can be either a Beagle, a GermanShepherd, or a GoldenRetriever.

Named unions

Named unions allow you to assign a name to the union and provide explicit variant references. Named unions are somewhat similar to enums, but instead of having string or numeric values, they use record models.

A named union can be declared with the union keyword. Its name must be an identifier.

union Breed {
  beagle: Beagle,
  shepherd: GermanShepherd,
  retriever: GoldenRetriever,
}

The above example is equivalent to the Breed alias mentioned earlier, with the difference that emitters can recognize Breed as a named entity and also identify the beagle, shepherd, and retriever names for the options. This format also allows the application of decorators to each of the options.