Unions

Unions represent a set of possible types for a value. By default, unions in Boa are tagged (like in Haskell, OCaml or Rust). It is however possible to mark them as untagged, mainly for C compatibility. It is also possible to specify the location of the tag, to map a C struct containing a tag and an union to a Boa union.

Simple example

struct FileEntry:
    name: string
    details: FileEntryDetails

union FileEntryDetails:
    File(contents: []u8, page_count: u32)
    Directory(children: []*FileEntry)
    SymLink(refers_to: *FileEntry)

Creating an instance of an union

let entry = FileEntry(
    name="hello_world.boa",
    details=File( // no need to write FileEntryDetails.File thanks to type inference
        contents=[],
        page_count=...
    )
)

Accessing the contents of a tagged union

This section is only applicable to tagged unions. Untagged unions' fields can be accessed directly with the . operator, without any form of type checking.