Enums, Flags, Variants, and Unions

The concept of "this value can be X or Y or Z" can be expressed in several ways depending on the context.

Enum

The enum keyword can be used to introduce a C-style enum. This is a type of named constant where each constant has its own value.

enum ordering {
    less-than,
    equal-to,
    greater-than,
}

For more details, consult the Item: enum section (opens in a new tab) in the *.wai format.

Flags

The flags keyword can be used to introduce a bitflags variable. The easiest way to think of this is as a "bag of bools" where each variant can be set independently.

flags text-style {
    bold,
    italics,
    underline,
    strikethrough,
}

The flags type is a separate concept from enum because multiple flag variants can be set at a time, whereas an enum can be only one thing at a time. Different languages are often able to express this in a very efficient form, typically an integer where each bit represents a different flag.

For more details, consult the Item: flags section (opens in a new tab) in the *.wai format.

Variant

A variant lets you express something that is one of a set of types. This is similar to an enum, except each variant may have some associated data.

variant error {
    file-not-found(string),
    parse(parse-failed),
    other,
}

record parse-failed {
    message: string,
    line-number: u32,
}

Variants are implemented very differently based on what is idiomatic for a particular language.

In Rust, a variant is just a normal enum.

In TypeScript, variants are implemented as tagged unions.

type error = { type: "file-not-found", value: string }
    | { type: "parse", value: ParseFailed }
    | { type: "other" };

For more details, consult the Item: variant section (opens in a new tab) in the *.wai format.

Union

A union is very similar to a variant, except it drops the type tag.

union configuration {
    string,
    list<string>,
}

This is distinct from a variant because some languages may be able to represent a union in a way that is more efficient or idiomatic.

For more details, consult the Item: union section (opens in a new tab) in the *.wai format.