closed-structs.md

Closed Structs and Struct Traits

Structs can be marked with the closed modifier. This transforms them into discriminated unions and allows them to effectively be subtyped. The "subtypes" become members of the discriminated union.

A closed struct can only have a fixed set of subtype values or structs. These are declared with value and struct declarations, though see the Cases section for a shorthand syntax. Typically, these are declared inside the closed struct, though technically they can also be declared outside the struct. Private initializers can be used to prevent that if desired. The individual subtype values or structs can override members of the struct and add fields.The standard mechanisms of sealed, overrides, and hides can be used to control this process. A closed struct is effectively always abstract. That is, there can be no instances of the struct itself created. Only values and instances of the subtypes.

public closed struct release_stage
{
    let is_prerelease: bool;
    public init(.is_prerelease = false) { }

    public value Alpha: release_stage(true);
    public value Beta: release_stage(true);
    public value ReleaseCandidate: release_stage;
    public value Full: release_stage(true);
}

// Value declared outside of struct
public value PreAlpha: release_stage(true);

public closed struct token
{
    public value plus_token: token;
    public struct Identifier(value: string): token;
    public struct StringLiteral(value: string): token;
}

Closed Struct Value Optimization

While a normal value declaration effectively declares a constant for the struct type and therefore has the size of the struct type the value is declared for, closed structs work differently. Since the compiler knows that their is a fixed set of values, it does not have to pass the field values around when using instances of the closed struct. Instead, it passes only an id from which it can determine which value it is. Field values are stored separately in a constant table. This makes them equivalent to enums in other languages like C# and Java where they are just an integer identifying which value is used. Note, this does not apply to subtype structs. They do have space for any fields of the closed struct.