Structure

A structure (struct, also called a message) is a named data shape. It carries typed fields.

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struct Message {
    sender: str
    recipient: str
    body: string
    priority: u8
}

Fields

A field is name: Type, one per line, in declaration order. A field may be marked optional and may carry a default.

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struct Message {
    body: string
    optional subject: str
    priority: u8 = 1
    language: Language = default
}

• optional — the field may be absent. Without it, the field is required.
• = <value> — a default. Any value form a constant accepts — a literal, another constant by name, a ::-path, or an f-string — plus the bare word default (the type's own default, e.g. an enum's first variant). Non-literals are recorded but not resolved yet.

Field types

Use str for text. string is an accepted synonym the compiler steers you away from; String is generated-Rust, not IDL — see str vs string vs String.

Docstrings and annotations

A docstring (/// lines) and @key=value annotations attach to a struct or an individual field.

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/// A message routed through the mail protocol.
struct Message {
    body: string

    /// Shown in the client's list view.
    optional subject: str
}

Annotation values are simple — an integer, string or identifier (@internal=true). Richer forms such as @validators=[…] are still being designed.