Structure

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

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struct Message {
    sender: char[max 64]
    recipient: char[max 64]
    body: char[max 4096]
    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: char[max 4096]
    optional subject: char[max 128]
    priority: u8 = 1
    language: Language = default
}

• optional — the field may be absent. Without it, the field is required.
• = <value> — a default. Any value a constant accepts — a literal, a constant by name, or an integer type's bound (u16::MAX) — plus the bare word default (the type's own default, e.g. an enum's first variant). It has to fit the field's type, through any type alias: a u8 = 300 or a u8 = LIMIT where LIMIT is 300 is an error.

Field types

A field can be any type: a primitive (u32, bool, char[max 64]), a struct, enum or type alias by name (scoped as pkg::module::Type), an array (Message[max 100], u8[16]) or a union (union(char[max 64] u32)).

Docstrings and annotations

A docstring (/// lines) documents the struct or a field. Annotations (@key = value) above a field attach to it, as @validators does; above the struct, they attach to the struct, as @deprecated and @settings do.

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/// A message routed through the mail protocol.
@deprecated = "use Envelope"
struct Message {
    /// Plain-text body, UTF-8.
    @validators = [StringBounds(min_chars = 1)]
    body: char[max 4096]

    /// Shown in the client's list view.
    optional subject: char[max 128]
}