Events (Zero-Cost Multicast)

Model

Events provide multicast subscription with static, allocation-aware lowering.

16.1 Declaration form

Events are declared as type fields with an optional inline capacity.

type Window {
    event{4} OnResize(i32 width, i32 height)
    event OnFocus()
}

Rules:

• event{N} uses explicit compile-time positive capacity,
• event without {N} uses the default event capacity,
• event signature is declared as parameter list after the event name,
• event members are part of type layout via lowering expansion.

16.2 Subscription model

• += adds a handler.
• -= removes a handler.
• handlers must match event signature.

Invocation rule:

• only the owning type implementation may invoke the event (this.EventName(...)).

16.3 Lowering contract

Events do not lower to a general-purpose runtime event object. They lower to structural fields in HIR/type layout:

• __<EventName>_count
• __<EventName>_handlers: [FatPointer; N]

Invocation lowers to static conditional dispatch up to N.

16.4 Performance and safety goals

• predictable bounded dispatch,
• no hidden per-invocation heap allocation,
• closure/lambda handlers respect capture non-escape constraints.

16.5 Diagnostics

Compile-time diagnostics include:

• capacity errors,
• signature mismatch on subscription,
• illegal invocation from non-owner scope,
• unsupported escaping capture patterns.