Optional¶

Wraps DSL words to allow a reaction to proceed when their data is unavailable, rather than dropping the task.

Syntax¶

on<Trigger<T>, Optional<With<U>>>()

Parameters¶

Parameter	Description
`DSLWords...`	One or more DSL words that provide data via `get`

Behavior¶

By default, if a word like With<T> references data that has not yet been emitted, the task is dropped — the callback never executes. Optional changes this: the task proceeds regardless, but the data may be absent.

Wrapping a word in Optional changes the corresponding callback parameter type:

Expression	Callback Parameter Type	On Missing Data
`With<T>`	`const T&`	Task dropped
`Optional<With<T>>`	`std::shared_ptr<const T>`	`nullptr`

Optional can wrap any word whose fusion operation provides data via get, including With, Trigger, and Last.

Warning

You **must** check for `nullptr` before dereferencing the shared pointer.

Accessing a null pointer is undefined behavior.

Example¶

// Without Optional: task dropped if Config hasn't been emitted
on<Trigger<Data>, With<Config>>().then([](const Data& d, const Config& c) {
    // Only runs when both Data and Config exist
});

// With Optional: task runs regardless, check for nullptr
on<Trigger<Data>, Optional<With<Config>>>().then(
    [](const Data& d, std::shared_ptr<const Config> c) {
        if (c) {
            // Config is available
        }
        else {
            // Config hasn't been emitted yet — use defaults
        }
    });

Notes¶

Multiple words can be wrapped independently: Optional<With<A>>, Optional<With<B>>
Optional only affects data availability checks — triggering semantics are unchanged
A triggered reaction with all-optional data words will always execute when triggered