precondition¶

Called before a task is queued. If any precondition returns false, the task is dropped.

Signature¶

template <typename DSL>
static bool precondition(threading::ReactionTask& task)

Details¶

Aspect	Detail
When	After task creation, before `get` and submission
Thread	The emitter's thread (the thread that called `emit()`)
Returns	`bool` — `true` to allow, `false` to drop
Fusion	Logical AND with short-circuit evaluation. First `false` stops evaluation and drops the task.

Context & Arguments¶

task — the ReactionTask being evaluated. Access to task.reaction gives you the parent Reaction's state (e.g., active_tasks count, enabled state).

Runs on the emitter's thread. The precondition check happens synchronously during the emit loop — before the task is submitted to any scheduler queue. This means precondition checks should be fast and non-blocking.

When a task is dropped by a precondition, its status is set to BLOCKED.

Example¶

struct Single {
    template <typename DSL>
    static bool precondition(threading::ReactionTask& task) {
        // Only allow one instance of this reaction at a time
        return task.reaction->active_tasks.load() == 0;
    }
};

Built-in Words Using precondition¶

Single — blocks if the reaction already has an active task
Buffer<N> — blocks if N tasks are already active/queued
Reaction enable/disable — disabled reactions always fail precondition