Extension Points¶

Extension points are static template methods that a DSL word can implement to hook into different stages of the reaction lifecycle. The Fusion Engine discovers which points each word implements and combines them according to point-specific strategies.

Every extension point receives a DSL template parameter — the full fused type of all words in the on<>() statement. This gives each word access to the complete DSL context.

Lifecycle Overview¶

The extension points are invoked at specific stages of a reaction's lifecycle:

flowchart TD
    REG["on< Words... >().then(callback)"] --> BIND["<strong>bind</strong><br/>Register reaction with event sources"]
    BIND --> WAIT(["Waiting for trigger..."])
    WAIT --> EMIT["Data emitted / event fires"]
    EMIT --> TASK["Task generated"]
    TASK --> PRE{"<strong>precondition</strong><br/>Should task run?"}
    PRE -->|false| DROP[Task dropped]
    PRE -->|true| GET["<strong>get</strong><br/>Retrieve callback data"]
    GET --> META["<strong>priority / pool / group / run_inline</strong><br/>Determine scheduling"]
    META --> QUEUE["Submit to scheduler queue"]
    QUEUE --> PICKUP["Thread picks up task"]
    PICKUP --> SCOPE["<strong>scope</strong><br/>Acquire RAII locks"]
    SCOPE --> PRERUN["<strong>pre_run</strong>"]
    PRERUN --> CB["User callback executes"]
    CB --> POSTRUN["<strong>post_run</strong>"]
    POSTRUN --> RELEASE["~scope — release locks"]

Extension Point Pages¶

Each extension point is documented in detail on its own page:

Extension Point	When	Thread Context	Purpose
bind	Registration	Main thread (during construction)	Connect reaction to event sources
get	Task creation	Emitter's thread	Provide data to the callback
precondition	Task creation	Emitter's thread	Gate whether the task should run
priority	Task creation	Emitter's thread	Set scheduling priority
pool	Task creation	Emitter's thread	Choose which thread pool runs the task
group	Task creation	Emitter's thread	Limit concurrent execution
run_inline	Task creation	Emitter's thread	Control inline vs queued execution
scope	Task execution	Executor thread	Acquire RAII locks around callback
pre_run	Task execution	Executor thread	Hook before callback
post_run	Task execution	Executor thread	Hook after callback

Quick Reference¶

Point	Signature	Returns	Fusion Strategy
`bind`	`bind<DSL>(reaction, args...)`	void	Arg-distributing (FunctionFusion)
`get`	`get<DSL>(task)`	data	Tuple concatenation
`precondition`	`precondition<DSL>(task)`	bool	AND (short-circuit)
`priority`	`priority<DSL>(task)`	int	Maximum
`pool`	`pool<DSL>(task)`	descriptor	Exactly one
`group`	`group<DSL>(task)`	set	Set union
`run_inline`	`run_inline<DSL>(task)`	Inline enum	Must agree
`scope`	`scope<DSL>(task)`	RAII lock	All held
`pre_run`	`pre_run<DSL>(task)`	void	Sequential
`post_run`	`post_run<DSL>(task)`	void	Sequential