Adopt Context Receivers

[idugalic]

Context receivers enable context-dependent declarations in Kotlin.

It enables composing contexts without using inheritance, which can simplify the application layer of the systems / orchestrate the logic's execution.

It will eliminate the need of application interfaces, for example interface EventSourcingAggregate<C, S, E> : IDecider<C, S, E>, EventRepository<C, E> or interface StateStoredAggregate<C, S, E> : IDecider<C, S, E>, StateRepository<C, S>

/**
 * Event-sourced algorithm
 * Computes new Events based on the previous Events and the Command.
 */
context (Decider<C, S, E>, C)
private fun <C, S, E> Flow<E>.computeNewEvents(): Flow<E> = flow {
    val currentState = fold(initialState) { s, e -> evolve(s, e) }
    val resultingEvents = decide(this@C, currentState)
    emitAll(resultingEvents)
}

/**
 * State-stored algorithm
 * Computes new State based on the previous State and the Command.
 */
context (Decider<C, S, E>, C)
private suspend fun <C, S, E> S?.computeNewState(): S {
    val currentState = this ?: initialState
    val events = decide(this@C, currentState)
    return events.fold(currentState) { s, e -> evolve(s, e) }
}

/**
 * Publish command - Event-sourced decider
 */
context (Decider<C, S, E>, EventRepository<C, E>)
fun <C, S, E> C.publish(): Flow<E> = fetchEvents().computeNewEvents().save()

/**
 * Publish command - State-stored decider
 */
context (Decider<C, S, E>, StateRepository<C, S>)
suspend fun <C, S, E> C.publish(): S = fetchState().computeNewState().save()

https://gist.github.com/idugalic/232bc21490f4a8976bc31e0acf548831

A plan is to alternatively implement context-dependent declarations by keeping current interfaces as a valid option: EventSourcingAggregate, StateStoredAggregate, ...