Kotlin’s context receivers - context-dependent declarations in Kotlin

DeciderApplicationExtensions.kt

package com.fraktalio.application.demo

import com.fraktalio.fmodel.application.EventRepository
import com.fraktalio.fmodel.application.StateRepository
import com.fraktalio.fmodel.domain.Decider
import kotlinx.coroutines.flow.Flow
import kotlinx.coroutines.flow.emitAll
import kotlinx.coroutines.flow.flow
import kotlinx.coroutines.flow.fold

// ##########################################################################
// ####################### Kotlin’s context receivers #######################
// ##########################################################################
// Context receivers enables context-dependent declarations in Kotlin.
// It enables composing contexts without using inheritance, which can simplify the `application` layer of the systems / orchestrates the execution of the logic.
// This demo is using FModel library which provides tactical Domain-Driven Design patterns, optimised for Event Sourcing, CQRS and streaming.
// https://github.com/fraktalio/fmodel
// https://fraktalio.com/blog/the-template.html

/**
 * 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()


// ##############################################################
// ###################### build.gradle.kts ######################
// ##############################################################
//import org.jetbrains.kotlin.gradle.tasks.KotlinCompile
//
//plugins {
//    kotlin("jvm") version "1.6.20-M1"
//}
//
//tasks.withType<KotlinCompile>().configureEach {
//    kotlinOptions {
//        freeCompilerArgs = freeCompilerArgs + "-Xcontext-receivers"
//    }
//}
//
//group = "com.fraktalio.application.demo"
//version = "1.0.0-SNAPSHOT"
//
//repositories {
//    mavenCentral()
//}
//
//dependencies {
//    implementation(kotlin("stdlib"))
//    implementation("org.jetbrains.kotlinx:kotlinx-coroutines-core:1.6.0")
//    implementation("com.fraktalio.fmodel:application:3.0.0")
//}

// #####################################################################
// ########## Latest Kotlin EAP release 1.6.20-M1 is required ##########
// #####################################################################
// - https://kotlinlang.org/docs/install-eap-plugin.html

// ######################################################################
// ################### References and further reading ###################
// ######################################################################
// - https://blog.jetbrains.com/kotlin/2022/02/kotlin-1-6-20-m1-released/
// - https://github.com/Kotlin/KEEP/blob/master/proposals/context-receivers.md#detailed-design