CC&M

This is a continuation of series about coroutines internals. In the first part I explained how the compiler turns sequential code into suspendable by turing it into state-machines and how suspension works. This part covers continuation passing style - the reason for two-world model, and the counterpart of suspension - resumption. Continuation Passing Style In the first blogpost I touched upon the COROUTINE_SUSPENDED marker and said that suspending functions and lambdas return the marker when they suspend. Consequently, every suspend function return returnType | COROUTINE_SUSPENDED union type. However, since neither Kotlin nor JVM support union types, every coroutine's return type is Any? (also known as java.lang.Object ) at runtime. Let's now look to resume process closely. Suppose we have a couple of coroutines, one of them calls the other: fun main () { val a: suspend () -> Unit = { suspendMe() } val b: suspend () -> Unit = { a() } builder { b()

Introduction Throughout 2020 I wrote a document on coroutines codegen, which aimed mostly at my colleagues at JetBrains and Google to help them understand the pitfalls of the codegen and which design decisions we made. These blog posts are a simplified and trimmed down version of the documentation for a larger audience of experienced Kotlin programmers, who just want to know how the compiler turns sequential code into suspendable and become more proficient at their jobs. The blogposts will not cover kotlinx.coroutines internals. However, I think, with the knowledge of coroutines internals, it will be easier to understand, for example, how Flow's backpressure works and why it is essential for Flow operators to be inline with crossinline lambda parameter (spoilers: this is an optimization). I decided to write the blogposts now, before Kotlin 1.5 release, since Kotlin 1.5 brings new JVM_IR back-end with a lot of improvements in code generations, tail-call optimization of suspend fu