knowledge/content/docs/programming/kotlin.adoc

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== Kotlin

https://kotlinlang.org/[Kotlin] is a modern language that can run on
the JVM (as well as compiling to JavaScript or to native binaries).
I use Kotlin for any Android app I write, and most desktop apps that
require a GUI.

=== Assorted coroutine notes

* *launched* in the context of a scope, e.g. `GlobalScope`
* `launch` and `runBlocking` are both builders
** Can parameterise `runBlocking` for e.g. tests - `runBlocking<Unit> {...}`
** Builders return a job, can call `job.join()` to wait, `job.cancel()` to stop
** Every builder adds a `CoroutineScope` to the scope of a code block
** `launch`/`runBlocking` operate on current scope
* `coroutineScope` is a builder to manually create a scope
* `suspend` functions don't carry the scope
** keep a reference in the class/fun
** make them an extension method on CoroutineScope
* Coroutines launched in `GlobalScope` don't keep the process alive if `main()` returns
* Cancellation is co-operative - must throw `CancellationException`
** Can check `isAlive` property
** `withContext(NonCancellable)` if job *can't* be cancelled (e.g. suspend fun in a finally block)
* `withTimeout` and `withTimeoutOrNull` cancel automatically
* Children can be cancelled with `coroutineContext.cancelChildren()`
* `async` is like `launch` but returns a `Deferred`
* Contexts are job + dispatcher
* Dispatcher specifies thread to run on
* Launchers take an optional context
** `launch(newSingleThreadContext("name")) {...}`
* `GlobalScope`'s dispatcher uses a shared threadpool
* Can get own job from `CoroutineContext[Job]`
* `CoroutineName` gives named context
** Can concat contexts with `+`
* `select` can receive data from multiple channels, like Go