My favorite Kotlin feature

Simon Vergauwen · January 18, 2022

Over the last several years I’ve seen many discussions on Kotlin’s features. Nullability is always a very high ranked one. I like it, but it’s definitely not my favorite feature.

What I love most about Kotlin are some small features, that make the language very elegant. Such as smart-casting (contracts), suspend, receiver/extension functions & delegation.

But at this point in time my favorite feature is the combination of inline and suspend.

Inline recap

The inline feature is a feature everyone in Kotlin is using regularly, knowingly or not. A function can be marked inline when it has lambdas as parameters, also known as Higher Order Functions. A Higher Order Function is a function that takes other functions as parameters, or returns a new function. We call map a HOF since it takes another function transform as a parameter.

inline fun <A, B> Iterable<A>.map(transform: (A) -> B): List<B> is probably the best known one. It maps all elements in a List of type A to a List of type B using the transform function (A) -> B. When these patterns were first introduced, there was some concern about performance due to for-loops being more efficient at byte-code level.

inline is a Kotlin compiler feature that can completely get rid of that concern. If we look at the function body of we see a for-loop.

So let’s see what the bytecode of listOf(1, 2).map(Int::toString) looks like:

val destination = ArrayList<String>()
for (item in listOf(1, 2)) {

It looks the same as if we’d have written the for-loop by hand. Awesome, we can write high-level code without losing any of the benefits of writing low-level code!

However, inlining is not unique to Kotlin. Many other languages also support inlining.

suspend recap

suspend in Kotlin is extremely powerful, and it has many interesting use-cases. Suspension allows us to wrap callbacks, and write imperative code over callbacks.

suspend fun example(): Int = suspendCoroutine { continuation ->
    callbackCode { int, error ->
      if(error == null) continuation.resume(int)
      else continuation.resumeWithException(error)

suspend fun twice() = example() + example()

In concurrency that means we can work with JVM Future or JS Promise without having to write callback based code. This is very powerful, as we can now write “regular imperative” to describe powerful async workflows, but it also allows for very interesting use-cases outside of concurrency. A great example is discussed at length in my previous blogpost.

The compiler also verifies that suspending code can only be called from other suspending code, so that you can never accidentally call suspending code from places that do not support it.

suspend fun example(): Int = suspendCoroutine { continuation ->
  callbackCode { int, error ->
    if(error == null) continuation.resume(int)
    else continuation.resumeWithException(error)

fun once() = example()
// Cannot call suspending fun example from non-suspending fun once

The suspend functionality is also known as Continuation Passing Style, which means that the compiler can automatically pass Continuation between function for us. It does so in an extremely efficient manner, and that allows us to write callback based code in an immediate manner.

suspend + inline

Now that we’ve discussed how suspend and inline works, we can take a look at my favourite Kotlin feature which is support between suspend + inline!

Let’s start from a non-inline example.

fun <A, B> Iterable<A>.map(transform: (A) -> B): List<A> = TODO()

suspend fun fetchUser(id: Int): Result<Int> = Result.success(id)

suspend fun example5() = listOf(1, 2, 3).map { fetchUser(it) }

This fails to compile with Suspension functions can be called only within coroutine body even though our example function is marked as suspend. This is because we’re creating a non-suspending lambda transform to pass to the map function, and from within that non-suspending lambda we cannot call the suspend function fetchUser.

But when we mark this function inline it suddenly works. That’s because it’s not creating a non-suspending lambda anymore, but instead in our example we now have again our optimised for-loop and the resolve function is being called from within.

Since the compiler knows that the non-suspending lambda will not exist due to inline, it allows suspend to be called from inside. This is similar to smart-casting, the compiler has certain knowledge about the program that it allows us to do something that is regularly not allowed.

When designing libraries, or higher order functions it’s important to keep inline in mind. Not only for performance reasons, but also for suspend.

It’s one of the features of the Kotlin Compiler that is easily overseen, since it works so transparently.

More examples

Let’s see some other interesting examples, and some caveats.

fun process(i: Int): Either<String, Int> = i.right()

val res: List<Either<String, Int>> = (0..9).map { process(it) }

In the above example we have a list of 10 items, and want to process every element. When we use we get List<Either<String, Int>>, which is not that interesting. Actually we’d want Either<String, List<Int>>, either the first error, or all processed results.

If you’re familiar with Arrow, or the Cont blogpost you’ve seen a DSL where we can use suspend to work over Either.Right in a nicer way. Since we can combine inline with suspend, we can extract the value out of Either inside

fun process(i: Int): Either<String, Int> = i.right()

val res: Either<String, List<Int>> = either.eager<String, Int> {
  (0..9).map { process(it).bind() }

Now we get the result we’re interested in Either<String, List<Int>>, either the first error, or all processed results. If you’re familiar with functional programming, you might see that this replaces the Traverse and Monad hierarchy. So when practicing FP in Kotlin, we can forget about a lot of complexity since it’s possible to provide powerful semantic sugar through suspension. Note: here we use either.eager which uses suspend but doesn’t allow any foreign suspending code.

Another interesting example is Sequence from Kotlin Std, where suspend is used to offer a powerful DSL for building lazy sequences. sequence offers suspend fun yield(a: A): Unit which can be used to emit/yield a value to the Sequence. Below we can use it to implement flatMap through suspend and inline.

fun <A, B> Sequence<A>.flatMap(transform: (A) -> Sequence<B>): Sequence<B> =
  sequence {
    forEach { a: A ->
      f(a).forEach { b: B -> yield(b) }
  1. Open a sequence DSL block
  2. Loop over all elements in Sequence<A>
  3. transform A into Sequence<B>
  4. Loop over all elements of Sequence<B> and yield them

The reason we can call yield from the double nested forEach is because both are marked as inline, and thus we can call yield safely from inside.


This pattern is sadly not foolproof, there are some gotchas in terms of resource-safety and cancellation.

coroutineScope {
  listOf(1, 2, 3)
    .map { async { ... } }
    .map { it.await() }

This code is not cancellation safe, if a task fails it can never cancel the other tasks ahead of it in the list. Due to awaiting the Deferred in the order of the list instead of all at the same time. To await all the Deferred at the same time you should use awaitAll.

coroutineScope {
  listOf(1, 2, 3)
    .map { async { ... } }


The way these two features work so seamlessly together is in my opinion an underappreciated feature of the Kotlin compiler and one of my favourites.

An upcoming feature in Kotlin, Context receivers, is going to make this pattern more powerful. More on that in a next post.

Thanks for reading my blog on my favorite Kotlin feature(s). I hope you enjoyed it, and a Happy New Year!

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