Hey people, this is more of a type system question...

# coroutines

[Maciej S]

Hey people, this is more of a type system question rather than a coroutines question per se, but are the suspend lambdas function types non-denotable types in Kotlin?

[Maciej S]

Or if alternatively someone could point me to the correct channel to ask this type of question 🙏

[Joffrey]

Lambdas are not really types themselves. Lambdas are one way to write expressions that represent values of a function type. Function types are denotable, and suspending function types as well. You just need to add suspend in front of a regular function type:

suspend () -> Unit is an example of a suspending function type.

You can see this sort of types in the declarations of some higher-order functions in the coroutines library, typically coroutine builders like launch or `async`:
https://kotlinlang.org/api/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines/launch.html

[Maciej S]

> Lambdas are not really types themselves
Yeah, fair, I was referring to suspending function type. Sorry, I should’ve expressed my initial question better.

Here’s the reason why I ask. Consider the following code:

@JvmInline
value class DummySuspendValue(val block: suspend () -> Unit)

...

val a = DummySuspendValue {}
val classifier = a::class.declaredMemberProperties.first().returnType.classifier

In the example above, classifier is null . According to the docs, `classifier`:

Returns null if this type is not denotable in Kotlin, for example if it is an intersection type.

If I were to go by the reflection behaviour and the docs, I’d have to conclude that a suspending function type is in fact non-denotable.

If I remove the suspend modifier like so:

@JvmInline
value class DummySuspendValue(val block: () -> Unit)

the classifier is no longer null.