A question about Kotlin Function Type cast

Question

Code fragment in Kotlin

  public actual fun <R, T> (suspend R.() -> T).createCoroutineUnintercepted(
    receiver: R,
    completion: Continuation<T>
): Continuation<Unit> {
    val probeCompletion = probeCoroutineCreated(completion)
    return if (this is BaseContinuationImpl)
        create(receiver, probeCompletion)
    else {
        createCoroutineFromSuspendFunction(probeCompletion) {
            (this as Function2<R, Continuation<T>, Any?>).invoke(receiver, it)
        }
    }
}

What I want to know is How (suspend R.() -> T) AKA Function Type can be recognized as BaseContinuationImpl which is a Classify Type even to cast as `Function2'.

Any help will be appreciated.

Answer 1

There are no separate function types and class types. Function types are just types that can be executed with specific arguments and specific return type. They are interchangeable with Function0, Function1, etc. interfaces and they contain a single invoke() function.

We can implement a function type by our class:

class MyClass : (suspend () -> Unit) {
    override suspend fun invoke() {}
}

Now, let's get this code:

val lambda: (suspend () -> Unit) = {}

After disassembling we see that our lambda is compiled to:

final class FooKt$foo$lambda$1 extends kotlin/coroutines/jvm/internal/SuspendLambda implements kotlin/jvm/functions/Function1 {
    ...
    public final invoke(Ljava/lang/Object;)Ljava/lang/Object;
    ...
}

It extends SuspendLambda which is a subtype of BaseContinuationImpl. It also implements Function1 and contains invoke function which makes it a function type.