what’s the type of a lambda expression when seen from Java? My actual question: is it possible to pa...

# compiler

[August Lilleaas]

what’s the type of a lambda expression when seen from Java? My actual question: is it possible to pass lambdas to Java code and retrieve them based on the type? For example, store a lambda in a Spring Context and then retrieve it using the type of the lambda expression somehow? The little digging I’ve been able to do gives me the impression that it’s not easily doable. For example, suspend is lost to the Java type system and seems to be compiled into coroutine-friendly code with no type info about it.

[shikasd]

It is Function0-22 or FunctionN, depending on number of arguments

[ephemient]

yes, they get translated to functions taking an additional continuation parameter https://github.com/JetBrains/kotlin/blob/master/spec-docs/function-types.md

[ephemient]

suspend lambdas have an additional marker interface SuspendFunction
https://github.com/JetBrains/kotlin/blob/master/compiler/backend/src/org/jetbrains/kotlin/codegen/coroutines/coroutines-codegen.md

[August Lilleaas]

ah, i see, so the continuation is just an argument to a FunctionN?

[August Lilleaas]

I suppose there’s no way to get a lambda by its type then, type erasure etc, and there’s no way to create a “named” FunctionN?

[ephemient]

you can create your own

fun interface MyLambda {
    suspend operator fun invoke(args...)
}

which isn't a Function but can be used in the same ways

[August Lilleaas]

hmm, good point, it’s not a raw lambda, but that would probably work