Hello, I’ve a question regarding

inline functions

. How can I make an inline function stopping/returning the calling function?

Basically, I would like this :

//This method should return the calling function
inline fun onUserNull() = run {
  Timber.e("User is null")
  return
}

fun fetchUserIdentity() {
  val currentUser = userDataSource.currentUser ?: onUserNull()

  presenter.displayUserIdentity(currentUser)
}

Would do the same as:

fun fetchUserIdentity() {
  val currentUser = userDataSource.currentUser ?: run {
    Timber.e("User is null")
    return
  }

  presenter.displayUserIdentity(currentUser)
}

I don't think it's possible (except for throwing) I also don't think it's a good idea Why not do

val currentUser = userDataSource.currentUser ?: return Timber.e("User is null")

You need to take a function that returns

Nothing

, then at use site, you can put

return

in that lambda argument.

@Alowaniak I could, but this code is used in a lot of places, and I wanted to factorize it. 😕 From what I understood,

inline functions

content are more or less just copied in the call site. So, why couldn’t I do

return

from an inline function?

@Melvin

return

evaluates to

Nothing

too 😉

@louiscad I tried this code:

inline fun reportUserNull(): Nothing {
    Timber.e("User is null")

    return
    error("user is null")
}

But, it tells me

This function must return a value of type Nothing

I think I can’t just use

return

😕

@louiscad I don't follow how you suggest to achieve what @Melvin Biamont wants. I don't see how it's possible to call a function which will return the caller's function (without

return

in the caller via for example a lambda) other than throwing an exception? The reason why I would say you shouldn't want it in the first place is because IMO it would get totally unclear what happens at call-site Also I'm not sure but I don't think an

inline

function is always inlined (for example when calling from java, or when function is virtual) I really think you should just do

val x = foo ?: return onBar()

or

val x = foo ?: onBar().also { return }

@Melvin Biamont I don't think you understood what I said. Here's an example :

inline fun returnFromMyLambda(returnYourself: () -> Nothing) {
    stuff()
    if (shouldDoIt) returnYourself()
}

fun computeNumber(): Int {
    if (Random.nextBoolean()) {
        return 1 + 1
    } else {
        returnFromMyLambda {
            return 0 + 3 // returns from computeNumber
        }
    }
}

Yeah, but can I do it without passing lambda?

No, it's impossible. It'd be dangerous to allow a function to go beyond its boundaries like that. inline higher order functions is the best we got, and IMHO, the best we can get, unless a new thing is introduced in the language to allow functions that make their caller return (which would anyway be unpractical because the return type is not necessarily

Unit

, so would put return type constraints on the caller).

from a readability point of view- it would be a nightmare if seemingly innocuous method calls exited my method

You’re probably right. I gave up and did it using lambda. Thanks @louiscad @Alowaniak @czuckie 🙂