Hello, I’ve a question regarding `inline functions...

# announcements

[Melvin Biamont]

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)
}

[Alowaniak]

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")

[louiscad]

You need to take a function that returns Nothing, then at use site, you can put return in that lambda argument.

[Melvin Biamont]

@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 Biamont]

@louiscad

This is what I wanted to do, but how can I return Nothing? From what I read, the only way is to throw an exception, which is not what I wanted.

[louiscad]

@Melvin return evaluates to Nothing too 😉

[louiscad]

If it didn't, you could just throw after the return expression, since it'd never be executed anyway, being unreachable. That's why return expression already evaluates to `Nothing`: it makes code below unreachable.

[Melvin Biamont]

@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 😕

[Melvin Biamont]

Hmmm,

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

    return error("user is null")
}

Like this seems okay.
(I’ll try it)

[Melvin Biamont]

Arrgh, but it crashes. Looks like the error("user is null") is reachable ˆˆ

[Alowaniak]

@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 }

[louiscad]

@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
        }
    }
}

[Melvin Biamont]

Yeah, but can I do it without passing lambda?

[louiscad]

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).

[czuckie]

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

[Melvin Biamont]

You’re probably right.

I gave up and did it using lambda.

Thanks @louiscad @Alowaniak @czuckie 🙂