Is there any documentation on Kotlin `external` fu...

# getting-started

[Tianyu Zhu]

Is there any documentation on Kotlin external functions? How do they work? Is there a Java equivalent?

[Kirill Grouchnikov]

https://jonnyzzz.com/blog/2019/12/15/jni-kotlin/

[Kirill Grouchnikov]

So something like https://github.com/JetBrains/skiko/blob/master/skiko/src/commonMain/kotlin/org/jetbrains/skia/Canvas.kt that defines a bunch of external functions, that are implemented in C++ and accessed via the JNI bridge

[Kirill Grouchnikov]

Java’s equivalent is native

[Tianyu Zhu]

🤯

[Tianyu Zhu]

Is it bad practice to implement an external function with... not a native function?

I want to declare an external function in an API module, and then implement it in different ways in subsequent modules.

[Kirill Grouchnikov]

That would be expect and actual most likely

[Tianyu Zhu]

It's not a multiplatform thing where the function is declared in a common module and then the JVM/JS/other implementations are declared in their own module

[Kirill Grouchnikov]

image.png

[Tianyu Zhu]

Yeah, it's not a multiplatform thing. I'm working strictly in the JVM.

[Kirill Grouchnikov]

Hence, the use of external might not be relevant

[Paul Griffith]

Just use a ServiceLoader? Or even a plain ol’ interface?

[Goetz Markgraf]

If you just want to implement some functionality somewhere else but in still in kotlin, you could use the normal interface and “somewhere else” create a class that extends that interface .

[Klitos Kyriacou]

If you're talking about package-level functions (i.e. interfaces are not relevant), and you're asking about something equivalent to the extern keyword in C++, then the Kotlin equivalent is... nothing. You just have to either import the function name or just use its fully qualified name (including package name) at the place where you use it. Kotlin will know about this function if it is in the classpath. So all you have to do, if you have a number of alternative jars containing alternative implementations of the function, is to include one of those jars in your classpath when you compile the Kotlin file that uses the function.