Newby question: why was kotlinx.serialization desi...

# getting-started

[obadz]

Newby question: why was kotlinx.serialization designed as a compiler plugin rather than an annotation processor?

[diesieben07]

Annotation processors are:

• Limited to the JVM platform
• Limited to only generating new classes instead of modifying existing ones

[obadz]

Are annotation processors really limited to the JVM even if you generate Kotlin code rather than Java code?

[diesieben07]

Yes. Annotation processors are a javac feature, not a kotlinc feature

[diesieben07]

kapt (the thing that makes annotation processing work for Kotlin at all) is basically a giant hack.

[diesieben07]

It has to generate Java-stubs for all Kotlin classes so that javac can deal with them at all and run the annotation processors on them.

[obadz]

I see. And this hack somehow invokes javac?

[obadz]

Ah. Having to write bytecode+llvm emmiters seems a bit of a steep cost (which if I understood correctly you have to pau when writing a compiler plugin).

Is there a reason why annotation processing isn't made first class in kotlinc? (multiplatform etc.)

[diesieben07]

Because the API is fundamentally tied to Java, emitting Java classes and/or Java bytecode.

[diesieben07]

But a true Kotlin compiler plugin API is apparently planned.

[diesieben07]

There is one already (which is what the serialization plugin uses), but it requires separate code for all targets and it's not documented at all

[diesieben07]

And not API-stable

[obadz]

Thanks for answering all my questions!

[obadz]

Actually one more: is there any forums where the design of the stable compiler plugin API is being discussed?

[diesieben07]

I haven't seen any public discussion about how it will look, no. Just "it will come".

[Ruckus]

I believe the #compose team is working closely with the Kotlin team on this, so you could try asking there.

[Robert Jaros]

You can also be interested in looking at https://github.com/Foso/MpApt project. It contains a lot of info about Kotlin compiler plugins.

[Casey Brooks]

You can also take a look at the work the Arrow folks have been working on to simplify writing compiler plugins with Arrow Meta https://github.com/arrow-kt/arrow-meta

[obadz]

Thank you all for these pointers!