Hi, I'm trying to write a plugin using arrow meta that injects some code at the beginning of some fu...

# arrow

[Marcello Lelio Albano]

Hi, I'm trying to write a plugin using arrow meta that injects some code at the beginning of some functions. Is there a way to modify a KtNamedFunction bodyBlockExpression?

[Patrick Louis]

Hey @Marcello Lelio Albano,
it should be possible to do this using Transform.replace . You can reference all elements of the original KtNamedFunction in the newDeclaration while also injecting additional code.

This should work as inspiration for what you're trying to do.

[raulraja]

@Marcello Lelio Albano just a friendly heads up that Quotes and in general the PSI layer will be replaced with Frontend IR once this is stable around Kotlin 1.6 or 1.7. Performing body replacements in the AST instead of generating new sources with Transform.newSources has also disadvantages as whatever declaration you introduce won’t be visible to the IDEA plugin.
For example if you intercept

fun foo() {
  val x = 1
}

and replace it for

fun foo() {
  val x = 2
}

There is no visual indication in the IDE that X was replaced by a compiler plugin or instrumented in any way and it becomes confusing to the end user. We had the alternative to push our own IDE plugin as well but that also requires all users of metaprogramming create IDE plugins extending the Kotlin plugin and it becomes extremely complex very quick for any simple feature. We hope FIR addresses these issues and then the Quote system will be based on FIR instead of PSI+compiler descriptors.

[Marcello Lelio Albano]

Thanks for answering. Is there a way to do this in arrow without touching the function signature (i.e. keeping same arguments, annotations, generics, etc) ? I think it's ok there is no visual indication right now but I may want to revisit this in future.

[raulraja]

If you don’t need visual indications then you can perform this transformation in the backend IR tree directly.
You can intercept and transform any node in the IR tree. In this case you would be watching for functions and replacing the statements in their body to include additional statements or rewrite the function body

val Meta.myPlugin: CliPlugin
  get() =
    "Intercepts functions and replaces their body" {
      meta(
        irFunction { 
          it.body...         
        }
      )
    }

[raulraja]

that will get called on each declared function

[raulraja]

If you know something inside the body you are looking for, for example a call then you can use irCall or whatever node you are after to identify the transformation. Returning null from that lambda is a no op.

[raulraja]

It filters and transforms in the same block.

[Marcello Lelio Albano]

This is super helpful. Will try it. Thank you!

[Marcello Lelio Albano]

Can you make an example of manipulation in the lamba function? Maybe even adding a simple println() - I can't seem to find any example of this

[Marcello Lelio Albano]

I think I figured out!