I really want to be able to do this in kotlin :( `...

# language-proposals

[efemoney]

I really want to be able to do this in kotlin :(

val Application.cronUsername by this.environment.config

this will refer to the receiver ie the Application . Is there any limitation I’m missing why its not possible right now?

[gildor]

you cannot delegate directly, what it should do exactly?”

[gildor]

just declare getter

[gildor]

but anyway this syntax is not supported

[natpryce]

Why the annotation? I’d say it’s clearer as the following, unless there’s a good reason to have the annotation.

val Application.cronUsername get() =
  environment.config[“app.cron.username”]

[efemoney]

Sorry, the annotation is not included/ necessary to what I’m proposing which is being able to access the receiver when delegating an extension property.

[efemoney]

Basically I want to be able to delegate the implementation of an extension property to another (publicly accessible) property/method of the receiver of the extension property. My example is pretty contrived but above is the core of what I wanted

[efemoney]

If this was allowed, for instance, the config keys could be “figured out” from the property name and you can write

val Application.app_cron_username by environment.config

and the getValue will transform the property.name into key

[efemoney]

Or we could abstract out type conversion by making the getValue generic on reified return type and have typesafe extension properties like

val Application.port: Int by environment.config

where getValue will have if (T::class == Int::class) getPropertyString("...").toInt()

[efemoney]

Part of both of the above examples are supported right now of course. Whats not is being able to access the receiver of the extension property within the delegate expression. I would like to know if it was something considered before and if so what are the limitations that are not immediately obvious to me right now.

[efemoney]

<EDITED OP to remove annotation @natpryce>

[natpryce]

Ah I see. The receiver is passed to getValue (but as an untyped Any? value). You could write your own class or object that implements getValue and passes the receiver and property name & type to a function that calculates the property value.