Extension delegation properties It should be poss...

# language-proposals

[guenther]

Extension delegation properties

It should be possible, to extend a class with an extension property using delegation. Of course only, when the delegated object (in this case the map) is accessible.

class Foo(val map: MutableMap<String, String> = mutableMapOf()) {
    var bar1: String by map
}
var Foo.bar2: String by map

[orangy]

It is not possible right now, because there is no storage for an extension property to keep a value of map per Foo instance. In your example it is not obvious why it is needed, but consider:

val Foo.bar2: String by map.filter { it.key.isNotEmpty() }

i.e. when expression is not so simple

[guenther]

hm, actually the problem I get is Unresolved reference: map. Also for the extension property you suggested.

[guenther]

Workaround of course is possible with e.g.

var Foo.bar3: String
    get() = map[Foo::bar3.name] as String
    set(value) { map[Foo::bar3.name] = value }

But as long as the map is public accessible, I guess there is no reason why the the extension property can't support that, is it?

[orangy]

Yes, if you write it like this.map it will highlight this with a proper error. But in general, when something “can obviously work” or “there is no reason why it couldn’t” it is quite often a lot of issues arising as soon as you start to actually design a language feature.

[orangy]

Filed an issue for diagnostics: https://youtrack.jetbrains.com/issue/KT-24289

[guenther]

Ok, but improving the error message wasn't quite what I expected. What I want to "discuss" is, if Extension delegation properties are useful for the language.