I have a variety of base types that I add extensions too e g kotlinlang #announcements

I have a variety of base types that I add extensions too (e.g. a foobar property), and I’d like to fetch foobar from a heterogenous collection of Any’s when/if they support it, otherwise do a default/fallback thing

Shawn

08/28/2019, 5:22 PM

it kinda sounds like you want traits?

which aren’t a thing in Kotlin

karelpeeters

08/28/2019, 5:22 PM

Or duck typing.

Shawn

08/28/2019, 5:22 PM

definitely not a thing in Kotlin lol

Travis Griggs

08/28/2019, 5:23 PM

well… what I want is to emulate something I did in Swift (definitely not duck typed or traited)

in Swift, I can create a protocol for said foobar property, and then I can extend base types with said protocol and add implementations. then I can write code like: if let foobarable = thing as? Foobarable { foobarable.foobar } else { do_the_fallback_thing }

Shawn

08/28/2019, 5:24 PM

protocol extensions are definitely very trait-like, and is still not a thing you can do in Kotlin if you don’t own the class 😕

Travis Griggs

08/28/2019, 5:26 PM

fair enough. is there an alternate pattern that would be workable? Is there enough reflection available in Kotlin on top of Java to determine if said static extension exists and invoke it if it does?

Shawn

08/28/2019, 5:26 PM

relevant discussion thread: https://discuss.kotlinlang.org/t/extensions-function-to-implement-interface/411

hmm I suppose I can’t rule out a reflection-based approach but I know it’s probably going to be kinda messy

would have to defer to someone with more reflect experience than I

streetsofboston

08/28/2019, 5:30 PM

There’s a KEEP out there from the Arrow folks that would enable constructs like this: https://quickbirdstudios.com/blog/keep-87-typeclasses-kotlin/ https://github.com/Kotlin/KEEP/pull/87

Travis Griggs

08/28/2019, 5:36 PM

thanks guys

James Richardson

08/29/2019, 7:40 AM

Why is this not just a normal interface? Must be missing something.

karelpeeters

08/29/2019, 7:41 AM

Generally because you don't control the classes themselves.

James Richardson

08/29/2019, 7:43 AM

Well, in this case I think he does?

when (bob) is Foobarable -> bob.foobar() else -> whatever()

Matteo Mirk

08/29/2019, 10:23 AM

maybe what you’re after could be realized with the Abstract Document pattern: http://ageofjava.com/java/Type-Safe-Views-using-Abstract-Document-Pattern (Java implementation but no-brainer to port it over Kotlin)

James Richardson

08/30/2019, 1:24 PM

Well, that is just interfaces! 😄

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