Hello all I am looking for a way to extend the interface of

Question

Hello all I am looking for a way to extend the interface of an existing class without modifying the class itself I was reading about receivers thinking there might be a way to achieve the desired result but I am having trouble wrapping my head around the concept Concrete example in thread

Billy Newman · Answer

I have a few room database entities ``` Entity data class FooEntity Entity data class BarEntity ``` I would like each entity to implement some interface without polluting the entity class itself ```interface Writeable fun write ``` I am currently creating a wrapper object that implements the interface for each entity ```class FooWriteable private val foo Foo Writeable class BarWriteable private val bar Bar Writeable``` Combing through the discussion here <https discuss kotlinlang org t implementing an interface for an existing type 20967 11> It mentions receivers as a possible solution Currently having trouble fully understanding Am I heading down the right path with receivers Could someone possibly at least point me in the right direction Thanks

Johann Pardanaud · Answer

Unfortunately this is not possible with Kotlin <https discuss kotlinlang org t extensions function to implement interface 411>

Johann Pardanaud · Answer

Swift supports this and its beautiful

Johann Pardanaud · Answer

~Wait its seems like context receivers could allow to do this~ not sure at all about that I ll let context receiver experts talk about this

Youssef Shoaib [MOD] · Answer

Is the interface set in stone Or is it your own custom thing Basically you can change the interface to be ```interface Writeable lt T gt fun T write ``` And have implementations of it defined ```object FooWriteable Writeable lt Foo gt fun Foo write = TODO ``` and then any function that takes a Writeable should instead be defined like so ```context Writeable lt W gt fun lt W gt myCustomWrite writeable W writeable write ``` And as you can see you can t tell the difference on the callsite

Billy Newman · Answer

Thank you I guess the part I am still missing is how to convert my entity pojos ```val foos = fooRepository getFoos val bars = barRepository getBars val writeables = foos + bars myCustomWrite writables ```

Youssef Shoaib [MOD] · Answer

No need to convert you just do `with FooWriteable ` Edit oh in this case you might run into issues because Foo and bar have different types Maybe `myCustomWrite` could just take 2 different list

Billy Newman · Answer

That is when I tell you I don t just have foo and bar I have 12 different types

Youssef Shoaib [MOD] · Answer

Do they all need to be placed together into lists If so I m afraid wrapping might be the best solution for now However soon enough we ll have union types and then maybe you could write this in a nicer way by having a list of Union of the 12 types and having a `UnionWriteable` that implements writeable for all those types