@AutoReducer & generics in Kotlin

[jhavatar]

When using Kotlin, My simple todo list reducer fails to build/generate its implementation with the following error:

error: First parameter arg0 of method addItem(java.util.List,io.chthonic.template.kotlin.data.model.TodoItem) should have the same type as state (java.util.List<? extends io.chthonic.template.kotlin.data.model.TodoItem>) 1 error

The reducer:

    @AutoReducer.Action(
            value = TodoListActions.ADD_ITEM,
            from = TodoListActions::class)
    fun addItem(state: List<TodoItem>, item: TodoItem): List<TodoItem>  {
        return state.plus(item)
    }

The Action:

    @ActionCreator.Action(ADD_ITEM)
    fun addItem(item: TodoItem): Action

Note, that the following horrible version of the reducer does build in Kotlin:

    @AutoReducer.Action(
            value = TodoListActions.ADD_ITEM,
            from = TodoListActions::class)
    fun addItem(state: List<Any>, item: TodoItem): List<TodoItem>  {
        return (state as List<TodoItem>).plus(item)
    }

Since "List<? extends X>" parameter does not have an equivalent in Kotlin, as far as I know, do you know a better solution to get my reducer to build?

[Yarikx]

@jhavatar Thank you for this issue. To be honest, I didn't spend much effort on Kotlin support :), but that was my next big step.

I need to investigate why Kotlin represents type differently from Java while compilation.

One ad-hoc solution that comes to mind: you can still declare your args as List<? extends X>:

    @AutoReducer.Action(
            value = TodoListActions.ADD_ITEM,
            from = TodoListActions::class)
    fun <T> addItem(state: T, item: TodoItem): List<TodoItem>  where T : List<TodoItem> {
        return state.plus(item)
    }

As that is a dirty and ugly solution, this should work. I will look into it and add first class support for Kotlin types soon.

[Yarikx]

@jhavatar No, sorry, the last example doesn't work either with the production version of Reductor, sorry. Another workaround is to wrap List into some other class which doesn't have generics.

[jhavatar]

cool, thanks Yarikx. I'll do a hack for now. Please reply on this thread when a version is released with updated kotlin support.

[Yarikx]

Opened discussion thread for this issue here: https://discuss.kotlinlang.org/t/declaration-site-variance-and-java-interop/2331

[Yarikx]

@jhavatar Ok, I found a solution! http://kotlinlang.org/docs/reference/java-to-kotlin-interop.html#variant-generics

So as the issue is caused by kotlin-specific declaration-site variance and Java interop, it cannot be really solved from the reductor side. Instead, you can tweak type declarations to help compiler a bit. The options are:

• Get rid of wildcard in Reducer type parameter

  @AutoReducer
  abstract class KotlinReducer : Reducer<List<@JvmSuppressWildcards TodoItem>> {
  ...
  }

  That will make Kotlin represent the class as Reducer<List<TodoItem>> and not as Reducer<List<? extends TodoItem>>

• Leave Reducer state as is, but annotate state argument in each action handler with @JvmWildcard

    fun addItem(state: List<@JvmWildcard TodoItem>, item: TodoItem): List<TodoItem>  {
        return state.plus(item)
    }

Hope that helps.

[jhavatar]

Yes, solves the problem. Thanks, will come in handy.