Can I build, using Arrow, a custom, generic sum ty...

# arrow

[Lukasz Kalnik]

Can I build, using Arrow, a custom, generic sum type similar to Either but with more subtypes?
I need something like this:

sealed class DataState<T> {

    class Loading<T> : DataState<T>()

    data class Error<T>(val error: CallError) : DataState<T>()

    data class DataLoaded<T>(
        val data: T
    ) : DataState<T>()
}

And I want only the DataLoaded subtype to behave as Either.Right (e.g. when using flatMap()) and all other subtypes behave as Either.Left, i.e. having early returns from flatMap().
I especially would like to have the monad comprehension syntax analogous to the either {} DSL (using .bind()).

[Lukasz Kalnik]

Currently when doing something with data I have to typecheck / cast everytime, which doesn't look elegant:

fun workWithData() {
    val dataLoaded = (dataState as? DataLoaded) ?: return
    val data = dataLoaded.data
}

[Youssef Shoaib [MOD]]

You can have the Left-y states have a common supertype, and then just use Either normally with Left being the supertype and Right being DataLoaded

[Lukasz Kalnik]

Thanks, that's a brilliant solution! I will try it out 🙏

[simon.vergauwen]

Here is an example @Lukasz Kalnik, https://github.com/nomisRev/ktor-arrow-example/blob/main/src/main/kotlin/io/github/nomisrev/DomainError.kt

I use sealed interface to define "layers", and then I can unite them with a common parent to closer they come to my API edge.

[Lukasz Kalnik]

Thank you for the examples!