For some reason the `bind()` on the `Either` contained in the `MutableStateFlow`'s `value` here cann...

# arrow

[Lukasz Kalnik]

For some reason the bind() on the Either contained in the `MutableStateFlow`'s value here cannot be resolved. commonConnectorApi.renameControlDevice() returns Either<CallError, Unit>.

[Stylianos Gakis]

Is DataMissing a CallError?

[Lukasz Kalnik]

No, this is a separate sealed class hierarchy

[Lukasz Kalnik]

sealed class DataState<T> {

    sealed class DataMissing : DataState<Nothing>()

    object NotInitialized : DataMissing() {

        operator fun invoke() = Left(NotInitialized)
    }

    data class Error private constructor(val error: CallError) : DataMissing() {
        companion object {

            operator fun invoke(error: CallError) = Left(Error(error))
        }
    }

    data class DataLoaded<T> private constructor(val data: T) : DataState<T>() {
        companion object {

            operator fun <T> invoke(data: T) = Right(DataLoaded(data))
        }
    }
}

[Lukasz Kalnik]

Can bind() be used only if the Left type argument is the same for all calls?

[Alejandro Serrano Mena]

each either block can only have one error type, so all the calls to bind need to be over the same type, or share a parent class

[Stylianos Gakis]

bind does exactly that, continues with the right if it is right, if it is not it needs to know how to exit and what the result of your either block will be. Since the return type of your function is Either<CallError, Unit>, if you could bind in there with the left being a DataMissing, what would you expect the return value to be? It has to confront to the type you said you’re returning.

[Lukasz Kalnik]

Yes, that makes sense - either block with bind() basically a flatMap, so left type always has to be the same (only right type can be changed through the flatmapping).

[Lukasz Kalnik]

Thank you for the explanations! 🙏

[Stylianos Gakis]

You could mapLeft { dataMissing -> dataMissing.toSomeCallError }.bind() if there is such a mapping, or in general rethink what kind of error you want to return there and have it be of the same type

[Lukasz Kalnik]

I think I actually should not bind() them both in the same either block.

[Lukasz Kalnik]

I want to return the result of the call but also, as a side effect, update the controlDevices.value. But controlDevices.value being in the state of DataMissing is the result of the previous calls. They are not related to each other.
I just wanted to oversimplify things and the bind() not resolving actually saved me from conflating a pure function call and a side effect.