Error Handling

Terminology

• positive value - successfully function/process result
• recovery value - positive value produced by error recovery process
• error value - value describing some error

error value can not be positive value at the same time.

Error passing

To express that function can result in an error you should return Either<Error, V> type. Error should be conventionally on left.

UseCase

UseCases should return either GenericError or some custom sealed class inheriting ErrorType and Throwable.

Logging

Errors should be logged at the positive value consumption point.

For example if you are loading some data for UI in controller you should log error at the point where you extract concrete value to set into state.

Handling

Recovery

No recovery policy specified, just show error to user.

Display

1. Pass error via viewmodel to UI layer.
2. Display error via ErrorDialog

class MyViewModel(...) {

    init {
        state = when (val data = getData(id).classify(MyClass::class)) {
            is Either.Left -> state.copy(error = data.value)
            is Either.Right -> state.copy(value = data.value)
        }
    }

    fun hideError() {
        state = state.copy(error = null)
    }
}

@Composable
fun MyComposable(vm: MyViewModel) {
    ErrorDialog(vm.state.error) {
        vm.hideError()
    }
}