/** @return a [ErrorStateGenerator] created by merging 2 [ErrorStateGenerator] lambdas */
private fun mergeErrorStateGenerators(
    first: ErrorStateGenerator,
    second: ErrorStateGenerator
): ErrorStateGenerator = lambda@ { throwable ->

    val firstError = first( throwable )
    if ( firstError !is ViewState.DefaultError ) return@lambda firstError

    val secondError = second( throwable )
    if ( secondError !is ViewState.DefaultError ) return@lambda secondError

    default
}

/** @return [ErrorStateGenerator] from 2 [ErrorStateGenerator] */
operator fun ErrorStateGenerator.plus( other: ErrorStateGenerator ) = mergeErrorStateGenerators( this, other )