Higher-order Observables

When talking about the prerequisites for functional programming we mentioned higher-order functions. These are functions that return other functions. A very similar concept is applied in RxPHP as well, when using Observables.

A higher-order Observable is an Observable that emits other Observables. To illustrate how higher-order Observables differ from first-order Observables, consider the following simple example:

// higher_order_01.php 
use Rx\Observable; 
Observable::range(1, 3) 
    ->subscribe(new DebugSubject()); 

This example just prints three values and completes as expected. This is what we expect from any first-order Observable:

$ php higher_order_01.php 
22:54:05 [] onNext: 1 (integer)
22:54:05 [] onNext: 2 (integer)
22:54:05 [] onNext: 3 (integer)
22:54:05 [] onCompleted

Now, we can make this more complicated by adding the map() operator that, instead of returning an integer, returns another Observable:

// higher_order_02.php 
...