In a new post to his site Sergey Zhuk takes a look at generators in PHP and shows how they're integrated into coroutines and promises to help with the chunked asynchronous processing.

Generators become available in PHP since version 5.5.0. The main idea is to provide a simple way to create iterators but without creating a class that implements Iterator interface. A generator function looks like a normal function, except that instead of returning the result once, a generator can yield as many times as it needs to in order to provide the values to be iterated over.

[...] So, let’s wrap up what we know about generators:

• interruptible/resumable functions
• use yield rather than (or in addition to) return
• values/exceptions can be send/thrown into
• behave like iterator

All these generator features are used in <a href="https://amphp.org/>Amp to implement coroutines.

The post starts out with a pretty detailed look at generators in PHP and the functionality they offer. It covers sending values into the generator, exception handling and the use of the yield statement. It then moves over to describing coroutines, promises and deferred handling. Generators make it easier for them to chunk up the operation one piece at a time rather than requiring all data up front.