Four or five years ago, the most popular talk that I gave at conferences was entitled “A Functional Guide to Cat Herding with PHP Generators” (the cats proved a very enjoyable talking point at every event where I gave it, because who doesn’t enjoy a technical talk featuring cats); and I even wrote a blog post here about that self-same topic. That presentation described how I was building up enormous volumes of GPX tracking data showing where my cats roamed each day. In order to analyse that data without using equally enormous amounts of memory, I read it directly from the GPX files, using Generators to process each trackpoint one at a time; and and took a map, filter, reduce approach to resolving the questions I wanted to answer about my cats movement habits. Because PHP’s standard array_map(), array_filter() and array_reduce() functions only work with arrays of data, which are expensive in terms of their memory usage, I wrote my own versions of those functions to work with Traversable objects (like Iterators and Generators) as well as with arrays. And the technical body of the talk was how I wrote and used my versions of these functions.
I chose to write the functions myself as a learning exercise, to better understand how they (and Traversables like Generators) worked; but for map and filter, I could just as easily have used the Iterators that already existed in the Standard PHP Library (SPL).
