Advent of Code 2021

today's challenge asks us to interpret a list of numbers. In the first part, our goal is to determine how many elements in the list are larger than their immediate predecessor.

The imperative way would be to toss it in a `foreach()` loop and track some state along the way. But we want to be functional and avoid "track some state," because the whole point of functional programming is to avoid tracking state, as tracking state is error prone.

When looking at a foreach-style operation that has some state, my first inclination is to look at a reduce operation. A reduce operation walks over a list and performs the same operation (function) on each item, using the output of the previous iteration as an input. That is, each step takes the output of the previous operation and the next element, and produces an output. It's quite elegant.

I am planning to participate in Advent of Code this year. For those not familiar with it, it's a daily coding challenge that runs through December, until Christmas. Mostly it's just for fun, but some people take it as an opportunity to either push themselves (by solving the puzzles in a language they're unfamiliar with) or to show off some feature of a language they like, which they then blog about.

In my case, I'll be solving puzzles in PHP, of course, but specifically using functional techniques. My goal is to demonstrate how functional programming in PHP is not just viable but creates really nice solutions. At least, I hope it works out that way; I haven't seen any of the challenges yet. :-)