Functional programming

Advent of Functional PHP: Day 5

Submitted by Larry on 5 December 2021 - 9:23pm

After the last two days, Day 5 of Advent of Code is almost mundane in comparison. Today we're asked to read in the definition for a series of lines and compute how many times they intersect.

The process is much the same as the previous days: Parse the incoming data into some sort of data model, then run some computations on it. And both parts will consist primarily of `pipe()` operations, since we're really just shuffling data from one form to another.

Our input data looks like this (albeit with a much larger range of coordinates):

Continue reading this post on PeakD.

Advent of Functional PHP: Day 4

Submitted by Larry on 4 December 2021 - 7:41pm

Day 4 of Advent of Code has us playing bingo against a giant squid. (Don't ask; I don't understand it either.) More specifically, we want to take an input file that consists of a series of numbers that will get called, followed by a series of boards. We then need to compute which board will be the first to win, following the standard rules of bingo (although with no free space in the middle, the cheating squid...).

This sort of problem is inherently very stateful, and thus, frankly, not a good fit for functional code. It absolutely can be done in a functional way, but it's not the best fit. We're not interested in the best fit in this series, though, just how it could be done functional-style. So let's do it functional style just to say we did. Along the way we will really exercise the function composition concept, and show a few other tricks along the way.

Onwards!

Advent of Functional PHP: Day 3

Submitted by Larry on 4 December 2021 - 6:15pm

The third challenge in this year's Advent of Code is all about bit manipulation. We're asked to read in a series of binary numbers and interpret them in various entirely illogical ways as a form of diagnostics. (Incidentally, if you ever write a system that requires this kind of logic to debug its output, you're fired.)

In any case, we're given a file with a list of 12 digit binary numbers and asked to compute various values. In the first part, we are asked to find the most common bit (0 or 1) in each position, and the result is known as "gamma." Then we have to find the least common bit in each position, and the result is known as "epsilon." (I don't know why you would want to do this; it's all Greek to me.)

Advent of Functional PHP: Day 2

Submitted by Larry on 2 December 2021 - 11:46am

In today's challenge, we're asked to interpret a series of basic command lines from a file and update the position of our submarine accordingly. It's basically a graph walk of sorts, with instructions of up, down, and forward. (Apparently you cannot go backward, as in life.)

As with yesterday's challenge, we could do it imperatively with a foreach() loop and a couple of state variables floating around, but that conflates a whole bunch of different behaviors into one blob of code. We don't want to do that, so let's step back and consider the problem more clearly.

Advent of Functional PHP: Day 1

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.

Larry 1 December 2021 - 11:24am
Advent of Code 2021: Functional PHP

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. :-)

Larry 29 November 2021 - 6:45pm

The case for partials and pipes in PHP

Submitted by Larry on 23 June 2021 - 3:07pm

The Partial Function Application RFC is currently in voting, and right now it's a close vote to the negative. I wanted to take this opportunity to try and make the broader case for partial application and for its related RFC, the pipe operator, in a way that is more appropriate for a blog post than the RFC body (which is, by design, more concerned with the finer details of "what").

The main pushback on the RFC so far is that the benefits don't outweigh the cost of yet-more-syntax in the language. Which is a fair position to hold, albeit one I hope to convince you is incorrect. That is, I believe the benefits vastly outweigh the syntax and implementation cost.

Continue reading this post on PeakD.

One year of functional PHP; now in Russian!

Submitted by Larry on 11 May 2021 - 8:14pm

A year ago, I published my first solo book, Thinking Functionally in PHP. The reception has been extremely positive; almost everyone that's read it (that has bothered to talk to me about it) has found it clear, helpful, and enlightening. Mission accomplished!

To celebrate the one year anniversary of the book's publication, I am happy to make two announcements.

First, Thinking Functionally in PHP is now available in Russian! The translation is by Alexey Pyltsyn, who is responsible for the Russian translation of the PHP documentation as well as numerous other tech book translations.

Byte-sized functional programming: Composition over inheritance for functions, too

Submitted by Larry on 15 August 2020 - 10:07am

A popular refrain in object-oriented code is to favor object composition over inheritance. It offers more flexibility, less overhead, and ends up being easier to reason about. The same concept applies to functions, too!

A common pattern is to have a function that does some work and then calls another function, which does some work and calls another function, and so on. The problem is that the first function then cannot be used or tested without the entire chain of other functions it calls. This is the function equivalent of "inheritance."

Instead, we can compose functions, that is, pipe them together. Instead, take the output of the first function and pass it to the second, then take the second's output and pass it to the third, etc. That way, each of the functions can be reused, tested, and understood in isolation, then we can stick them together like LEGO blocks to build whatever series of steps we want.

That is, instead of this:

<?php
function A($in)
{
  
// ...
  
return B($out);
}

function

B($in)
{
  
// ...
  
return C($out);
}

function

C($in)
{
  
// ...
  
return $out;
}
?>

Structure it like this:

<?php
function A($in)
{
   
// ...
   
return $out;
}

function

B($in)
{
   
// ...
   
return $out;
}

function

C($in)
{
   
// ...
   
return $out;
}

function

doit($in) {
   
$out = A($in);
   
$out = B($out);
   
$out = C($out);
    return
$out;
}
?>

Now `A()`, `B()`, and `C()` are all easier to read, understand, and test, and we can more easily add a step B2 or D if we want. So powerful is this concept that many languages have a native operator for piping functions together like that. PHP doesn't, yet, but it's straightforward enough to do in user space anyway.


Want to know more about functional programming and PHP? Read the whole book on the topic: Thinking Functionally in PHP.


Thinking Functionally in PHP

Byte-sized functional programming: Filter first

Submitted by Larry on 3 August 2020 - 1:47pm

Often when working with a list, we only want to work with a subset of a list that meets some criteria. All non-zero values, for example, or all users that have a given role. The procedural way to do that is to stick an if statement inside a foreach loop:

<?php
foreach ($list as $value) {
    If (!
meets_criteria($value)) {
        continue;
    }
   
// ...
}
?>

That mixes up the filtering with the iteration, though. It also doesn't work if we're using `array_map()`.

Instead, we can make stripping down the list a separate operation called "filter." PHP offers the array_filter() function for that purpose.

<?php
$criteria
= fn(User $user): bool => $user->hasRole('moderator');

$filtered = array_filter($users, $criteria);
?>

Now we can work with the `$filtered` list, which has only the values we want. That could be a simple foreach<code> loop, or, better, it's now ideally suited for use with <code>array_map().


Want to know more about functional programming and PHP? Read the whole book on the topic: Thinking Functionally in PHP.


Thinking Functionally in PHP