Don't Give Up Your PHP Code for Compiler Passes so Easily

This mini-series started in Why Config Coding Sucks. There we learned to move ~~weakly~~ un-typed strings to strict-typed PHP code. It's not only about YAML or NEON files, but about any config-like syntax in general (XML, in...).

Today we move to PHP-only land, that suffers a similar problem.

What We Talk About?

So we talk about Compiler Passes in Symfony? Well, yes and no. Not only about them, but about any PHP code that moves around services in the DI container.

• in Nette it's a class that extends CompilerExtension
• in Symfony it's class that implements CompilerPassInterface or extends Extension
• in Laravel it can be service providers

They have their useful use-cases, but people tend them to use as a bazooka to mouse. Just look at answers under this StackOverflow question.

Let's look at an example that is not far from the reality of your work with. But still it's only an example, it could be apples in a basket instead.

Make Price Calculation easy to Extend and Maintain without Changing it

Based on my experience with my clients, this is the biggest problem in e-commerce projects. The ideal wishes of company owners clash with limits programmers and architecture:

• "The price calculation must be ready for use"
• "I need to add different price to product B if they're in combination with product A"
• "It must be easy to maintain"
• "The business must be able to update dependency on our code"
• "We can't predict how the price will develop"

~~This not possible!~~ - How can we do it as close as possible now?

Let's say the solution is fairly easy. Same as Voters are to Security, we introduce 1 service PriceCalculator that collects all the little one PriceModifierInterface.

How would such implementations look like in framework-way?

1. In Symfony

<?php

namespace App\DependencyInjection;

use Symfony\Component\DependencyInjection\Compiler\CompilerPassInterface;
use Symfony\Component\DependencyInjection\ContainerBuilder;

final class PriceCompilerPass implements CompilerPassInterface
{
    public function process(ContainerBuilder $containerBuilder)
    {
        $priceCalculator = $containerBuilder->get(PriceCalculator::class);

        foreach ($containerBuilder->findTaggedServiceIds('price_modifier') as $service => $tags) {
            $priceCalculator->addMethodCall('add', [new Reference($service)]);
        }
    }
}

Again, we need to create some legacy code that is hard to maintain:

• add tagging in extension/bundle or better type resolution ❌
• register this in Kernel ❌
• remember the tag name (don't remember anything/) ❌
• use the tag everywhere (YAML) ❌
• vendor-lock the metadata in the config (YAML) ❌
• people maintaining project after you leave have to learn this Symfony way ❌

2. In Nette

<?php

namespace App\DI;

use Nette\DI\CompilerExtension;

final class PriceExtension extends CompilerExtension
{
    public function beforeCompile()
    {
        $containerBuilder = $this->getContainerBuilder();
        $priceCalculator = $containerBuilder->get(PriceCalculator::class);

        $priceModifiers = $containerBuilder->findByType(PriceModifierInterface::class);
        foreach ($priceModifiers as $service) {
            $priceCalculator->addSetup('add', [$service]);
        }
    }
}

Also, we create legacy code that is hard to maintain:

• register extension to a config (Neon) ❌
• vendor-lock the metadata in the config (Neon) ❌
• people maintaining project after you leave have to learn this Nette way ❌

I need to take a break, my brain is tired just by making up this complicated and non-sense code. I mean, I used to write this code in my every project for 5 years in Symfony and Nette projects, because it was "the best practice" and I didn't question it, but there was always something scratching in the back of my head.

Keep Simple Things Simple

Now imagine you've ended in a train crash, hit your head and forget all the frameworks you know. All you have left is actually the best you can achieve in any mastery - a mind of the beginner.

• "How would you get all services of a certain type in a certain order or key name?"

In our specific example:

• "How would you get all PriceModifierInterface services into PriceCalculator sorted by priority?"

<?php

final class PriceCalculatorFactory
{
    /**
     * @var PriceModifierInterface[]
     */
    private $priceModifiers = [];

    /**
     * @param PriceModifierInterface[] $priceModifiers
     */
    public function __construct(array $priceModifiers)
    {
        $this->priceModifiers = $priceModifiers;
    }

    public function create(): PriceCalculator
    {
        $priceModifiersByPriority = [];
        foreach ($this->priceModifiers as $priceModifier) {
            $priority = $priceModifier->getPriority(); // this could be "getKey()" or any metadata
            $priceModifiersByPriority[$priority] = $priceModifier;
        }

        // sort them in any way
        ksort($priceModifiersByPriority);

        return new PriceCalculator($priceModifiersByPriority);
    }
}

In some framework we have still have to add 1 config vendor-lock ❌ :

services:
    App\Price\PriceCalculator:
        factory: ['@App\Price\PriceCalculatorFactory', 'create']

I use compiler pass for now, but if you know how to remove it, let me know.

How we get $priceModifiers is not that important now, it's an implementation detail.

Durable & Readable

The important thing is we got a code that:

• is strictly typed ✅
• we all understand it ✅
• will not be affected by any syntax/architecture changes in our favorite framework ✅
• can be checked by coding standard tools ✅
• can be analysed by static analysis tools ✅
• and refactored by instant upgrade tools ✅

Happy coding!