Sensible Interfaces

How did we get from 'Program to an interface, not to an implementation' to 'Just slap an interface on there, it's the fashionable thing to do'?

Published on 24 September 2013 by @mathiasverraes

Here are some guidelines to help you live in friendly cohabitation with your fellow rockstar ninja code monkeys. I’ll update them as my ideas about the matter evolve.

Naming

In my team, suffixing your interface with the word Interface, is a sackable offence. Well, maybe it’s not that bad, but don’t bother trying it. The same goes for prefixing it with I – looking at you Microsoft. Here’s the reasoning.

If you have an interface, then you are suggesting that multiple implementations of that interface are possible. Usually, when you see something like TranslatorInterface, chances are there’s an implementation called Translator implements TranslatorInterface. It makes me wonder: what makes the Translator so special, that it has the unique right of being called Translator? Every other implementation needs a descriptive name, such as XmlTranslator or CachedTranslator, but that one is somehow “the default”, as suggested by it’s preferential treatment in being named Translator without a description.

Is that bad? I believe it is. It confuses people, as they tend to misunderstand whether they should typehint for Translator or TranslatorInterface. So both are being used in the client code. Program sometimes to an interface, sometimes to an implementation?

Just as bad, is that it suggests that the implementation is the real thing, and the interface is a label. It should be the other way around. Look at it from the point of view of the client code:

<?php
class KlingonDecoder {
   public function __construct(TranslatorInterface $translator)

This constructor definition is saying: “I need a translator interface to operate”. But that would be silly. It needs an object that is a Translator. It does not need an interface. And that object has a certain role, a certain contract, namely that of a Translator. I hope I’m making this clear. The interface Translator is the essential concept, the thing that clients use. They don’t care whether Translator is a concrete class or an interface, and they don’t care how it’s implemented. The client wants to be decoupled from all those details. That’s the power of interfaces.

Default implementations

The burden of having a descriptive name then lies with the implementations. If we rename TranslatorInterface to Translator, our former Translator class needs a new name. People tend to solve this problem by calling it DefaultTranslator. Again, what makes it so special to be called Default? Don’t be lazy, think really hard about what it does, and why that’s different from other possible implementations. You might even discover a thing or two about that class, such as having too many responsibilities.

Nameable

Another bad habit, is using the -able suffix for interface names. I guess I can live with something like Translatable, or maybe Serializable. But Timestampable? Jsonable`? Is that the world we want our children to inherit? English motherfucker, do you speak it? Try making a sentence, it’s so much nicer.

<?php
class Product implements CastsToJson, HasTimestamp

Say it out loud: “Product casts to json and has a timestamp”. It’s beautiful, it’s – dare I say it? – Shakespearian.

Respect the contract

PHP, having grown organically (to put it politely), is rather permissive when it comes to interfaces. Look at this code:

<?php
interface Animal {
    public function makeNoise();
}
class Dog implements Animal {
    public function makeNoise() {}
    public function fetchStick() {}
}
// elsewhere:
public function myClient(Animal $animal) {
    $animal->fetchStick();
}

Even though myClient() accepts an Animal, and should have no knowledge whether the $animal is a Dogb, PHP allows you to call fetchStick() if $animal has a method by that name. That flexibility can be very useful, in a very limited set of cases. For all normal cases: never call a method on an object that is not part of the interface you are typehinting for.

Interface segregation

If you find that your client code depends on an interface with many methods that your client doesn’t care about, your interface may be too big. The same is true if your classes implementing an interface, have a lot of unused stub methods:

<?php
class Fish implements Animal {
    public function makeNoise() {
        throw new NotImplemented("Fish don't make noise");
    }
}

This is a good sign that you need to split off the makeNoise() method into a separate interface. Perhaps MakesNoise, or Noisy?

Roles

Interfaces can be a nice way to share code without the client knowing, by seeing them as roles. Say the product prices are in a database. You have some logic in OrderBuilder, but you don’t want OrderBuilder to know that the prices are in the database, because that might change in the future. You could solve this with composition.

<?php
interface ProductRepository {
    /* defines find(), add()... */
}
interface ProductPricer {
   public function priceProduct(Product $product);
}
class DbProductPriceRepository implements ProductRepository {
    /* implements find(), add()... */
}
class DbProductPricer implements ProductPricer {
    public function __construct(ProductRepository $productRepository){ /* ... */ }
    public function priceProduct(Product $product) { /* ... */ }
}

To save a bit of typing, you can give the ProductPricer role to DbProductPricer. Sure, it’s doing double duty, but the clients don’t know that. In this example, it’s probably not a good long term solution when your application grows, but if nothing else, it’s a great timesaver when prototyping.

<?php
class DbProductPriceRepository implements ProductRepository, ProductPricer {
    /* implements find(), add()... */
    public function priceProduct(Product $product) { /* ... */ }
}

Roles of course work great with entities:

<?php
class Teacher implements User {}
class Pupil implements User {}
class Parent implements User {}

One implementation

ProductPricer is a great use case for an interface, because it’s easy to imagine different business rules applying in different circumstances: GermanProductPricer, BelgianProductPricer. There could be different technical implementations: DbProductPricer, SoapProductPricer, or a CachedProductPricer that wraps one of the other implementations.

But often, it’s less clear. If your business has only One True Way to calculate prices, and One True Datasource to store them. My rule of thumb would be that if you can imagine that there could be more implementations than just the one, it’s good to have an interface. If you can’t imagine different implementations, don’t have an interface. An example could be OrderTotalCalculator: There’s only one valid way to sum the different prices, so an interface does not make sense.

Aware

I’m sort of undecided about the Aware suffix. I don’t think it’s a problem per se. But of course ContainerAware is evil and should never have been in Symfony. But that is a Dependency Injection anti-pattern, not an interface naming problem. Thoughts?

Read next: Interface discovery with PHPUnit’s Mock objects - March 21, 2011

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