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.
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
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
CachedTranslator, but that one is somehow “the default”, as suggested by it’s preferential treatment in being named
without a description.
Is that bad? I believe it is. It confuses people, as they tend to misunderstand whether they should typehint for
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:
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
I hope I’m making this clear. The interface
Translator is the essential concept, the thing that clients use. They don’t care
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.
The burden of having a descriptive name then lies with the implementations. If we rename
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.
Another bad habit, is using the
-able suffix for interface names. I guess I can live with something like
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.
Say it out loud: “Product casts to json and has a timestamp”. It’s beautiful, it’s – dare I say it? – Shakespearian.
PHP, having grown organically (to put it politely), is rather permissive when it comes to interfaces. Look at this code:
myClient() accepts an
Animal, and should have no knowledge whether the
$animal is a
Dogb, PHP allows you to call
$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.
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:
This is a good sign that you need to split off the
makeNoise() method into a separate interface. Perhaps
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.
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.
Roles of course work great with entities:
ProductPricer is a great use case for an interface, because it’s easy to imagine different business rules applying in
BelgianProductPricer. There could be different technical implementations:
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
There’s only one valid way to sum the different prices, so an interface does not make sense.
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
Follow @mathiasverraes on Twitter.
|Advanced Domain-Driven Design||DDD Europe||workshop||Brussels, Paris||2018|
|Design Heuristics||DDD eXchange||keynote||London||April 2018|
|DDD for Messaging Architectures||ExploreDDD||workshop||Denver||Sep 2018|
|Design Heuristics||Kandddinsky||talk||Berlin||Oct 2018|
|Tactical DDD||Kandddinsky||workshop||Berlin||Oct 2018|
This work by Mathias Verraes is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 License.