A feature that often irks me in object-oriented code is the prevalence of useless interfaces. Interface isn’t meant literally here: this applies to traits of Rust/Scala and protocols of Clojure as well.

The advice of planning for the interface is just and solid, but people tend to follow this tip to the extreme. It is not uncommon to see people design a module or class by defining its interface first, with an actual implementation following later.

One eagerly designs an interface as follows:

trait Mediator {
  def validate(r: Request): Boolean
  def postpone(r: Request, d: Duration): Postponed[Request]
  def frobnicate(a: Request, b: Request): Frobnicated[Request]

Then, the implementation, in a class called MainMediator or DefaultMediator, in a separate directory, implements the Mediator interface:

class MediatorImpl extends Mediator {
  def validate(r: Request): Boolean = { ... }
  def postpone(r: Request, d: Duration): Postponed[Request] = { ... }
  def frobnicate(a: Request, b: Request): Frobnicated[Request] = { ... }

Dependents of the Mediator trait then naturally get their dependency provided with a constructor argument:

class Resequencer(m: Mediator, c: Clock) {
  def resequence(requests: Seq[Request]): Seq[Postponed[Request]] = 
    requests map { r => m.postpone(r, c.randomDelay()) }

val m = Mediator()
val c = Clock()
val foo = new Foo(m)

This pattern is older than the sun, and has been a characteristic of modern, inheritance-based object-oriented programming for ages. Fundamentally, it is alright to separate implementation from specification, but where it goes wrong is the overuse of this paradigm, or when this separation is superfluous.

This separation is superfluous when it serves no purpose. You could as well call it dependency injection on the toilet. There is no fundamental reason why a class or module like Mediator warrants an interface when it is likely that there will never be an alternative implementation.

At a glance, Guy Steele’s influential plan for growth idea from his “Growing a Language” talk seems to contradict what I just said. Shouldn’t defining an interface help us plan for future, alternative implementations of the Mediator? Perhaps a different kind of a Mediator?

Removing the Mediator trait and simply renaming its only implementation will still keep the code working, with the benefit that there are fewer lines of code now, and it isn’t any harder to extend.

This is actually more in line with Steele’s idea. It doesn’t say anywhere a trait or interface cannot be distilled from a set of basic implementations. In other words, when our intuition says to prepare for repetition, we should identify them. The Gang of Four book was never a recipe book for building great programs. It was a catalog! They observed several kinds of large-scale systems and programs and extracted repetetive behaviours in the code, patterns. They never said that to do things right, one ought to use the visitor pattern, or this other pattern, otherwise your programs will be bad.

Back to interface distillation. Programming is about getting rid of repetition. The more experienced the programmer, the better they get at noticing patterns of repetition. The downside is that this may also lead to overengineering for repetition.

So, an experienced programmer thinks, this behaviour I have specificed may be repetitive, let me first create a construct that lets me share the repetition (an interface), and then proceed with the implementation. This is fine if the actual code is known to be repeated, but by seeing interfaces as a hammer and every bit of code as a nail, you will soon bury yourself in pointless dependency injection scenarios.

It may be just as easy to first create a base implementation and once you must duplicate its behaviour, only then create the abstract implementation. You might actually need to spend less total time wiring up the interface, since you observed the repetition. Creating an abstract implementation first always involves a deal of speculation and this is not reliable.

The more experienced programmer understands that you don’t always need to plan for repetition. In fact, repetition is good sometimes. Not every shared functionality needs to be extracted to its own module, because sometimes, shared dependencies will be bad.

The approach I suggest is to in order to produce modularity as a side effect, structure your program into small, reusable pieces. Don’t create huge, monolithic interfaces. Functional programming shows us that dependency injection can be done just by passing a function.

class Foo(postponer: Request => Postponed[Request], delayer: =>Duration) {
  def resequence(requests: Seq[Request]): Seq[Postponed[Request]] = 
    requests map { r => postponer(r, delayer()) }

// ...
val m = Mediator()
val c = Clock()
val foo = new Foo(m.postpone, c.randomDelay)

One sees that a higher-order function like the above can be just as well represented by a trait with a single method. If you only need that method, you should depend only on that. With a structural type system it is easy to decompose types. An alternative is to stack traits, and in languages like Scala this is fairly easy. You could as well decompose Mediator into Validator, Postponer, et cetera, ideally interfaces should be fairly homogenous in their purpose: if your interface defines methods for reading, keep that interface separate from the writing interface, and if you need read and write just compose the interfaces together, and so on.

It also helps if your language is powerful enough to do without excessive DI – the reason why horrors like Spring exist is that Java simply wasn’t expressive enough to do dependency injection the traditional way without setting your hair on fire. That, and for some odd reason, people thought writing constructor arguments was so painful it warranted a gigantic framework for it.

Overall, it’s usually a good idea to toy around first with the concrete, and then extract the abstraction. Going the other way around is a dangerous swamp. It’s certainly something I’ve used to do – overengineer for patterns a priori – but I found better results by getting my hands dirty, by writing repetitive code first and then cleaning it up.


If you have any comments or questions, feel free to email me. Comments or questions may be posted as updates.