Boundaries in Object Oriented Design - Made Tech Blog
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Boundaries in Object Oriented Design

As responsible programmers we like to write programs to the best of our ability. As our profession has evolved so too have our languages and the tools we use. We identify around concepts, patterns and principles.

Today I want to explore 4 common programming concepts and highlight how they can help to draw boundaries within your application to make it easier to maintain. I will illustrate how they relate to each other and then draw conclusions on how they can work together to build better programs.

The four concepts I am talking about are:

  • Data
  • Behaviour
  • Inheritance
  • Functional composition

Data and Behaviour

Object oriented languages, such as Ruby, focus on structuring programs as objects. These languages bring together data and behaviour or rather fields and methods into one unit, the object.

A traditional way to structure the idea of a bank account in an OOP language would be to use a single objectBankAccount. Before going straight into this example, let's first think about a bank account. It has a balance which is a number that represents how much money is held within the account. You can also perform actions or behaviours on the bank account, such as withdrawing and depositing money from and to it. In an object, the balance would be data, and the actions of withdrawing and depositing would be methods.

In Ruby, a bank account might look something like this:

In this example we have the @balance field and three methods #initialize, #withdraw and #deposit. The balance is a number that can be increased or decreased by withdrawing or depositing an amount into the bank account.

Let's use the example above to do some withdrawing and depositing.

In this usage example we assign an instance of the BankAccount class to a variable, lukes_account, and then perform actions upon the account.

You can see why object oriented languages are so attractive. The ability to think of real world objects as objects in your programming language is useful for having a mental model of your program. Languages like Smalltalk took this a lot further, providing an IDE for the language which literally draws the objects and their relationships with each other on the programmer's screen.

So data and behaviour: both useful concepts traditionally encapsulated together in one object.


Inheritance is another common object oriented concept. By using hierarchy behvaiour can be shared and adapted. This hierarchy is declared by one class extending another.

Coming back to the bank account example, you might use inheritance to convey the relationship between a child's bank account and a standard one. The child's account may have a lower limit on the amount of money that may be taken out at one time.

First of all let's apply a limit to the standard account:

And now let's extend our bank account to represent a child account with a lower limit:

Here our ChildBankAccount extends BankAccount and overrides the #initialize method to change the account limit after calling the original implementation of #initialize with #super.

Immediately you see the continued attractiveness of object oriented languages. You can represent classes of things and their hierarchy. Descendants share properties with their ancestors but can also diverge from their data and behaviours.

Functional composition

Functional composition is often a foreign concept to object oriented programmers, and rightly so, as it's a functional language concept.

Function composition breaks away from the idea of keeping behaviour in one real world object. In fact, in functional languages, behaviour is contained within functions only. The composition of these functions then build up the behaviour of the program. Going back to our bank account example, instead of having the behaviour within the bank account object you would instead have other objects representing the actions that can be taken on a bank account.

In our Ruby example we might reduce our bank account object to have just its initialize and balance accessor:

The BankAccount becomes a container for data only. No longer is the bank account in charge of withdrawing and depositing. Its role now is to hold the balance and withdraw limit of the account.

We should also create our child bank account whilst we're at it:

Nothing new here but it's worth noting this ChildBankAccount is behaviourless and just a container for data likeBankAccount.

So we have our data described, what about our behaviour? To represent the withdraw and deposit behaviour we need two new composable objects:

Both Withdrawer and Depositor do not store any data and instead only act on data given to them as arguments. Lonesome, standalone behaviour. Your eyebrows may well be raising but let me run the examples course a little longer.

In order to enact on the BankAccount object we would have to use the objects in concert:

Now we are functionally equivalent to our pure inheritance example. The example admittedly involves more objects than before.

To fully explore functional composition, we need to actually compose something. One real life example would be the act of transferring money from one account to another. The act of transferring money involves withdrawing from one account and depositing in another.

Representing the transferral of money in Ruby with functional composition would probably look something like this:

We have now composed two functions into a third function. That third function is now more complex than the other two, but looks simple since it delegates out to well named pieces of functionality.

Now we can use the Transferrer object to move money from my account to that of a friend:

The Transferrer object sums up the power of functional composition. It isn't a crescendo of a summary, but you have to take a step back a think about it.

We have composed two units of work into a third larger concept. This kind of composition gives us the ability to write small units of work and compose them together into bigger ideas. We can continue to stack concepts on top of one another to create an entire application.

Unifying the two worlds

Unless you're a functional flirter or fanatic you may not yet see the advantages of using composable functions over wrapping larger concepts in objects.

If you're a Rails programmer you will have seen some rather large objects in your time. You'll know that classes become harder and harder to reason about the more lines they have. Ask yourself for a moment:

Why are objects with many methods harder to reason about?

The answer to this question lies in the boundaries or lack thereof in large objects. Remember, objects are encapsulations of ideas. The greater that idea gets the harder it will eventually be to grasp. The more data an object is responsibile for, the more behaviour it has, the harder it is to fit into your brain.

Functional composition is a remedy for when things get too tough. I wouldn't argue you should build your Rails apps with everything split out into tiny units of work, I would argue that when things get tough functional composition can make things easier for you.

The more data an object has, the more moving parts, the harder it is to get under test. If it's hard to get under test, it's probably hard for your head to think about too. It's a sign that you've pushed your object too far.

Let's go back to our example one more time. I want to implement the transfer functionality without composition:

As our object gets larger and, let's be honest, this object isn't that large yet, it will have an increasing amount of responsibilities. The more responsibilities the more edge cases that object may have. The more complicated your tests will be to setup and tear down.

Let's use our remedy to keep the same interface above, but move our logic out into composable units that can be individually tested away from the BankAccount:

We have fused our two ideas together. Now we have one object, a BankAccount that still encapsulates all the behaviours associated with a bank account. However we've drawn some boundaries. We now have other objects responsible for carrying out the behaviour. These composable units only accept data via their method parameters, no other complexity can travel through into them.

Lines are important for our mental well being as programmers. Knowing when and where to draw a line can help you reason about your program that much easier.

When you draw a line with an interface you now have two separate concerns that are individually testable. Not only that but your mental model only has to concern itself up to the line. This allows you to mentally step through your program a lot better than if everything were in one file.

Without these lines both you and your program are essentially evaluating in a global scope. We all know how bad globals are, don't we?

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