Structure and Interpretation of Computer Programmers

On receiving a message with a parameter, sometimes an object just reverses the sense of what just happened and sends another message to the parameter object with itself as the parameter of this message. That’s a pretty hard sentence to read, but I couldn’t work out how to rewrite it, so here’s an example. A Smalltalk class might have a method like this:

Object>>#printOn:
printOn:aStream
  "Append to the argument, aStream, a sequence of characters
  that describes the receiver."
  |description|
  description := self description.
  aStream nextPutAll:description.

I would do this as a matter of design: I can make it clear that I’m messaging one object and using another simply in a supporting role. It takes this:

foo doTheThing.
aStream nextPutAll:foo description.

and turns it into this:

foo doTheThing;
    printOn:aStream.

In Objective-C, this pattern has an additional use due to the nil sink for messages. Imagine formatting a string and appending the result to another string, but if the initial string is nil then you don’t want to add anything. It might look like this:

if (emailText) {
  NSString *quotedString = [NSString stringWithFormat:@"> %@", emailText];
  [output appendString:quotedString];
}

because I don’t want the phrase > (null) to appear in the output. But, with a bit of category magic, we can avoid the condition and make it clear that the email text is the important object.

[[emailText quote] appendTo:output];

I haven’t just hidden the conditional; it never appears. If emailText is nil then all the other messages will just get dropped, so neither quote nor appendTo: need to test for nil.

Sometimes, the messages are just flowing the wrong way, and clarity can be gained by reversing the polarity.

[Update: oops! I had read Kent Beck’s “Smalltalk Best Practice Patterns”, and apparently absorbed his Reversing Method pattern. This post is basically a discussion of that pattern, and credit should go to Kent for originally creating it. My addition is the Objective-C specific nil sink extension. Sorry, Kent.]

Everybody knows that the best way to sound intellectual and demonstrate the superiority of your approach to that thing you do is to wrap it in a fancy-schmancy noun term. This works particularly well with a term that can be expressed as a three-letter acronym, or TLA. In programming we have a whole raft of different [A-Z]DD disciplines, the UML, CRC, and your DBA probably does SQL. It also explains why functional programming has taken so many decades to take hold: FP is 33% less appealing as a mere acronym of two letters (or ATL).

When your term is sufficiently arcane or abstract, you don’t even need to actually do the three-letter nouny thing. Just tell people that what you’re doing is the three-letter nouny thing, and let everybody fill in the gaps. In fact, they’ll probably agree that not only are you doing that thing but then so are they. Soon, everybody will be doing the nouny thing.

So it goes with Object-Oriented Programming. We can construct all sorts of definitions for Object-Oriented Programming the term, so that everybody can get in on this highfalutin concept without having to do anything. The best time to arrange for this wide collection of meanings is as early as possible. Let’s look at August 1981.

1. It’s all about messages.

The Smalltalk-80 system represents the current state of the object-oriented point of view as it has been reduced to practice by the Xerox Learning Research Group. The Smalltalk-80 system is composed of objects that interact only by sending and receiving messages. The programmer implements a system by describing messages to be sent and describing what happens when messages are received.

(“The Smalltalk-80 System”, Dave Robson and Adele Goldberg, BYTE Volume 6 Number 8 p.36)

2. Except when it’s not.

SIMULA allows users to create object-oriented systems, but uses the standard data/ procedure-oriented ALGOL language to provide numbers, booleans, basic data structures, and control structures.

(op. cit.)

3. Or maybe it is.

Instead of two types of entity that represent information and its manipulation independently, an object-oriented system has a single type of entity, the object,
that represents both. Like pieces of data, objects can be manipulated. However, like procedures, objects describe manipulation as well. Information is manipulated by sending a message to the object representing the information.

(“Object-Oriented Software Systems”, Dave Robson, BYTE Volume 6 Number 8 p.78)

4. Actually, maybe it means whatever you want it to mean.

The words “object-oriented” mean different things to different people. Although the definition given in this article may exclude systems that should rightfully be called object-oriented, it is a useful abstraction of the idea behind many software systems.

(op. cit., p.74)

5. But it definitely means garbage collection.

To be truly “object-oriented”, a computer system must provide automatic storage management.

(“Design Principles Behind Smalltalk”, Dan Ingalls, BYTE Volume 6 Number 8 p.289)

6. And definitely (this time, maybe) means message-sending.

This suggests a third principle for object-oriented design:

Messages: Computing should be viewed as an intrinsic capability of objects that can be uniformly invoked by sending messages.

[…]In most computer systems, the compiler figures out what kind of number it is and generates code to add 5 to it. This is not good enough for an object-oriented system because the exact kind of number cannot be determined by the compiler[…]

(op. cit., p.290)

7. This message-sending capability must be exposed to the UI.

An enabling condition for adequate flexibility of a user interface can be stated as an object-oriented principle:

Reactive Principle: Every component accessible to the user should be able to present itself in a meaningful way for observation and manipulation.

(op. cit., p.296)

[note: reactive programming woo]

So, you know what? Whatever you’re doing, it’s probably object-oriented. And whenever I say object-oriented, it may possibly mean whatever your meaning is.