First, I want to link to this post by Stuart Halloway. It was one of the first posts I read on Clojure (and potentially the first time I had heard of Clojure). Without it I may not have even bothered with Clojure. I'm not a big fan of OOP, and don't use it that much, but I didn't think I could completely give it up.
Encapsulation
I didn't address the issue of encapsulation at all in my previous post. That's because, to me, encapsulation has never been a big issue.* In C++, you declare the inner workings of your classes to be public, private, friends and probably some others that I don't know. It's similar to running a web service. You can't open everything to anyone who wants it.
To be honest, I've never been in a situation where it would have been worth the work to set up an elaborate system in which I define who gets access to what. If there is a piece of data that should be accessible to the outside world, why can't I just use getters and setters? If there's no "get" defined, then I shouldn't be accessing the data.
Maybe to better summarize my experience, I'm not disagreeing with the concept of restricting access to data. What I don't see is how declaring public, private, friends, and whatever, and all the complexity it adds, plus all the work it creates (you have to do it for every class) is better than getter and setter functions combined with common sense and a few simple rules. The C++ approach seems no more reasonable than Haskell's purity requirement. Trying to debug a complicated OOP program is no fun. Maybe I work on smaller projects and don't have ten million lines of code and a hundred programmers of varying quality working with me, so my observations don't carry over to those situations.
For what it's worth, here is Rich Hickey's answer to an interview question by Fogus: "Following that idea—some people are surprised by the fact that Clojure does not engage in data-hiding encapsulation on its types. Why did you decide to forgo data-hiding?"
(Warning: partial quote) "...And if you have a notion of “private”, you need corresponding notions of privilege and trust. And that adds a whole ton of complexity and little value, creates rigidity in a system, and often forces things to live in places they shouldn’t.... If people don’t have the sensibilities to desire to program to abstractions and to be wary of marrying implementation details, then they are never going to be good programmers."
I see nothing controversial about that perspective.
Clojure and Learning OOP
I included the word "learning" in the title of my earlier post. I would argue that Clojure is a much easier way to learn about the underlying concepts of objects (but using maps), polymorphism, and inheritance. These are concepts that every programmer should understand and use.
I would go further, though. Even if you want to use OOP in Java or C++, you're better off to start with Clojure. The advantage of Clojure over Java/C++ is that you can learn one thing at a time. It's very easy for anyone to understand what's going on when you introduce a single concept. With Clojure, the concepts are well-motivated.
In Java or C++ you get hit with everything at one time. There's a lot of coupling. The degree of difficulty rises very quickly with lines of code when someone is trying to learn something. Introducing the grouping of related data, methods, and the system of privileges all at once is not only too difficult, there's no motivation. I've been programming for decades, and I don't see a need for C++-style encapsulation. How is a new programmer supposed to understand?
Only then do you get into polymorphism and inheritance, concepts they have a reason to learn. Give me an hour with a new programmer and I'll have him doing polymorphism and inheritance, and he'll have at least a small understanding of what they are and why he'd want to use them. Then put him in a class on Java or C++ and he'll have no problems. That's why you're better off to start with Clojure even if the goal is to train them in a more "practical" language.
Parenthesis Whining
As an aside, for those who think the use of parenthesis makes Lisp unsuitable for teaching programming, let me present you with Hello World! in Java:
class HelloWorldApp {
public static void main(String[] args) {
System.out.println("Hello World!");
}
}
In Clojure, Hello World! is
(println "Hello World!")
One set of parenthesis vs two in the Java program. Plus the Java version has two sets of {}, class, public, static, void, System.out, and String[]. If that doesn't make a programming newbie vomit, nothing will.
As with my other post, I'll conclude by admitting I'm not an OOP expert. I appreciate any comments you might have.
* If it were, I would have to stop programming in R, because I'm not aware that R offers real encapsulation.
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