Thursday, December 6, 2007

Pedantically Speaking

Spending your days programming a computer -- if you do it long enough -- will start to have a noticeable effect on your personality. Not that it is a big surprise, one's profession has always -- no matter how complex or simple -- gradually morphed their personality. If you think that all of the accountants you've met are basically the same, you're not that far off base.

Programming is a discipline where we bury ourselves in the tiniest of details. Perfection is necessary, at least to the degree that your work won't run on the computer if it is too sloppy, and it is exceptionally difficult to manage if it is even a bit sloppy. This drives in us, the need to be pedantic. We get so caught up in the littlest of things that we have trouble seeing the whole.

Most often this is a negative, but for this post I wanted to let it loose into its full glory. It is my hope that from the tiniest of things, we may be able to draw the greatest of conclusions.

Originally, the thoughts for this blog entry were a part of a comment I was going to make for another entry, but on reflection I realized that there was way too much of it to fit into a simple comment field. This post may be a bit long winded with lots of detail, but it is the only real way to present this odd bit of knowledge. You get some kinda award if you make it to the end in one piece.

I was going to be rigorous in my use of a programming language for any examples. Then I thought I might use pseudo code instead. But after I had decided on the opening paragraph, I figured that the last thing I actually needed was to be more rigorous. If I am going to be pedantic then the examples should be wantonly sloppy. Hopefully it all balances out. The best I can do for that is to mix and match my favorite idioms from various different programming languages. The examples might be messy -- they certainly won't run -- but hopefully beyond their usefulness there should be minimal extra artificial complexity. Straight to the point, and little else.

For me the classic example of encapsulation is the following function:

string getBookTitle() {
    return "The Programmer's Paradox";
}


This is an example of cutting out a part of the code into a function that acts as an indirect reference to the title of a book. The function getBookTitle is the only thing another programmer needs to add into their code in order to access its contents. Inside, encapsulated away from the rest of the world is a specific instance of 'book title' information that references a specific title of a book. It may also happen to be the title of a blog, but that is rather incidental to the information itself. What it has in common with other pieces of similar information out there is possibly interesting, but not relevant.

The title in this case both explains the underlying action for the function -- based on the verb: get -- but it also expresses a categorization of the encapsulated information: a book title. It may have been more generalized, as in getTitle, or less generalized, as in getMyBookTitle, but that is more about our degree of abstraction we are using then it is about the way we are encapsulating the information.

In a library, if the code were compiled and the source wasn't available, the only way other programmers could access this information is by directly running this function. This 'information hiding', while important is just one attribute of encapsulation. In this case my book title is hidden, but it is also abstracted to one degree as a generic book title. Used throughout a system, there is no reference to the author or any necessity to know about the author.

Now this "slice and dice" of the code is successful if and only if all of the other locations in the code access the getBookTitle function not the actual string itself. If all of the programmers agree on using this function, and they are consistent in applying that agreement, then we get a special attribute from this. If you consider that this is more work than having the programmers just type in the string each time they needed it, then that extra work should be offset by getting something special in return. In this case, the actual title string itself exists in one and only one place in the code, so it is a) easy to look up, b) easy to change consistently for the whole program and c) easy to verify correctness visually. All strong points that make this more than just a novel approach for coding.

So far, so simple. Now consider the following modification to the function:

string getBookTitle(string override) {
    if(override is NULL)
        return "The Programmer's Paradox";
    else
        return override;
}


This is a classic example of what I often refer to as partial encapsulation. While the base function is identical, the programmer has given the option to any other programmers to override it with their own individual title. Not a realistic example, you say? Well, if you consider how many libraries out there allow you to override core pieces of their information, you'll see that this little piece of code is actually a very common pattern in our software. In a little example like this, it seems silly to allow an override, but it is done so often that it always makes me surprised that people can't see that what they are doing is really isomorphic to this example. It happens frequently.

The beauty of encapsulation is that we can assume that if all is good in coding-land and everyone followed the rules, that we can easily change the book title to something else. If we rebuild then the impact of that change will be obvious and consistent. More great attributes of the work we did. The problem with partially encapsulating it however, is that these great attributes are no longer true. We no longer know were in the code programmers used the original string and where they chose to override it with one of their own. While we could use a search program like grep to list out all of the instances of the call and check each one manually, that is a lot of extra time required in order to fully understand the impact of a change to the code. At 4am in the morning, that makes partially encapsulating something a huge pain in the ass. We lose the certainty of knowing the impact of the change.

With the exception of still having the ability to at least search for all of the calls for the getBookTitle function, the partially encapsulated version is barely better than just having all of the programmers brute force the code by typing in the same string each time, over and over again. This is still one advantage left, but consider how many great attributes we lost by opening up the call. If it was just to be nice to the calling programmers and give them a 'special' feature then the costs do not justify the work.

We can move on to the next iteration of the function:

string getBookTitle() {
    file = openFile("BookInformation.cfg");
    hash = readPropertiesFromFile(file);
    closeFile(file);
    return hash{"BookTitle"};
}


Now in this example, we moved our key information out of the code into a location that is far more configurable. In a file it is easy for programmers, system administrators and sometimes the users themselves to access the file and change the data. I ignored any issues of missing files or corrupt data, they aren't relevant to this example, other than to say that the caller high up the stack is concerned about them. The BookTitle functionality is really only concerned with returning the right answer. Presumably the processing stops if it is unavailable.

When we push out the information to a file, we open up an new interface for potential users. This includes anyone who can or will access the data. A configuration file is an interface in the same way that a GUI is one. It has all of the same problems requiring us to check all of the incoming data in a bullet proof manner to make sure we are protecting ourselves from garbage.

More to the point, we've traded away our BookTitle information, which we've encapsulated into the file definition in exchange for the location of the file itself and the ability to correctly parse it. We've extended our core information.

In the outside world, the other programmers don't know about the file and if we are properly encapsulating this information, they never will. If we open up the ability to pass in an overriding config file, we are back to partially encapsulating the solution, but this time it is the config file access info itself, not the original title info that has been opened up.

Now the BookTitle is an accessible file that even if we set read only it is hard to actually determine if it was edited or not, so in essence it is no longer encapsulated at all. It should be considered to be publicly available. But, even though its actual value can no longer be explicitly controlled, its usage in the system is still fully encapsulated. The information is no longer hidden, but most of the encapsulation properties still hold. We have the freedom to handle any value, but the encapsulation on how it is used within the system. The best of both worlds.

Now we could consider what happens when we are no longer looking at a single piece of information, for example:

(title, font, author, date) = getBookInformation();

In this case, the one function is the source for multiple pieces of related information for a book. This has the properly of 'clumping' together several important pieces of information into the same functionality that in itself tends towards ensuring that the information is all used together and consistently. If we open it up:

(title, font, author, date) = getBookInformation(book_identification);

Then a programmer could combine the output of multiple different calls together and get them mixed up, but given that that would take extra work it is unlikely to end up being that way in production code. Sometimes the language can enforce good behavior, but sometimes all you can do is lead the programmer to the right direction and make it hard, but not impossible for them to be bad.

In this last case, the passed in key acts as a unique way of identifying each and every book. It is itself an indirect reference in the same way that the function name was in the very first example. Assuming that no real information was 'overloaded' into the definition of the key itself, then the last example is still just as encapsulated as the very first example in this post.

Passing around a key does no real harm. Particularly if it is opaque. If the origin of the key comes from navigating directly around some large data source, then the program itself never knows either the key or the book information.

It turns out that sticking information in a relational database is identical to sticking it in a file. If we get the key from a database, along with the rest of the information then it is all encapsulated in the calling program. There are still multiple ways to manipulate it in the database, but if at 4am we see the value in the database we should be safe to fully assume that wherever that key is used in the code it is that specific value, and that property is also true of all other values. We have enough encapsulation to know that the value of the data itself is not significant.

That is, if I see a funny value on the screen then it should match what is in the database, if not then I can make some correct assumptions about how the code disrupted it along the way.

There are many more examples that are similar, but to some degree or another they are all similar to encapsulation or partial encapsulation. We just need to look at whether or not the information is accessible in some manner and how much of it is actually accessible. Is all comes down to whether or not we are making our jobs harder or easier.

So far this has been fairly long piece, so I'll need to wrap it up for now. Going back over it slightly, we slice and dice our code to make it easier to build, fix and extend. Encapsulation is a way in which we can take some of the information in the system and put it out of harm's reach so that our development problems are simpler and easier to manage. Partial encapsulation undoes most of that effort, making our work far less productive. We seek to add as many positive attributes to our code as possible so that we can achieve high quality without having to strain at the effort.

We can construct programs that have little knowledge of the underlying data beyond its elementary structure. For that little extra in complexity, we can add many other great attributes to the the program, making it more dynamic and encapsulating the underlying information. The best code possible is always extremely readable, simple and easy to fix. This elegance is not accidental, it must be built into the code line by line. With the correct focus and a bit of practice programmers can easily build less complex and more robust systems. PS. I was only kidding about the reward, all you'll get from me is knowledge. Try the shopping channel for a real reward...