There is an often unresolved question in software about comments and documentation, about in what measure they should be used, and what should be commented and documented. About comments in particular it is often said that code should strive be "self-documenting", or that the tests should be (and are) the documentation, thus comments can be a sign of a lack of clarity in the code, a way to compensate for a weak or bad design, or an inadecuate test suite.
I'm not sure what the True Answer might look like (maybe it all depends on each project and there is no silver bullet), but in general I find documentations and comments in most systems sorely lacking. To show why, first I'd like to introduce a pair of concepts that need to be taken into account in this analysis.
Unitary understanding vs Integral understanding
Look at this small code example:
def mysterious_method i = 0 while i < 10 puts i + 1 i = i + 1 end end
We have here a small piece of code, a method called mysterious_method that for any programmer is not mysterious at all, and certainly requires no comments. I want you to look at the last significant line, "i = i + 1", and ask yourself "what does this line does?"
Most programmers would answer "it increments i by 1". While this is not wrong, I want you to look to another way you might answer this: "The line increments i plus 1 and allows the while condition to get from 0 to 9 while the "puts i + 1" sentence spits the numbers from 1 to 10. In other words the sentences 'moves the loop forward'".
What did I just do? Many would question that I've just explained the whole method, not just the line. But does this line exist cut off from the method, or does it exists in the context of the method?
By "unitary understanding" I refer to the understanding of a piece of a system (in this case, a method is a system of interrelated expressions) by looking it only by itself, observing what it does from the point of view of the pure computation. And by "integral understanding" I mean to the understanding of the piece of the system not by itself but by looking at how it relates to rest of the system.
So, by the point of view of unitary undersanding, "i = i + 1" does what it reads: it increments i by 1. But from the point of view of integral understanding, we've still need to describe how does this line relates to the rest of the lines in the system. That's why I talked earlier about "moving the loop forward", I was relating the sentence to the while loop above.
Note that I'm not talking about anything technical, I'm merely elaborating on the understanding of the system parts in the minds of the programmers. These two levels of understanding exists, and every programmmer handles intuitively that the result of the method arises from the interaction of all the elements in the method, what I did is just give two different names to the different "levels" of understanding, which mirror unitary and integral testing.
Comments and integral understanding
Well, when we talk about adding a comment in a line to explain what the lines does, while the line might be self-documenting in the sense of an unitary understanding, it might very well not be self-documenting at all in the sense of an integral understanding. In the example given above, the code is self-documenting because it's short and simple: Any programmer can read it and have an integral understanding. But try to integrally understand what i = i + 1 does in this example:
class QuickSort def self.sort!(keys) quick(keys,0,keys.size-1) end private def self.quick(keys, left, right) if left < right pivot = partition(keys, left, right) quick(keys, left, pivot-1) quick(keys, pivot+1, right) end keys end def self.partition(keys, left, right) x = keys[right] i = left-1 for j in left..right-1 if keys[j] <= x i = i + 1 keys[i], keys[j] = keys[j], keys[i] end end keys[i+1], keys[right] = keys[right], keys[i+1] i+1 end end
In the same way, if we see a piece of code that does an SQL INSERT, to understand it integrally, we need to understand how and when the table is updated, queried, and inserted elsewhere. In general, stateful code is more complex to understand than stateless code precisely because integral understanding is harder to achieve there, you need to have a clear view of the evolution of the system state through time. In those cases, comments are very useful to help you achieve integral understanding of what something does, something like "this insert here will trigger a sending of an sms because there is a supervisor process polling the table".
A quick top level documentation of the system can also help inmensely to have an integral understanding, it does not need to be complete or explain every little detail of the system. Your code might be very nicely structured, and have great namings and interfaces, but still, when faced with a pile of code that no one knows, I think no programmer would dislike from a little top level exposition of how the system works. Because the code might be self documenting... but not integrally. And in general I see that a lot of software projects lack documentation and/or comments, the code is expected to be self-explanatory, but code rarely (if ever) is self-explanatory in an integral way.
Automated tests and integral understanding
Tests can help you work through the integral understanding because they verify the integral assumptions of how the code will work. In general what automated tests do is to code the integral behaviour of your code so that you don't need to have it all in your head: good tests allow you to try a change, and if you broke the supposed integral behaviour, they will let you know. In some sense, that's why people will argue that in self documenting code, "the tests are the documentation".
For example, suppose you figured out an SQL INSERT inside a class was superfluous, you rewrite the unitary test, then rewrite the class, and you're done....only when you run the full suite and you break another test you figure out that this insert leaved a trail that was meant to be queried elsewhere. So even if the sql insert did not have a comment telling you why this insert was there, and you did not have the integral understanding to know what the insert did, the test suite had a way to tell you.
From this, one can easily conclude that in general, code should be tested as integrally as possible: It's dangerous to test separately two integrated components, at the very least if you test them separately you should test the integration itself in some place.
An example: If you have a component, let's call it Producer, that produces a change in the state of the system, and you have another component, let's call it Consumer, that reads such a change in the state of the system, many programmers will hardcode the change in the state of the system as would have made by the Producer component in the setup of the Consumer unit test, instead of calling the Producer component. If you do that then one could change the Producer component and the test suite will pass, without realizing the the relationships between the two components are broken.
Grades of understanding
Take into account that what we get to call "integral understanding" eventually depends on what we consider the "rest of the system", and this might not always be obvious. A good example is a public API: Are the callers of a public api part of the system, thus part of what we need to take into account as integral understanding? As soon as an API becomes public, we lose control of who uses, and eventually, we lose understanding of how the API interacts with the rest of the system, when inside the limits of the system we include the callers of the API. A painful side effect of this can be seen in Operating Systems and Browsers who preserve "quirks" and even "bugs" that turn into "features" because a set of sites or drivers depend on a behaviour that now cannot be changed.
In a sense, what you consider is the rest of the system can be as big as you want it to be. Another example, to understand a line within a method integrally, you need to understand how that line relates to the rest of the lines in the method. Is that enough? Well, no, because to understand the method, you need to understand how and when it is called and with which kind of parameters. Is that enough? Well, have you considered what happens if the system breaks down because it has no memory in the middle of the method? In the end, the rest of the system includes the compiler/interpreter, the machine that it is running on, basically any abstraction that can leak.
Human beings in the end are also an often ignored part of the systems, even if in the end, software is meaningless unless it impacts a human being in some way. And true integral understanding eventually relies on human beings, and while explaning why and in what sense is beyond the scope of this article, I think we can easily see that even if the code is self-documenting, even if the tests are beautifully written, the system gives away the what is doing but often lacks a why, as in why does the system behaves like this instead of that. In enterprise systems, where business rules can often get convoluted and seem arbitrary, it applies even more. In the end the code plus the test suite can have that why lacking and sometimes there are good reasons to document the why, if the why is non obvious.
As we can see, integral understanding thus in the end is hard if not impossible, and no one has complete integral understanding, yet another reason why tests are helpful, they cover up for this deficiency.
Can code be self documenting?
Fans of the "code should be self documenting" idea will point out that your code needs to have an easy, self documenting structure. A simple, well written system will be easy to understand integrally compared to spaghetti code. So you still need to try to write code that needs the least amount of comments, code where the integral structure is as self evident as possible, where you can figure out the impact of a line of the code by tracking (with a little help from the IDE) the scope of the full integral impact of the line. And they are correct.
And while it's true, we need to take into account those two levels of understanding, and try to be as helpful as possible to explain to a fellow programmer, or maybe yourself in the future, what may not be so obvious to understand from a glance.