Phil Nash from level of indirection
Following Peter Hilton's excellent ACCU talk, at last week's conference in Bristol, "How to name things - the hardest problem in programming", a few of us were discussing some of the points raised - and some not raised.
He had discussed identifier length without any mention of Uncle Bob's guideline, whereby the length of a variable name should be proportional to it's scope (i.e. large or global scopes need longer, descriptive, names whereas in smaller, local, scopes shorter, more concise - even single letter - names are appropriate). This seemed all the more of an omission given that he later referenced the book, Clean Code.
It wasn't that Peter disgreed with Uncle Bob (who doesn't, half the time?) that surprised me but that he didn't even mention it in passing. I thought it was fairly well known. Actually I double checked and it is not discussed fully in the book, which only says, "The length of a name should correspond to the size of its scope". This is expanded considerably in Clean Coders (video) episode 2. Also, of course, this is not really "Uncle Bob's rule". Kevlin Henney recalls that he first heard of it in the 90s and it may well have been kicking around before that. Bob calls it "The Scope Rule".
Kevlin was one of those discussing this afterwards. After initially toying with The Scope Rule in the 90s he came to consider it not particularly useful. This, too, surprised me as I had found it worked quite well for me. Or so I thought. Further discussion with Kevlin led to the conclusion that I had read more of my own interpretation into The Scope Rule than I had realised! So I started musing over exactly what my interpretation was.
A transparent reference
As it happened a concept key to clarifying matters came from another great talk at the same conference just the day before. Didier Verna's "Referential transparency is overrated". In this talk Didier discussed various ways that useful idioms in Lisp required violating referential transparency. At one point he explained how "hidden" variables may be introduced by one macro that were then used by another. This worked because the thing being referred to was named very generically - so both macros agreed on the name. He drew on the term "Anaphora" from linguistics, which is where one part of an expression - usually a pronoun - stands in for a more specialised part - such as a person's name - introduced earlier in the context. For example, just now I used the word, "he" to refer to Didier Verna. It was clear who I was talking about because his was the most recently specified name in the current context. In fact I used this anaphoric term a couple of times - and many, many, times in this article. If I had had to fully qualify "Didier Verna" every time writing would quickly become very cumbersome. Anaphora is used very frequently in natural language - usually to good effect.
I believe this is key to understanding why and when shorter identifiers can be used too. When I had been talking with Kevlin it had become apparent that we had different interpretations of the word, "scope". I realised that I had subconsciously expanded the specific technical meaning to include a more general idea of "context" - including the anaphoric context.
To make this clear I might write some (C++) code like this:
std::string s = getNextString();
if( !s.empty() )
std::cout << "received string: " << s << std::endl;
Many corporate, or personal, coding standards would balk at such practice! Single character identifiers? Way to obfuscate the code!
But how has it obfuscated anything? Look at it as an anaphoric entity. In this case the variable name 's' is anaphoric. We know it is "the next string" because we saw it being introduced by the function call, "getNextString()". We then use it twice on the next couple of lines. There are no other strings being introduced in the same vicinity to confuse it with, and the context in which it is used is kept small. There is no ambiguity and the full identity is revealed in the immediate vicinity.
But what if we add more code, or move parts of this elsewhere? Certainly code evolves over time in ways that can make things less clear if we don't change them. That's true regardless. Naming of the entities at play should always form part of your consideration when refactoring or otherwise modifying existing code. Does it make this code less "sustainable" (to reference another property that Kevlin likes to talk about)? I don't think so. In the worst case, if you don't immediately notice that a short name has become unclear because it's usage has drifted out of anaphoric range, you'll notice the next time you look at it and, momentarily, think "why is there an free variable called 's' here? What on earth is that. You'll take a moment to find it's original declaration, work out what it is, then decide to encode that in the name by renaming the variable at that point. Variable renaming is one of the safest and most ubiquituous refactorings around so I have no qualms about deferring such identity expansion to such a time as it is needed.
But what about the other side of the argument? Is there any advantage to using a short, even single character, variable name in the first place?
This is often cast as a matter of optimising for typing speed - in world where we typically read these names many many times more than we write them.
While introducing, even small, speed bumps to writing code might discourage spending more time than necessary writing code (which in turn may discourage certain refactorings) it's not really about typing performance at all - It's about readability!
Consider again the linguistic definition of anaphora: substituting an, unambiguous, subsequent reference to an entity with a shorter form (e.g. a pronoun) that means the same thing. We do this all the time in natural speech and the written word. Why? Because it would sound unnatural and cumbersome to fully qualify every entity we talk about all the time!
The same applies in programming. Where it is perfectly clear from the immediate context what an identifier refers to then using greater verbosity actually increases the cognitive friction! The more unnecessary and redundant noise and ceremony we can strip away from our code the easier it will be to read, in a shorter period of time. That fact that anaphora is so common in natural language should give us a clue as to our ability to code with it's use in a natural and efficient way.
Now I've only mentally organised my thoughts around this as a result of ruminating on those two talks - and some of the offshoot discussions - but I realise this is essentially how I had interpreted The Scope Rule. Now I've worked it through when I go back and compare it with what Mr Martin actually said his version sounds like a poor proxy for the anaphoric interpretation.
So naming - good naming - is still hard. We've only just discussed one narrow aspect here. But perhaps this has made some of it that little bit easier.