Tuesday, June 9, 2015

The devil is in the details...

Dijkstra: "The purpose of abstraction is not to be vague, but to create a new semantic level in which one can be absolutely precise". It's a favourite quote of a good friend. I'm growing to like it more and more by the day.

Perhaps I'm getting old, or perhaps it's because I'm working more and more on larger teams, with code that I did not write, but I find that I cannot deal with 50+ details at the same time any more.

If the details bleed from another part of the code, and I was forced to do a depth first reading of the code at every function call, I might be somewhat uncharitable towards the original author.

Imagine a circular buffer - in my world, it should have an IsEmpty() function. Few would argue this. If I had to wander into the internals of CBuffer and start comparing first, and last, and remembering if it's Empty when they are the same, or when they are 1 away from each other, don't forget to MOD by the size of the buffer, where was I going with this, hold on, let me go back over that again...

The abstraction IsEmpty() is very precise. In a sense it's far more precise than any comparison of first and last pointers, since it expresses exactly what I want to know, not how to go about finding out. And I can simply trust IsEmpty(), because it will (should) have unit tests. But if I am repeatedly adding this comparison code to my class, then every time I add it, I can goof it.

You would think that this is software 101, and nobody needs reminding about this. But again and again it creeps into code, usually more complex code than a simple buffer, but it does creep in.

It creeps in in little ways, like the train wreck antipattern. It creeps in when there are comparisons that imply something else. The little comment along side "// only set if" is a giveaway that we have been far too intimate with another class, when we should be maintaining a professional distance.

Thursday, June 4, 2015

Do one thing... sort of

This was always confusing advice. Do one thing.

So how do I open a file, read it, close it, parse it, and then process the input.
That's a lot more than one thing. Something has got to "do several things"

Like most soundbites, Do One Thing is correct, but incomplete. You have to take into account levels of abstractions. So do one thing at a given level of abstraction.

A call to ProcessConfigFile can reasonably kick off calls to open, read, close, parse and process.
But it should not actually _do_ the Reading, Parsing and the Processing for example. You should call someone else to _do_ these.

If it does Reading, Parsing and Processing, then you have them bound together. So now if you want to process already parsed data from an block of memory or from a network connection, your Process code needs to be "fished out" of the Read Parse code.

So at one level ProcessConfigFile does indeed do one thing. At a more detailed level, each function or class it calls or uses will also do one thing.

If you aggregate the functionality of others classes or functions, then you don't want to mix in any functionality of you own.  Mixing low level details and high level structures tends to hide what is important.  If you have a 30 line function which contains 20 lines of string formatting and 4 lines of structurally important code, it starts to become easy to miss the big stuff for the detail.