Meta-analysis of three different notions of software complexity
8 comments
·June 14, 2025perrygeo
chii
The hickey definition is from the POV of the engineer - the creation of software with simplicity. Code that is simple, doesn't necessarily result in a piece of software that is simple for the end-user.
The tellman's version is for the end user - simplicity of use, by the user, with their existing expectation, culture and pre-knowledge. It's basically describing skeuomorphism in software (but not limited to just UI). It might be enormously complicated to create for the engineer, while it remains simple for the end-user.
motorest
Quantifying software complexity is still largely an open problem. There are approaches that focus on quantifying but are arguably of limited use due to their disconnect with mental models, and there are approaches that focus on evaluating mental models but can't be objectively quantified. The best output from the latter group is that they can serve as guidelines on how to make simple software systems,and how to keep them simple.
_benton
I like Ousterhout's the best because I think it's the most concrete out of the three. The other two are more subjective imo, of course still very useful and important.
Although I think "dependency" is an overloaded term. There are dependencies as in libraries or shared code, and then there are "dependencies" which are like assumptions about other parts of the system. Hickey also mentions this when he talks about interfaces. If you break a class into two classes, but they both make assumptions about the other, in effect they are still the same class.
Perhaps another way to think about complexity is the number of assumptions baked into a system?
0xbadcafebee
When you see the words "complex", "simple", and "easy", keep this in mind: these are words that exist to help humans communicate their difficulty in interacting with the world.
This whole post is a discussion of philosophy. There are three different ways you can think about software:
- philosophy
- science
- function
The science of software is computer science, which is more about math than anything else. This is the only place you're gonna find objectivity or consensus, because you can't really argue with a number. Discussions around CS are less poetic and more boring, and require a lot more knowledge, so you won't typically find these on HN.
The function of software is the day-to-day practice of writing and using software. This is the grunt work, the meat and potatoes. No need for philosophy or science. Just put the legos together and kick the tires. This is 99% of what 99% of software developers deal with. Most people don't think about the science or philosophy of the coffee they're ordering or the bike they're riding; they're just ordering it, riding it.
readthenotes1
I read a research article long ago that said that most of the complexity measures investigated at the time were highly correlated with loc.
For example, it's nice to measure McCabe cyclomatic complexity, and it generally rings true, but if you can't be bothered - just look at LOC and you'll get pretty close
0xbadcafebee
Yes, in the same way that large forces are highly correlated with bigger things. You don't have to know anything about design or engineering to know that an 18-wheeler will deal with larger forces than a sub-compact car. Similarly, a whole lotta software is gonna be more complex.
n0n0n4t0r
For those who will stumble as me: Loc = lines of code. (He he, we were talking about complexity, weren't we?)
I respect all three authors deeply but this is my first time reading Tellman's take. His idea that "simplicity is not intrinsic ... Simplicity is a fitness between software and our expectations" has a very nice ring to it.
Contrast that with Hickey's notion of simplicity as an objective count of interleaved concerns or "folds". The quibble with this is, how do you delineate the concerns? Depending on your style and knowledge of the system, you'll get a different number. So it's hard to call it objective.
Tellman's definition is nice because it acknowledges the subjectivity, and puts it front and center. IOW the "style and knowledge of the system" form a mental model of the software system. What's important is not the cardinality, OR the interleaving. It's the ability for that model to make good predictions about the software's behavior. Accurate model held in the minds of humans that operate it == simple software.