Split over join

Ryan James Spencer

If you're a software developer, you've likely been there; the manic, not to mention frantic, working through complex input cases and systems interactions, all to fix or extend some potentially tiny bit of code. This sort of "global reasoning" is often considered business as usual for software systems at a certain size, and I'm genuinely surprised there isn't more active discussion around the tenants of local reasoning. In fact, some time back I was part of a meeting discussing what we, software developers, each felt was important for software quality. When it came my turn, I said "local reasoning" only to confused faces.

All that local reasoning means is that you can infer your code is correct using local information, without having to think about every input permutation or complex ordering of system processes or interfaces all fitting together. I find this to be a huge part of my toolkit of writing software of reasonable quality, and I believe in it enough that I started writing some articles and a talk on it.

Part of local reasoning is being able to do what's called case analysis where we can implement and reason about individual situations; for example, if we had a vending machine, we might want to process the steps individually, such that if we're in a "vendoring" stage, we wouldn't want to think about the "payment processing" stage sets, or vice versa.

And while I love local reasoning, this article isn't where I go over those foundations. Instead, I wanted to talk about the software design pattern of favoring splitting out cases and over trying to prematurely generalize. I like to call this "joining" code as generalized cases are better thought as trying to join together several special cases by finding the common points, but when we rush into a general implementation, we may not understand enough of that common surface area, and mistake the one or two cases we have as being representative of the larger population.

Why do we generalize in the first place? Many developers are told "don't repeat yourself" but forget this advice is for refactoring after the code is in place; identifying the places where we've split things out, where the same thing is largely being said, but in insiduously different ways that can lead to bugs. We are also ushered to consider joint abstractions early on, where we can model multiple things under a canonical notion or concept, but, as with software, splitting and joining is cheaper than prematurely joining and having to tease apart ex post facto. Teasing apart is a sharp corner that leads us down complicated, highly orchestrated migrations, the kind that take months, if not years, to finalise, if and only if they manage to not get stalled! This is partly why, although joining is cheap, joining is not always a reversible decision.


  • Splitting is cheaper than teasing apart premature general abstractions
  • Joining is cheap and easy once we have enough specific cases to identify the common points between many cases. Some suggest three as a good number for when to trigger this conversion.
  • Local reasoning is awesome, and case analysis is but one part of the foundations that which allow us to reason about code sanely without having to tear our hair out about every imaginable configuration of state, inputs, or system interactions. Each specific case we reason about is isolated from other cases. Splitting out is similarly about identifying the concepts that can be isolated and, well, isolating them.