Side effects are what makes programming hard — all the non-trivial pure code has already been written by smart people with degrees, all the trivial pure code can be generated with LLMs; and in side-effect free programs you can’t even print anything to the console so any pure program is just equivalent to the program that does nothing.

To do anything useful in a program always involves invoking side-effects, to some extent.

But then if I design an API for others to use, I will try to design it in such a way that the consuming party doesn’t know anything about my database, or perhaps upstream limits imposed onto me by a third party. I’ll add a cache so you can do as many API requests as you want!

Essentially I’m trying to hide all the side-effects so to you the API you’re consuming looks as pure as plausible.

This leads to an effect I call “side-effect decoherence” (stolen from quantum physics), where the further you are removed from a side-effect, the less you need to worry about it, and the more you can pretend like it does not exist, making impure code pure again, or at least “pure enough”.

A small scale example would be, everytime I write a “pure” program in javascript like const x = 1 + 2;, and then run it, I’m actually causing heaps of side effects.. in my CPU there are registers that I’m setting and overwriting and probably I’m also producing a lot of heat. Some programmer in the chain of programmers that allowed me to write cute javascript programs in my fancy electron-based editor must have had to worry about CPU heat but don’t have to care about it and that’s great!

Even in corporate projects with tight deadlines where I don’t have the time to tidy up my API for you that tends to happen. If I’m calling a neighbouring microservice then I might need to worry about doing API requests in the right order and with enough delay in between because oh my god what are these guys doing why is their API so bad. But if I’m dependent on data that worked it’s way through three microservices to get to me then these issues never seem to pop up; and that’s because the devs in between me and the buggy API have added their own fixes and workarounds and abstractions.

On the bigger scale, the side-effects have decohered.