The most recent offenders were nodejs and kustomize used as part of a test flow orchestrated by a Makefile, run both locally and in CircleCI.

People will just install the latest version and start hacking away, and now you've got all this code that depends on that version. Backwards compatibility ain't perfect, so maybe several years later the original author doesn't work here anymore, so tests are breaking in subtle ways when you install what's the now latest version and there's nobody to ask what the "right" version is.

But since we're a culture that uses these tools (though I wish we weren't), this story has played out several times so different projects need different versions--you can't just discover the right one once and leave it installed in your system, you have to install the right one for your project and change it when you switch to a new project.

For the most part these are go projects, so even though there is language-specific dependency locking via go.mod and such, dependencies which aren't go libraries but which are nonetheless needed to work with the project (e.g. make) are left as an exercise to the reader. Make is pretty well behaved, I haven't had to do much version antics with it, but I wouldn't say that's the norm.

When I find one of these repos I put my archaeologist hat on and write a flake.nix to provide whatever the dependency is, and then I walk the version backwards until it starts working. That way next time I'm in that project I don't have to go through that exercise again.

To make matters worse, people often try to help by adding entries to the makefiles which download the correct version for that project, but some people have the newer arm chips and others are still on x86 so confusion of a new kind ensues. Of course it's easy to fix these scripts to detect the local architecture, but that's a whole extra step.

And then maybe you're trying to make this stuff work in CircleCI or somesuch, and you don't want the workflow to just be reaching out via https and blindly running whatever comes across the wire because who knows if it'll be the same thing tomorrow, so you add hash checks, but once you've got the hash checks and the architecture checks and you're checking the right hash for your architecture... we'll you've basically got a poor man's flake.lock at that point, might as well use the same nix config in both places rather than use homebrew or apt or whatever locally and then figure out how to do the same thing via circleci orbs in yaml and god forbid you have to do it in prod too so now there's a Dockerfile... Having a single source of truth for dependencies and using it everywhere is super handy.

That's work. Another example is in my personal projects. I use helix, so there's a .helix/languages.toml in my repo which defines the language servers that I'd like it to use for that project. But merely pointing helix at the language server isn't enough, it also has to be on the PATH. My older projects are using mypy, and my newer ones are using pyright (python type checkers).

Sure I guess I could just install both at the system level everywhere I go, but when I clone the project on a new machine I want to have everything work right away--I don't want to start coding and then wonder why my editor sucks and then go discover and install the right LSP and then resume coding. I'd end up with a smattering of different versions installed across all of my devices, even for the same project. If I find a bug which happens on this machine but not that one, I'd have a much harder time knowing where to start re: debugging it.

Finally there's this idea that maybe you don't even have to clone the repo, you can just reference the app you want to run from anywhere. I invoke some of my tools like:

nix run git+ssh://git@github.com/myorg/myrepo -- mycli --best-arg

Nix knows how to set up the environment for running the app (the one I have in mind is written in python, so a certain version of poetry is involved etc...) so I really don't need the caller to think about that environment at all. I like this because it decouples the orchestrator from the executor. So if I can manage to get something working locally with one of these commands, then I can go put the command someplace weird like in an Airflow DAG (kubernetes pod operator, NixOS image) and I have a pretty strong assurance that it'll work the same in the remote environment as it does locally.

From the perspective of a nix user, these problems are all the same problem and they're everywhere and nix is the only thing that solves all of them at once. My feeling is that from the outside, they look like separate problems, and it's not clear just how many of them are solved by nix--so the juice doesn't appear to be worth the squeeze.