This might show my ignorance, but can Nix replace something like asdf (https://github.com/asdf-vm/asdf)?
Nix stores all builds of its packages in a separate “store” on the machine, each identified by the hash of the build (“derivation”). When you activate your config, it basically symlinks the version you want into your PATH.
If your project needs a specific version, when you activate a Nix shell or script it will check your store for that version and activate it. If it doesn’t exist, it will fetch it (or even compile it from scratch if you need to).
Does Nix allow for a similar workflow? It's really nice to be able to move between projects and automatically have the correct tool versions configured without having to run any special command.
I think the nice thing about project-specific tooling like this is it allows getting started with the project very easily. (Rather than copy-pasting "apt-get <whatever>").
However, if you really want to make sure that you're not including anything from your system, you can have direnv pass the `--pure` flag, which will then include only the dependencies from the project and nothing else. This is helpful to make sure that you're not accidentally relying on something already on your system when you declare the project dependencies.