[throwaway894345]

I don't think this is any more true for this problem space than other problem spaces. The "pure functional package manager" property doesn't come from the functional expression language, but from the thing that takes the static build plan and executes it. You could make these build plans in Python (or Starlark https://go.starlark.net) very elegantly and with the massive added bonus that they are intelligible to a much broader audience, and Python still allows for a very functional style. The only caveat I would add is that the Nix expression language has really nice support for multiline strings and Python still hasn't figured that out. Lastly, Python also supports type annotations and even a type checker (although the type checker leaves a lot to be desired, such as basic types or callbacks that take kwargs so it might be better off to implement one's own type checker).

But 100% agreement that Nix needs a type checker!