[yjftsjthsd-h]

I'm not saying that I want library/module integration; I want nix to be able to build a package by creating a working directory with a set of input directories and an output directory, and then running an arbitrary script to convert the inputs into the output.

[throwaway894345]

If I understand correctly you can do that today (nix builds with bash scripts by convention but there's no reason you can't ask it to use Python or whatever or call into Python/etc from bash). But that's all orthogonal to the expression language.