Ok so, calling it a script was wrong. It's a nix function which takes arguments specifying various configurations about the project I'm working on, such as any dependency overrides. It then provides a nix derivation which can define shells and compile to whatever platforms I need. No bash scripting is necessary to use this machinery except a convenience script to install Nix if you don't have it.

All of the functionality is just leveraging nix's powerful declarative model which easily defines the build artifacts of a project along with the environments necessary to hack on or compile them.

This sits on top of cabal and stack, actually. Within a project shell provided by Nix, I'll use cabal to get a repl, for example. But since Nix is handling all the dependency management, cabal doesn't have to do any work in that regard.