I’ve thought a lot about a type system for Nix. Typing the mix expression language would improve some things, but since so much of what people do with the Nix expression language is writing scripts that take in files and transform them into other files, you would really want a type system for the derivations which could describe the “shape” of the input and output files. I’m not aware of any such type system, but it would be a really interesting area of research. Would love to know if others have thought about this or not.