yeah I've also already thought about how to model the evaluation/build process of Nix derivarions as a monadic data structure

basically you wanna have a monad like Derivation a, where a is the type of the value produced by the build of the derivation (currently, Nix derivations always produce files or directories, but theoretically nothing speaks against derivations producing strings, integers, etc as their build result too)

Derivation FileOrDirectory would then be equivalent to our current .drv files