The Nix properties fit ideally with the domain it tries to solve:
- functional with no side effects (for the same imput you always supposed to get the exact same output); derivation is function of its inputs (e.g. source code, dependencies, compiler flags, architecture and so on) if none of them changes you're expecting to get the same file. Introducing side effects would remove this guarantee, and introduce bugs.
- lazily evaluated - you can specify multiple things but Nix will process only the ones that are referenced, this makes weird experience that the code isn't executed from top to bottom (as most people would expect)
Those two things essentially makes the derivation declarative. You specify what derivation you want and what it needs (like dependencies) you can also use the language to transform the inputs to the way you like and still preserving the guarantee that if one dependency changes the project needs to be rebuilt.
- functional with no side effects (for the same imput you always supposed to get the exact same output); derivation is function of its inputs (e.g. source code, dependencies, compiler flags, architecture and so on) if none of them changes you're expecting to get the same file. Introducing side effects would remove this guarantee, and introduce bugs. - lazily evaluated - you can specify multiple things but Nix will process only the ones that are referenced, this makes weird experience that the code isn't executed from top to bottom (as most people would expect)
Those two things essentially makes the derivation declarative. You specify what derivation you want and what it needs (like dependencies) you can also use the language to transform the inputs to the way you like and still preserving the guarantee that if one dependency changes the project needs to be rebuilt.