Stability promise, reprodubility properties about nix HOT 2 OPEN

This issue has been mentioned on NixOS Discourse. There might be relevant details there:

https://discourse.nixos.org/t/stability-of-function-return-values-in-nix/45124/2

from nix.

Triaged in Nix team meeting

• @edolstra such documentation is always welcome
• @fricklerhandwerk have to slice it somehow, it's a large surface to cover
• @roberth could start with the language
  • @fricklerhandwerk: why the language? it has the largest surface area
  • @roberth: the store seems hard to talk about clearly, Nix is mostly experienced through the language
  • @fricklerhandwerk: ideally we'd have a minimal spec or "eduational" implementation to have something that conveys intent. otherwise it's almost impossible to draw the line to say what's a bug and what is not
  • @roberth: "correct" behavior is somewhere between such an ideal spec and the Nix we have (and has been used by many people for a long time). I don't think it's feasible that we can make the decision of correct behavior objectively, mechanically testable, but we should at least state our goals
• @edolstra: probably would be best if somebody writes a proposal as a starting point, so we can discuss that

from nix.