You write it in vim and then copy paste it once you are done.

BTW, why do you need a markdown editor?

A package is reproducible if you use the same inputs, run the build, and get the same outputs.

The issue is that the build can produce different outputs given the same inputs. So you need to modify the build or patch the outputs. This is something that is being worked on by most distributions: https://reproducible-builds.org/who/projects/

NixOS is not special in that regard nor are all NixOS packages reproducible.

Nope, nix doesn’t ensure or require that the builds are deterministic. It’s not any better in that regard than other package managers.

It’s not really fully reproducible either.

The implementations mostly don’t matter. The only thing that you need to get right are the interfaces.

Some people consider working on programming languages fun, so they create new ones.

I randomly generated mine using a Markov model. It’s just pronounceable nonsense.