Option to create shell.nix from nix-shell arguments

[wchresta]

It is very easy to initiate a nix-environment through nix-shell that has a given set of derivations active. This is relatively easy to understand for new users and use right away. There are also a lot of tips and comments that rely on nix-shell's arguments.

The step from giving a derivation name in the command line (nix-shell -p python37) to creating a well formatted shell.nix is not small. It would be helpful for new users if there was an option or command for the nix-shell command that turns the current arguments into an equivalent shell.nix that has the same effect.

That way, a user can not only quickly recreate a previous environment, but can also learn how shell.nix files are supposed to look like and how packages and environment are connected together. This can be seen as both, a quality of life feature, as well as a mini-tutorial for nix-derivations.

Example:

$ nix-shell -p "haskellPackages.ghcWithPackages (p: [p.lens p.JuicyPixels])"
[environment with ghc and lens, JuicyPixels]
$ nix-shell init -p "haskellPackages.ghcWithPackages (p: [p.lens p.JuicyPixels])"
Created shell.nix. You can use nix-shell without arguments now.
$ cat shell.nix
# nix-shell init -p "haskellPackages.ghcWithPackages (p: [p.lens p.JuicyPixels])"
{ pkgs ? import <nixpkgs> {} }:
with pkgs;

# Insert some useful and helpful comments explaining what the expressions do
stdenv.mkDerivation rec {
  name = "env";
  buildInputs = [ (haskellPackages.ghcWithPackages (p: [p.lens p.JuicyPixels]))
                ];
}
$ nix-shell
[environment with ghc and lens, JuicyPixels]

Ref: nixpkgs #36634

[prusnak]

It might be also worth considering whether we want to edit the shell.nix file from currently running shell.

For example, if I run pipenv install foo it will install package into currently running pipenv and update the Pipfile file, so next-time when I run pipenv in this directory, the environment will contain this new package as well.

[stale]

I marked this as stale due to inactivity. → More info

[PierreR]

Sound like a great idea to me. I am constantly copy/paste the same basic shell.nix which is not that nice.

[stale]

I marked this as stale due to inactivity. → More info

[nixos-discourse]

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

https://discourse.nixos.org/t/how-do-nix-profiles-and-flakes-fit-together/28139/4