generate-versions.nix

# Usage: $ nix eval "(builtins.attrNames (import (builtins.fetchurl "https://matthewbauer.us/generate-versions.nix") {}).emacs)"
#        $ nix run "(import (builtins.fetchurl "https://matthewbauer.us/generate-versions.nix") {}).emacs.\"24.3\"" -u LANG -c emacs
{}: 

let

  channels = [ "nixos-20.09" "nixos-20.03" "nixos-19.09" "nixpkgs-unstable" ];

  getSet = channel: (import (builtins.fetchTarball "channel:${channel}") {inherit (builtins.currentSystem);}).pkgs;

  getPkg = name: channel: let
    pkgs = getSet channel;
    pkg = pkgs.${name};
    version = (builtins.parseDrvName pkg.name).version;
  in if builtins.hasAttr name pkgs && pkg ? name then {
    name = version;
    value = pkg;
  } else null;

  attrs = builtins.attrNames (import <nixpkgs> {});

in builtins.listToAttrs (map (name: {
  inherit name;
  value = builtins.listToAttrs
    (builtins.filter (x: x != null)
      (map (getPkg name) channels));
}) attrs)