If you're tracking a branch of some kind of repo and don't really care about purity, trust the supply chain and just want to get the latest stuff you can use "fetchTree" in impure evaluation to fetch the the hottest shit out there. By default it uses a 1 hour cache to not fetch every time.

      testFetchTree = builtins.fetchTree {
        type = "github";
        repo = "nixpkgs";
        owner = "NixOS";
        ref = "master";
      };

My understanding from reading github comments and whatnot is that it's using the same paths as flakes use for it's inputs.

https://asciinema.org/a/QW02t4oWnM8zBI8JSov4VysmF

I AI'd if --impure can be enabled by default and just now stumbled on a feature I hadn't heard about before! The option allow-unsafe-native-code-during-evaluation gives you a new builtin function called builtins.exec which can be used like this:

nix-repl> :p builtins.fromJSON (builtins.exec [ ./test.fish ])
{
  active = true;
  age = 25;
  name = "test-user";
  tags = [
    "developer"
    "nix"
    "json"
  ];
}

# test.fish
#! /usr/bin/env fish

echo "''"
echo '
{
  "name": "test-user",
  "age": 25,
  "active": true,
  "tags": ["developer", "nix", "json"]
}'
echo "''"

(It parses output as a Nix expression so i had to wrap the json in '' '' so it's treated as a string by Nix)

I'm definitely gonna dive deeper into usecases for this, as everything Nix it'll stall evaluation so don't do extremely slow things :)

In reply to @lillecarl:matrix.org
For home/personal use --impure should be the default IMO. You can use builtins.getEnv to read environment variables and you can read files anywhere on the filesystem, not just the sacred repo.
(Non-flake users don't have this problem since everything is --impure by default :))