Quickly reload nix expression in nix-shell

Looking at the source it seems very likely you have another nixpkgs directory somewhere in ~.

nix-env searches (recursively) all directories for default.nix adding the parent directory as a top level attribute.

Using nix-env -f ~ seems like a bad idea - very much to search and chances for such collisions. Unless there's something I'm missing symlinking ~/nixpkgs into ~/.nix-defexpr seems like a good solution.

There's nothing wrong with simply using nix-env -f nixpkgs -iA exercism. In this form nix sees a default.nix directly in the -f argument and uses that as the top level attribute set.

If you can't find another nixpkgs directory, I'd try using the --show-trace option.

This file is indeed specific to NixOS and it is created automatically when installing NixOS. That said, there are workarounds.

• One of them is described in https://nixos.org/nixpkgs/manual/#sec-declarative-package-management in the Nixpkgs manual. (Added in this PR.)

  Please note that the overlays mechanism came along since then, and according to Chapter 12. Overlays: "Overlays are similar to other methods for customizing Nixpkgs, in particular the packageOverrides attribute described in Section 6.5, “Modify packages via packageOverrides”. Indeed, packageOverrides acts as an overlay with only the super argument. It is therefore appropriate for basic use, but overlays are more powerful and easier to distribute."

• It seems that there was also a work-in-progress for a user-side declarative configuration but it hasn't been updated in a while: https://github.com/NixOS/nixpkgs/pull/9250

• See also the Declarative package management for normal users discussion on the NixOS Discourse.