Hey! 
I’ve got some questions on moving a NixOS system configuration to a flake. I’m currently not able to make it work and I have some question about it in general.
Edit: I may have been able to solve the first issue by using a higher home.stateVersion (don’t think I had one before(?) and used the system.stateVersion). This seems to get past the first issue, but I’m now stuck at using overlays again 
Having seen this video on moving your NixOS system configuration to a flake (which made it look real easy), I thought I’d give it a go myself. But in doing so, I encountered a few issues. I’ve managed to solve (I think?) some of them, but am now stuck. I was hoping I could get some help.
For reference, here’s the current state of the flake I’m using (tracking my efforts, so it may change).
For reference, here's the current state of the flake I'm using.
{
description = "My Nixos system config";
inputs =
# let
# systemVersion = "22.05";
# # if system version is "unstable" use "master"; otherwise use "release-${systemVersion}"
# # homeManagerVersion = if systemVersion == "unstable" then "master" else "release-${systemVersion}";
# in
{
# nixpkgs.url = "nixpkgs/nixos-${systemVersion}";
nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
home-manager.url = "github:nix-community/home-manager/master";
home-manager.inputs.nixpkgs.follows = "nixpkgs";
musnix.url = "github:musnix/musnix";
emacsOverlay.url = "github:nix-community/emacs-overlay";
};
outputs = { nixpkgs, home-manager, musnix, emacsOverlay, ... }@inputs:
let
system = "x86_64-linux";
pkgs = import nixpkgs {
inherit system;
config = {
allowUnfree = true;
};
};
lib = nixpkgs.lib;
in
{
homeManagerConfigs = {
phaaze = home-manager.lib.homeManagerConfiguration {
pkgs = nixpkgs.legacyPackages.${system};
# pkgs = import nixpkgs {
# overlays = [emacsOverlay.overlay];
# inherit system;
# };
modules = [
({
nixpkgs.overlays = [ emacsOverlay.overlay ];
})
# ({ pkgs, ... }: {
# nixpkgs.overlays = [ emacsOverlay.overlay ];
# })
./phaaze/home.nix
{
home = {
username = "thomas";
homeDirectory = "/home/thomas";
stateVersion = "18.09";
};
}
];
};
};
nixosConfigurations = {
phaaze = lib.nixosSystem {
inherit system;
modules = [
musnix.nixosModules.musnix
./phaaze/configuration.nix
];
};
};
};
}
The current issue is that when I try to build the home manager package (using nix build .#homeManagerConfigs.phaaze.activationPackage), I get this message:
error: cannot look up '<nixpkgs>' in pure evaluation mode (use '--impure' to override)
at /nix/store/bdk5advb7w9kfnv25z9j6xmmmqfy9sga-source/modules/modules.nix:301:11:
300| else
301| <nixpkgs>);
| ^
302| _module.args.pkgs = lib.mkDefault pkgs;
(use '--show-trace' to show detailed location information)
For most of the error messages I have found, I have been able to go and change the source file to adapt to the error message, but this doesn’t seem to be (directly) in my files at all. Following the file it points to, I do indeed find the line in question, but it looks auto-generated to me, and I don’t know how or if I can change it:
`modules.nix`, the file that errors
{ pkgs
# Note, this should be "the standard library" + HM extensions.
, lib
# Whether to enable module type checking.
, check ? true
# If disabled, the pkgs attribute passed to this function is used instead.
, useNixpkgsModule ? true }:
with lib;
let
modules = [
./accounts/email.nix
./config/home-cursor.nix
./config/i18n.nix
./files.nix
./home-environment.nix
./i18n/input-method/default.nix
./launchd/default.nix
./manual.nix
./misc/dconf.nix
./misc/debug.nix
./misc/fontconfig.nix
./misc/gtk.nix
./misc/lib.nix
./misc/news.nix
./misc/numlock.nix
./misc/pam.nix
./misc/qt.nix
./misc/specialization.nix
./misc/submodule-support.nix
./misc/tmpfiles.nix
./misc/version.nix
./misc/vte.nix
./misc/xdg-desktop-entries.nix
./misc/xdg-mime-apps.nix
./misc/xdg-mime.nix
./misc/xdg-system-dirs.nix
./misc/xdg-user-dirs.nix
./misc/xdg.nix
./programs/abook.nix
./programs/afew.nix
./programs/alacritty.nix
./programs/alot.nix
./programs/aria2.nix
./programs/astroid.nix
./programs/atuin.nix
./programs/autojump.nix
./programs/autorandr.nix
./programs/bash.nix
./programs/bat.nix
./programs/beets.nix
./programs/bottom.nix
./programs/broot.nix
./programs/browserpass.nix
./programs/chromium.nix
./programs/command-not-found/command-not-found.nix
./programs/dircolors.nix
./programs/direnv.nix
./programs/eclipse.nix
./programs/emacs.nix
./programs/eww.nix
./programs/exa.nix
./programs/feh.nix
./programs/firefox.nix
./programs/fish.nix
./programs/foot.nix
./programs/fzf.nix
./programs/getmail.nix
./programs/gh.nix
./programs/git.nix
./programs/gitui.nix
./programs/gnome-terminal.nix
./programs/go.nix
./programs/gpg.nix
./programs/helix.nix
./programs/hexchat.nix
./programs/himalaya.nix
./programs/home-manager.nix
./programs/htop.nix
./programs/hyfetch.nix
./programs/i3status-rust.nix
./programs/i3status.nix
./programs/info.nix
./programs/ion.nix
./programs/irssi.nix
./programs/java.nix
./programs/jq.nix
./programs/just.nix
./programs/kakoune.nix
./programs/keychain.nix
./programs/kitty.nix
./programs/kodi.nix
./programs/lazygit.nix
./programs/less.nix
./programs/lesspipe.nix
./programs/lf.nix
./programs/librewolf.nix
./programs/lieer.nix
./programs/lsd.nix
./programs/man.nix
./programs/mangohud.nix
./programs/matplotlib.nix
./programs/mbsync.nix
./programs/mcfly.nix
./programs/mercurial.nix
./programs/micro.nix
./programs/mpv.nix
./programs/msmtp.nix
./programs/mu.nix
./programs/mujmap.nix
./programs/navi.nix
./programs/ncmpcpp.nix
./programs/ncspot.nix
./programs/ne.nix
./programs/neomutt.nix
./programs/neovim.nix
./programs/newsboat.nix
./programs/nix-index.nix
./programs/nnn.nix
./programs/noti.nix
./programs/notmuch.nix
./programs/nushell.nix
./programs/obs-studio.nix
./programs/octant.nix
./programs/offlineimap.nix
./programs/opam.nix
./programs/pandoc.nix
./programs/password-store.nix
./programs/pazi.nix
./programs/pet.nix
./programs/pidgin.nix
./programs/pistol.nix
./programs/piston-cli.nix
./programs/powerline-go.nix
./programs/pubs.nix
./programs/pylint.nix
./programs/qutebrowser.nix
./programs/rbw.nix
./programs/readline.nix
./programs/rofi-pass.nix
./programs/rofi.nix
./programs/rtorrent.nix
./programs/sagemath.nix
./programs/sbt.nix
./programs/scmpuff.nix
./programs/senpai.nix
./programs/sioyek.nix
./programs/skim.nix
./programs/sm64ex.nix
./programs/sqls.nix
./programs/ssh.nix
./programs/starship.nix
./programs/swaylock.nix
./programs/taskwarrior.nix
./programs/tealdeer.nix
./programs/terminator.nix
./programs/termite.nix
./programs/texlive.nix
./programs/timidity.nix
./programs/tint2.nix
./programs/tiny.nix
./programs/tmux.nix
./programs/topgrade.nix
./programs/urxvt.nix
./programs/vim.nix
./programs/vscode.nix
./programs/vscode/haskell.nix
./programs/pywal.nix
./programs/watson.nix
./programs/waybar.nix
./programs/xmobar.nix
./programs/z-lua.nix
./programs/zathura.nix
./programs/zellij.nix
./programs/zoxide.nix
./programs/zplug.nix
./programs/zsh.nix
./programs/zsh/prezto.nix
./services/barrier.nix
./services/betterlockscreen.nix
./services/blueman-applet.nix
./services/caffeine.nix
./services/cbatticon.nix
./services/clipmenu.nix
./services/devilspie2.nix
./services/dropbox.nix
./services/dunst.nix
./services/dwm-status.nix
./services/easyeffects.nix
./services/emacs.nix
./services/etesync-dav.nix
./services/espanso.nix
./services/flameshot.nix
./services/fluidsynth.nix
./services/fnott.nix
./services/fusuma.nix
./services/getmail.nix
./services/git-sync.nix
./services/gnome-keyring.nix
./services/gpg-agent.nix
./services/grobi.nix
./services/gromit-mpx.nix
./services/home-manager-auto-upgrade.nix
./services/hound.nix
./services/imapnotify.nix
./services/kanshi.nix
./services/kbfs.nix
./services/kdeconnect.nix
./services/keepassx.nix
./services/keybase.nix
./services/keynav.nix
./services/lieer.nix
./services/lorri.nix
./services/mako.nix
./services/mbsync.nix
./services/mopidy.nix
./services/mpd.nix
./services/mpdris2.nix
./services/mpd-discord-rpc.nix
./services/mpris-proxy.nix
./services/muchsync.nix
./services/network-manager-applet.nix
./services/nextcloud-client.nix
./services/notify-osd.nix
./services/opensnitch-ui.nix
./services/owncloud-client.nix
./services/pantalaimon.nix
./services/parcellite.nix
./services/pass-secret-service.nix
./services/password-store-sync.nix
./services/pasystray.nix
./services/pbgopy.nix
./services/picom.nix
./services/plan9port.nix
./services/playerctld.nix
./services/plex-mpv-shim.nix
./services/polybar.nix
./services/poweralertd.nix
./services/pulseeffects.nix
./services/random-background.nix
./services/recoll.nix
./services/redshift-gammastep/gammastep.nix
./services/redshift-gammastep/redshift.nix
./services/rsibreak.nix
./services/screen-locker.nix
./services/sctd.nix
./services/spotifyd.nix
./services/stalonetray.nix
./services/status-notifier-watcher.nix
./services/swayidle.nix
./services/sxhkd.nix
./services/syncthing.nix
./services/systembus-notify.nix
./services/taffybar.nix
./services/tahoe-lafs.nix
./services/taskwarrior-sync.nix
./services/trayer.nix
./services/twmn.nix
./services/udiskie.nix
./services/unclutter.nix
./services/unison.nix
./services/volnoti.nix
./services/window-managers/awesome.nix
./services/window-managers/bspwm/default.nix
./services/window-managers/herbstluftwm.nix
./services/window-managers/i3-sway/i3.nix
./services/window-managers/i3-sway/sway.nix
./services/window-managers/i3-sway/swaynag.nix
./services/window-managers/spectrwm.nix
./services/window-managers/xmonad.nix
./services/wlsunset.nix
./services/xcape.nix
./services/xembed-sni-proxy.nix
./services/xidlehook.nix
./services/xscreensaver.nix
./services/xsettingsd.nix
./services/xsuspender.nix
./systemd.nix
./targets/darwin
./targets/generic-linux.nix
./xresources.nix
./xsession.nix
./misc/nix.nix
(pkgs.path + "/nixos/modules/misc/assertions.nix")
(pkgs.path + "/nixos/modules/misc/meta.nix")
] ++ optional useNixpkgsModule ./misc/nixpkgs.nix
++ optional (!useNixpkgsModule) ./misc/nixpkgs-disabled.nix;
pkgsModule = { config, ... }: {
config = {
_module.args.baseModules = modules;
_module.args.pkgsPath = lib.mkDefault
(if versionAtLeast config.home.stateVersion "20.09" then
pkgs.path
else
<nixpkgs>);
_module.args.pkgs = lib.mkDefault pkgs;
_module.check = check;
lib = lib.hm;
} // optionalAttrs useNixpkgsModule {
nixpkgs.system = mkDefault pkgs.stdenv.hostPlatform.system;
};
};
in modules ++ [ pkgsModule ]
Does anyone know what this is about or what might be going on?
Extra questions
I have a few more questions relating to using flakes for system configuration. Some of them may or may not relate to the issue: I genuinely don’t know.
Nix expressions outside of outputs
From my understanding, you can’t do any sort of evaluation outside of the outputs attribute, is that correct? I wanted to put my system version into a variable and use that to update all paths in the inputs attribute, but using a let block, I was told that it got a thunk but expected a set. I think I found a github thread about this when searching, but it was definitely surprising. Is there any way about this? I’d like to set system version once and then populate the nixos and home-manager channels with the correct url based on that.
How do you use overlays?
(This may relate to the issue I’m having, but I don’t know.)
Up until now, I’ve used the Nix community emacs overlay to get my emacs version. I’ve imported it as an overlay with fetchTarball in my home-manager config. However, I thought I’d try and add the overlay to the flake instead (seems like the right thing to do), but I can’t find any good documentation on how to add overlays in flakes. I did manage to find this response in a thread here on discourse which gives a brief explanation on how to set it up. That seems to work, but it doesn’t say anything about how you reference the overlay in any of the other modules.
What I ended up doing was this:
phaaze = home-manager.lib.homeManagerConfiguration {
pkgs = nixpkgs.legacyPackages.${system};
modules = [
({
nixpkgs.overlays = [ emacsOverlay.overlay ];
})
./phaaze/home.nix
{
home = {
username = "thomas";
homeDirectory = "/home/thomas";
stateVersion = "18.09";
};
}
];
};
and then referencing the version of emacs in my home manager module params:
{ pkgs, config, emacsNativeComp, ... }: # adding emacsNativeComp to the top of the file
let
# ... rest of the config
Is that how it’s supposed to be done? Or is there another way?
Update: The overlay method does not work
After updating the home.stateVersion I used, I got past the initial hurdle mentioned above. It now gets to the point where it seems to try and evaluate my home config, but it can’t find emacsNativeComp. So the overlay question is now the main one: how do you use them and reference them in pre-existing modules?
Thanks a lot for making it all the way down here! Any input (on any of the questions) would be very much appreciated 
If you need additional info, I would of course be happy to provide that too 
