Rebuild fails "error: cannot look up '<nixos-wsl/modules>' in pure evaluation mode" with flakes enabled

[JackTheMico]

Hi, I can run sudo nixos-rebuild switch to rebuild system successfully. But after I enabled the flakes feature and write a simple /etc/nixos/flake.nix configuration file, the same command returned the error message.
My flake.nix file's content is simple:

{
  description = "A very basic flake";

  outputs = { self, nixpkgs }: {
     nixosConfigurations.nixos = nixpkgs.lib.nixosSystem {
       system = "x86_64-linux";
       modules = [
        ./configuration.nix
       ];
     };
  };
}

so as my configuration.nix file:

# Edit this configuration file to define what should be installed on
# your system. Help is available in the configuration.nix(5) man page, on
# https://search.nixos.org/options and in the NixOS manual (`nixos-help`).

# NixOS-WSL specific options are documented on the NixOS-WSL repository:
# https://github.com/nix-community/NixOS-WSL

{ config, lib, pkgs, ... }:

{
  imports = [
    # include NixOS-WSL modules
    <nixos-wsl/modules>
  ];

  wsl.enable = true;
  wsl.defaultUser = "nixos";
  nix.settings.experimental-features = [ "nix-command" "flakes" ];
  environment.systemPackages = with pkgs; [
    git
    neovim
    wget
    # helix.packages."${pkgs}".helix
  ];


  # This value determines the NixOS release from which the default
  # settings for stateful data, like file locations and database versions
  # on your system were taken. It's perfectly fine and recommended to leave
  # this value at the release version of the first install of this system.
  # Before changing this value read the documentation for this option
  # (e.g. man configuration.nix or on https://nixos.org/nixos/options.html).
  system.stateVersion = "23.11"; # Did you read the comment?
}

The full error output are:

[nixos@nixos:~]$ sudo nixos-rebuild switch --option eval-cache false --show-trace
error:
       … while calling the 'seq' builtin

         at /nix/store/jnk1mfi594y0dsmzla4nmkbr4rnrqa71-source/lib/modules.nix:322:18:

          321|         options = checked options;
          322|         config = checked (removeAttrs config [ "_module" ]);
             |                  ^
          323|         _module = checked (config._module);

       … while evaluating a branch condition

         at /nix/store/jnk1mfi594y0dsmzla4nmkbr4rnrqa71-source/lib/modules.nix:261:9:

          260|       checkUnmatched =
          261|         if config._module.check && config._module.freeformType == null && merged.unmatchedDefns != [] then
             |         ^
          262|           let

       … in the left operand of the AND (&&) operator

         at /nix/store/jnk1mfi594y0dsmzla4nmkbr4rnrqa71-source/lib/modules.nix:261:72:

          260|       checkUnmatched =
          261|         if config._module.check && config._module.freeformType == null && merged.unmatchedDefns != [] then
             |                                                                        ^
          262|           let

       … in the left operand of the AND (&&) operator

         at /nix/store/jnk1mfi594y0dsmzla4nmkbr4rnrqa71-source/lib/modules.nix:261:33:

          260|       checkUnmatched =
          261|         if config._module.check && config._module.freeformType == null && merged.unmatchedDefns != [] then
             |                                 ^
          262|           let

       … while evaluating a branch condition

         at /nix/store/jnk1mfi594y0dsmzla4nmkbr4rnrqa71-source/lib/modules.nix:254:12:

          253|
          254|         in if declaredConfig._module.freeformType == null then declaredConfig
             |            ^
          255|           # Because all definitions that had an associated option ended in

       … from call site

         at /nix/store/jnk1mfi594y0dsmzla4nmkbr4rnrqa71-source/lib/modules.nix:242:28:

          241|           # For definitions that have an associated option
          242|           declaredConfig = mapAttrsRecursiveCond (v: ! isOption v) (_: v: v.value) options;
             |                            ^
          243|

       … while calling 'mapAttrsRecursiveCond'

         at /nix/store/jnk1mfi594y0dsmzla4nmkbr4rnrqa71-source/lib/attrsets.nix:734:5:

          733|     # Attribute set to recursively map over.
          734|     set:
             |     ^
          735|     let

       … from call site

         at /nix/store/jnk1mfi594y0dsmzla4nmkbr4rnrqa71-source/lib/modules.nix:234:33:

          233|           ({ inherit lib options config specialArgs; } // specialArgs);
          234|         in mergeModules prefix (reverseList collected);
             |                                 ^
          235|

       … while calling 'reverseList'

         at /nix/store/jnk1mfi594y0dsmzla4nmkbr4rnrqa71-source/lib/lists.nix:510:17:

          509|   */
          510|   reverseList = xs:
             |                 ^
          511|     let l = length xs; in genList (n: elemAt xs (l - n - 1)) l;

       … from call site

         at /nix/store/jnk1mfi594y0dsmzla4nmkbr4rnrqa71-source/lib/modules.nix:229:25:

          228|       merged =
          229|         let collected = collectModules
             |                         ^
          230|           class

       … while calling anonymous lambda

         at /nix/store/jnk1mfi594y0dsmzla4nmkbr4rnrqa71-source/lib/modules.nix:445:37:

          444|
          445|     in modulesPath: initialModules: args:
             |                                     ^
          446|       filterModules modulesPath (collectStructuredModules unknownModule "" initialModules args);

       … from call site

         at /nix/store/jnk1mfi594y0dsmzla4nmkbr4rnrqa71-source/lib/modules.nix:446:7:

          445|     in modulesPath: initialModules: args:
          446|       filterModules modulesPath (collectStructuredModules unknownModule "" initialModules args);
             |       ^
          447|

       … while calling 'filterModules'

         at /nix/store/jnk1mfi594y0dsmzla4nmkbr4rnrqa71-source/lib/modules.nix:413:36:

          412|       # modules recursively. It returns the final list of unique-by-key modules
          413|       filterModules = modulesPath: { disabled, modules }:
             |                                    ^
          414|         let

       … while calling anonymous lambda

         at /nix/store/jnk1mfi594y0dsmzla4nmkbr4rnrqa71-source/lib/modules.nix:439:31:

          438|           disabledKeys = concatMap ({ file, disabled }: map (moduleKey file) disabled) disabled;
          439|           keyFilter = filter (attrs: ! elem attrs.key disabledKeys);
             |                               ^
          440|         in map (attrs: attrs.module) (builtins.genericClosure {

       … from call site

         at /nix/store/jnk1mfi594y0dsmzla4nmkbr4rnrqa71-source/lib/modules.nix:400:22:

          399|           let
          400|             module = checkModule (loadModule args parentFile "${parentKey}:anon-${toString n}" x);
             |                      ^
          401|             collectedImports = collectStructuredModules module._file module.key module.imports args;

       … while calling anonymous lambda

         at /nix/store/jnk1mfi594y0dsmzla4nmkbr4rnrqa71-source/lib/modules.nix:359:11:

          358|         then
          359|           m:
             |           ^
          360|             if m._class != null -> m._class == class

       … from call site

         at /nix/store/jnk1mfi594y0dsmzla4nmkbr4rnrqa71-source/lib/modules.nix:400:35:

          399|           let
          400|             module = checkModule (loadModule args parentFile "${parentKey}:anon-${toString n}" x);
             |                                   ^
          401|             collectedImports = collectStructuredModules module._file module.key module.imports args;

       … while calling 'loadModule'

         at /nix/store/jnk1mfi594y0dsmzla4nmkbr4rnrqa71-source/lib/modules.nix:336:53:

          335|       # Like unifyModuleSyntax, but also imports paths and calls functions if necessary
          336|       loadModule = args: fallbackFile: fallbackKey: m:
             |                                                     ^
          337|         if isFunction m then

       … from call site

         at /nix/store/jnk1mfi594y0dsmzla4nmkbr4rnrqa71-source/lib/modules.nix:337:12:

          336|       loadModule = args: fallbackFile: fallbackKey: m:
          337|         if isFunction m then
             |            ^
          338|           unifyModuleSyntax fallbackFile fallbackKey (applyModuleArgs fallbackKey m args)

       … while calling 'isFunction'

         at /nix/store/jnk1mfi594y0dsmzla4nmkbr4rnrqa71-source/lib/trivial.nix:458:16:

          457|   */
          458|   isFunction = f: builtins.isFunction f ||
             |                ^
          459|     (f ? __functor && isFunction (f.__functor f));

       error: cannot look up '<nixos-wsl/modules>' in pure evaluation mode (use '--impure' to override)

       at «none»:0: (source not available)

I couldn't find out what's going wrong. Anyone please take a look and help me out.
Let me know if you need more details. Thank you.

[SuperSandro2000]

You want to use a flake input instead of the channel.

[JackTheMico]

Thank you for getting back to me. Could you be more specific?

[573]

I think where you have <nixos-wsl/modules> you're using the channel and using inputs.nixos-wsl.nixosModules would be what is meant depending on what your flake input for nixos-wsl is called.

[nzbr]

It seems like you haven't added a nixos-wsl input to your flake. Channels do not work with flakes, because the flake inputs replace them. Your source files need to look like this:

flake.nix:

{
  description = "A very basic flake";
  
  inputs = {
    nixpkgs.url = "github:nixos/nixpkgs/nixos-23.11";
    nixos-wsl.url = "github:nix-community/nixos-wsl";
  };

  outputs = { self, nixpkgs, nixos-wsl }: {
     nixosConfigurations.nixos = nixpkgs.lib.nixosSystem {
       system = "x86_64-linux";
       modules = [
        ./configuration.nix
        nixos-wsl.nixosModules.wsl
       ];
     };
  };
}

configuration.nix:

# Edit this configuration file to define what should be installed on
# your system. Help is available in the configuration.nix(5) man page, on
# https://search.nixos.org/options and in the NixOS manual (`nixos-help`).

# NixOS-WSL specific options are documented on the NixOS-WSL repository:
# https://github.com/nix-community/NixOS-WSL

{ config, lib, pkgs, ... }:

{
  wsl.enable = true;
  wsl.defaultUser = "nixos";
  nix.settings.experimental-features = [ "nix-command" "flakes" ];
  environment.systemPackages = with pkgs; [
    git
    neovim
    wget
    # helix.packages."${pkgs}".helix
  ];


  # This value determines the NixOS release from which the default
  # settings for stateful data, like file locations and database versions
  # on your system were taken. It's perfectly fine and recommended to leave
  # this value at the release version of the first install of this system.
  # Before changing this value read the documentation for this option
  # (e.g. man configuration.nix or on https://nixos.org/nixos/options.html).
  system.stateVersion = "23.11"; # Did you read the comment?
}

[benjaminrich]

Thank you, this worked for me. It would be nice if this could be made more prominent in the documentation, as it wasn't really obvious.

[timblaktu]

https://nix-community.github.io/NixOS-WSL/howto.html

[Yakumo-zi]

It seems like you haven't added a nixos-wsl input to your flake. Channels do not work with flakes, because the flake inputs replace them. Your source files need to look like this:

flake.nix:

{
  description = "A very basic flake";
  
  inputs = {
    nixpkgs.url = "github:nixos/nixpkgs/nixos-23.11";
    nixos-wsl.url = "github:nix-community/nixos-wsl";
  };

  outputs = { self, nixpkgs, nixos-wsl }: {
     nixosConfigurations.nixos = nixpkgs.lib.nixosSystem {
       system = "x86_64-linux";
       modules = [
        ./configuration.nix
        nixos-wsl.nixosModules.wsl
       ];
     };
  };
}

configuration.nix:

# Edit this configuration file to define what should be installed on
# your system. Help is available in the configuration.nix(5) man page, on
# https://search.nixos.org/options and in the NixOS manual (`nixos-help`).

# NixOS-WSL specific options are documented on the NixOS-WSL repository:
# https://github.com/nix-community/NixOS-WSL

{ config, lib, pkgs, ... }:

{
  wsl.enable = true;
  wsl.defaultUser = "nixos";
  nix.settings.experimental-features = [ "nix-command" "flakes" ];
  environment.systemPackages = with pkgs; [
    git
    neovim
    wget
    # helix.packages."${pkgs}".helix
  ];


  # This value determines the NixOS release from which the default
  # settings for stateful data, like file locations and database versions
  # on your system were taken. It's perfectly fine and recommended to leave
  # this value at the release version of the first install of this system.
  # Before changing this value read the documentation for this option
  # (e.g. man configuration.nix or on https://nixos.org/nixos/options.html).
  system.stateVersion = "23.11"; # Did you read the comment?
}

it works for me.