If I just add

let
  compiledLayout = pkgs.runCommand "keyboard-layout" {} ''
    ${pkgs.xorg.xkbcomp}/bin/xkbcomp ${/path/to/file//ergol.xkb} $out
  '';
in
  services.xserver.displayManager.sessionCommands = "${pkgs.xorg.xkbcomp}/bin/xkbcomp ${compiledLayout} $DISPLAY";

on top of my configuration.nix file, I get an error

In reply to @thatoo:matrix.org
Hello,
How can I add this keyboard layout to my configuration.nix file while I'm using flakes : https://github.com/Nuclear-Squid/ergol/raw/main/0_7_0/ergol.xkb_custom ?

services.xserver = {
    layout = "fr";
    xkbVariant = "";
    xkb.extraLayouts = {
      mine = {
        description = "ergol xkb layout.";
        languages = [ "fr-ergol" ];
        symbolsFile = /path/to/file/ergol.xkb_custom;
      };
    };
  };

should work?
Do I need to reboot or logout/login?

mewpjust to let you know that I'm typing with this ergol.xkb_custom keaybord layout. It has work exactly as you said. After a logout/login, I could choose and use it.
Thank you.