shared/modules/hjem: init environment module

shared/modules/hjem/environment.nix

@@ -0,0 +1,64 @@
+{
+  config,
+  lib,
+  ...
+}: let
+  inherit (lib) concatStringsSep mapAttrsToList;
+  inherit (lib.modules) mkIf;
+  inherit (lib.options) mkOption mkEnableOption;
+  inherit (lib.types) attrsOf listOf oneOf int path str;
+
+  envFile = "99-user-env.conf";
+  cfg = config.environment;
+
+  toString = env:
+    if builtins.isList env
+    then concatStringsSep ":" env
+    else builtins.toString env;
+
+  toConf = attrs:
+    concatStringsSep "\n"
+    (mapAttrsToList (name: value: "${name}=\"${toString value}\"") attrs);
+in {
+  options.environment = {
+    enable = mkEnableOption "environment management";
+    variables = mkOption {
+      default = {};
+      example = {
+        EDITOR = "nvim";
+        VISUAL = "nvim";
+      };
+      description = ''
+        A set of environment variables used in the user environment.
+        These variables will be set as systemd environment
+        variables, using `environment.d`. The value of each
+        variable can be either a string or a list of strings. The
+        latter is concatenated, interspersed with colon
+        characters.
+      '';
+      type = attrsOf (oneOf [(listOf (oneOf [int str path])) int str path]);
+    };
+  };
+
+  config =
+    mkIf cfg.enable
+    {
+      files.".config/environment.d/${envFile}".text = toConf cfg.variables;
+
+      warnings = let
+        overlappingVars = builtins.filter (x: builtins.elem x (builtins.attrNames config.environment.variables)) (builtins.attrNames cfg.variables);
+      in
+        if overlappingVars != []
+        then
+          map (name: ''
+            The environment variable '${name}' is defined in both
+            `hjem.users.<name>.environment.variables` and
+            `config.environment.variables`. This may lead to conflicts.
+
+            If you want the one defined in hjem to take precedence, make sure you
+            unset it manually in `environment.extraInit` (`unset ${name}`).
+          '')
+          overlappingVars
+        else [];
+    };
+}