lib: init XDG-specific subset

This is arguably more suitable for certain environment but Nix ecosystem
is already largely Unix-adjacent anyways.

lib/default.nix

@@ -13,6 +13,7 @@ in pkgs.lib.makeExtensible (self:
     trivial = callLib ./trivial.nix;
     data = callLib ./data.nix;
     math = callLib ./math.nix;
+    xdg = callLib ./xdg.nix;
 
     # For future references, these are the only attributes that are going to be
     # exported as part of nixpkgs overlay.
@@ -43,6 +44,7 @@ in pkgs.lib.makeExtensible (self:
     inherit (self.trivial) countAttrs filterAttrs';
     inherit (self.data) importYAML renderTeraTemplate renderMustacheTemplate;
     inherit (self.fetchers) fetchInternetArchive fetchUgeeDriver;
+    inherit (self.xdg) getXdgDesktop;
   } // lib.optionalAttrs (builtins ? fetchTree) {
     flake = callLib ./flake.nix;
 

lib/xdg.nix

@@ -0,0 +1,31 @@
+{ pkgs, lib, self }:
+
+rec {
+  /**
+    Naively get the absolute path of a `.desktop` file given a derivation and a
+    name.
+
+    # Arguments
+
+    drv
+    : The derivation.
+
+    name
+    : The name of the `.desktop` file (without the `.desktop` extension).
+
+    # Type
+
+    ```
+    getXdgDesktop :: Derivation -> String -> Path
+    ```
+
+    # Example
+
+    ```nix
+    getXdgDesktop pkgs.wezterm "org.wezfurlong.wezterm"
+    => /nix/store/$HASH-wezterm-org.wezterm.wezterm.desktop
+    ```
+  */
+  getXdgDesktop = drv: name:
+    "${drv}/share/applications/${name}.desktop";
+}

tests/lib/default.nix

@@ -27,6 +27,7 @@ in {
   trivial = callLib ./trivial.nix;
   data = callLib ./data;
   math = callLib ./math.nix;
+  xdg = callLib ./xdg.nix;
 
   # Environment-specific subset.
   home-manager = callLib ./home-manager.nix;

tests/lib/xdg.nix

@@ -0,0 +1,14 @@
+{ pkgs, lib, self }:
+
+lib.runTests {
+  testGetXdgDesktop = {
+    expr = self.xdg.getXdgDesktop pkgs.wezterm "org.wezfurlong.wezterm";
+    expected = "${pkgs.wezterm}/share/applications/org.wezfurlong.wezterm.desktop";
+  };
+
+  # This should be a naive function so it should just naively get things.
+  testGetXdgDesktop2 = {
+    expr = self.xdg.getXdgDesktop pkgs.hello "non-existing-desktop";
+    expected = "${pkgs.hello}/share/applications/non-existing-desktop.desktop";
+  };
+}