fontconfig: add defaultFonts.* options

modules/misc/fontconfig.nix

@@ -1,3 +1,7 @@
+# This module is heavily inspired by the corresponding NixOS module. See
+#
+#   https://github.com/NixOS/nixpkgs/blob/23.11/nixos/modules/config/fonts/fontconfig.nix
+
 { config, lib, pkgs, ... }:
 
 with lib;
@@ -32,6 +36,50 @@ in {
           {command}`nix-env`.
         '';
       };
+
+      defaultFonts = {
+        monospace = mkOption {
+          type = with types; listOf str;
+          default = [ ];
+          description = ''
+            Per-user default monospace font(s). Multiple fonts may be listed in
+            case multiple languages must be supported.
+          '';
+        };
+
+        sansSerif = mkOption {
+          type = with types; listOf str;
+          default = [ ];
+          description = ''
+            Per-user default sans serif font(s). Multiple fonts may be listed
+            in case multiple languages must be supported.
+          '';
+        };
+
+        serif = mkOption {
+          type = with types; listOf str;
+          default = [ ];
+          description = ''
+            Per-user default serif font(s). Multiple fonts may be listed in
+            case multiple languages must be supported.
+          '';
+        };
+
+        emoji = mkOption {
+          type = with types; listOf str;
+          default = [ ];
+          description = ''
+            Per-user default emoji font(s). Multiple fonts may be listed in
+            case a font does not support all emoji.
+
+            Note that fontconfig matches color emoji fonts preferentially,
+            so if you want to use a black and white font while having
+            a color font installed (eg. Noto Color Emoji installed alongside
+            Noto Emoji), fontconfig will still choose the color font even
+            when it is later in the list.
+          '';
+        };
+      };
     };
   };
 
@@ -71,27 +119,53 @@ in {
       fi
     '';
 
-    xdg.configFile = {
+    xdg.configFile = let
-      "fontconfig/conf.d/10-hm-fonts.conf".text = ''
+      mkFontconfigConf = conf: ''
         <?xml version='1.0'?>
 
         <!-- Generated by Home Manager. -->
 
         <!DOCTYPE fontconfig SYSTEM 'urn:fontconfig:fonts.dtd'>
         <fontconfig>
-          <description>Add fonts in the Nix user profile</description>
+        ${conf}
-
-          <include ignore_missing="yes">${config.home.path}/etc/fonts/conf.d</include>
-          <include ignore_missing="yes">${config.home.path}/etc/fonts/fonts.conf</include>
-
-          <dir>${config.home.path}/lib/X11/fonts</dir>
-          <dir>${config.home.path}/share/fonts</dir>
-          <dir>${profileDirectory}/lib/X11/fonts</dir>
-          <dir>${profileDirectory}/share/fonts</dir>
-
-          <cachedir>${config.home.path}/lib/fontconfig/cache</cachedir>
         </fontconfig>
       '';
+    in {
+      "fontconfig/conf.d/10-hm-fonts.conf".text = mkFontconfigConf ''
+        <description>Add fonts in the Nix user profile</description>
+
+        <include ignore_missing="yes">${config.home.path}/etc/fonts/conf.d</include>
+        <include ignore_missing="yes">${config.home.path}/etc/fonts/fonts.conf</include>
+
+        <dir>${config.home.path}/lib/X11/fonts</dir>
+        <dir>${config.home.path}/share/fonts</dir>
+        <dir>${profileDirectory}/lib/X11/fonts</dir>
+        <dir>${profileDirectory}/share/fonts</dir>
+
+        <cachedir>${config.home.path}/lib/fontconfig/cache</cachedir>
+      '';
+
+      "fontconfig/conf.d/52-hm-default-fonts.conf".text = let
+        genDefault = fonts: name:
+          optionalString (fonts != [ ]) ''
+            <alias binding="same">
+              <family>${name}</family>
+              <prefer>
+              ${
+                concatStringsSep "" (map (font: ''
+                  <family>${font}</family>
+                '') fonts)
+              }
+              </prefer>
+            </alias>
+          '';
+      in mkFontconfigConf ''
+        <!-- Default fonts -->
+        ${genDefault cfg.defaultFonts.sansSerif "sans-serif"}
+        ${genDefault cfg.defaultFonts.serif "serif"}
+        ${genDefault cfg.defaultFonts.monospace "monospace"}
+        ${genDefault cfg.defaultFonts.emoji "emoji"}
+      '';
     };
   };
 }