flake: Add overlay

flake.nix

@@ -6,124 +6,127 @@
   };
 
   outputs = { self, nixpkgs, flake-utils, }:
-    with builtins;
     with nixpkgs.lib;
-    flake-utils.lib.eachDefaultSystem (system:
+    {
+      overlays.default = (final: prev:
+        let
+          localFonts = [
+            {
+              # Popular font, used on NYC Subway
+              name = "Akzidenz-Grotesk Pro";
+              dir = ./Akzidenz-Grotesk-Pro;
+              license = licenses.unfree;
+              author = "Berthold Type Foundry";
+            }
+            {
+              # Free handwriting font
+              name = "Caveat";
+              files = [ ./Caveat.ttf ];
+              license = licenses.ofl;
+              author = "Pablo Impallari and Alexi Vanyashin";
+            }
+            {
+              # Deutsche Bahn website font
+              name = "DB Screen";
+              dir = ./DB-Screen;
+              license = licenses.unfree;
+              author = "URW Type Foundry";
+            }
+            {
+              # Popular font, used on Amtrak
+              name = "Frutiger";
+              dir = ./Frutiger;
+              license = licenses.unfree;
+              author = "Adrian Frutiger";
+            }
+            {
+              # Discord custom font
+              name = "gg sans";
+              dir = ./ggsans;
+              license = licenses.unfree;
+              author = "Colophon Foundry";
+            }
+            {
+              # Also called "NR Brunel, used on UK railways in 2010s
+              name = "New Brunel";
+              dir = ./New-Brunel;
+              license = licenses.unfree;
+              author = "David Quay";
+            }
+            {
+              # Modern expansion of machine-readable font
+              name = "OCR-B";
+              files = [ ./OCR-B-Regular.otf ];
+              license = licenses.cc-by-40;
+              author = "Matthew Anderson";
+            }
+            {
+              # Modern expansion of Johnston Sans, used on London Underground
+              name = "P22 Underground";
+              dir = ./P22-Underground;
+              license = licenses.unfree;
+              author = "P22 Type Foundry";
+            }
+          ];
+          buildLocalFont = { name, files ? [ ], dir ? null, license, author }:
+            let
+              types = {
+                ttf = "truetype";
+                otf = "opentype";
+              };
+
+              pathEndsWith = suffix: path:
+                let
+                  str = toString path;
+                  sufLen = stringLength suffix;
+                  sLen = stringLength str;
+                in sufLen <= sLen && suffix
+                == substring (sLen - sufLen) sufLen str;
+
+              copyType = extension: pathName:
+                let
+                  filteredFiles = filter (pathEndsWith ("." + extension)) files;
+                  filename = file:
+                    lists.last (strings.splitString "/" (toString file));
+                in (optionalString (filteredFiles != [ ]) ''
+                  mkdir -p $out/share/fonts/${pathName}
+                '') + concatStringsSep "\n" (map (file:
+                  "cp -v ${file} $out/share/fonts/${pathName}/${filename file}")
+                  filteredFiles);
+
+              copyTypeDir = extension: pathName: ''
+                if [ -n "$(shopt -s nullglob; echo ${dir}/*.${extension})" ]; then
+                  mkdir -p $out/share/fonts/${pathName}
+                  cp -v -t $out/share/fonts/${pathName} ${dir}/*.${extension}
+                fi
+              '';
+
+            in final.stdenvNoCC.mkDerivation {
+              inherit name;
+              dontUnpack = true;
+              installPhase = mapAttrsToList copyType types
+                ++ optional (dir != null) (mapAttrsToList copyTypeDir types);
+              meta = {
+                inherit license;
+                platforms = platforms.all;
+                description = "${name} font by ${author}";
+              };
+            };
+        in {
+          extraFonts = listToAttrs (map (font: {
+            name = replaceStrings [ " " ] [ "-" ] (toLower font.name);
+            value = buildLocalFont font;
+          }) localFonts);
+        });
+    } // flake-utils.lib.eachDefaultSystem (system:
       let
         pkgs = import nixpkgs {
           inherit system;
+          overlays = [ self.overlays.default ];
           config.allowUnfree = true;
         };
-
-        pathEndsWith = suffix: path:
-          let
-            str = toString path;
-            sufLen = stringLength suffix;
-            sLen = stringLength str;
-          in sufLen <= sLen && suffix == substring (sLen - sufLen) sufLen str;
-
-        buildFont = { name, files ? [ ], dir ? null, license, author }:
-          let
-            types = {
-              ttf = "truetype";
-              otf = "opentype";
-            };
-
-            copyType = extension: pathName:
-              let
-                filteredFiles = filter (pathEndsWith ("." + extension)) files;
-                filename = file:
-                  lists.last (strings.splitString "/" (toString file));
-              in (optionalString (filteredFiles != [ ]) ''
-                mkdir -p $out/share/fonts/${pathName}
-              '') + concatStringsSep "\n" (map (file:
-                "cp -v ${file} $out/share/fonts/${pathName}/${filename file}")
-                filteredFiles);
-
-            copyTypeDir = extension: pathName: ''
-              if [ -n "$(shopt -s nullglob; echo ${dir}/*.${extension})" ]; then
-                mkdir -p $out/share/fonts/${pathName}
-                cp -v -t $out/share/fonts/${pathName} ${dir}/*.${extension}
-              fi
-            '';
-          in pkgs.stdenvNoCC.mkDerivation {
-            inherit name;
-
-            dontUnpack = true;
-            installPhase = mapAttrsToList copyType types
-              ++ optional (dir != null) (mapAttrsToList copyTypeDir types);
-
-            meta = {
-              inherit license;
-              platforms = platforms.all;
-              description = "${name} font by ${author}";
-            };
-          };
-
-        fonts = [
-          {
-            # Popular font, used on NYC Subway
-            name = "Akzidenz-Grotesk Pro";
-            dir = ./Akzidenz-Grotesk-Pro;
-            license = licenses.unfree;
-            author = "Berthold Type Foundry";
-          }
-          {
-            # Free handwriting font
-            name = "Caveat";
-            files = [ ./Caveat.ttf ];
-            license = licenses.ofl;
-            author = "Pablo Impallari and Alexi Vanyashin";
-          }
-          {
-            # Deutsche Bahn website font
-            name = "DB Screen";
-            dir = ./DB-Screen;
-            license = licenses.unfree;
-            author = "URW Type Foundry";
-          }
-          {
-            # Popular font, used on Amtrak
-            name = "Frutiger";
-            dir = ./Frutiger;
-            license = licenses.unfree;
-            author = "Adrian Frutiger";
-          }
-          {
-            # Discord custom font
-            name = "gg sans";
-            dir = ./ggsans;
-            license = licenses.unfree;
-            author = "Colophon Foundry";
-          }
-          {
-            # Also called "NR Brunel, used on UK railways in 2010s
-            name = "New Brunel";
-            dir = ./New-Brunel;
-            license = licenses.unfree;
-            author = "David Quay";
-          }
-          {
-            # Modern expansion of machine-readable font
-            name = "OCR-B";
-            files = [ ./OCR-B-Regular.otf ];
-            license = licenses.cc-by-40;
-            author = "Matthew Anderson";
-          }
-          {
-            # Modern expansion of Johnston Sans, used on London Underground
-            name = "P22 Underground";
-            dir = ./P22-Underground;
-            license = licenses.unfree;
-            author = "P22 Type Foundry";
-          }
-        ];
       in {
-        packages = listToAttrs (map (font: {
+        packages = pkgs.extraFonts;
-          name = replaceStrings [ " " ] [ "-" ] (toLower font.name);
-          value = buildFont font;
-        }) fonts);
-
         formatter = pkgs.nixfmt;
       });
 }