add elpa-packages.nix

pkgs/applications/editors/emacs-modes/elpa-packages.nix

@@ -0,0 +1,42 @@
+pkgs: with pkgs;
+
+let
+
+  inherit (stdenv.lib) makeScope mapAttrs;
+
+  json = builtins.readFile ./elpa-packages.json;
+  manifest = builtins.fromJSON json;
+
+  mkPackage = self: name: recipe:
+    let drv =
+          { elpaBuild, stdenv, fetchurl }:
+          let fetch = { inherit fetchurl; }."${recipe.fetch.tag}"
+                or (abort "emacs-${name}: unknown fetcher '${recipe.fetch.tag}'");
+              args = builtins.removeAttrs recipe.fetch [ "tag" ];
+              src = fetch args;
+          in elpaBuild {
+            pname = name;
+            inherit (recipe) version;
+            inherit src;
+            deps =
+              let lookupDep = d:
+                    self."${d}" or (abort "emacs-${name}: missing dependency ${d}");
+              in map lookupDep recipe.deps;
+            meta = {
+              homepage = "http://elpa.gnu.org/packages/${name}.html";
+              license = stdenv.lib.licenses.free;
+            };
+          };
+    in self.callPackage drv {};
+
+  packages = self:
+    let
+      elpaPackages = mapAttrs (mkPackage self) manifest;
+
+      elpaBuild = import ../../../build-support/emacs/melpa.nix {
+        inherit (pkgs) lib stdenv fetchurl texinfo;
+        inherit (self) emacs;
+      };
+    in elpaPackages // { inherit elpaBuild elpaPackages; };
+
+in makeScope pkgs.newScope packages