elisp.nix: add a param to include user config as a default init file

README.org

@@ -91,6 +91,17 @@ required in a user's config via =use-package= or =leaf=.
         # config = ./emacs.org;
         config = ./emacs.el;
 
+        # Whether to include your config as a default init file.
+        # If being bool, the value of config is used.
+        # Its value can also be a derivation like this if you want to do some
+        # substitution:
+        #   defaultInitFile = pkgs.substituteAll {
+        #     name = "default.el";
+        #     src = ./emacs.el;
+        #     inherit (config.xdg) configHome dataHome;
+        #   };
+        defaultInitFile = true;
+
         # Package is optional, defaults to pkgs.emacs
         package = pkgs.emacsGit;
 

elisp.nix

@@ -12,6 +12,8 @@ let
 
 in
 { config
+# bool to use the value of config or a derivation whose name is default.el
+, defaultInitFile ? false
 # emulate `use-package-always-ensure` behavior (defaulting to false)
 , alwaysEnsure ? null
 # emulate `#+PROPERTY: header-args:emacs-lisp :tangle yes`
@@ -64,5 +66,27 @@ emacsWithPackages (epkgs:
     overridden = override epkgs;
     usePkgs = map (name: overridden.${name} or (mkPackageError name)) packages;
     extraPkgs = extraEmacsPackages overridden;
+    defaultInitFilePkg =
+      if !((builtins.isBool defaultInitFile) || (lib.isDerivation defaultInitFile))
+      then throw "defaultInitFile must be bool or derivation"
+      else
+        if defaultInitFile == false
+        then null
+        else
+          let
+            # name of the default init file must be default.el according to elisp manual
+            defaultInitFileName = "default.el";
+          in
+          epkgs.trivialBuild {
+            pname = "default-init-file";
+            src =
+              if defaultInitFile == true
+              then pkgs.writeText defaultInitFileName configText
+              else
+                if defaultInitFile.name == defaultInitFileName
+                then defaultInitFile
+                else throw "name of defaultInitFile must be ${defaultInitFileName}";
+            packageRequires = usePkgs;
+          };
   in
-  usePkgs ++ extraPkgs)
+  usePkgs ++ extraPkgs ++ [ defaultInitFilePkg ])