Add support for installing packages temporarily via Nix.

This patch creates a new macro on fpkg called `try!` which
is similar to `use!` but only for the running session.

flake.nix

@@ -50,14 +50,16 @@
           '';
         };
 
-        fg42 = pkgs.callPackage ./nix/fg42 {
+        app = pkgs.callPackage ./nix/fg42 {
           inherit nixpkgs;
           extraPackages = {
             noether = inputs.noether.outputs.packages.${system}.default;
           };
         };
 
-        fg42-wm = pkgs.callPackage ./nix/fg42 {
+        fg42 = app.drv;
+        fg42-wm = pkgs.callPackage ./nix/fg42
+          {
             inherit nixpkgs;
             modules = [
               ./nix/modules/editor
@@ -69,7 +71,7 @@
             extraPackages = {
               noether = inputs.noether.outputs.packages.${system}.default;
             };
-        };
+          }.drv;
 
         run-test-wm = pkgs.writeShellApplication {
           name = "run-test-wm";
@@ -82,7 +84,7 @@
 
       in
       {
-        packages = {
+        packages = app.emacsPkgs // {
           default = fg42;
           wm = fg42-wm;
         };

nix/fg42/derivation.nix

@@ -64,10 +64,12 @@ let
   };
 
 in
-stdenv.mkDerivation rec {
+{
+  emacsPkgs = (emacsPackagesFor emacs);
+  drv =
+    stdenv.mkDerivation rec {
       inherit version src pname;
 
-
       buildPhase = ''
         runHook preBuild
 
@@ -138,6 +140,7 @@ stdenv.mkDerivation rec {
         export PATH=${pathsStr}:$PATH
         export FONTCONFIG_FILE="$LISPDIR/fonts.conf"
         export STARTUP_FILE=$LISPDIR/lisp/fg42_init.el
+        export FG42_PATH=$out
 
         LIBRARY_PATH="\$(${stdenv.cc}/bin/cc -print-file-name=libgccjit.so):\$LIBRARY_PATH" \
           FG42_WM=fales ${emacsBundle}/bin/emacs \
@@ -162,6 +165,7 @@ stdenv.mkDerivation rec {
         export PATH=${pathsStr}:\$PATH
         export FONTCONFIG_FILE="$LISPDIR/fonts.conf"
         export STARTUP_FILE=$LISPDIR/lisp/fg42_init.el
+        export FG42_PATH=$out
 
         # Disable access control for the current user.
         ${xorg.xhost}/bin/xhost +SI:localuser:\$USER
@@ -192,6 +196,12 @@ stdenv.mkDerivation rec {
         runHook postBuild
       '';
 
+      postInstall = ''
+        nixDir=$out/share/fg42/;
+        mkdir -p $nixDir
+        cp -rv flake.nix flake.lock nix/ $nixDir
+      '';
+
       buildInputs = [ emacs emacsBundle git texinfo gcc bash ];
 
       addEmacsNativeLoadPath = true;
@@ -211,4 +221,6 @@ stdenv.mkDerivation rec {
         '';
         license = lib.licenses.gpl3Plus;
       };
+    };
+
 }

nix/modules/unit/lisp/fg42.el

@@ -29,11 +29,18 @@
   (defvar package-archives)
   (defvar use-package-ensure-function))
 
+
 (defun fg42-version ()
   "Return FG42's version."
   (interactive)
   (message "FG42 Version %s" (fg42/config-get "version")))
 
+
+(defun fg42-path ()
+  "Return the path to FG42's installation."
+  (or (getenv "FG42_PATH") (error "'FG42_PATH' is not set!")))
+
+
 (defun defer-garbage-collection ()
   "Disable garbage collection."
   (setq gc-cons-threshold fg42/-gc-cons-threshold))

nix/modules/unit/lisp/fpkg.el

@@ -74,5 +74,41 @@ same NAME is set to t."
      (use! ,name ,docs ,@details)))
 
 
+(defun add-to-emacs-load-path (p)
+  "Add the path P to Emacs's load path."
+  (when (file-directory-p p)
+    (add-to-list 'load-path p)))
+
+
+(defun add-emacs-vars (p)
+  "Make the path P available to Emacs."
+  (require 'f)
+  (require 'seq)
+
+  (seq-reduce
+   (lambda (acc x)
+     (when (not (member x acc))
+       (when (f-glob "*.el" x)
+         (cons x acc)
+         (add-to-emacs-load-path x))))
+   (append
+    (f-directories (format "%s/share/emacs/site-lisp" p) nil t)
+    (f-directories (format "%s/share/emacs/native-lisp" p) nil t))
+   nil))
+
+
+(defmacro try! (pkg &rest details)
+  "Try the PKG with the given DETAILS in the running session."
+  (let ((flake-dir (format "%s/share/fg42" (fg42-path)))
+        (cwd (getenv "PWD")))
+    (cd flake-dir)
+    (let* ((output (shell-command-to-string
+                    (format "nix build '.#%s' --no-link --print-out-paths" pkg)))
+           (out (car (string-split output "\n" t))))
+      (add-emacs-vars out)
+      (cd cwd)
+      out)))
+
+
 (provide 'fpkg)
 ;;; fpkg.el ends here