emacs: Add "edit" command to a list of packages.

* emacs/guix-base.el (guix-edit-package): New function.
* emacs/guix-list.el (guix-list-edit-package): New command.  Bind it to
  "e" key.
* emacs/guix-main.scm (package-location-string): New procedure.
* doc/emacs.texi (Emacs List buffer): Document "e" key binding.
* doc/guix.texi (Invoking guix edit): Mention "package list" buffer.

doc/emacs.texi

@@ -268,6 +268,10 @@ Mark the current package for deletion.
 Mark the current package for upgrading.
 @item ^
 Mark all obsolete packages for upgrading.
+@item e
+Edit the definition of the curent package (go to its location).  This is
+similar to @command{guix edit} command (@pxref{Invoking guix edit}), but
+for opening a package recipe in the current Emacs instance.
 @item x
 Execute actions on the marked packages.
 @end table

doc/guix.texi

@@ -3577,9 +3577,9 @@ launches the program specified in the @code{EDITOR} environment variable
 to edit the recipe of GCC@tie{}4.8.4 and that of Vim.
 
 If you are using Emacs, note that the Emacs user interface provides
-similar functionality in the ``package info'' buffers created by
-@kbd{M-x guix-search-by-name} and similar commands (@pxref{Emacs
-Commands}).
+similar functionality in the ``package info'' and ``package list''
+buffers created by @kbd{M-x guix-search-by-name} and similar commands
+(@pxref{Emacs Commands}).
 
 
 @node Invoking guix download

emacs/guix-base.el

@@ -172,6 +172,14 @@ If PATH is relative, it is considered to be relative to
       (move-to-column col)
       (recenter 1))))
 
+(defun guix-edit-package (id)
+  "Edit (go to location of) package with ID."
+  (let ((loc (guix-eval-read (guix-make-guile-expression
+                              'package-location-string id))))
+    (if loc
+        (guix-find-location loc)
+      (message "Couldn't find package location."))))
+
 
 ;;; Buffers and auto updating.
 

emacs/guix-list.el

@@ -467,6 +467,11 @@ With prefix (if ARG is non-nil), describe entries marked with any mark."
                  (list (guix-list-current-id)))))
     (guix-list-describe-maybe guix-entry-type ids)))
 
+(defun guix-list-edit-package ()
+  "Go to the location of the current package."
+  (interactive)
+  (guix-edit-package (guix-list-current-package-id)))
+
 
 ;;; Displaying packages
 
@@ -505,6 +510,7 @@ likely)."
   :group 'guix-package-list)
 
 (let ((map guix-package-list-mode-map))
+  (define-key map (kbd "e")   'guix-list-edit-package)
   (define-key map (kbd "x")   'guix-package-list-execute)
   (define-key map (kbd "i")   'guix-package-list-mark-install)
   (define-key map (kbd "d")   'guix-package-list-mark-delete)
@@ -662,6 +668,7 @@ The specification is suitable for `guix-process-package-actions'."
 
 (let ((map guix-output-list-mode-map))
   (define-key map (kbd "RET") 'guix-output-list-describe)
+  (define-key map (kbd "e")   'guix-list-edit-package)
   (define-key map (kbd "x")   'guix-output-list-execute)
   (define-key map (kbd "i")   'guix-output-list-mark-install)
   (define-key map (kbd "d")   'guix-output-list-mark-delete)

emacs/guix-main.scm

@@ -887,6 +887,12 @@ GENERATIONS is a list of generation numbers."
   (with-store store
     (delete-generations store profile generations)))
 
+(define (package-location-string package-id)
+  "Return a location string of a package PACKAGE-ID."
+  (and-let* ((package  (package-by-id package-id))
+             (location (package-location package)))
+    (location->string location)))
+
 (define (package-source-derivation->store-path derivation)
   "Return a store path of the package source DERIVATION."
   (match (derivation-outputs derivation)