emacs: Add code to run 'dot' program.

* emacs/guix-external.el: New file.
* emacs.am (ELFILES): Add it.

emacs.am

@@ -23,6 +23,7 @@ ELFILES =					\
   emacs/guix-base.el				\
   emacs/guix-command.el				\
   emacs/guix-emacs.el				\
+  emacs/guix-external.el			\
   emacs/guix-help-vars.el			\
   emacs/guix-history.el				\
   emacs/guix-info.el				\

emacs/guix-external.el

@@ -0,0 +1,72 @@
+;;; guix-external.el --- External programs  -*- lexical-binding: t -*-
+
+;; Copyright © 2015 Alex Kost <alezost@gmail.com>
+
+;; This file is part of GNU Guix.
+
+;; GNU Guix is free software; you can redistribute it and/or modify
+;; it under the terms of the GNU General Public License as published by
+;; the Free Software Foundation, either version 3 of the License, or
+;; (at your option) any later version.
+
+;; GNU Guix is distributed in the hope that it will be useful,
+;; but WITHOUT ANY WARRANTY; without even the implied warranty of
+;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+;; GNU General Public License for more details.
+
+;; You should have received a copy of the GNU General Public License
+;; along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+;;; Commentary:
+
+;; This file provides auxiliary code for running external programs.
+
+;;; Code:
+
+(defgroup guix-external nil
+  "Settings for external programs."
+  :group 'guix)
+
+(defcustom guix-dot-program (executable-find "dot")
+  "Name of the 'dot' executable."
+  :type 'string
+  :group 'guix-external)
+
+(defcustom guix-dot-default-arguments
+  '("-Tpng")
+  "Default arguments for 'dot' program."
+  :type '(repeat string)
+  :group 'guix-external)
+
+(defcustom guix-dot-file-name-function #'guix-png-file-name
+  "Function used to define a file name of a temporary 'dot' file.
+The function is called without arguments."
+  :type '(choice (function-item guix-png-file-name)
+                 (function :tag "Other function"))
+  :group 'guix-external)
+
+(defun guix-dot-arguments (output-file &rest args)
+  "Return a list of dot arguments for writing a graph into OUTPUT-FILE.
+If ARGS is nil, use `guix-dot-default-arguments'."
+  (or guix-dot-program
+      (error (concat "Couldn't find 'dot'.\n"
+                     "Set guix-dot-program to a proper value")))
+  (apply #'list
+         guix-dot-program
+         (concat "-o" output-file)
+         (or args guix-dot-default-arguments)))
+
+(defun guix-dot-file-name ()
+  "Call `guix-dot-file-name-function'."
+  (funcall guix-dot-file-name-function))
+
+(defun guix-png-file-name ()
+  "Return '.png' file name in the `temporary-file-directory'."
+  (concat (make-temp-name
+           (concat (file-name-as-directory temporary-file-directory)
+                   "graph-"))
+          ".png"))
+
+(provide 'guix-external)
+
+;;; guix-external.el ends here