33/37: emacs: Return nil instead of "" in completing readers.

[Alex Kost]

alezost pushed a commit to branch wip-refactor-emacs-ui
in repository guix.

commit 25b5ee273ccc037eba9ce28f1df4473ae8f95123
Author: Alex Kost <address@hidden>
Date:   Fri Dec 11 12:36:56 2015 +0300

    emacs: Return nil instead of "" in completing readers.
    
    * emacs/guix-utils.el (guix-completing-read): New procedure.
      (guix-define-readers): Use it.
---
 emacs/guix-utils.el |   11 ++++++++++-
 1 files changed, 10 insertions(+), 1 deletions(-)

diff --git a/emacs/guix-utils.el b/emacs/guix-utils.el
index a67d0e7..a8942f6 100644
--- a/emacs/guix-utils.el
+++ b/emacs/guix-utils.el
@@ -212,6 +212,15 @@ If NO-MESSAGE? is non-nil, do not display a message about 
it."
 See also `guix-copy-as-kill'."
   (guix-copy-as-kill (guix-command-string args) no-message?))
 
+(defun guix-completing-read (prompt table &optional predicate
+                             require-match initial-input
+                             hist def inherit-input-method)
+  "Same as `completing-read' but return nil instead of an empty string."
+  (let ((res (completing-read prompt table predicate
+                              require-match initial-input
+                              hist def inherit-input-method)))
+    (unless (string= "" res) res)))
+
 (defun guix-completing-read-multiple (prompt table &optional predicate
                                       require-match initial-input
                                       hist def inherit-input-method)
@@ -458,7 +467,7 @@ keywords are available:
             `(defvar ,completions-var nil))
 
          ,(when single-reader
-            `(guix-define-reader ,single-reader completing-read
+            `(guix-define-reader ,single-reader guix-completing-read
                                  ,completions ,single-prompt))
 
          ,(when multiple-reader