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bug#18175: files.el: use mapc in (mapcar 'switch-to-buffer ...)


From: Ivan Shmakov
Subject: bug#18175: files.el: use mapc in (mapcar 'switch-to-buffer ...)
Date: Sun, 03 Aug 2014 08:55:21 +0000
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/24.3 (gnu/linux)

>>>>> Drew Adams <address@hidden> writes:

 >> Given that switch-to-buffer returns its argument, /and/ given that
 >> mapc returns the sequence it’s given, I suggest that the
 >> (mapcar 'switch-to-buffer LIST) forms in lisp/files.el be replaced
 >> with (mapc 'switch-to-buffer LIST), – if only to avoid the
 >> unnecessary consing when the list is effectively copied in the
 >> mapcar case.

 >> The lists mapcar is applied to in such cases are returned from
 >> find-file-noselect, and so, as it seems, are “fresh” ones anyway.

 > Not a good idea, IMHO.

 > It's not just about performance; it's about coding style.

 > By using `mapcar' you are signaling that you are interested in the
 > return values of the argument function (and of course the resulting
 > list of them).

 > By using `mapc' you are signaling that the values returned by the
 > argument function are unimportant (only its side effects are
 > significant).

        How do you signal that the values returned by the argument
        function are unimportant, /and/ that you’re interested in the
        /original/ list instead?

 > If you want to improve the performance, and that is the only change
 > you want to make, then please consider another approach.

        Please consider the patch MIMEd.  FWIW, it avoids one more cons
        in both find-file-other-window and find-file-other-frame.

-- 
FSF associate member #7257  http://boycottsystemd.org/  … 3013 B6A0 230E 334A
diff --git a/lisp/files.el b/lisp/files.el
index 9272e98..c3a6f0d 100644
--- a/lisp/files.el
+++ b/lisp/files.el
@@ -1428,7 +1428,7 @@ automatically choosing a major mode, use 
\\[find-file-literally]."
                         (confirm-nonexistent-file-or-buffer)))
   (let ((value (find-file-noselect filename nil nil wildcards)))
     (if (listp value)
-       (mapcar 'switch-to-buffer (nreverse value))
+       (mapc 'switch-to-buffer (nreverse value))
       (switch-to-buffer value))))
 
 (defun find-file-other-window (filename &optional wildcards)
@@ -1448,10 +1448,10 @@ expand wildcards (if any) and visit multiple files."
                         (confirm-nonexistent-file-or-buffer)))
   (let ((value (find-file-noselect filename nil nil wildcards)))
     (if (listp value)
-       (progn
-         (setq value (nreverse value))
-         (cons (switch-to-buffer-other-window (car value))
-               (mapcar 'switch-to-buffer (cdr value))))
+       (prog1
+           (setq value (nreverse value))
+         (switch-to-buffer-other-window (car value))
+         (mapc 'switch-to-buffer (cdr value)))
       (switch-to-buffer-other-window value))))
 
 (defun find-file-other-frame (filename &optional wildcards)
@@ -1471,10 +1471,10 @@ expand wildcards (if any) and visit multiple files."
                         (confirm-nonexistent-file-or-buffer)))
   (let ((value (find-file-noselect filename nil nil wildcards)))
     (if (listp value)
-       (progn
-         (setq value (nreverse value))
-         (cons (switch-to-buffer-other-frame (car value))
-               (mapcar 'switch-to-buffer (cdr value))))
+       (prog1
+           (setq value (nreverse value))
+         (switch-to-buffer-other-frame (car value))
+         (mapc 'switch-to-buffer (cdr value)))
       (switch-to-buffer-other-frame value))))
 
 (defun find-file-existing (filename)

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