visible-whitespace-mode

[Miles Bader]

A while back during one of the usual bouts of discussions about an
emacs analogue of vi's `:set list', I whipped up the following package,
which implements `visible-whitespace-mode'.  It tries to be slightly
more clever than usual by changing the color of the whitespace indicators
characters so that they are easily distinguishable from ordinary text,
and using somewhat more iconic characters if the terminal supports them.

Also (unlike `:set list'), it attempts to put the whitespace indicators
`in context'; that is, whereas `:set list' will change all tabs to `^I',
thereby making all your indented code look wonky, visible-whitespace-mode
actually displays tabs as an indicator character (say `\', rendered in
an apropriately muted color) followed by an actual tab.  This means
that most of the time, indentation won't be affected.

Anyone have any comments on this?  Is it an appropriate thing to add
to emacs?

-Miles


Here's the source:

;;; visws.el --- Make whitespace visible
;;
;; Copyright (C) 2001 Free Software Foundation, Inc.
;;
;; Author: Miles Bader <address@hidden>
;; Keywords: convenience

;; GNU Emacs 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 2, or (at your option)
;; any later version.

;; GNU Emacs 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 GNU Emacs; see the file COPYING.  If not, write to the
;; Free Software Foundation, Inc., 59 Temple Place - Suite 330,
;; Boston, MA 02111-1307, USA.

;;; Commentary:
;;
;; Defines the mode `visible-whitespace-mode'.
;;
;; When active, normally invisible whitespace characters are made visible.

;;; Code:

(require 'latin1-disp)

(defface visible-whitespace '((t :foreground "blue" :bold t))
  "Face for control-characters revealed by `visible-whitespace-mode'.")

(defcustom visible-whitespace-mappings
  '((?\n [?$ ?\n])
    ;; Note that TAB still tabs, but with a graphic indicator before the
    ;; tab; we only use single-character graphic indicator to reduce the
    ;; number of cases where the indicator causes the tabbing to be
    ;; screwed up.
    (?\t [?» ?\t] [?\\ ?\t])
    (?   [?·] [?.]))
  "An alist of mappings for displaying whitespace in `visible-whitespace-mode'.

The car of each mapping is a whitespace character, and the cdr is a list of
display vectors (a vector of characters).  The first display vector the
list that can be displayed is used; if no display vector for a mapping can
be displayed, then that character is displayed unmodified.

The characters in are displayed using the `visible-whitespace' face."
  :type 'list)

(defun visws-legal-display-vector-p (vec)
  "Return true if every character in the display vector VEC can be displayed."
  (let ((i 0) (len (length vec)))
    (while (and (< i len) (latin1-char-displayable-p (aref vec i)))
      (setq i (1+ i)))
    (= i len)))

;; Buffer local variable used to remember whether a buffer initially had
;; a local display table or not.
(defvar visws-display-table-was-local nil)

(define-minor-mode visible-whitespace-mode
  "Toggle Visible Whitespace mode
When active, normally invisible whitespace characters are made visible.

With prefix argument ARG, turn on if positive, otherwise off.
Returns non-nil if the new state is enabled."
  nil
  " VisWS"
  nil
  (if visible-whitespace-mode
      (let ((face-bits (ash (face-id 'visible-whitespace) 19)))
        (set (make-local-variable 'visws-display-table-was-local)
             buffer-display-table)
        (unless buffer-display-table
          (setq buffer-display-table (make-display-table)))
        (dolist (entry visible-whitespace-mappings)
          (let ((vecs (cdr entry)))
            (while (and vecs (not (visws-legal-display-vector-p (car vecs))))
              (setq vecs (cdr vecs)))
            (when vecs
              (let ((vec (copy-sequence (car vecs))))
                (dotimes (i (length vec))
                  (when (not (eq (aref vec i) (car entry)))
                    (aset vec i (logior (aref vec i) face-bits))))
                (aset buffer-display-table (car entry) vec))))))
    (if visws-display-table-was-local
        (dolist (entry visible-whitespace-mappings)
          (aset buffer-display-table (car entry) nil))
      (setq buffer-display-table nil))))

;;; visws.el ends here

-- 
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                        -- Walter Hines Page