toggle 80 columns edittable

toggle80.el

(defun toggle-80-editting-columns ()
  "Set the right window margin so the edittable space is only 80 columns."
  (interactive)
  (let ((margins (window-margins)))
    (if (or (car margins) (cdr margins))
        (set-window-margins nil 0 0)
      (set-window-margins nil 0 (max (- (window-width) 80) 0)))))

(defun toggle-80-editting-columns-balanced ()
  "Set both window margins so the edittable space is only 80 columns."
  (interactive)
  (let ((margins (window-margins)))
    (if (or (car margins) (cdr margins))
        (set-window-margins nil 0 0)
      (let* ((change (max (- (window-width) 80) 0))
             (left (/ change 2))
             (right (- change left)))
        (set-window-margins nil left right)))))