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Programming in Emacs Lisp

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Point and Mark

Before discussing save-excursion, however, it may be useful first to review what point and mark are in GNU Emacs. Point is the current location of the cursor. Wherever the cursor is, that is point. More precisely, on terminals where the cursor appears to be on top of a character, point is immediately before the character. In Emacs Lisp, point is an integer. The first character in a buffer is number one, the second is number two, and so on. The function point returns the current position of the cursor as a number. Each buffer has its own value for point.

The mark is another position in the buffer; its value can be set with a command such as C-SPC (set-mark-command). If a mark has been set, you can use the command C-x C-x (exchange-point-and-mark) to cause the cursor to jump to the mark and set the mark to be the previous position of point. In addition, if you set another mark, the position of the previous mark is saved in the mark ring. Many mark positions can be saved this way. You can jump the cursor to a saved mark by typing C-u C-SPC one or more times.

The part of the buffer between point and mark is called the region. Numerous commands work on the region, including center-region, count-lines-region, kill-region, and print-region.

The save-excursion special form saves the locations of point and mark and restores those positions after the code within the body of the special form is evaluated by the Lisp interpreter. Thus, if point were in the beginning of a piece of text and some code moved point to the end of the buffer, the save-excursion would put point back to where it was before, after the expressions in the body of the function were evaluated.

In Emacs, a function frequently moves point as part of its internal workings even though a user would not expect this. For example, count-lines-region moves point. To prevent the user from being bothered by jumps that are both unexpected and (from the user's point of view) unnecessary, save-excursion is often used to keep point and mark in the location expected by the user. The use of save-excursion is good housekeeping.

To make sure the house stays clean, save-excursion restores the values of point and mark even if something goes wrong in the code inside of it (or, to be more precise and to use the proper jargon, "in case of abnormal exit"). This feature is very helpful.

In addition to recording the values of point and mark, save-excursion keeps track of the current buffer, and restores it, too. This means you can write code that will change the buffer and have save-excursion switch you back to the original buffer. This is how save-excursion is used in append-to-buffer. (See section The Definition of append-to-buffer.)

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