37.13.6 The Overlay Arrow

The overlay arrow is useful for directing the user’s attention to a particular line in a buffer. For example, in the modes used for interface to debuggers, the overlay arrow indicates the line of code about to be executed. This feature has nothing to do with overlays (see Overlays).

Variable: overlay-arrow-string

This variable holds the string to display to call attention to a particular line, or nil if the arrow feature is not in use. On a graphical display the contents of the string are ignored; instead a glyph is displayed in the fringe area to the left of the display area.

Variable: overlay-arrow-position

This variable holds a marker that indicates where to display the overlay arrow. It should point at the beginning of a line. On a non-graphical display the arrow text appears at the beginning of that line, overlaying any text that would otherwise appear. Since the arrow is usually short, and the line usually begins with indentation, normally nothing significant is overwritten.

The overlay-arrow string is displayed in any given buffer if the value of overlay-arrow-position in that buffer points into that buffer. Thus, it is possible to display multiple overlay arrow strings by creating buffer-local bindings of overlay-arrow-position. However, it is usually cleaner to use overlay-arrow-variable-list to achieve this result.

You can do a similar job by creating an overlay with a before-string property. See Overlay Properties.

You can define multiple overlay arrows via the variable overlay-arrow-variable-list.

Variable: overlay-arrow-variable-list

This variable’s value is a list of variables, each of which specifies the position of an overlay arrow. The variable overlay-arrow-position has its normal meaning because it is on this list.

Each variable on this list can have properties overlay-arrow-string and overlay-arrow-bitmap that specify an overlay arrow string (for text terminals) or fringe bitmap (for graphical terminals) to display at the corresponding overlay arrow position. If either property is not set, the default overlay-arrow-string or overlay-arrow fringe indicator is used.