50.3.2 Buffer Glyphs

Creating a glyph using make-glyph does not specify where the glyph will be used, it only specifies what the glyph will look like. Once you have created a glyph, you specify where it will be used by attaching it to an extent as a begin-glyph or end-glyph.

buffer text

To insert a glyph into a buffer, create an extent in the buffer and then use set-extent-begin-glyph or set-extent-end-glyph to set a glyph to be displayed at the corresponding edge of the extent. (It is common to create zero-width extents for this purpose.)

margins

To insert a glyph into the left or right margin of a buffer, first make sure the margin is visible by setting a value for the specifiers left-margin-width or right-margin-width. (Not strictly necessary when using margin glyphs with layout policy whitespace.) Follow the same procedure above for inserting a glyph in a buffer, then set a non-default layout policy for the glyph using set-extent-begin-glyph-layout or set-extent-end-glyph-layout. Alternatively, use the high-level annotations API (see make-annotation). (In fact, you can also use the annotations API for glyphs in a buffer, by setting a layout policy of text.)