Overlay category set properties

Question

From the Elisp manual:

If an overlay has a category property, we call it the category of the overlay. It should be a symbol. The properties of the symbol serve as defaults for the properties of the overlay.

How can I set the properties of the overlay category?

Answer 1

Use put to set a property of a symbol. Use get to retrieve one property and symbol-plist to retrieve them all. The properties of a symbol are an attribute of it, parallel to its value as a variable (symbol-value) and its function definition (symbol-function).

For example, the following snippet causes overlay to be shown in the default face, but in bold italic when the mouse cursor is over it, because the mouse-face property of the category applies but the face property of the category is overridden by the same property on the overlay itself.

(put 'my-category 'face 'bold)
(put 'my-category 'mouse-face 'bold-italic)
(let ((overlay (make-overlay (point) (1+ (point)))))
  (overlay-put overlay 'category 'my-category)
  (overlay-put overlay 'face 'default)
  ...)