7.1 Property Lists

These functions augment the standard Emacs Lisp functions get and put for operating on properties attached to objects. There are also functions for working with property lists as first-class data structures not attached to particular objects.

Function: getf place property &optional default

This function scans the list place as if it were a property list, i.e., a list of alternating property names and values. If an even-numbered element of place is found which is eq to property, the following odd-numbered element is returned. Otherwise, default is returned (or nil if no default is given).

In particular,

(get sym prop)  ≡  (getf (symbol-plist sym) prop)

It is legal to use getf as a setf place, in which case its place argument must itself be a legal setf place. The default argument, if any, is ignored in this context. The effect is to change (via setcar) the value cell in the list that corresponds to property, or to cons a new property-value pair onto the list if the property is not yet present.

(put sym prop val)  ≡  (setf (getf (symbol-plist sym) prop) val)

The get function is also setf-able. The fact that default is ignored can sometimes be useful:

(incf (get 'foo 'usage-count 0))

Here, symbol foo’s usage-count property is incremented if it exists, or set to 1 (an incremented 0) otherwise.

When not used as a setf form, getf is just a regular function and its place argument can actually be any Lisp expression.

Special Form: remf place property

This macro removes the property-value pair for property from the property list stored at place, which is any setf-able place expression. It returns true if the property was found. Note that if property happens to be first on the list, this will effectively do a (setf place (cddr place)), whereas if it occurs later, this simply uses setcdr to splice out the property and value cells.