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These functions augment the standard Emacs Lisp functions get
and put
for operating on properties attached to symbols.
There are also functions for working with property lists as
first-class data structures not attached to particular symbols.
Function: get* symbol property &optional default
This function is like get
, except that if the property is
not found, the default argument provides the return value.
(The Emacs Lisp get
function always uses nil
as
the default; this package's get*
is equivalent to Common
Lisp's get
.)
The get*
function is setf
-able; when used in this
fashion, the default argument is allowed but ignored.
Function: remprop symbol property
This function removes the entry for property from the property
list of symbol. It returns a true value if the property was
indeed found and removed, or nil
if there was no such property.
(This function was probably omitted from Emacs originally because,
since get
did not allow a default, it was very difficult
to distinguish between a missing property and a property whose value
was nil
; thus, setting a property to nil
was close
enough to remprop
for most purposes.)
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
and get*
functions are 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.
@secno=2
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