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push 416
I
push item reference &key area localize Function
If the list held in reference is viewed as a push-down stack, then push
pushes an element onto the top of the stack. The value of the argument
item can be any lisp object. The value of the argument reference can be
the name of any generalized variable containing a list. item is consed onto
the front of the list, and the augmented list is stored back into reference
and returned. The value of reference can be any form acceptable as a
generalized variable to setf.
The effect of (push item place) is the same as (setf place (cons item place),
except that the setf form evaluates any subforms of place twice, while push
evaluates them only once. Moreover, for certain place forms, push can be
significantly more efficient than the setf form.
The optional keyword arguments :area and :localize are Symbolics extensions
to Common Lisp.
:area
:localize
Examples:
An integer that specifies the area in which to store the
augmented list. See the section "Areas" in Internals,
Processes, and Storage Management.
Can be nil, t, or a positive integer, which behave as follows:
nil
t
integer
Do not change the behavior of push.
Localize the top level of list structure
by calling sys:localize-list or
sys:localize-tree on the list before
returning it.
Localize integer levels of list structure
by calling sys:localize-list or
sys:localize-tree on the list before
returning it.
(setq alist '«a. b) (c . d))) => «A. 8) (C . D))
(push '(1.2) (cdr alist)) => «1.2) (C . D))
alist => «A. 8) (1 . 2) (C . D))
(push '(3 . 4) alist :localize 2) =>
( (3 . 4) (A . ",8) (1 . 2) (C . D))
alist => «3 . 4) (A . 8) (1
. 2) (C . D))
For a table of related items: See the section "Functions for Constructing
Lists and Conses" in Symbolics Common Lisp: Language Concepts.