4.4 Value Cell Functions

The contents of the value cell of an id is usually accessed by eval (Chapter 11) or valuecell (below) and changed by setq, setf or sometimes set.

is equivalent to

Where setq may be used to set any type of variable (fluid, global or local) the function set is restricted to fluid and global variables.

If EXP evaluates to t or nil an error occurs.

defines the value of A to be (1), B to be 2, and C to ((3) 4).

The application of setf can take on any one of the following forms:

(setf id data) expands into (setq id data)

(setf (eval form) data) expands into (set form data)

The same effect is obtained by substituting value in place of eval.

(setf (car pair) data) expands into (rplaca pair data)

(setf (cdr pair) data) expands into (rplacd pair data)

(setf (getv vector index) data) expands into
(putv vector index data)

(setf (indx form index) data) expands into
(setindx form index data)

(setf (sub form start size) data) expands into
(setsub form start size data)

(setf (nth pair index) data) expands into an expression similar to
(rplaca (pnth pair index) data)

If the first argument to setf is a macro then it will be expanded before setf is. For example, if first is defined as

(ds first (p) (car p)) then

(setf (first p) data) is equivalent to (setf (car p) data)

The USEFUL module contains an expanded version of setf. The basic definition of setf is not consistent with that of setq. The value returned from an application of setq is always the value assigned. For example, the expression

(setf (car '(a b)) 'd) expands into (rplaca '(a b) 'd)

The value returned after evaluating this second expression is (d b). The extended version of setf will always return the value assigned.

An application of the extended version of setf will accept the additional following forms:

(setf (cons left right) pair) will expand into an expression similar to
(progn (setf left (car pair)) (setf right (cdr pair)))

(setf (cXYr pair) data) expands into an expression similar to
(rplacX (cYr pair) data)

where X is either a or d and Y is either a, d, aa, ..., or ddd

(setf (flagp id name) data) expands into an expression similar to
(flag (list id) name), if data is non-nil otherwise
(remflag (list id) name)

(setf (get id name) data) expands into (put id name data)

(setf (getd name) data) expands into an expression similar to
(putd name (car data) (cdr data))

This expansion assumes that data is similar to an expression which would be returned by a call on getd. If data is a code-pointer or a lambda expression then ’expr is used in place of (car data).

(setf (lastcar pair) data) expands into an expression similar to
(rplaca (lastpair pair) data).

(setf (list a b c ...) pair)
the expansion of this expression is very similar to the expansion of
(desetq (a b c ...) pair)

(setf (pnth pair index) data) expands into an expression similar to
(rplacd (pnth pair (sub1 index)) data)

(setf (vector al bl cl ...) [ar br cr ...])
expands into an expression similar to
(progn (setf al ar)(setf bl br)(setf cl cr)...)

The setf function is extensible to permit additional operators on the left hand side. If there is an assign-op property on the property list of the operator then the value of that property (either a lambda expression or the name of a function) is used to build the expansion of the macro. The effect is similar to

The property setf-expand is searched for when their is no assign-op property. If there is such a property its value is applied to the two arguments passed to setf. The effect is similar to

If the left hand side operator is flagged as setf-safe, then it is assumed that the expansion of the macro will yeild an expression which will return the value of the right hand side. Otherwise the expansion will take one of the forms listed below. Within the second expansion, references to RHS are replaced with references to VAR. The second form is used when RHS is a list, the assumption being that it is effecient to evaluate an application only once.