[R7RS+ base][SRFI-17] First, expression is evaluated. In the first form, the binding of symbol is modified so that next reference of symbol will return the result of expression. If symbol is not locally bound, the global variable named symbol must already exist, or an error is signaled.
The second form is a “generalized set!” specified in SRFI-17. It is a syntactic sugar of the following form.
((setter proc) arg … expression)
Note the order of the arguments of the setter method
differs from CommonLisp’s
(define x 3) (set! x (list 1 2)) x ⇒ (1 2) (set! (car x) 5) x ⇒ (5 2)
Sets values of multiple variables at once. Expr must yield as many values as var …. Each value is set to the corresponding var.
(define a 0) (define b 1) (set!-values (a b) (values 3 4)) a ⇒ 3 b ⇒ 4 (set!-values (a b) (values b a)) a ⇒ 4 b ⇒ 3
[SRFI-17] Returns a setter procedure associated to the procedure proc. If no setter is associated to proc, its behavior is undefined.
A setter procedure g of a procedure f is such that when used
(g a b … v), the next evaluation of
(f a b …)
To associate a setter procedure to another procedure, you can use
the setter of
setter, like this:
(set! (setter f) g)
A procedure’s setter can be “locked” to it. System default setters,
car, is locked and can’t be set by
the above way. In order to lock a setter to a user defined procedure,
If proc is not a procedure, a setter generic function
object-apply is returned;
it allows the applicable object extension to work
seamlessly with the generalized
See Applicable objects, for the details.
#t if a setter is associated to proc.
[SRFI-17] Takes two procedure get and set. Returns a new procedure which does the same thing as get, and its setter is locked to set.
The intention of this procedure is, according to the SRFI-17 document, to allow implementations to inline setters efficiently. Gauche hasn’t implement such optimization yet.
A few macros that adopts the same semantics of generalized
are also provided. They are built on top of
and the value of place, then sets the result to place.
place is either a variable or a form
(proc arg …),
as the second argument of
set!. The result of this form
(define x (list 2)) (push! x 3) x ⇒ (3 2) (push! (cdr x) 4) x ⇒ (3 4 2)
When place is a list, it roughly expands like the following.
(push! (foo x y) item) ≡ (let ((tfoo foo) (tx x) (ty y)) ((setter tfoo) tx ty (cons item (tfoo tx ty))))
Note: Common Lisp’s
push macro takes its argument reverse
order. I adopted this order since it is consistent with other
destructive operations. Perl’s
push function takes the same
argument order, but it appends item at the end of the array
unshift is closer to
You can use a queue (see Queue) if you need a behavior of Perl’s
Retrieves the value of place, sets its
cdr back to place
and returns its
(define x (list 1 2 3)) (pop! x) ⇒ 1 x ⇒ (2 3) (define x (vector (list 1 2 3))) x ⇒ #((1 2 3)) (pop! (vector-ref x 0)) ⇒ 1 x ⇒ #((2 3))
Note: This works the same as Common Lisp’s
pop pops value from the end of the sequence;
shift does the same thing as
Evaluates the value of place. It should be a number.
inc!) or subtracts (
dec!) delta to/from it,
and then stores the result to place. The default value of
delta is 1.
This is like Common Lisp’s
that you can’t use the result of
Generalized form of
push! etc. Proc must be a procedure
which takes one argument and returns one value. The original value of
place is passed to the proc, then its result is set to
(define a (cons 2 3)) (update! (car a) (lambda (v) (* v 3))) a ⇒ (6 . 3) (update! (cdr a) (cut - <> 3)) a ⇒ (6 . 0)