4.4 Assignments

Special Form: set! symbol expression

Special Form: set! (proc arg ...) expression

[R5RS][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 signalled.

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 setf.

Some examples:

(define x 3) (set! x (list 1 2)) => undefined x => (1 2) (set! (car x) 5) => undefined x => (5 2)

Macro: set!-values (var ...) expr

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)) => undefined a => 3 b => 4 (set!-values (a b) (values b a)) => undefined a => 4 b => 3

Function: setter proc

[SRFI-17] Returns a setter procedure associated to the procedure proc. If no setter is associated to proc, an error is signalled.

A setter procedure g of a procedure f is such that when used as (g a b ... v), the next evaluation of (f a b ...) returns v.

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, like set-car! for car, is locked and can't be set by the above way. In order to lock a setter to a user defined procedure, use getter-with-setter below.

Function: getter-with-setter get set

[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 set! are also provided. They are built on top of set!.

Macro: push! place item

Conses item 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 is undefined.

(define x (list 2)) (push! x 3) => undefined x => (3 2) (push! (cdr x) 4) => undefined 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 (Perl's unshift is closer to push!). You can use a queue (See section 11.22 util.queue - Queue) if you need a behavior of Perl's push.

Macro: pop! place

Retrieves the value of place, sets its cdr back to place and returns its car.

(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. Perl's pop pops value from the end of the sequence; its shift does the same thing as pop!.

Macro: inc! place &optional delta

Macro: dec! place &optional delta

Evaluates the value of place. It should be a number. Adds (inc!) or subtracts (dec!) delta to/from it, and then stores the result to place. The default value of delta is 1.

This is Common Lisp's incf and decf.

Macro: update! place proc

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 place.

(define a (cons 2 3)) (update! (car a) (lambda (v) (* v 3))) => undefined a => (6 . 3) (update! (cdr a) (cut - <> 3)) a => (6 . 0)