locative in scheme

locative.scm

(define (make-locative accessor modifier)
  (cons accessor modifier))

(define (locative-ref loc)
  ((car loc)))

(define (locative-set! loc value)
  ((cdr loc) value))

(set! (setter locative-ref) locative-set!)                   ; srfi-17: generalized set!

; This macro used to be very long but @dico_leque improved. Thanks!
(define-syntax locative
  (syntax-rules ()
    ((_ v)
     (make-locative
      (lambda () v)
      (lambda (x) (set! v x))))))    ; set! is from srfi-17


;-------

; test


; Suppose we have ! reader macro for locative-ref.

;;; catch reference to variable

(define var 1)

(define varloc (locative var))

!varloc ;=> 1

(set! !varloc 4)

!varloc ;=> 4



;;; catch reference to vector element

(define v (vector 1 2 3))

(define a (locative (vector-ref v 1)))

!a ;=> 1

(set! !a 4)

!a ;=> 4


;;; `map` procedure without extra consing

(define (map f list)
  (do ((x list (cdr x))
       (ans '())
       (ansloc (locative ans) (locative (cdr !ansloc))))
      ((null? x) ans)
    (set! !ansloc (list (f (car x))))))