Arc Forum | ; Eli's code to modify mzscheme-4's immutable pairs. ;; to avoid a malloc o...

 ; Eli's code to modify mzscheme-4's immutable pairs.
 ;; to avoid a malloc on every call, reuse a single pointer, but make
 ;; it thread-local to avoid races
 (define ptr (make-thread-cell #f))
 (define (get-ptr)
 (or (thread-cell-ref ptr)
 (let ([p (malloc _scheme 1)]) (thread-cell-set! ptr p) p)))
 ;; set a pointer to the cons cell, then dereference it as a pointer,
 ;; and bang the new value in the given offset
 (define (set-ca/dr! offset who p x)
 (if (pair? p)
 (let ([p* (get-ptr)])
 (ptr-set! p* _scheme p)
 (ptr-set! (ptr-ref p* _pointer 0) _scheme offset x))
 (raise-type-error who "pair" p)))
 (define (n-set-car! p x) (set-ca/dr! 1 'set-car! p x))
 (define (n-set-cdr! p x) (set-ca/dr! 2 'set-cdr! p x))