update-if-true!, syntax, scheme

update-if-true-bang.scm

;; update-if-true!

(define-syntax cond-set!
  (syntax-rules ()
    ((_ pred var init)
     (if pred
         (set! var init)))
    ((_ pred var1 init1 var2 init2 ...)
     (if pred
         (begin
           (set! var1 init1)
           (set! var2 init2) ...)))))

(define-syntax update-if-true!
  (syntax-rules ()
    ((_ (pred var))
     (let ((p pred))
       (cond-set! p  var p)))
    ((_ (pred1 var1)(pred2 var2) ...)
     (begin
       (update-if-true! (pred1 var1))
       (update-if-true! (pred2 var2))
       ...))))

(define (test x y z)
  (let ((x-is-big #f)
        (y-is-zero #f)
        (z-is-string #f))
    (update-if-true!
     ((> x 5) x-is-big)
     ((zero? y) y-is-zero)
     ((string? z) z-is-string))
    (values x-is-big y-is-zero z-is-string)))