mutual.scm

;; extending the y combinator to handle mutual recursion, for zacts

(define (y . funcs)
  ((lambda (x) (x x))
   (lambda (x)
     (map (lambda (g)
            (lambda args
              (apply (apply g (x x)) args)))
          funcs))))

(define-syntax-rule (my-letrec ((var val) ...) body ...)
  (apply (lambda (var ...)
           body ...)
         (y (lambda (var ...)
              val)
            ...)))

(my-letrec
   ((odd? (lambda (x)
           (if (zero? x)
               #f
               (even? (- x 1)))))
    (even? (lambda (x)
            (if (zero? x)
                #t
                (odd? (- x 1))))))
   (list (even? 10)
         (odd?  10)
         (even? 9)
         (odd?  9)))