Scheme `let-optionally`

let-optionally.scm

(define-syntax let-optionally
  (syntax-rules (_)
    ((_ vs () e f ...)
     (let () e f ...))
    ((_ vs (_ xv ...) e f ...)
     (if (null? vs)
         (let-optionally '() (xv ...) e f ...)
         (let-optionally (cdr vs) (xv ...) e f ...)))
    ((_ vs ((x v) xv ...) e f ...)
     (if (null? vs)
         (let ((x v))
           (let-optionally '() (xv ...) e f ...))
         (let ((x (car vs)))
           (let-optionally (cdr vs) (xv ...) e f ...))))))

(import (chibi test))

(test-begin "LET-OPTIONALLY")
(test '(0 1 3 4)
      (let-optionally '(0 1 2 3 4 5)
                      ((x #f) (y #f) _ (z #f) (w #f) _ _ _)
                      (list x y z w)))
(test-end)