top-prog.scm

;;; list
(define (filter proc xs)
  (if (pair? xs)
      (if (proc (car xs))
          (cons (car xs) (filter proc (cdr xs)))
          (filter proc (cdr xs)))
      '()))

(define (map f ls . more)
  (if (null? more)
        (let map1 ([ls ls])
          (if (null? ls)
              '()
              (cons (f (car ls))
                    (map1 (cdr ls)))))
        (let map-more ([ls ls] [more more])
          (if (null? ls)
              '()
              (cons
                (apply f (list (car ls) (map car more)))
                (map-more (cdr ls) (map cdr more)))))))

(define (fold-left proc obj xs)
  (if (pair? xs)
      (fold-left proc (proc obj (car xs)) (cdr xs))
      obj))