Custom map implementation written in Scheme.

my-map.scm

;; my/map
;; -- proc: A function
;; -- lss: All other arguments. They must be lists

;; This is a naive implementation of map, probably
;; It does a matrix transposition on lss first
;; e.g. given (4 6 7) and (2 1 1), we get ((4 2) (6 1) (7 1))
;; Then we use apply to... apply proc to each sublist
;; e.g. given + as proc, and using the list from the previous example,
;; we get ((+ 4 2) (+ 6 1) (+ 7 1)) => (6 7 8)
(define (my/map proc . lss)
  (letrec ((transpose (lambda (result1 result2 result3 lss)
                       (cond
                         ((null? lss) (transpose (cons (reverse result2) result1)
                                                 '()
                                                 '()
                                                 (reverse result3)))
                         ((null? (car lss)) (reverse result1))
                         (else (transpose result1
                                          (cons (car (car lss)) result2)
                                          (cons (cdr (car lss)) result3)
                                          (cdr lss)))))))
    (let loop ((result '()) (ls (transpose '() '() '() lss)))
      (if (null? ls)
          (reverse result)
          (let ((v (apply proc (car ls))))
            (loop (cons v result) (cdr ls)))))))