;;  ------------------------------------------------------------------------
;; |   FILE           :  list-index.rkt                                     |
;; |   AUTHOR         :  Eugene Wallingford                                 |
;; |   CREATION DATE  :  2023/03/28                                         |
;; |   DESCRIPTION    :  list-index returns the 0-based position of an      |
;; |                     item in a list, or -1 if not found.  We last       |
;; |                     saw this function in Session 17 when learning      |
;; |                     about letrec.                                      |
;;  ------------------------------------------------------------------------

#lang racket
(provide list-index)

(define list-index
  (lambda (target los)
    (letrec ((list-index-helper
               (lambda (los pos)
                 (cond ((null? los) -1)
                       ((eq? target (first los)) pos)
                       (else (list-index-helper (rest los)
                                                (add1 pos)))))))
      (list-index-helper los 0))))

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