#lang racket

(define (first-uniq chars)
  (when (empty? chars)
    (raise "There are no unique letters"))
  (let ([c (first chars)])
    (if (memq c (rest chars)) ; MEMQ is like MEMBER with EQ?
      (first-uniq (remq* (list c) chars)) ; N.B the tail-recursion here
      c)))

(require rackunit)
(check-equal? (first-uniq (string->list "aabcbcdeef"))
              #\d)
(check-equal? (first-uniq (string->list "faabcbcdeef"))
              #\d)
(check-equal? (first-uniq (string->list "aabcbcdeefd"))
              #\f)