https://okmij.org/ftp/Algorithms/grasping-all-apples-at-once.html

apples.rkt

#lang racket/base

(define (apples a-string)
  (define string-size (string-length a-string))
  (define (once current count i)
    (cond
      [(= i string-size)
       (if current
           (list (cons current count))
           null)]
      [else
       (define ch (string-ref a-string i))
       (cond
         [(not current)       (once ch 1 (add1 i))]
         [(char=? ch current) (once current (add1 count) (add1 i))]
         [else
          (cons (cons current count)
                (once ch 1 (add1 i)))])]))
  (once #f #f 0))

(apples "aaaabbbcca")