a.rkt

#lang racket

(require racket/generic)

;; ---------------------------------------------------------------------------------------------------

;; An iterator is a structure that actually implements iteration over some iterable sequence. The
;; iterator method of gen:iterable fetches an iterator to be used in iteration.

(define-generics iterator
  (iterator-empty? iterator)
  (iterator-value iterator)
  (iterator-next iterator))

(define-generics iterable
  (iterator iterable)
  #:defaults
  ([iterator? ; calling iterator on an iterator just returns itself
    (define iterator values)]
   [sequence?
    (define (iterator seq)
      (sequence-iterator seq))]))

;; ---------------------------------------------------------------------------------------------------

;; This is a simplified version of gen:sequence that only supports constructed sequences (sequences
;; which use cons, first, and rest, such as lists and streams).

(define-generics sequence
  (cons value sequence)
  (first sequence)
  (rest sequence)
  (empty? sequence))

;; For constructed sequences, implementing an iterator is trivial, since the basic operations are
;; already supported by the sequence type.

(struct sequence-iterator (seq)
  #:methods gen:iterator
  [(define (iterator-empty? s-i)
     (empty? (sequence-iterator-seq s-i)))
   (define (iterator-value s-i)
     (first (sequence-iterator-seq s-i)))
   (define (iterator-next s-i)
     (rest (sequence-iterator-seq s-i)))])

b.rkt

#lang racket

(require racket/generic)

;; ---------------------------------------------------------------------------------------------------

;; An iterator is a structure that actually implements iteration over some iterable sequence. The
;; iterator method of gen:iterable fetches an iterator to be used in iteration.

(define-generics iterator
  (iterator-empty? iterator)
  (iterator-value iterator)
  (iterator-next iterator))

(define-generics iterable
  (iterator iterable)
  #:defaults
  ([iterator? ; calling iterator on an iterator just returns itself
    (define iterator values)]))

;; ---------------------------------------------------------------------------------------------------

;; This is a simplified version of gen:sequence that only supports constructed sequences (sequences
;; which use cons, first, and rest, such as lists and streams).

(define-generics sequence
  (cons value sequence)
  (first sequence)
  (rest sequence)
  (empty? sequence)

  ; all constructed sequences are iterable
  #:implements gen:iterable
  [(define (iterator seq)
     (sequence-iterator seq))])

;; For constructed sequences, implementing an iterator is trivial, since the basic operations are
;; already supported by the sequence type.

(struct sequence-iterator (seq)
  #:methods gen:iterator
  [(define (iterator-empty? s-i)
     (empty? (sequence-iterator-seq s-i)))
   (define (iterator-value s-i)
     (first (sequence-iterator-seq s-i)))
   (define (iterator-next s-i)
     (rest (sequence-iterator-seq s-i)))])