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---

title: SICP exercise 3.19

date: 2021-09-02

layout: post

lang: en

ref: sicp-exercise-3-19

---

```scheme
(define (cycle? l)
  (define (rec l x)
    (cond
      ((null? x) false)
      ((eq? l x) true)
      (true (rec l (cdr x)))))
  (rec l (cdr l)))
```

Sample interactive session:

```scheme
scheme@(guile-user)> (define true #t)
scheme@(guile-user)> (define false #f)
scheme@(guile-user)>
(define (cycle? l)
  (define (rec l x)
    (cond
      ((null? x) false)
      ((eq? l x) true)
      (true (rec l (cdr x)))))
  (rec l (cdr l)))
scheme@(guile-user)> (cycle? '(1 2 3))
$9 = #f
scheme@(guile-user)> (cycle? (make-cycle '(1 2 3)))
$10 = #t
```