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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
```
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