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----
-
-title: SICP persistent O(n) queue
-
-date: 2021-09-03
-
-layout: post
-
-lang: en
-
-ref: sicp-persistent-on-queue
-
----
-
-```scheme
-(define (queue)
-  (cons '()
-	'()))
-
-(define (enqueue x q)
-  (cons (car q)
-	(cons x (cdr q))))
-
-(define (flush q)
-  (cons (reverse (cdr q))
-	'()))
-
-(define (dequeue q)
-  (if (null? (car q))
-    (dequeue (flush q))
-    (cons (caar q)
-	  (cons (cdar q)
-		(cdr q)))))
-
-(define (empty? q)
-  (and (null? (car q))
-       (null? (cdr q))))
-
-(define (peek q)
-  (car (dequeue q)))
-
-(define (print-queue q)
-  (define (rec l leading-space?)
-    (when (not (null? l))
-      (when leading-space?
-	(display " "))
-      (display (car l))
-      (rec (cdr l) #t)))
-
-  (display "#q(")
-  (rec (car q) false)
-  (rec (reverse (cdr q)) (not (null? (car q))))
-  (display ")")
-  (newline))
-```
-
-Sample interactive session:
-```scheme
-scheme@(guile-user)> (define true #t)
-scheme@(guile-user)> (define false #f)
-scheme@(guile-user)> (define q (queue))
-scheme@(guile-user)> (print-queue q)
-#q()
-scheme@(guile-user)> (print-queue (enqueue 'a q))
-#q(a)
-scheme@(guile-user)> (print-queue q)
-#q()
-scheme@(guile-user)> (set! q (enqueue 'a q))
-scheme@(guile-user)> (print-queue q)
-#q(a)
-scheme@(guile-user)> (set! q (enqueue 'e (enqueue 'd (enqueue 'c (enqueue 'b q)))))
-scheme@(guile-user)> (print-queue q)
-#q(e d c b a)
-scheme@(guile-user)> (peek q)
-$28 = a
-scheme@(guile-user)> (define ret (dequeue q))
-scheme@(guile-user)> (define value (car ret))
-scheme@(guile-user)> (set! q (cdr ret))
-scheme@(guile-user)> value
-$29 = a
-scheme@(guile-user)> (print-queue q)
-#q(b c d e)
-scheme@(guile-user)> (print-queue (cdr (dequeue (cdr (dequeue (enqueue 'g (enqueue 'f q)))))))
-#q(d e f g)
-```