Initial data commit

Task/Queue-Definition/Racket/queue-definition-2.rkt

@@ -0,0 +1,32 @@
+#lang racket
+;; Invariants:
+;; The elements in the queue are (append front (reverse back)).
+;; Front is always non-empty (except for the empty queue).
+(struct queue (front back))
+
+(define empty (queue '() '()))
+
+(define (push x q)
+  (if (null? (queue-front q))
+      (queue (reverse (cons x (queue-back q))) '())
+      (queue (queue-front q) (cons x (queue-back q)))))
+
+(define (empty? q)
+  (null? (queue-front q)))
+
+(define (pop q)
+  (cond [(empty? q) (error 'pop "the queue is empty")]
+        [(not (null? (queue-front q)))
+         (if (null? (rest (queue-front q)))
+             (queue (reverse (queue-back q)) '())
+             (queue (rest (queue-front q)) (queue-back q)))]
+        [else (queue (reverse (queue-back q)) '())]))
+
+(define (first q)
+  (cond [(empty? q) (error 'first "the queue is empty")]
+        [(car (queue-front q))]))
+
+;; Example:
+(first (pop (pop (for/fold ([q empty]) ([x '(1 2 3 4)])
+                   (push x q)))))
+;; => 3