Initial data commit

Task/History-variables/Racket/history-variables.rkt

@@ -0,0 +1,29 @@
+#lang racket
+
+(require rackunit)
+
+(struct hvar (current history) #:mutable)
+
+(define (make-hvar v) (hvar v empty))
+
+(define (hvar-set! hv new)
+  (match-define (hvar cur hist) hv)
+  (set-hvar-history! hv (cons cur hist))
+  (set-hvar-current! hv new))
+
+(define (hvar-undo! hv)
+  (match-define (hvar cur (cons old hist)) hv)
+  (set-hvar-current! hv old)
+  (set-hvar-history! hv hist))
+
+;; unit tests
+(define hv (make-hvar 0))
+(hvar-set! hv 1)
+(check-equal? (hvar-current hv) 1)
+(hvar-set! hv 2)
+(hvar-set! hv 3)
+(check-equal? (hvar-history hv) '(2 1 0))
+(hvar-undo! hv)
+(hvar-undo! hv)
+(hvar-undo! hv)
+(check-equal? (hvar-current hv) 0)