June 2018 Update

Task/History-variables/Elena/history-variables.elena

@@ -0,0 +1,51 @@
+import extensions.
+import system'collections.
+import system'routines.
+import extensions'routines.
+
+class HistoryVariable
+{
+    object prop Value :: _value.
+    stack  _previous := Stack new.
+
+    set Value:v
+    [
+        if ($nil != _value)
+        [
+            _previous push(_value)
+        ].
+
+        _value := v
+    ]
+
+    undo
+    [
+        ifnot (_previous isEmpty)
+        [
+            _value := _previous pop
+        ];
+        [
+            _value := $nil
+        ]
+    ]
+
+    enumerator => _previous.
+
+    dispatch => _value.
+}
+
+program =
+[
+    var o := HistoryVariable new.
+    o Value := 5.
+    o Value := "foo".
+    o Value := o Value + "bar".
+
+    console printLine(o).
+
+    o forEach:printingLn.
+
+    o undo; undo; undo.
+
+    console printLine(o Value).
+].