added __with__ function according to proposed syntax in issue #29

1 files changed, 18 insertions, 1 deletions

diff --git a/lib/__set__.js b/lib/__set__.js
index 0fa88ac..ad0e5b0 100644
--- a/lib/__set__.js
+++ b/lib/__set__.js
@@ -46,4 +46,21 @@ function __set__() {
     };
 }
 
-module.exports = __set__;
+function __with__() {
+  var args = arguments;
+  return function(callback) {
+    if (typeof callback !== "function") {
+      throw new TypeError("__with__ expects a callback function")
+    }
+
+    var undo = __set__.apply(null, args)
+    try {
+      callback();
+    }
+    finally {
+      undo();
+    }
+  }
+}
+
+module.exports = {"__set__": __set__, "__with__": __with__}