scala doc 中对 future 和promise的描述

http://docs.scala-lang.org/overviews/core/futures.html


easy 在群里的描述：
While futures are defined as a type of read-only placeholder object created for a result which doesn’t yet exist, a promise can be thought of as a writable, single-assignment container, which completes a future. That is, a promise can be used to successfully complete a future with a value (by “completing” the promise) using the success method. Conversely, a promise can also be used to complete a future with an exception, by failing the promise, using the failure method.