Futures and Promises - Read-only Views

Read-only Views

In some programming languages such as Oz, E, and AmbientTalk, it is possible to obtain a read-only view of a future, which allows reading its value when resolved, but does not permit resolving it:

• In Oz, the !! operator is used to obtain a read-only view.
• In E and AmbientTalk, a future is represented by a pair of values called a promise/resolver pair. The promise represents the read-only view, and the resolver is needed in order to set the future's value.
• In C++11 a std::future provides a read-only view. The value is set directly by using a std::promise, or set to the result of a function call using std::packaged_task or std::async.
• In the Dojo Toolkit's Deferred API as of version 1.5, a consumer-only promise object represents a read-only view.
• In Alice ML, futures only provide a read-only view, whereas a promise contains both a future and the ability to resolve the future
• In .NET 4.0 System.Threading.Tasks.Task represents a read-only view. Resolving the value can be done through System.Threading.Tasks.TaskCompletionSource.

Support for read-only views is consistent with the Principle of Least Authority, since it enables the ability to set the value to be restricted to subjects that need to set it. In a system that also supports pipelining, the sender of an asynchronous message (with result) receives the read-only promise for the result, and the target of the message receives the resolver.