In [[computer programmingscience]], a '''promise''' (also known as a '''future''' in some languages) is a placeholder for a result that is not yet known, usually because a computation has not yet finished or a message from a remote party has not yet arrived. They were introduced in [[1977]] in the paper by Henry Baker and [[Carl Hewitt]]. The use of promises (futuures) can dramatically reduce latency in distributed systems because it enables pipelining of messages, called '''promise pipelining''' [http://www.erights.org/elib/distrib/pipeline.html] [http://c2.com/cgi/wiki?PromisePipelining].
== Known implementations ==
ThisPromises [[concurrent programming]] technique was invented by the MIT Actors group in the 1970s. It(futures) became more well known by its inclusion in [[MultiLisp]]. The use of logic variables for communication in concurrent logic programming languages is quite similar. These started with "Prolog with Freeze" and "IC Prolog", and became a true concurrency primitive with Concurrent Prolog, Flat Concurrent Prolog, Parlog, Vulcan, Janus, Mozart/Oz, Flow Java, and Alice. The single assignment "I-var" from data flow languages, included in Reppy's "Concurrent ML", is much like the concurrent logic variable.
The pipelining technique (using promises/futures to overcome latency) was invented independently twice in the 1980s. It was invented at [[Project Xanadu]] (circa 1989) and by [[Barbara Liskov]] in 1988.
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* [[Joule programming language|Joule]]
* [[E programming language|E]]
==Reference==
*Henry Baker and Carl Hewitt '''The Incremental Garbage Collection of Processes''' Proceeding of the Symposium on Artificial Intelligence Programming Languages. SIGPLAN Notices 12, August 1977.
