In computer programming, 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. The use of promises 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].
This [[concurrent programming]] technique of promises/futures was invented by the MIT Actors group in the 1970s. It became more well known by its inclusion in [[MultiLisp programming language|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.
This pipelining technique (of 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.
Languages containing "promises/futures/concurrent logic variables/I-vars" include [[Alice programming language|Alice]] and [[MultiLisp programming language|MultiLisp]].
Languages containing promise pipelining include [[Joule programming language|Joule]] and [[E programming language|E]].
==External references==
* [http://c2.com/cgi/wiki?PromisePipelining PromisePipelining] on the [http://c2.com/cgi/wiki?FrontPage C2 wiki]
