Futures and Promises - History

History

The future and/or promise constructs were first implemented in programming languages such as MultiLisp and Act 1. The use of logic variables for communication in concurrent logic programming languages was quite similar to futures. These started with Prolog with Freeze and IC Prolog, and became a true concurrency primitive with Relational Language, Concurrent Prolog, Guarded Horn Clauses (GHC), Parlog, Vulcan, Janus, Mozart/Oz, Flow Java, and Alice ML. The single-assignment I-var from dataflow programming languages, originating in Id and included in Reppy's Concurrent ML, is much like the concurrent logic variable.

The promise pipelining technique (using futures to overcome latency) was invented by Barbara Liskov and Liuba Shrira in 1988, and independently by Mark S. Miller, Dean Tribble and Rob Jellinghaus in the context of Project Xanadu circa 1989.

The term promise was coined by Liskov and Shrira, although they referred to the pipelining mechanism by the name call-stream, which is now rarely used.

Both the design described in Liskov and Shrira's paper, and the implementation of promise pipelining in Xanadu, had the limitation that promise values were not first-class: an argument to, or the value returned by a call or send could not directly be a promise (so the example of promise pipelining given earlier, which uses a promise for the result of one send as an argument to another, would not have been directly expressible in the call-stream design or in the Xanadu implementation). It appears that promises and call-streams were never implemented in any public release of Argus (the programming language used in the Liskov and Shrira paper); Argus development stopped around 1988. The Xanadu implementation of promise pipelining only became publicly available with the release of the source code for Udanax Gold in 1999, and was never explained in any published document. The later implementations in Joule and E support fully first-class promises and resolvers.

Several early Actor languages, including the Act series of languages, supported both parallel message passing and pipelined message processing, but not promise pipelining. (Although it is technically possible to implement the last of these features in terms of the first two, there is no evidence that the Act languages did so.)