Futures and promises

In computer science, future, promise, and delay refer to constructs used for synchronizing in some concurrent programming languages. They describe an object that acts as a proxy for a result that is initially unknown, usually because the computation of its value is yet incomplete.
The term promise was proposed in 1976 by Daniel P. Friedman and David Wise, and Peter Hibbard called it eventual. A somewhat similar concept future was introduced in 1977 in a paper by Henry Baker and Carl Hewitt.
The terms future, promise, and delay are often used interchangeably, although some differences in usage between future and promise are treated below. Setting the value of a future is also called resolving, fulfilling, or binding it.


Use of futures may be implicit (any use of the future automatically obtains its value, as if it were an ordinary reference) or explicit (the user must call a function to obtain the value, such as the get method of java.util.concurrent.Future in Java). Obtaining the value of an explicit future can be called stinging or forcing. Explicit futures may be implemented as a library, whereas implicit futures need language support.
The original Baker and Hewitt paper described implicit futures, which are naturally supported in the Actor model of computation and pure object-oriented programming languages like Smalltalk. The Friedman and Wise paper described only explicit futures, probably reflecting the difficulty of efficiently implementing implicit futures on stock hardware. The difficulty is that stock hardware does not deal with futures for primitive data types like integers. For example, an add instruction does not know how to deal with 3 + future factorial(100000). In pure object or Actor languages this problem can be solved by sending future factorial(100000) the message +, which asks the future to add 3 to itself and return the result. Note that the message passing approach works regardless of when factorial(100000) finishes computation and that no stinging/forcing is needed.