Difference between revisions of "Futures and promises"

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In computer science, future, promise, delay, and deferred refer to constructs used for synchronizing program execution 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.

Description

The term promise was proposed in 1976 by Daniel P. Friedman and David Wise; 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, delay, and deferred are often used interchangeably, although some differences in usage between future and promise are treated below. Specifically, when usage is distinguished, a future is a read-only placeholder view of a variable, while a promise is a writable, single assignment container which sets the value of the future.

Notably, a future may be defined without specifying which specific promise will set its value, and different possible promises may set the value of a given future, though this can be done only once for a given future.

In other cases a future and a promise are created together and associated with each other: the future is the value, the promise is the function that sets the value – essentially the return value (future) of an asynchronous function (promise). Setting the value of a future is also called resolving, fulfilling, or binding it.

See also

• jQuery Deferred object

External links

• Futures and promises @ Wikipedia.org
• Promises by Forbes Lindesay
• Promises are first-class objects for function calls by Terry Jones