How do Clojure futures and promises differ?

Both futures and promises block until they have calculated their values, so what is the difference between them?

Answering in Clojure terms, here are some examples from Sean Devlin's screencast:

(def a-promise (promise))
(deliver a-promise :fred)

(def f (future (some-sexp)))
(deref f)

Note that in the promise you are explicitly delivering a value that you select in a later computation (:fred in this case). The future, on the other hand, is being consumed in the same place that it was created. The some-expr is presumably launched behind the scenes and calculated in tandem (eventually), but if it remains unevaluated by the time it is accessed the thread blocks until it is available.

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edited to add

To help further distinguish between a promise and a future, note the following:

promise

1. You create a promise. That promise object can now be passed to any thread.
2. You continue with calculations. These can be very complicated calculations involving side-effects, downloading data, user input, database access, other promises -- whatever you like. The code will look very much like your mainline code in any program.
3. When you're finished, you can deliver the results to that promise object.
4. Any item that tries to deref your promise before you're finished with your calculation will block until you're done. Once you're done and you've delivered the promise, the promise won't block any longer.

future

1. You create your future. Part of your future is an expression for calculation.
2. The future may or may not execute concurrently. It could be assigned a thread, possibly from a pool. It could just wait and do nothing. From your perspective you cannot tell.
3. At some point you (or another thread) derefs the future. If the calculation has already completed, you get the results of it. If it has not already completed, you block until it has. (Presumably if it hasn't started yet, derefing it means that it starts to execute, but this, too, is not guaranteed.)

While you could make the expression in the future as complicated as the code that follows the creation of a promise, it's doubtful that's desirable. This means that futures are really more suited to quick, background-able calculations while promises are really more suited to large, complicated execution paths. Too, promises seem, in terms of calculations available, a little more flexible and oriented toward the promise creator doing the work and another thread reaping the harvest. Futures are more oriented toward automatically starting a thread (without the ugly and error-prone overhead) and going on with other things until you -- the originating thread -- need the results.

Both Future and Promise are mechanisms to communicate result of asynchronous computation from Producer to Consumer(s).

In case of Future the computation is defined at the time of Future creation and async execution begins "ASAP". It also "knows" how to spawn an asynchronous computation.

In case of Promise the computation, its start time and [possible] asynchronous invocation are decoupled from the delivery mechanism. When computation result is available Producer must call deliver explicitly, which also means that Producer controls when result becomes available.

For Promises Clojure makes a design mistake by using the same object (result of promise call) to both produce (deliver) and consume (deref) the result of computation. These are two very distinct capabilities and should be treated as such.

There are already excellent answers so only adding the "how to use" summary:

Both

Creating promise or future returns a reference immediately. This reference blocks on @/deref until result of computation is provided by other thread.

Future

When creating future you provide a synchronous job to be done. It's executed in a thread from the dedicated unbounded pool.

Promise

You give no arguments when creating promise. The reference should be passed to other 'user' thread that will deliver the result.

In Clojure, promise, future, and delay are promise-like objects. They all represent a computation that clients can await by using deref (or @). Clients reuse the result, so that the computation is not run several times.

They differ in the way the computation is performed:

• future will start the computation in a different worker thread. deref will block until the result is ready.

• delay will perform the computation lazily, when the first client uses deref, or force.

• promise offers most flexibility, as its result is delivered in any custom way by using deliver. You use it when neither future or delay match your use case.

I think chapter 9 of Clojure for the Brave has the best explanation of the difference between delay, future, and promise.

The idea which unifies these three concepts is this: task lifecycle. A task can be thought of as going through three stages: a task is defined, a task is executed, a task's result is used.

Some programming languages (like JavaScript) have similarly named constructs (like JS's Promise) which couple together several (or all) of the stages in the task lifecycle. In JS, for instance, it is impossible to construct a Promise object without providing it either with the function (task) which will compute its value, or resolveing it immediately with a constant value.

Clojure, however, eschews such coupling, and for this reason it has three separate constructs, each corresponding to a single stage in the task lifecycle.

1. delay: task definition
2. future: task execution
3. promise: task result

Each construct is concerned with its own stage of the task lifecycle and nothing else, thus disentangling higher order constructs like JS's Promise and separating them into their proper parts.

We see now that in JavaScript, a Promise is the combination of all three Clojure constructs listed above. Example:

const promise = new Promise((resolve) => resolve(6))

Let's break it down:

1. task definition: resolve(6) is the task.
2. task execution: there is an implied execution context here, namely that this task will be run on a future cycle of the event loop. You don't get a say in this; you can't, for instance, require that this task be resolved synchronously, because asynchronicity is baked into Promise itself. Notice how in constructing a Promise you've already scheduled your task to run (at some unspecified time). You can't say "let me pass this around to a different component of my system and let it decide when it wants to run this task".
3. task result: the result of the task is baked into the Promise object and can be obtained by thening or awaiting. There's no way to create an "empty" promised result to be filled out later by some yet unknown part of your system; you have to both define the task and simultaneously schedule it for execution.

PS: The separation which Clojure imposes allows these constructs to assume roles for which they would have been unsuited had they been tightly coupled. For instance, a Clojure promise, having been separated from task definition and execution, can now be used as a unit of transfer between threads.

Firstly, a Promise is a Future. I think you want to know the difference between a Promise and a FutureTask.

A Future represents a value that is not currently known but will be known in the future.

A FutureTask represents the result of a computation that will happen in future (maybe in some thread pool). When you try to access the result, if the computation has not happened yet, it blocks. Otherwise the result is returned immediately. There is no other party involved in the computing the result as the computation is specified by you in advance.

A Promise represents a result that will be delivered by the promiser to the promisee in future. In this case you are the promisee and the promiser is that one who gave you the Promise object. Similar to the FutureTask, if you try to access the result before the Promise has been fulfilled, it gets blocked till the promiser fulfills the Promise. Once the Promise is fulfilled, you get the same value always and immediately. Unlike a FutureTask, there is an another party involved here, one which made the Promise. That another party is responsible for doing the computation and fulfilling the Promise.

In that sense, a FutureTask is a Promise you made to yourself.