User Guide

A promise library for Clojure and ClojureScript.

Install

Leiningen:

[funcool/promesa "9.0.485"]

deps.edn:

funcool/promesa {:mvn/version "9.0.485"}

On the JVM platform promesa is built on top of completable futures (requires JDK >= 11). On JS engines it is built on top of the execution environment's built-in Promise implementation.

Introduction

A promise is an abstraction that represents the result of an asynchronous operation that has the notion of error.

This is a list of all possible states for a promise:

• resolved: means that the promise contains a value.
• rejected: means that the promise contains an error.
• pending: means that the promise does not have value.

The promise can be considered done when it is resolved or rejected.

Creating a promise

There are several different ways to create a promise instance. If you just want to create a promise with a plain value, you can use the polymorphic promise function:

(require '[promesa.core :as p])

;; creates a promise from value
(p/promise 1)

;; creates a rejected promise
(p/promise (ex-info "error" {}))

It automatically coerces the provided value to the appropriate promise instance: rejected when the provided value is an exception and resolved in all other cases.

If you already know that the value is either resolved or rejected, you can skip the coercion and use the resolved and rejected functions:

;; Create a resolved promise
(p/resolved 1)
;; => #object[java.util.concurrent.CompletableFuture 0x3e133219 "resolved"]

;; Create a rejected promise
(p/rejected (ex-info "error" {}))
;; => #object[java.util.concurrent.CompletableFuture 0x3e563293 "rejected"]

Another option is to create an empty promise using the deferred function and provide the value asynchronously using p/resolve! and p/reject!:

(defn sleep
  [ms]
  (let [p (p/deferred)]
    (future (p/resolve! p))
    p))

Another option is using a factory function. If you are familiar with JavaScript, this is a similar approach:

@(p/create (fn [resolve reject] (resolve 1)))
;; => 1

NOTE: the @ reader macro only works on JVM.

The factory will be executed synchronously (in the current thread) but if you want to execute it asynchronously, you can provide an executor:

(require '[promesa.exec :as exec])

@(p/create (fn [resolve reject] (resolve 1)) exec/default-executor)
;; => 1

Another way to create a promise is using the do macro:

(p/do
  (let [a (rand-int 10)
        b (rand-int 10)]
    (+ a b)))

The do macro works similarly to clojure's do block, so you can provide any expression, but only the last one will be returned. That expression can be a plain value or another promise.

If an exception is raised inside the do block, it will return the rejected promise instead of re-raising the exception on the stack.

If the do contains more than one expression, each expression will be treated as a promise expression and will be executed sequentially, each awaiting the resolution of the prior expression.

For example, this do macro:

(p/do (expr1)
       (expr2)
       (expr3))

Is roughtly equivalent to:

(p/let [_ (expr1)
        _ (expr2)]
  (expr3))

Finally, promesa exposes a future macro very similar to the clojure.core/future:

@(p/future (some-complex-task))
;; => "result-of-complex-task"

One difference from clojure.core/future is that if the return value of the future expression is itself a promise instance, then it will await and unwrap the inner promise:

@(p/future (p/future (p/future 1)))
;; => 1

Promise Chaining

then

The most common way to chain a transformation to a promise is using the general purpose then function:

@(-> (p/resolved 1)
     (p/then inc))
;; => 2

;; flatten result
@(-> (p/resolved 1)
     (p/then (fn [x] (p/resolved (inc x)))))
;; => 2

As you can observe in the example, then handles functions that return plain values as well as functions that return promise instances (which will automatically be flattened).

NOTE: If you know that the chained function will always return plain values, you can use the more performant then' variant of this function.

map

The map function works similarly to the then' function, the difference is the order of arguments:

(def result
  (->> (p/resolved 1)
       (p/map inc)))

@result
;; => 2

chain

If you have multiple transformations and you want to apply them in one step, there are the chain and chain' functions:

(def result
  (-> (p/resolved 1)
      (p/chain inc inc inc)))

@result
;; => 4

NOTE: these are analogous to then and then' but accept multiple transformation functions.

->, ->> and as->

NOTE: -> and ->> introduced in 6.1.431, as-> introduced in 6.1.434.

This threading macros simplifices chaining operation, removing the need of using then all the time.

Lets look an example using then and later see how it can be improved using the -> threading macro:

(-> (p/resolved {:a 1 :c 3})
    (p/then #(assoc % :b 2))
    (p/then #(dissoc % :c)))

Then, the same code can be simplified with:

(p/-> (p/resolved {:a 1 :c 3})
      (assoc :b 2))
      (dissoc :c))

The threading macros hides all the accidental complexity of using promise chaining.

The ->> and as-> are equivalent to the clojure.core macros, but they work with promises in the same way as -> example shows.

handle

If you want to handle rejected and resolved callbacks in one unique callback, then you can use the handle chain function:

(def result
  (-> (p/promise 1)
      (p/handle (fn [result error]
                  (if error :rejected :resolved)))))

@result
;; => :resolved

finally

And finally if you want to attach a (potentially side-effectful) callback to be always executed notwithstanding if the promise is rejected or resolved, there is a executed regardless of whether the promise is rejected or resolved, there is a finally function (very similar to try/finally):

(def result
  (-> (p/promise 1)
      (p/finally (fn [_ _]
                  (println "finally")))))

@result
;; => 1
;; => stdout: "finally"

Promise Composition

let

The promesa library comes with convenient syntactic-sugar that allows you to create a composition that looks like synchronous code while using the Clojure's familiar let syntax:

(require '[promesa.core :as p]
         '[promesa.exec :as exec])

;; A function that emulates asynchronos behavior.
(defn sleep
  [wait]
  (p/create (fn [resolve reject]
              (exec/schedule! wait #(resolve wait)))))

(def result
  (p/let [x (sleep 42)
          y (sleep 41)
          z 2]
    (+ x y z)))

@result
;; => 85

The let macro behaves identically to Clojure's let with the exception that it always returns a promise. If an error occurs at any step, the entire composition will be short-circuited, returning exceptionally resolved promise.

Under the hood, the let macro evalutes to something like this:

(p/then (sleep 42)
        (fn [x] (p/then (sleep 41)
                        (fn [y] (p/then 2 (fn [z]
                                            (p/promise (do (+ x y z)))))))))

all

In some circumstances you will want wait for completion of several promises at the same time. To help with that, promesa also provides the all helper.

(let [p (p/all [(do-some-io)
                (do-some-other-io)])]
  (p/then p (fn [[result1 result2]]
              (do-something-with-results result1 result2))))

plet

The plet macro combines syntax of let with all; and enables a simple declaration of parallel operations followed by a body expression that will be executed when all parallel operations have successfully resolved.

@(p/plet [a (p/delay 100 1)
          b (p/delay 200 2)
          c (p/delay 120 3)]
   (+ a b c))
;; => 6

The plet macro is just a syntactic sugar on top of all. The previous example can be written using all in this manner:

(p/all [(p/delay 100 1)
        (p/delay 200 2)
        (p/delay 120 3)]
  (fn [[a b c]] (+ a b c)))

any

There are also circumstances where you only want the first successfully resolved promise. For this case, you can use the any combinator:

(let [p (p/any [(p/delay 100 1)
                (p/delay 200 2)
                (p/delay 120 3)])]
  (p/then p (fn [x]
              (.log js/console "The first one finished: " x))))

race

The race function method returns a promise that fulfills or rejects as soon as one of the promises in an iterable fulfills or rejects, with the value or reason from that promise:

@(p/race [(p/delay 100 1)
          (p/delay 110 2)])
;; => 1

Error handling

One of the advantages of using the promise abstraction is that it natively has a notion of errors, so you don't need to reinvent it. If some computation inside the composed promise chain/pipeline raises an exception, the pipeline short-circuits and propagates the exception to the last promise in the chain.

The catch function adds a new handler to the promise chain that will be called when any of the previous promises in the chain are rejected or an exception is raised. The catch function also returns a promise that will be resolved or rejected depending on what happens inside the catch handler.

Let see an example:

(-> (p/rejected (ex-info "error" nil))
    (p/catch (fn [error]
               (.log js/console error))))

If you prefer map-like parameter ordering, the err function (and error alias) works in same way as catch but has parameters ordered like map:

(->> (p/rejected (ex-info "error" nil))
     (p/error (fn [error]
                (.log js/console error))))

Delays and Timeouts.

JavaScript, due to its single-threaded nature, does not allow you to block or sleep. But, with promises you can emulate that functionality using delay like so:

(-> (p/delay 1000 "foobar")
    (p/then (fn [v]
              (println "Received:" v))))

;; After 1 second it will print the message
;; to the console: "Received: foobar"

The promise library also offers the ability to add a timeout to async operations thanks to the timeout function:

(-> (some-async-task)
    (p/timeout 200)
    (p/then #(println "Task finished" %))
    (p/catch #(println "Timeout" %)))

In this example, if the async task takes more that 200ms then the promise will be rejected with a timeout error and then successfully captured with the catch handler.

Scheduling Tasks

In addition to the promise abstraction, this library also comes with a lightweight abstraction for scheduling tasks to be executed at some time in future:

(require '[promesa.exec :as exec])
(exec/schedule! 1000 (fn []
                       (println "hello world")))

This example shows you how you can schedule a function call to be executed 1 second in the future. It works the same way for both Clojure and ClojureScript.

The tasks can be cancelled using its return value:

(def task (exec/schedule! 1000 #(do-stuff)))

(p/cancel! task)

Execution model

NOTE: This section is mainly affects the JVM.

Lets consider this example::

@(-> (p/delay 100 1)
     (p/then' inc)
     (p/then' inc))
;; => 3

This will create a promise that will resolve to 1 in 100ms (in a separate thread); then the first inc will be executed (in the same thread), and then another inc is executed (in the same thread). In total only one thread is involved.

This default execution model is usually preferrable because it don't abuse the task scheduling and leverages function inlining on the JVM.

But it does have drawbacks: this approach will block the thread until all of the chained callbacks are executed. For small chains this is not a problem. However, if your chain has a lot of functions and requires a lot of computation time, this might cause unexpected latency. It may block other threads in the thread pool from doing other, maybe more important, tasks.

For such cases, promesa exposes an additional arity for provide a user-defined executor to control where the chained callbacks are executed:

(require '[promesa.exec :as exec])

@(-> (p/delay 100 1)
     (p/then inc exec/default-executor)
     (p/then inc exec/default-executor))
;; => 3

This will schedule a separate task for each chained callback, making the whole system more responsive because you are no longer executing big blocking functions; instead you are executing many small tasks.

The exec/default-executor is a ForkJoinPool instance that is highly optimized for lots of small tasks.

Performance overhead

promesa is a lightweight abstraction built on top of native facilities (CompletableFuture in the JVM and js/Promise in JavaScript).

Internally we make heavy use of protocols in order to expose a polymorphic and user friendly API, and this has little overhead on top of raw usage of CompletableFuture or Promise. This is the latest micro benchmark (2019-09-17) that shows the real overhead of this library in contrast to the use of native abstractions:

(run-bench (simple-promise-chain-5-raw))
;; => amd64 Linux 5.2.9-arch1-1-ARCH 4 cpu(s)
;; => OpenJDK 64-Bit Server VM 12.0.2+10
;; => Runtime arguments: -Dclojure.compiler.direct-linking=true
;; => Evaluation count : 687647820 in 60 samples of 11460797 calls.
;; =>       Execution time sample mean : 82.617649 ns
;; =>              Execution time mean : 82.606811 ns
;; => Execution time sample std-deviation : 2.348589 ns
;; =>     Execution time std-deviation : 2.365164 ns
;; =>    Execution time lower quantile : 78.787962 ns ( 2.5%)
;; =>    Execution time upper quantile : 86.941501 ns (97.5%)
;; =>                    Overhead used : 9.967315 ns
;; =>

(run-bench (simple-completable-chain-5-raw))
;; => amd64 Linux 5.2.9-arch1-1-ARCH 4 cpu(s)
;; => OpenJDK 64-Bit Server VM 12.0.2+10
;; => Runtime arguments: -Dclojure.compiler.direct-linking=true
;; => Evaluation count : 823532160 in 60 samples of 13725536 calls.
;; =>       Execution time sample mean : 62.267034 ns
;; =>              Execution time mean : 62.279349 ns
;; => Execution time sample std-deviation : 1.967931 ns
;; =>     Execution time std-deviation : 2.014908 ns
;; =>    Execution time lower quantile : 59.663843 ns ( 2.5%)
;; =>    Execution time upper quantile : 67.599822 ns (97.5%)
;; =>                    Overhead used : 9.967315 ns

The benchmarked functions are:

(defn simple-promise-chain-5-raw
  []
  @(as-> (CompletableFuture/completedFuture 1) $
     (p/then' $ inc)
     (p/then' $ inc)
     (p/then' $ inc)
     (p/then' $ inc)
     (p/then' $ inc)))

(defn simple-completable-chain-5-raw
  []
  @(as-> (CompletableFuture/completedFuture 1) $
     (.thenApply ^CompletionStage $ ^Function (pu/->FunctionWrapper inc))
     (.thenApply ^CompletionStage $ ^Function (pu/->FunctionWrapper inc))
     (.thenApply ^CompletionStage $ ^Function (pu/->FunctionWrapper inc))
     (.thenApply ^CompletionStage $ ^Function (pu/->FunctionWrapper inc))
     (.thenApply ^CompletionStage $ ^Function (pu/->FunctionWrapper inc))))

Contributing

Unlike Clojure and other Clojure contrib libs, this project does not have many restrictions for contributions. Just open an issue or pull request.

Get the Code

promesa is open source and can be found on github.

You can clone the public repository with this command:

git clone https://github.com/funcool/promesa

Run tests

To run the tests execute the following:

For the JVM platform:

clojure -X:dev:test

And for JS platform:

clojure -A:dev -T:build build-cljs-tests
node target/tests.js

You will need to have Node.js installed on your system.

License

promesa is licensed under MPL-2.0 license:

This Source Code Form is subject to the terms of the Mozilla Public
License, v. 2.0. If a copy of the MPL was not distributed with this
file, You can obtain one at http://mozilla.org/MPL/2.0/.

Copyright (c) Andrey Antukh <niwi@niwi.nz>