Working with Promises

Introduction

A promise is an abstraction that represents the result of an asynchronous operation that has the notion of error. Backed with CompletebleFuture on the JVM and Promise on JS.

NOTE: keep in mind that the vast majority of things work identically regardless of the runtime, but there are cases where the limitations of the platform implementation imply differences or even the omission of some functions.

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)
;; => #<CompletableFuture[resolved:1964677152]>

;; Create a rejected promise
(p/rejected (ex-info "error" {}))
;; => #<CompletableFuture[rejected:1153075015]>

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

(defn some-fn
  [ms]
  (let [p (p/deferred)]
    (p/resolve! p nil)
    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

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

Inspecting promise

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.

When a promise is created with no value (using p/deferred function), the initial promise state is pending, once has a value it will be considered resolved or rejected if the value is an exception.

(def p1 (p/deferred))
(def p2 (p/resolved 1))
(def p3 (p/rejected (ex-info "test" {})))

(p/pending? p1)
;; => true

(p/resolved? p2)
;; => true

(p/rejected? p3)
;; => true

(p/done? p1)
;; => false

(p/done? p2)
;; => true

You also can access the current value of the promise independently of the state with p/extract:

(p/extract p1 :no-val)
;; => :no-val

(p/extract p2 :no-val)
;; => 1

(p/extract p3 :no-val)
;; => #error {...}

You also can use deref or the @ reader macro for blocking access to the promise value. The blocking operation only works on JVM, on CLJS, deref has the same semantics as extract (it does not blocks and acces the current value independently of the state of the promise).

Chaining computations

This section explains the helpers and macros that promesa provides for chain different (high-probably asynchonous) operations in a sequence of operations.

It provides mainly two style of API:

1. one designed for use with -> threading macro and make it easy and familiar to someone that already know how JS promises works. The functions that are part of this style are: then, chain, catch, handle and finally.
2. one designed for use with ->> threading macro and focused on correctness and performance. The functions that are part of this style are: fmap, mcat, hmap, hcat, merr and fnly.

Lets look on detail on all of them.

then

The most common way to chain a transformation to a promise is using the general purpose then function. Consists on applying the 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 promise instances (which will automatically be flattened, in the same way as JS promises).

For performance sensitive code, consider using a more specific functions like fmap or mcat.

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: chain is analogous to then and then' but accept multiple transformation functions. The chain' variant does not auto-flattens the return value.

->, ->> and as-> (macros)

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

It works in the same way as then, if the function returns a promise instance it will be automatically unwrapped.

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:

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

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

The return value of the function will be ignored and new promise instance will be returned mirroning the original one.

fmap

Returns a new promise instance which will be completed with the return value of applying a function to the eventually successfully resolved promise.

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

@result
;; => 2

In contrast to then, there are no automatic unwrapping of neested promises. Use mcat (or mapcat) for for handle one level unwrapping.

Aliases: map.

mcat

Returns a new promise instance which will be completed with the same value as the returning promise instance of applying a function to eventually successfully resolved promise. The function must return a promise instance.

(def result
  (->> (p/resolved 1)
       (p/mapcat (fn [v] (p/resolved (inc v))))))

@result
;; => 2

Aliases: mapcat.

hmap

Applies a function in the same way as fmap to both possible results: value and exception. It returns a promise that will be completed with the return value of the function.

(def result
  (->> (p/resolved 1)
       (p/hmap (fn [v _] (inc v)))))

@result
;; => 2

hcat

Applies a function in the same way as mcat to both possible results: value and exception. Funciton must return a promise. It returns a mirrored promise returned by the applied function.

(def result
  (->> (p/resolved 1)
       (p/hmap (fn [v _] (p/resolved (inc v))))))

@result
;; => 2

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.

catch

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]
               (prn "erorr:" erorr))))

You also can filter by predicate or by class the possible exception to handle:

(-> (p/rejected (ex-info "error" nil))
    (p/catch clojure.lang.ExceptionInfo
             (fn [error]
               (prn "erorr:" erorr))))

Or

(defn ex-info?
  [o]
  (instance? clojure.lang.ExceptionInfo o))

(-> (p/rejected (ex-info "error" nil))
    (p/catch ex-info? (fn [error]
                        (prn "erorr:" erorr))))

merr

In the same way as catch allow apply a function to the promise rejection. This function has the parameters in inverse order, intended to be used with ->> in the same way as fmap and mcat.

The function must return a promise instance,

(def result
  (->> (p/rejected (ex-info "hello" nil))
       (p/merr (fn [error]
                 (p/resolved (ex-message error))))))

@result
;; => "hello"

Composition

Promse exposes a set of helpers and syntactic abstractions (macros) for facilitate working with compositions of asynchronous computations.

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:

(def result
  (p/let [x (p/delay 1000 42)
          y (p/delay 1200 41)
          z 2]
    (+ x y z)))

@result
;; => 85

The let macro behaves almost 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)))))))))

do

(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 let macro (explained below):

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

In fact, the let macro uses do internally.

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.

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

Is up to the user properly handle concurrency, p/all does not lauches additional threads of execution.

plet macro

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)))

The real parallelism strictly depends on the underlying implementation of the executed functions. If they does syncronous work, all the code will be executed serially, almost identical to the standard let. Is the user responsability of the final execution model.

any

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

(def result
  (p/any [(p/delay 125 1)
          (p/delay 200 2)
          (p/delay 120 3)]))

@result
;; => 3

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

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.

Promise chaining & execution model

Let's try to understand how promise chained functions are executed and how they interact with platform threads. **This section is mainly affects the JVM.

Lets consider this example:

@(->> (p/delay 100 1)
      (p/map inc)
      (p/map 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 px])

@(->> (p/delay 100 1)
      (p/fmap :default inc)
      (p/fmap :default inc))
;; => 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 :default keyword will resolve to px/*default-executor*, that is a ForkJoinPool instance that is highly optimized for lots of small tasks.

On JDK19 with Preview enabled you will also have the px/*vthread-executor* (:vthread keyword can be used) that is an instance of Virtual Thread per task executor.

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.

For performance sensitive code, prefer using functions designed to be used for ->>; they are more optimized because they don't perform automatic unwrapping handling (unlike the then or handle functions). This applies only to CLJ, on CLJS all they work the same way because of how the underlying implementation works.