[][src]Module sealrs::futures

Classic realization of Future / Promise paradigm

Table of Contents

  1. Introduction
  2. CompletablePromise
  3. AsyncPromise
  4. Failures control
  5. Future combinators
    1. map
    2. flat_map
    3. recover
    4. on_complete
    5. Example of chaining
  6. Requirements to V and E
  7. Blocking operations
  8. Futures conversion
  9. Pre completed futures


Future is a container for some value which will be received after a certain time but not right after creation. Promise is a function, which produce a value and pass it to a Future. Right after Future receives a value, she calls callback functions, attached by the user, which may transform the value and do some actions with finite result. How Promise calculate a value is not defined. Besides on this, Promise presented as abstract trait Promise<V, E>, which implement specific realizations. Type V presents some success result and E presents an error. In this library exists few predefined realizations of Promise.


This promise may be completed manually from the user side. Let's see on example.

let mut p: CompletablePromise<u32, TSafe<Fail + Send>> = CompletablePromise::new();
let mut fut = p.future();

fut.on_complete(|v| {
    println!("Result={}", v.as_ref().ok().unwrap());

thread::spawn(move || {

At first line, new Promise is created. You must explicitly specify him result and error types, because for now, compiler does not may infer it automatically. Next, we extract the Future stored in the Promise. This is exactly that Future which will completes the that promise. At line forth, we attach on_complete callback to the Future. We learn about it later in this doc. At the moment, you must know, that on_complite is called on finite result of the future's chain and accept Result value, which may be as is 'Ok' as is 'Err'. This value represents result of all commutation. For now, in the example code, error value is not produced, and we may simply unwrap 'Ok' result. Next we create new thread and move Promise into it. Further in this another thread, calls success method of Promise. This call, lead to that passed to it value, will be filled to the future, and right after than, on_complete callback will be called. You see 'Result=100' on the your screen.

This is not very useful code and it is written only for demonstration, how Promise / Future is works together. In the real world, you may want to perform some more very complicated actions in the on_complete callback. For example you may perform an http request to a remote server, based on the computation results.


This promise type accept a closure and executor. Closure will be planned on the specified executor and when it completes, promise will be automatically completes with result of its execution. See to example.

let task = Box::new(|| Ok(50 + 50));

let mut p: AsyncPromise<u32, TSafe<Fail + Send>> =
    AsyncPromise::new(task, executor);

let mut fut = p.future();

fut.on_complete(|v| {
    println!("Result={}", v.as_ref().ok().unwrap());

First we create the task. Then as in the previous example we create new promise, but now it is AsyncPromise. To the constructor we put task and executor. Right after that, task will be planned to execution on the executor. Next as is earlier, is set on_complete callback. But for now, we don't need to complete the promise manually. This will happen automatically after the task will be completed.

This type of promise is most of more usable for various needs. Therefore for him exists special syntactic sugar, which prevents creation of Promise by the user and simply returns prepared and extracted future.

let mut fut: WrappedFuture<u32, TSafe<Fail + Send>> =
    Future::asyncp(task, executor)

Failures control

Before we talk about futures combinators, need to say about its failures control mechanism. This realization of Future / Promise suggests that at any stage of execution, may occurs some error. It is not panic, but application level error, when programmers consciously creates error precedent. For this, used Result type, which represents the two possible situations - success or failure. Each piece of code executed in the promise / future context must always explicitly specify how it completed - success or with some error. On this is based logic of some combinators, which may be executed only for success or for failed result. Upper type of error specified as E type at promise creation stage.

Future combinators

Combinators is a functions which may be attached to the future. They will be called right after future was filled and may transform result with some algorithms. All combinators except of on_complete produce a new future, value of then depends on result of calling attached combinator on parent future. This construction is called - chain of futures. In all the examples, work with the following “future” will be assumed:

let mut fut: WrappedFuture<u32, TSafe<Fail + Send>> =
    Future::asyncp(|| Ok(500 + 500), executor);


Transforms Ok result of the current future to Ok result of some another type. Never calls if previous result was error.

fut.map(|v| {
      Ok(format!("{}", v))

In this example, value of u32 type will be transformed to the result of String type.


Transforms Err result of the current future to Err result of some another type. Never calls if previous result was Ok.

fut.map_err(|e| {


Transforms result of the current future to another future. Never calls if previous result was error.

fut.flat_map(move |v| {
    let vc = *v;
    let fut_inner: WrappedFuture<String, TSafe<Fail + Send>> =
    Future::asyncp(move || {
        let r = format!("{}", vc + 1000);
    }, executor1.clone());

In this example we create new future in the flat_map and return in as result. How it work is difficult to understand without understanding of chaining (read about it in the special paragraph). All next combinators which will ba attached after flat_map call, will be attached to the inner future which was returned as result from the flat_map. For example, if you attach map as next combinator, it will accept String type as result with value calculated in the inner future.


Recovers from error if it's occurs at previous stage. Never calls if previous result was success.

fut.map(|v| {
    let err = tsafe!(MyError::ExampleError { text: String::from("Oops!") });
}).recover(|e| {

In upper map was produced an error. Recovery combinator handle her and returns some default value as recovery result. Should be noted, than self combinator may produce error. This error will be lifted to a subsequent combinator.


Final combinator, because it does not return new future and as consequence of this, you can't unable to extend chain after it.

fut.on_complete(|v| {
    match v {
        Ok(result) => println!("Result={}", result),
        Err(error) => println!("Error={}", error.lock().unwrap())

Body of the combinator accepts a raw result of the last future. This may be both success and error and you mast detect who is this.

Example of chaining

Example of how combinators may be chained to work together.

let vst = 1000;

let mut fut: WrappedFuture<u32, TSafe<Fail + Send>> =
    Future::asyncp(move || {
        if vst > 500 {
            Ok(vst + 1000)
        } else {
            let err: TSafe<Fail + Send> =
            tsafe!(MyError::ExampleError { text: String::from("Vst must be > 500!") });
}, executor.clone());

fut.map(|v| {
    Ok(format!("{}", *v))
}).recover(|e| {
    println!("Recovered to default 100 because error occurs -> {}", e.lock().unwrap());
}).on_complete(|v| {
    println!("Result={}", v.as_ref().unwrap())

In this example, depends on value of the vst variable, chain will be executed in two ways. If vst is gross than 500, it will be converted to string and printed at finish. If less, it will case error, which will cause to drop map combinator and call recover. In him we print the message about occurred error, and return default converted to string value 100, which will also be printed at finish.

Requirements to V and E

To the types T and E exists some requirements:

  • Types must not conflict with marker trait Send (Another words, they must be sized)
  • Types must implements trait Clone

Obviously that not all types supports this requirements. Simplest way for get around this restrictions is use interior mutability. If your object is conflicts with Promise type's requirements, simply wrap it to Arc<Mutex<T>> and all will be work.

Blocking operations

Future have feature for work in fully synchronous way, when the calling thread blocks on waiting on that the future will be completed. Immediately want to warn that this option allowed only in test scenarios and when you need to interact with future from a synchronous code. In all other situations it is a very scary antipattern and his usage may cause to very serious errors in the asynchronous logic. WrappedFuture object presents two synchronous methods.

  • Ready - waits while future will be completed in a specified timeout. If future was completed before timeout was reached, true will be returned. If timeout occurs, returned false.
let mut fut1: WrappedFuture<u32, TSafe<Fail + Send>> =
    Future::asyncp(|| {
    }, executor.clone());

let ready = fut1.ready(Duration::from_secs(3));

match ready {
    true => {
        match fut1.get_value() {
            Ok(value) => {
                println!("fut1 completed with Ok({})", value);
            Err(error) => {
                println!("fut1 completed with Err({})", error.lock().unwrap());
    false => {
        println!("fut1 does not completed with timeout");
  • Result - waits while future will be completed in a specified timeout. If future was completed before timeout was reached, value of future packed in Ok will be returned. If timeout occurs, returned Err(TimeoutError).
let mut fut2: WrappedFuture<u32, TSafe<Fail + Send>> =
    Future::asyncp(|| {
    }, executor.clone());

let result = fut2.result(Duration::from_secs(3));

match result {
    Ok(value) => {
        match value {
            Ok(value) => {
                println!("fut2 completed with Ok({})", value);
            Err(error) => {
                println!("fut2 completed with Err({})", error.lock().unwrap());
    Err(_) => {
        println!("fut2 does not completed with timeout");

Full code of the example you may find in the examples submodule.

Futures conversion

Despite the fact that the library futures internally is very different from default Rust futures, it may be simply converted to they. This is possible because both futures type uses identical completion paradigm. For convert sealrs futures to the rust futures, you need implement simple converter. I intentionally did not turn it on to the library in order to don't creates the 'futures' crate versions incompatibility. You may copy this file to anywhere in your project and use it.

use sealrs::futures::future::WrappedFuture;
use futures::future;
use futures::{Async, Poll};
use std::time::Duration;

pub struct SealFuture<V: Send  + Clone + 'static, E: Send  + Clone + 'static> {
    fut: WrappedFuture<V, E>

impl <V: Send  + Clone + 'static, E: Send  + Clone + 'static> SealFuture<V, E> {
    pub fn new(fut: WrappedFuture<V, E>) -> SealFuture<V, E> {
        SealFuture {

impl <V: Send + Clone, E: Send + Clone> future::Future for SealFuture<V, E> {
    type Item = V;
    type Error = E;

    fn poll(&mut self) -> Poll<Self::Item, Self::Error> {

        let inner = self.fut.inner.lock().unwrap();
        let value = inner.value.as_ref().unwrap();
        if value.is_ok() {
            let ok_result = value.as_ref().ok().unwrap().clone();
        } else {
            let err_result = value.as_ref().err().unwrap().clone();

As example of usage of this converter may be presented json-rpc server based on the 'jsonrpc-http-server' crate. Here requests handler may work in asynchronous mode, in that it return a rust future with expected result instead of a result itself. You may interconnect sealrs future with this server with the following way:

let mut io = IoHandler::new();

io.add_method("say_hello", move |_: Params| {
    // Create sealrs future
    let fut = Future::asyncp(|| {
        Ok(Value::String(format!("hello", v)))
    }, (&executor).clone());

    // Convert sealrs future to 'futures' future

let server = ServerBuilder::new(io)
    .expect("Unable to start RPC server");


Need pay attention that future which will be passed to the converter must be unclosed, which means that the future may contains any sorts of a combinators (map, recover, flat_map and etc.), but not on_complete, because he never returns any result.

Pre completed futures

You can creates already completed futures. Such needs may occurs in situation when you needs return some result as future, but you already have expects result. You can create pre completed futures in this way:

let f1: WrappedFuture<u32, Error> = Future::ok(10);
let f2: WrappedFuture<u32, Error> = Future::err(Error::from("xxx"));



Asynchronous completed promise


Manually completable promise


Container for value which while does not calculated


Abstract promise definition