Futures and async/await for Crystal
0.2.0 Latest release released


Async operations for Crystal, taking inspiration from Python's asyncio.Future, JavaScript Promises, and async/await.


  1. Add the dependency to your shard.yml:

        github: protoncr/async
  2. Run shards install



Async contains futures, much like you'd find in Python's asyncio. Futures by default are not executed on creation, but are rather a building block to be used for other async operations.

require "async"

# This is an empty future. Doesn't do much right now.
future = Async::Future(Int32).new
# => #<Async::Future(Int32):0x1016f2f50 ...>

# => Idle

# To put the future to use, we can give it a callback
future.callback = -> { sleep 5; 5 }

# The future still hasn't executed yet. To make it execute, we can
# call the `#execute` method

# This will cause the callback to execute in the backround, allowing
# other tasks to happen concurrently. To wait for the execution to
# finish and retrieve the value, call `#wait`
# => 5

Futures are made a bit more powerful when you use the Future.execute constructor:

future = Async::Future.execute { sleep 5; 5 }
# => 5

Futures also have built in error handling. If an error occurs while your callback is executing, the exception will be stored in the error property and failure? will be true. If you want to throw a possible exception during wait, you can do so with wait!.

future = Async::Future.execute { sleep 5; 5_u8 + 251 }
# => Unhandled exception: Arithmetic overflow (OverflowError)

.all / .any / .race

Insipred by the JavaScript promise methods with the same names, the .all, .any, and .race methods can be used to wait for certain things to happen with your futures.

.all accepts N futures and will wait for them all to finish before returning. The return value of .all is an Array(T) where T is the type(s) of the futures. If ordered is set to true, futures will be resolved and returned in order.

.any accepts N futures and returns the value of the first future to complete successfully.

.race is similar to .all, but it returns the first time a future finishes, whether successful or not.


fut1 = Async::Future.execute do
  loop do
    num = rand(1..99)
    sleep num
    puts "f1 " + num.to_s
    break if num == 69

fut2 = Async::Future.execute do
  loop do
    num = rand(1..99)
    sleep num
    puts "f2 " + num.to_s
    break if num == 69

Async::Future.all(fut1, fut2)

async / await

Async also includes the loved and hated async / await.

The async macro basically works by wrapping the body of your function in a Future and executing that future immediately. Theoretically most functions should work just fine as async functions, but more complex function definitions are still untested. Most specifically functions with blocks.

await can be used on async methods and futures to wait for the return value. All it really does currently is call .wait!. If an exception is raised inside the future await will raise an Async::UncaughtException which contains the uncaught exception.

These are optional (so as not to pollute the global namespace with unwanted macros), so if you want to use it be sure to require future/macros.


  1. Fork it (
  2. Create your feature branch (git checkout -b my-new-feature)
  3. Commit your changes (git commit -am 'Add some feature')
  4. Push to the branch (git push origin my-new-feature)
  5. Create a new Pull Request


  github: protoncr/async
  version: ~> 0.2.0
License MIT
Crystal 0.35.1


Dependencies 0

Development Dependencies 0

Dependents 1

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