Module async_std::future

Asynchronous values.

Base Futures Concurrency

Often it’s desirable to await multiple futures as if it was a single future. The join family of operations converts multiple futures into a single future that returns all of their outputs. The race family of operations converts multiple future into a single future that returns the first output.

For operating on futures the following functions can be used:

Name	Return signature	When does it return?
Future::join	(T1, T2)	Wait for all to complete
Future::race	T	Return on first value

Fallible Futures Concurrency

For operating on futures that return Result additional try_ variants of the functions mentioned before can be used. These functions are aware of Result, and will behave slightly differently from their base variants.

In the case of try_join, if any of the futures returns Err all futures are dropped and an error is returned. This is referred to as “short-circuiting”.

In the case of try_race, instead of returning the first future that completes it returns the first future that successfully completes. This means try_race will keep going until any one of the futures returns Ok, or all futures have returned Err.

However sometimes it can be useful to use the base variants of the functions even on futures that return Result. Here is an overview of operations that work on Result, and their respective semantics:

Name	Return signature	When does it return?
Future::join	(Result<T, E>, Result<T, E>)	Wait for all to complete
Future::try_join	Result<(T1, T2), E>	Return on first Err, wait for all to complete
Future::race	Result<T, E>	Return on first value
Future::try_race	Result<T, E>	Return on first Ok, reject on last Err

Structs

• TimeoutError
  An error returned when a future times out.

Traits

• Future
  A future represents an asynchronous computation obtained by use of async.
• IntoFutureunstable
  Convert a type into a Future.

Functions

• pending
  Never resolves to a value.
• poll_fn
  Creates a new future wrapping around a function returning Poll.
• ready
  Resolves to the provided value.
• timeout
  Awaits a future or times out after a duration of time.