Essential combinators

We saw a few of the most important combinators in the futures and streams overviews. Here we’ll take a look at a few more. It’s also worth spending some time with the trait documentation to familiarize yourself with the full range of combinators available (cheatsheet).

Some concrete futures and streams

Any value can be turned into an immediately complete future. There are a few functions in the future module for creating such a future:

  • ok, which is analogous to Result::Ok: it treats the value you give it as an immediately successful future.
  • err, which is analogous to Result::Err: it treats the value you give it as an immediately failed future.
  • result, which lifts a result to an immediately-complete future.

For streams, there are a few equivalents of an “immediately ready” stream:

  • iter, which creates a stream that yields the same items as the underlying iterator. The iterator produces Result values, and the first error terminates the stream with that error.
  • once, which creates a single-element stream from a Result.

In addition to these constructors, there’s also a function, lazy, which allows you to construct a future given a closure that will produce that future later, on demand.


A crucial API to know about is the IntoFuture trait, which is a trait for values that can be converted into futures. Most APIs that you think of as taking futures actually work with this trait instead. The key reason: the trait is implemented for Result, allowing you to return Result values in many places that futures are expected.


Like Iterator, the Future, Stream and Sink traits all come equipped with a broad range of “adapter” methods. These methods all consume the receiving object and return a new, wrapped one. For futures, you can use adapters to:

  • Change the type of a future (map, map_err)
  • Run another future after one has completed (then, and_then, or_else)
  • Figure out which of two futures resolves first (select)
  • Wait for two futures to both complete (join)
  • Convert to a trait object (Box::new)
  • Convert unwinding into errors (catch_unwind)

For streams, there are a large set of adapters, including:

The Sink trait currently has fewer adapters

Finally, an object that is both a stream and a sink can be broken into separate stream and sink objects using the split adapter.

All adapters are zero-cost, meaning that no memory is allocated internally and the implementation will optimize to what you would have otherwise written by hand.

Error handling

Futures, streams and sinks all treat error handling as a core concern: they are all equipped with an associated error type, and the various adapter methods interpret errors in sensible ways. For example:

  • The sequencing combinators then, and_then, or_else, map, and map_err all chain errors similarly to the Result type in the standard library. So, for example, if you chain futures using and_then and the first future fails with an error, the chained future is never run.

  • Combinators like select and join also deal with errors. For select, the first future to complete in any way yields an answer, propagating the error, but also giving access to the other future should you want to keep working with it. For join, if any future produces an error, the entire join produces that error.

By default, futures don’t have any special handling for panics. In most cases, though, futures are ultimately run as tasks within a thread pool, where you’ll want to catch any panic they produce and propagate that elsewhere. The catch_unwind adapter can be used to reify a panic into a Result without taking down the worker thread.