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10.4 Futures for Parallelism

+Parallelism with Futures in Guide: Racket introduces futures.

The bindings documented in this section are provided by the racket/future and racket libraries, but not racket/base.

Currently, parallel support for future is enabled by default for Windows, Linux x86/x86_64, and Mac OS X x86/x86_64. To enable support for other platforms, use --enable-futures with configure when building Racket.

The future and touch functions from racket/future provide access to parallelism as supported by the hardware and operation system. In contrast to thread, which provides concurrency for arbitrary computations without parallelism, future provides parallelism for limited computations. A future executes its work in parallel (assuming that support for parallelism is available) until it detects an attempt to perform an operation that is too complex for the system to run safely in parallel. Similarly, work in a future is suspended if it depends in some way on the current continuation, such as raising an exception. A suspended computation for a future is resumed when touch is applied to the future.

“Safe” parallel execution of a future means that all operations provided by the system must be able to enforce contracts and produce results as documented. “Safe” does not preclude concurrent access to mutable data that is visible in the program. For example, a computation in a future might use set! to modify a shared variable, in which case concurrent assignment to the variable can be visible in other futures and threads. Furthermore, guarantees about the visibility of effects and ordering are determined by the operating system and hardware – which rarely support, for example, the guarantee of sequential consistency that is provided for thread-based concurrency. At the same time, operations that seem obviously safe may have a complex enough implementation internally that they cannot run in parallel. See also Parallelism with Futures.

(future thunk)  future?
  thunk : (-> any)
(touch f)  any
  f : future?
The future procedure returns a future value that encapsulates thunk. The touch function forces the evaluation of the thunk inside the given future, returning the values produced by thunk. After touch forces the evaluation of a thunk, the resulting values are retained by the future in place of thunk, and additional touches of the future return those values.

Between a call to future and touch for a given future, the given thunk may run speculatively in parallel to other computations, as described above.

  > (let ([f (future (lambda () (+ 1 2)))])
      (list (+ 3 4) (touch f)))

  '(7 3)

(current-future)  (or/c #f future?)
Returns the descriptor of the future whose thunk execution is the current continuation. If a future thunk itself uses touch, future-thunk executions can be nested, in which case the descriptor of the most immediately executing future is returned. If the current continuation is not a future-thunk execution, the result is #f.

(future? v)  boolean?
  v : any/c
Returns #t if v is a future value, #f otherwise.

Returns the number of parallel computation units (e.g., processors or cores) that are available on the current machine.