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12.6 control.future - Futures

Module: control.future

A future is a simple construct for concurrent computation.

It encloses an expression, and compute its value concurrently. The result of computation can be retrieved later with future-get.

Futures are introduced in MultiLisp, in which retrieval of the computed value is implicit—a future is substituted with the result automatically. Racket and Guile have futures as a library, though the primitive to retrieve the result is called touch. We avoided to use the name since touch is too generic.

Macro: future expr

{control.future} Returns a future object, which run the compuatation of expr in a separate thread. The result(s) of expr can be retrieved by future-get. Note that expr can yield multiple values.

The expr is evaluated in the same environment as future appears, though if an exception raised within expr is not caught, its delivery is delayed until you call future-get.

The following exmaple runs HTTP access concurrently with other computation, and retrieves the result later.

(use control.future)
(use rfc.http)

(let1 f (future (http-get "example.com" "/"))
  ... some computation ...
  (receive (code headers body) (future-get f)
    ...))
Function: make-future thunk

{control.future} Returns a future that calls thunk in a separate thread.

(future expr) ≡ (make-future (lambda () expr))
Function: future? obj

{control.future} Returns #t if obj is a future, #f otherwise.

Function: future-get future :optional timeout timeout-val

{control.future} The argument must be a future. Retrieve the result(s) of future.

If the result of future is already available, it is returned immediately. If future is still computing, future-get blocks until the result is ready by default. You can limit how long you will wait by timeout argument, which can be #f (default, no timeout), a nonnegative real numebr (relative time in seconds), or a <time> object (absolute timepoint). If the timeout reaches before the result is available, timeout-val is returned, which defaults to #f. Calling future-get more than once returns the same result.

If an uncaught exception is raised during computation in the future, it is kept and reraised from future-get. It is handled in the dynamic environment of future-get (not the one in the original future call). If you call future-get again on such future, the effect is undefined (currently it returns #<undef> without raising an exception, but it may change in future).

Function: future-done? future

{control.future} Returns #t if computation in future is finished, #f otherwise.


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