A promise encapsulates an expression to be evaluated on demand via force. After a promise has been forced, every later force of the promise produces the same result.
Returns #t if v is a promise, #f otherwise.
Creates a promise that, when
forced, evaluates the
bodys to produce its value. The result is then cached, so further uses of
forceproduce the cached value immediately. This includes multiple values and exceptions.
Like
delay, if the last
bodyproduces a promise when forced, then this promise is
forced, too, to obtain a value. In other words, this form creates a composable promise, where the computation of its body is “attached” to the computation of the following promise, and a single
forceiterates through the whole chain, tail-calling each step.
Note that the last body of this form must produce a single value, but the value can itself be a delay promise that returns multiple values.
The lazy form is useful for implementing lazy libraries and languages, where tail calls can be wrapped in a promise.
If
vis a promise, then the promise is forced to obtain a value. If the promise has not been forced before, then the result is recorded in the promise so that future
forces on the promise produce the same value (or values). If forcing the promise raises an exception, then the exception is similarly recorded so that forcing the promise will raise the same exception every time.
If v is forced again before the original call to force returns, then the exn:fail exception is raised.
If v is not a promise, then it is returned as the result.
Returns #t if promise has been forced.
Returns #t if promise is currently being forced. (Note that a promise can be either running or forced but not both.)
10.3.1 Additional Promise Kinds🔗ℹCreates a “call-by-name” promise that is similar to
delay-promises, except that the resulting value is not cached. This kind of promise is essentially a thunk that is wrapped in a way that
forcerecognizes.
If a delay/name promise forces itself, no exception is raised, the promise is never considered “running” or “forced” in the sense of promise-running? and promise-forced?.
Returns
#tif
promiseis a promise created with
delay/name.
Added in version 6.3 of package base.
Creates a “strict” promise: it is evaluated immediately, and the result is wrapped in a promise value. Note that the body can evaluate to multiple values, and forcing the resulting promise will return these values.
Produces a promise where an attempt to
forcethe promise by a thread other than one currently running the promise causes the
forceto block until a result is available. This kind of promise is also a
synchronizable eventfor use with
sync;
syncing on the promise does not
forceit, but merely waits until a value is forced by another thread. The
synchronization resultis
#<void>.
If a promise created by delay/sync is forced on a thread that is already running the promise, an exception is raised in the same way as for promises created with delay.
body/option = body | #:group thread-group-expr(delay/thread body/option ...+)
Like
delay/sync, but begins the computation immediately on a newly created thread. The thread is created under the
thread groupspecified by
thread-group-expr, which defaults to
(make-thread-group). A
#:groupspecification can appear at most once.
Exceptions raised by the bodys are caught as usual and raised only when the promise is forced. Unlike delay/sync, if the thread running body terminates without producing a result or exception, force of the promise raises an exception (instead of blocking).
body/option = body | #:wait-for wait-evt-expr | #:work-while while-evt-expr | #:tick tick-secs-expr | #:use use-ratio-expr(delay/idle body/option ...+)
the computation does not start until the event produced by wait-evt-expr is ready, where the default is (system-idle-evt);
the computation thread gets to work only when the process is otherwise idle as determined by while-evt-expr, which also defaults to (system-idle-evt);
the thread is allowed to run only periodically: out of every tick-secs-expr (defaults to 0.2) seconds, the thread is allowed to run use-ratio-expr (defaults to 0.12) of the time proportionally; i.e., the thread runs for (* tick-secs-expr use-ratio-expr) seconds.
If the promise is forced before the computation is done, it runs the rest of the computation immediately without waiting on events or periodically restricting evaluation.
A #:wait-for, #:work-while, #:tick, or #:use specification can appear at most once.
maybe-group = | #:group thread-group-expr
Iterates like
for/list, but the bodies (following any
#:breakor
#:finalclauses) are wrapped in
delay/thread. Each
promiseis forced before the result list is returned.
Threads are created under thread-group-expr, which defaults to (make-thread-group). An optional #:group clause may be provided, in which case the threads will be created under that thread group.
This form does not support returning multiple values.
Example:
thread 2 slept for 10.0 milliseconds
thread 1 slept for 10.0 milliseconds
thread 3 slept for 13.0 milliseconds
thread 0 slept for 16.0 milliseconds
thread 4 slept for 20.0 milliseconds
cpu time: 22 real time: 20 gc time: 0
'(0 1 2 3 4)
Added in version 8.6.0.4 of package base.
Added in version 8.6.0.4 of package base.
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