10.3 Delayed Evaluation

A promise encapsulates an expression to be evaluated on demand via force . After a promise has been force d, every later force of the promise produces the same result.

procedure
( promise? v)→boolean?
v:any/c

Returns #t if v is a promise, #f otherwise.

syntax
( delay body...+)

Creates a promise that, when force d, evaluates the bodys to produce its value. The result is then cached, so further uses of force produce the cached value immediately. This includes multiple values and exceptions.

syntax
( lazy body...+)

Like delay , if the last body produces a promise when forced, then this promise is force d, 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 force iterates 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.

procedure
( force v)→any
v:any/c

If v is 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 force s 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 force d 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.

procedure
( promise-forced? promise)→boolean?
promise:promise?

Returns #t if promise has been forced.

procedure
( promise-running? promise)→boolean?
promise:promise?

Returns #t if promise is currently being forced. (Note that a promise can be either running or forced but not both.)

10.3.1Additional Promise Kinds🔗 i

syntax
( delay/name body...+)

Creates 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 force recognizes.

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? .

procedure
( promise/name? promise)→boolean?
promise:any/c

Returns #t if promise is a promise created with delay/name .

Added in version 6.3 of package base.

syntax
( delay/strict body...+)

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.

syntax
( delay/sync body...+)

Produces a promise where an attempt to force the promise by a thread other than one currently running the promise causes the force to block until a result is available. This kind of promise is also a synchronizable event for use with sync ; sync ing on the promise does not force it, but merely waits until a value is forced by another thread. The synchronization result is #<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 .

syntax
( delay/thread body/option...+)

body/option = body
| #:groupthread-group-expr

Like delay/sync , but begins the computation immediately on a newly created thread. The thread is created under the thread group specified by thread-group-expr, which defaults to (make-thread-group ). A #:group specification can appear at most once.

Exceptions raised by the bodys are caught as usual and raised only when the promise is force d. 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).

syntax
( delay/idle body/option...+)

body/option = body
| #:wait-forwait-evt-expr
| #:work-whilewhile-evt-expr
| #:ticktick-secs-expr
| #:useuse-ratio-expr

Like delay/thread , but with the following differences:

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-expruse-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.

syntax
( for/list/concurrent maybe-group(for-clause...)
body-or-break...body)

maybe-group =
| #:groupthread-group-expr

thread-group-expr : thread-group?

Iterates like for/list , but the bodies (following any #:break or #:final clauses) are wrapped in delay/thread . Each promise is 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:

> (time
(for/list/concurrent ([i(in-range 5)])
(define duration(/ 1.0(random 50100)))
(sleep duration)
(printf "thread ~a slept for ~a milliseconds~n"i(truncate (* duration1000)))
i))

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.

syntax
( for*/list/concurrent maybe-group(for-clause...)
body-or-break...body)

Like for/list/concurrent , but with the implicit nesting of for*/list .

Added in version 8.6.0.4 of package base.

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