10.3 Delayed Evaluation
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
Creates a promise that, when force
d, evaluates the
s 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.
, 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.
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 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
Creates a “call-by-name” promise that is similar to
-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
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?.
is 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
Produces a promise where an attempt to force
the promise by a
thread other than one currently running the promise causes the
to block until a result is available. This kind of
promise is also a synchronizable event
for use with
ing on the promise does not force
it, but merely waits until a value is forced by another thread.
The synchronization result
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.
(delay/thread body/option ...+)
|body/option|| ||=|| ||body|
| || ||||| ||#:group thread-group-expr|
, but begins the computation immediately on a
newly created thread. The thread is created under the thread
specified by thread-group-expr
, which defaults to
. A #:group
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
(delay/idle body/option ...+)
|body/option|| ||=|| ||body|
| || ||||| ||#:wait-for wait-evt-expr|
| || ||||| ||#:work-while while-evt-expr|
| || ||||| ||#:tick tick-secs-expr|
| || ||||| ||#:use use-ratio-expr|
the computation does not start until the event produced by
wait-evt-expr is ready, where the default is
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.