►The Racket Reference
▼10 Control Flow
►10.3 Delayed Evaluation
On this page:
promise?
delay
lazy
force
promise-forced?
promise-running?
10.3.1 Additional Promise Kinds
delay/  name
promise/  name?
delay/  strict
delay/  sync
delay/  thread
delay/  idle
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10.3 Delayed Evaluation

 (require racket/promise) package: base
The bindings documented in this section are provided by the racket/promise and racket libraries, but not racket/base.

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.

procedure

(promise? v) → boolean?

  v : any/c
Returns #t if v is a promise, #f otherwise.

syntax

(delay body ...+)

Creates a promise that, when forced, 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 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 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 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.

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.)

10.3.1 Additional Promise Kinds

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; syncing 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
  | #:group thread-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 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).

syntax

(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
Like delay/thread, but with the following differences:

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.

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