60 lines
1.9 KiB
Racket
60 lines
1.9 KiB
Racket
#lang scribble/doc
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@(require "mz.ss"
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(for-label scheme/promise))
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@title{Delayed Evaluation}
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@note-lib[scheme/promise]
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A @deftech{promise} encapsulates an expression to be evaluated on
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demand via @scheme[force]. After a promise has been @scheme[force]d,
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every later @scheme[force] of the promise produces the same result.
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@defproc[(promise? [v any/c]) boolean?]{
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Returns @scheme[#t] if @scheme[v] is a promise, @scheme[#f]
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otherwise.}
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@defform[(delay expr)]{
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Creates a promise that, when @scheme[force]d, evaluates @scheme[expr]
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to produce its value.}
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@defform[(lazy expr)]{
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Like @scheme[delay], except that if @scheme[expr] produces a promise,
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then this promise is @scheme[force]d to obtain a value. In other
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words, this form creates a kind of a composable promise, which is
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mostly useful for implementing lazy libraries and languages. Also
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note that the @scheme[expr] in this case is restricted to one that
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produces a single value.}
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@defproc[(force [v any/c]) any]{
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If @scheme[v] is a promise, then the promise is forced to obtain a
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value. If the promise has not been forced before, then the result is
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recorded in the promise so that future @scheme[force]s on the promise
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produce the same value (or values). If forcing the promise raises an
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exception, then the exception is similarly recorded so that forcing
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the promise will raise the same exception every time.
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If @scheme[v] is @scheme[force]d again before the original call to
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@scheme[force] returns, then the @exnraise[exn:fail].
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If @scheme[v] is not a promise, then it is returned as the result.}
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@defproc[(promise-forced? [promise promise?]) boolean?]{
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Returns @scheme[#t] if @scheme[promise] has been forced.}
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@defproc[(promise-running? [promise promise?]) boolean?]{
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Returns @scheme[#t] if @scheme[promise] is currently being forced.
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(Note that a promise can be either running or forced but not both.)}
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