283 lines
11 KiB
Racket
283 lines
11 KiB
Racket
#lang scribble/doc
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@(require scribble/struct "mz.rkt" (for-label racket/async-channel))
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@title[#:tag "sync"]{Events}
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@section-index["select"]
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@section-index["poll"]
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A @deftech{synchronizable event} (or just @defterm{event} for short)
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works with the @racket[sync] procedure to coordinate synchronization
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among threads. Certain kinds of objects double as events, including
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ports and threads. Other kinds of objects exist only for their use as
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events.
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At any point in time, an event is either @deftech{ready for
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synchronization}, or it is not; depending on the kind of event and how
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it is used by other threads, an event can switch from not ready to
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ready (or back), at any time. If a thread synchronizes on an event
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when it is ready, then the event produces a particular
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@deftech{synchronization result}.
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Synchronizing an event may affect the state of the event. For example,
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when synchronizing a semaphore, then the semaphore's internal count is
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decremented, just as with @racket[semaphore-wait]. For most kinds of
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events, however (such as a port), synchronizing does not modify the
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event's state.
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Racket values that act as @tech{synchronizable events} include
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@tech{semaphores}, @tech{channels}, @tech{asynchronous channels},
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@tech{ports}, @tech{TCP listeners}, @tech{threads},
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@tech{subprocess}es, @tech{will executors}, and @tech{custodian
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box}es. Libraries can define new synchronizable events, especially
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though @racket[prop:evt].
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@;------------------------------------------------------------------------
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@defproc[(evt? [v any/c]) boolean?]{
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Returns @racket[#t] if @racket[v] is a @tech{synchronizable event},
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@racket[#f] otherwise.}
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@defproc[(sync [evt evt?] ...+) any]{
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Blocks as long as none of the @tech{synchronizable events}
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@racket[evt]s are ready, as defined above.
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When at least one @racket[evt] is ready, its @tech{synchronization
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result} (often @racket[evt] itself) is returned. If multiple
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@racket[evt]s are ready, one of the @racket[evt]s is chosen
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pseudo-randomly for the result; the
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@racket[current-evt-pseudo-random-generator] parameter sets the
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random-number generator that controls this choice.}
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@defproc[(sync/timeout [timeout (or/c #f (and/c real? (not/c negative?)) (-> any))]
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[evt evt?] ...+)
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any]{
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Like @racket[sync] if @racket[timeout] is @racket[#f]. If
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@racket[timeout] is a real number, then the result is @racket[#f]
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if @racket[timeout] seconds pass without a
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successful synchronization. If @racket[timeout] is a procedure, then
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it is called in tail position if polling the @racket[evt]s discovers
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no ready events.
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A zero value for @racket[timeout] is equivalent to @racket[(lambda ()
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#f)]. In either case, each @racket[evt] is checked at least once
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before returning @racket[#f] or calling @racket[timeout].
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See also @racket[alarm-evt] for an alternative timeout mechanism.}
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@defproc[(sync/enable-break [evt evt?] ...+) any]{
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Like @racket[sync], but breaking is enabled (see
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@secref["breakhandler"]) while waiting on the @racket[evt]s. If
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breaking is disabled when @racket[sync/enable-break] is called, then
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either all @racket[evt]s remain unchosen or the @racket[exn:break]
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exception is raised, but not both.}
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@defproc[(sync/timeout/enable-break [timeout (or/c #f (and/c real? (not/c negative?)) (-> any))]
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[evt evt?] ...+)
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any]{
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Like @racket[sync/enable-break], but with a timeout as for @racket[sync/timeout].}
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@defproc[(choice-evt [evt evt?] ...) evt?]{
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Creates and returns a single event that combines the
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@racket[evt]s. Supplying the result to @racket[sync] is the same as
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supplying each @racket[evt] to the same call.
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That is, an event returned by @racket[choice-evt] is @tech{ready for
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synchronization} when one or more of the @racket[_evt]s supplied to
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@racket[choice-evt] are @tech{ready for synchronization}. If the
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choice event is chosen, one of its ready @racket[_evt]s is chosen
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pseudo-randomly, and the @tech{synchronization result} is the chosen
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@racket[_evt]'s @tech{synchronization result}.}
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@defproc[(wrap-evt [evt (and/c evt? (not/c handle-evt?))]
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[wrap (any/c ... . -> . any)])
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evt?]{
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Creates an event that is @tech{ready for synchronization} when
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@racket[evt] is @tech{ready for synchronization}, but whose
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@tech{synchronization result} is determined by applying @racket[wrap]
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to the @tech{synchronization result} of @racket[evt]. The number
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of arguments accetped by @racket[wrap] must match the number of values
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for the synchronization result of @racket[evt].
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The call to @racket[wrap] is
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@racket[parameterize-break]ed to disable breaks initially. The
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@racket[evt] cannot be an event created by @racket[handle-evt] or any
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combination of @racket[choice-evt] involving an event from
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@racket[handle-evt].}
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@defproc[(handle-evt [evt (and/c evt? (not/c handle-evt?))]
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[handle (any/c ... . -> . any)])
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handle-evt?]{
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Like @racket[wrap], except that @racket[handle] is called in @tech{tail
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position} with respect to the synchronization request, and without
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breaks explicitly disabled.}
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@defproc[(guard-evt [maker (-> evt?)]) evt?]{
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Creates a value that behaves as an event, but that is actually an
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event maker.
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An event @racket[_guard] returned by @racket[guard-evt] generates an
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event when @racket[_guard] is used with @racket[sync]
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(or whenever it is part of a choice event used with @racket[sync],
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etc.), where the generated event is the result of calling
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@racket[maker]. The @racket[maker] procedure may be called by
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@racket[sync] at most once for a given call to @racket[sync], but
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@racket[maker] may not be called if a ready event is chosen before
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@racket[_guard] is even considered.
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If @racket[maker] returns a non-event, then @racket[maker]'s
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result is replaced with an event that is @tech{ready for
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synchronization} and whose @tech{synchronization result} is
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@racket[_guard].}
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@defproc[(nack-guard-evt [maker (evt? . -> . evt?)]) evt?]{
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Like @racket[guard-evt], but when @racket[maker] is called, it is
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given a NACK (``negative acknowledgment'') event. After starting the
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call to @racket[maker], if the event from @racket[maker] is not
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ultimately chosen as the ready event, then the NACK event supplied to
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@racket[maker] becomes @tech{ready for synchronization} with a
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@|void-const| value.
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The NACK event becomes @tech{ready for synchronization} when the event
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is abandoned when either some other event is chosen, the synchronizing
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thread is dead, or control escapes from the call to @racket[sync]
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(even if @racket[_nack-guard]'s @racket[maker] has not yet returned a
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value). If the event returned by @racket[maker] is chosen, then the
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NACK event never becomes @tech{ready for synchronization}.}
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@defproc[(poll-guard-evt [maker (boolean? . -> . evt?)]) evt?]{
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Like @racket[guard-evt], but when @racket[maker] is called, it is
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provided a boolean value that indicates whether the event will be used
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for a poll, @racket[#t], or for a blocking synchronization,
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@racket[#f].
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If @racket[#t] is supplied to @racket[maker], if breaks are
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disabled, if the polling thread is not terminated, and if polling the
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resulting event produces a @tech{synchronization result}, then the event
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will certainly be chosen for its result.}
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@defthing[always-evt evt?]{A constant event that is always @tech{ready
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for synchronization}, with itself as its @tech{synchronization result}.}
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@defthing[never-evt evt?]{A constant event that is never @tech{ready
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for synchronization}.}
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@defproc[(system-idle-evt) evt?]{
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Returns an event that is @tech{ready for synchronization} when the
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system is otherwise idle: if the result event were replaced by
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@racket[never-evt], no thread in the system would be available to run.
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In other words, all threads must be suspended or blocked on events
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with timeouts that have not yet expired. The system-idle event's
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@tech{synchronization result} is @|void-const|. The result of the
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@racket[system-idle-evt] procedure is always the same event.}
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@defproc[(alarm-evt [msecs nonnegative-number?]) evt]{
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Returns a @tech{synchronizable event} that is not @tech{ready for synchronization} when
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@racket[(current-inexact-milliseconds)] would return a value that is
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less than @racket[msecs], and it is @tech{ready for synchronization} when
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@racket[(current-inexact-milliseconds)] would return a value that is
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more than @racket[msecs]. @ResultItself{alarm event}.}
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@defproc[(handle-evt? [evt evt?]) boolean?]{
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Returns @racket[#t] if @racket[evt] was created by @racket[handle-evt]
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or by @racket[choice-evt] applied to another event for which
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@racket[handle-evt?] produces @racket[#t]. Such events are illegal as
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an argument to @racket[handle-evt] or @racket[wrap-evt], because they
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cannot be wrapped further. For any other event, @racket[handle-evt?]
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produces @racket[#f], and the event is a legal argument to
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@racket[handle-evt] or @racket[wrap-evt] for further wrapping.}
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@;------------------------------------------------------------------------
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@defthing[prop:evt struct-type-property?]{
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A @tech{structure type property} that identifies structure types whose
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instances can serve as @tech{synchronizable events}. The property value can
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be any of the following:
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@itemize[
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@item{An event @racket[_evt]: In this case, using the structure as an
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event is equivalent to using @racket[_evt].}
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@item{A procedure @racket[_proc] of one argument: In this case, the
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structure is similar to an event generated
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by @racket[guard-evt], except that the would-be guard
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procedure @racket[_proc] receives the structure as an argument, instead
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of no arguments; also, a non-event result from @racket[_proc]
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is replaced with an event that is already @tech{ready for synchronization}
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and whose @tech{synchronization result} is the structure.}
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@item{An exact, non-negative integer between @racket[0] (inclusive)
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and the number of non-automatic fields in the structure type
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(exclusive, not counting supertype fields): The integer identifies a
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field in the structure, and the field must be designated as
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immutable. If the field contains an object or an event-generating
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procedure of one argument, the event or procedure is used as
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above. Otherwise, the structure acts as an event that is never
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ready.}
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]
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Instances of a structure type with the @racket[prop:input-port] or
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@racket[prop:output-port] property are also @tech{synchronizable events} by virtue
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of being a port. If the structure type has more than one of
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@racket[prop:evt], @racket[prop:input-port], and
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@racket[prop:output-port], then the @racket[prop:evt] value (if any)
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takes precedence for determining the instance's behavior as an event,
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and the @racket[prop:input-port] property takes precedence over
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@racket[prop:output-port] for synchronization.
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@examples[
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(define-struct wt (base val)
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#:property prop:evt (struct-field-index base))
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(define sema (make-semaphore))
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(sync/timeout 0 (make-wt sema #f))
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(semaphore-post sema)
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(sync/timeout 0 (make-wt sema #f))
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(semaphore-post sema)
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(sync/timeout 0 (make-wt (lambda (self) (wt-val self)) sema))
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(semaphore-post sema)
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(define my-wt (make-wt (lambda (self) (wrap-evt
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(wt-val self)
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(lambda (x) self)))
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sema))
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(sync/timeout 0 my-wt)
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(sync/timeout 0 my-wt)
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]}
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@defparam[current-evt-pseudo-random-generator generator pseudo-random-generator?]{
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A @tech{parameter} that determines the pseudo-random number generator used by
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@racket[sync] for events created by @racket[choice-evt].}
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