suspension
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Serialized partial continuations
A suspension is a serialized partial continuation. This extension provides basic tools for working with suspensions and may be useful for implementing continuation-based modal behaviour in applications like web-servers.
Note that suspensions can only be reliably created for compiled code, as continuations created by the interpreter will contain more references to live data as needed - in particular the environment representation of the interpreter is simpler and retains more garbage. Continuations created by compiled code only reference the minimal set of live data required to continue execution and thus are much smaller.
Not all data is serializable, ports and foreign pointers are not, for example. To be able to create a suspension from a pending continuation, no references to such data may exist in the continuation (say, in local bindings which are accessed by code executed when the continuation is resumed).
The implementation of delimited execution contexts creates a separate thread so (current-thread) will not be eq? to the thread that created the delimited execution context. Parameters modified in the new execution context are propagated back to the parent thread, though. Also, any exceptions raised in the new context will be delivered to the parent thread.
Creates a limited execution context by spawning a new thread and suspending the current one. THUNK should be a procedure of no arguments and will be executed in th new context. Any results returned by THUNK will be returned by the with-limited-continuation form.
Captures the current continuation up to the point of the innermost enclosing with-limited-continuation form and invokes the 1-argument procedure STORE with the serialized representation (a string) of the continuation. The execution context will be exited and the enclosing with-limited-continuation form will return any results returned by STORE.
The effect of invoking continuation-suspend outside of the dynamic context of a with-limited-continuation is undefined.
Deserializes the continuation in the string SK and invokes it, continuing the execution suspended and stored in the serialized continuation. The given results will be returned from the continuation-suspend form that suspended SK.
The effect of invoking continuation-resume outside of the dynamic context of a with-limited-continuation is undefined.
Exits the execution context created by the innermost enclosing with-limited-continuation form and returns RESULTS ...
The effect of invoking continuation-drop outside of the dynamic context of a with-limited-continuation is undefined.
This example demonstrates how to use suspensions for continuation-based programming in the spiffy web-server:
;;;; webtest.scm
;
; Compile and start and point your browser to "http://localhost:8080/count".
; Click "Next" to invoke the continuation and try cloning the window.
; (Note how several windows share the global "j" but have private "i"
; bindings).
(use spiffy suspension posix utils)
(unless (file-exists? "continuations")
(create-directory "continuations") )
(define-http-resource (k id)
(with-limited-continuation
(lambda ()
(if id
(let ((k (with-input-from-file (make-pathname "continuations" id) read-all)))
(spiffy-debug "read: ~a" id)
(continuation-resume k #f) )
(error "missing `id' argument") ) ) ) )
(define (send/suspend proc)
(continuation-suspend
(lambda (k)
(let ((id (conc (current-seconds) (random 1000000))))
(with-output-to-file (make-pathname "continuations" id)
(cut display k) )
(spiffy-debug "written continuation ~a ..." id)
(proc (conc "k?id=" id)) ) ) ) )
(define j 1)
(define-http-resource (count)
(with-limited-continuation
(lambda ()
(let loop ((i 1))
(send/suspend
(lambda (kurl)
(conc "<h1>Count: " i " / " j "</h1><a href='" kurl "'>next</a>") ) )
(set! j (+ j 1))
(loop (+ i 1)) ) ) ) )
(start-server debug: #t root: "." port: 8080)Copyright (c) 2006, Felix L. Winkelmann. All rights reserved. Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the Software), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions: The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software. THE SOFTWARE IS PROVIDED ASIS, WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.