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From stev...@outerthought.org
Subject [WIKI-UPDATE] Main Gzip_XML_Serializer Cocoon_and_Apache_mod_gzip RhinoWithContinuations Wed Mar 19 23:00:06 2003
Date Wed, 19 Mar 2003 22:00:06 GMT
Page: http://wiki.cocoondev.org/Wiki.jsp?page=Main , version: 152 on Wed Mar 19 21:19:35 2003
by SylvainWallez

+ * March 19, 2003 ''-- [SW|SylvainWallez]''
+ ** Added [RhinoWithContinuations] which explains continuations used in Cocoon's flow engine.


Page: http://wiki.cocoondev.org/Wiki.jsp?page=Gzip_XML_Serializer , version: 9 on Wed Mar
19 21:06:53 2003 by 142.166.206.237

- XML is meant to be human-readable. Often this means that the markup is pointlessly verbose
from the machine's standpoint. The common solution is to compress the XML -- the SVG spec,
for instance, stipulates that SVG viewers need to be able to read gzipped files as well as
uncompressed equivalent; and numerous applications, from Gnumeric to OpenOffice, compress
their XML before saving. When files are being sent over the internet, compression can make
the difference between impracticality and practicality. Working with SVG maps at the Heml
Project, I found I was producing files well over 500k in size. These could never be served
to the modem-connected great unwashed, but their gzipped equivalent, weighing in at less than
100k, could.
+ XML is meant to be human-readable. Often this means that the markup is pointlessly verbose
from the machine's standpoint. The common solution is to compress the XML -- the SVG spec,
for instance, stipulates that SVG viewers need to be able to read gzipped files as well as
uncompressed equivalent; and numerous applications, from Gnumeric to OpenOffice, compress
their XML before saving. When files are being sent over the internet, compression can make
the difference between impracticality and practicality. Working with SVG maps at the [Heml
Project | http://heml.mta.ca], I found I was producing files well over 500k in size. These
could never be served to the modem-connected great unwashed, but their gzipped equivalent,
weighing in at less than 100k, could.
?                                                                                        
                                                                                         
                                                                                         
                                                                                         
                                                                                         
                                                                                         
 +            ++++++++++++++++++++++



Page: http://wiki.cocoondev.org/Wiki.jsp?page=Cocoon_and_Apache_mod_gzip , version: 2 on Wed
Mar 19 21:08:38 2003 by 142.166.206.237

+ __These ideas have been superceded by the How-To describing a [Gzip_XML_Serializer]__
+ 
- (More to come ...)


Page: http://wiki.cocoondev.org/Wiki.jsp?page=RhinoWithContinuations , version: 1 on Wed Mar
19 21:17:52 2003 by SylvainWallez

New page created:
+ ''Transcript of [an explanation|http://marc.theaimsgroup.com/?l=xml-cocoon-dev&m=102781075226697&w=2]
of [ChristopherOliver] on cocoon-dev in July 2002.''
+ 
+ Flow scripts in Cocoon are written in JavaScript. Why JavaScript ? Because it's a high-level
scripting language that looks a lot like Java, and also because we have a special version
of the Rhino JavaScript interpreter that has the ability to "capture" the current execution
state of a program.
+ 
+ This special version is hosted on [Cocoondev.org|http://cvs.cocoondev.org/cgi-bin/viewcvs.cgi/?cvsroot=rhino].
+ 
+ The {{org.mozilla.javascript.continuations}} package introduces an
+ additional interpreted mode for Rhino that supports tail-call elimination and
+ first-class continuations. Currently this mode is selected by setting the
+ optimization level of the current context to -2. It may also be selected by
+ passing "-opt -2" on the command line to the Rhino shell or debugger, for
+ example like this:
+ 
+ (shell:)
+ 
+ {{% java -cp js.jar org.mozilla.javascript.tools.shell.Main -opt -2 file.js}}
+ 
+ (debugger:)
+ 
+ {{% java -cp js.jar org.mozilla.javascript.tools.debugger.Main -opt -2 file.js}}
+ 
+ !!Features
+ 
+ !Tail-call elimination
+ 
+ You might think the following code is faulty since it apparently would
+ overflow the call-stack given a large enough value for 'limit':
+ 
+ {{{
+ function g(count, limit) {
+    if (count == limit) {
+       return "done";
+    }
+    return g(count + 1, limit);
+ }
+ }}}
+ 
+ In fact, with the "continuations" mode of Rhino this is not the case. The
+ interpreter detects that the recursive call to 'g()' is in so-called "tail
+ position" (meaning that there is no further code to execute after the call)
+ and simply overwrites the current call frame with the new call to 'g()'.
+ Thus no additional stack space is used in such cases.
+ 
+ !Continuations
+ 
+ Rhino now supports first-class continuations. In Rhino a continuation is a
+ JavaScript object that represents a snapshot of the state of an executing
+ Rhino script -- i.e the current call-stack -- including each call-frame's
+ program counter and local variables. The term "Continuation"
+ (borrowed from Scheme) is used because it represents the rest of,
+ or the "continuation" of, a program. Each time you call a function, there is
+ an implicit continuation -- the place where the function should return to.
+ A JavaScript Continuation object provides a way to bind that implicit
+ continuation to a name and keep hold of it. If you don't do anything with
+ the named continuation, the program will eventually invoke it anyway when
+ the function returns and passes control to its caller. Now that it's
+ named, however, you could invoke the continuation earlier than normal,
+ or you could invoke it later (after it has already been invoked by the
+ normal control flow). In the early case, the effect is a non-local exit.
+ In the later case, it's more like returning from the same function more
+ than once.
+ 
+ You can capture the continuation of a script by simply calling
+ 
+ {{{
+ new Continuation()
+ }}}
+ 
+ for example:
+ 
+ {{{
+ function someFunction(a, b) {
+     var kont = new Continuation();
+ }
+ }}}
+ 
+ The variable {{kont}} now represents the execution state of the current
+ caller of {{someFunction}}. (For those of you who are familiar with Scheme's
+ {{call-with-current-continuation}}, here is the Rhino equivalent:
+ 
+ {{{
+ function call_with_current_continuation(fun) {
+     var kont = new Continuation();
+     return fun(kont);
+ }
+ }}}
+ 
+ Since {{kont}} is a first-class JavaScript object you can return it,
+ store it in a variable, assign it to a property of another object -
+ whatever you like. In addition, a Continuation object is also a function,
+ which may be called. When you make such a call the current execution
+ state of the program is discarded and the snapshot of the program
+ represented by the Continuation object is resumed in its place. You may also
+ pass an argument to the call to a Continuation object (if you don't pass an
+ argument 'undefined' is implicitly passed instead). The value you pass as an
+ argument to the Continuation object becomes the return value of the function
+ in which the Continuation was captured. For example:
+ 
+ {{{
+ 01  function someFunction()  {
+ 02     var kont  = new  Continuation();
+ 03     print("captured: " + kont);
+ 04     return kont;
+ 05  }
+ 06
+ 07  var k = someFunction();
+ 08  if (k instanceof Continuation) {
+ 09     print("k is a continuation");
+ 10     k(200);
+ 11  } else {
+ 12     print("k is now a " + typeof(k));
+ 13  }
+ 14  print(k);
+ }}}
+ 
+ Evaluating the above script yields the following output:
+ 
+ {{{
+ captured: [object Continuation]
+ k is a continuation
+ k is now a number
+ 200
+ }}}
+ 
+ When the continuation {{k}} is invoked on line 10, the program "jumps" back to
+ the call to {{someFunction}} on line 7, but this time {{someFunction}} returns
+ with the value '200' (which was passed into the call to {{k}} on line 10).
+ 
+ In addition, a Continuation object may be called more than once. Each time it
+ is called it restarts execution at the return point of the function in which
+ it was captured. This means that the same function invocation can return
+ multiple times (and with different return values).
+ 
+ Finally, note that a Continuation created in a top-level script provides a
+ means to terminate any script immediately. Whenever such a Continuation is
+ invoked it simply terminates the interpreter, for example:
+ 
+ {{{
+ var suicide = new Continuation();
+ 
+ function foo(suicide) {
+     print("commiting suicide");
+     suicide();
+     print("never reached");
+ }
+ 
+ foo(suicide);
+ }}}
+ 
+ !ContinuationException
+ 
+ A Continuation can be thought of as representing the return from a function
+ invocation. In JavaScript, in addition to a normal return, a function
+ invocation may return due to an exception. In continuations mode, Rhino
+ provides a special built-in object {{ContinuationException}} which allows you to
+ throw an exception to a Continuation. ContinuationException's constructor
+ takes one argument - the object you want to throw. When an instance of
+ ContinuationException is passed to a Continuation, the value of that argument
+ is thrown in the context of the Continuation after the Continuation is
+ restored. For example:
+ 
+ {{{
+ function someFunction() {
+     var k = new Continuation();
+     return k;
+ }
+ 
+ try {
+     var k = someFunction();
+     print("k: " + k);
+     if (k instanceof Continuation) {
+         print("k is a continuation");
+         k(new ContinuationException("this is thrown from someFunction"));
+     }
+     print("never reached");
+ } catch (e) {
+     print("caught exception: " + e);
+ }
+ }}}
+ 
+ Evaluating the above script yields the following output:
+ 
+ {{{
+ k: [object Continuation]
+ k is a continuation
+ caught exception: this is thrown from someFunction
+ }}}
+ 
+ !Controlling what gets captured in a Continuation
+ 
+ In continuations mode, Rhino provides a special extended syntax to allow you
+ to control what gets captured in a continuation as follows:
+ 
+ {{{
+ catch (break) {
+     // a continuation has been captured - code to handle that
+     // goes here
+ }
+ 
+ catch (continue) {
+     // a continuation has been resumed - code to handle that
+     // goes here
+ }
+ }}}
+ 
+ Multiple such "catch" clauses may be present at any scope. All such clauses
+ contained within a script or function invocation captured or resumed as part
+ of a Continuation will be executed when that Continuation is captured
+ or resumed. For example, you might want to return a pooled JDBC connection to
+ its connection pool while a Continuation is suspended and recover the
+ connection when the Continuation is resumed:
+ 
+ {{{
+ var pool = ...;
+ 
+ function someFunction() {
+ 
+     var conn = pool.getConnection();
+     ...
+ 
+     catch (break) {
+         conn.close();
+         conn = null;
+     }
+ 
+     catch (continue) {
+         conn = pool.getConnection();
+     }
+ }
+ }}}
+ 
+ !Continuations are Serializable
+ 
+ This version of Rhino also contains experimental support for serializing
+ Continuations. Thus you may save the state of an executing script and
+ restore it later. Here is an example:
+ 
+ {{{
+ function capture(filename) {
+     var k = new Continuation();
+     serialize(k, filename);
+     java.lang.System.exit(0);
+ }
+ 
+ function foo(level) {
+     var now = new java.util.Date();
+     if(level > 5) {
+         print("run the file foo.ser");
+         capture("foo.ser");
+     } else {
+         print("next level");
+         foo(level + 1);
+     }
+     print("restarted("+level+"): " + now)
+ }
+ 
+ foo(1);
+ }}}
+ 
+ Evaluating the above script saves a Continuation to the file "foo.ser" and
+ prints the following output:
+ 
+ {{{
+ next level
+ next level
+ next level
+ next level
+ next level
+ run the file foo.ser
+ }}}
+ 
+ The following script deserializes the Continuation and executes it:
+ 
+ {{{
+ var k = deserialize("foo.ser");
+ k();
+ }}}
+ 
+ Evaluating the second script produces the following output:
+ 
+ {{{
+ restarted(6): Mon May 20 19:03:12 PDT 2002
+ restarted(5): Mon May 20 19:03:12 PDT 2002
+ restarted(4): Mon May 20 19:03:12 PDT 2002
+ restarted(3): Mon May 20 19:03:12 PDT 2002
+ restarted(2): Mon May 20 19:03:12 PDT 2002
+ restarted(1): Mon May 20 19:03:12 PDT 2002
+ }}}
+ 



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