Anton van Straaten  <anton@appsolutions.com> wrote:
+---------------
| Rob Warnock wrote:
| > But how do you provide *multiple* continuations using CALL/CC syntax?
| 
| To illustrate using your form example, you can define a macro along 
| these lines: (define-syntax form-let ...[elided]... )
|
| You can use it like this:
| (form-let ((form1 make-form1 handle-form1)
|             (form2 make-form2 handle-form2))
|    (send/suspend (make-page form1 form2)))
+---------------

Neat! By hoisting the building of the forms into the FORM-LET,
you get to generate multiple continuations. Now why didn't *I*
think of that...?  ;-}  ;-}

[Answer: Because I use an s-expr-based HTML-generator (HTOUT,
but it could be CL-WHO or HTMLGEN or others), I tend to generate
the forms sequentially inline, and thus was thinking that there
was only one possible continuation to SEND-SUSPEND.]

+---------------
| When either of the two forms is submitted by the client, the 
| corresponding continuation is invoked.  This restarts execution at the 
| point at which either form1 or form2 was bound, setting them to the 
| result of the form submission.  The form? test detects that it has been 
| given something other than a form (i.e. it has a result), and the 
| corresponding handler is invoked.  Once the handler completes, execution 
| would continue "after" the form-let expression, i.e. the form-let 
| expression's continuation is invoked with the result returned by the 
| form handler.
+---------------

I suspect another reason I didn't think of this approach is that,
using it, the usual default continuation created by SEND/SUSPEND is
*abandoned* completely, since that continuation is never invoked.

+---------------
| I set up this example for ease of illustration, but it's easy enough to 
| make a more integrated version to support e.g. constructing a page with 
| multiple forms as a single inline sexp, without requiring procedures 
| such as make-form.
+---------------

Sure. Just initialize a collection at the start of the page generation,
and as each form is generated push its continuation onto the collection,
and (SEND/SUSPEND (APPLY #'MAKE-PAGE COLLECTED-FORMS)) at the end.
[But as above, the default continuation created by SEND/SUSPEND is
abandoned if any of the forms is submitted.]

Thanks for the tip. Some of the ideas apply even if one *isn't*
using "real" continuations on the server...


-Rob

-----
Rob Warnock			<rpw3@rpw3.org>
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