Thanks for the detailed answer Philip! Some of it is definitely over my head. The reason I don't pass around the id is that I use the html field exactly to track the id and wanted to avoid having to pass around the argument left and right - and the only ways to pass it on are via forms or by sticking it into the requested URL or by using continuations. I will give the continuation-style solution a shot though, it's probably cleaner to pass the id around as an argument.
Cheers, Marc On Tue, Mar 14, 2017 at 1:34 PM, Philip McGrath <phi...@philipmcgrath.com> wrote: > In this case it will work despite being a hack, because you know that your > id argument affects only the display stage, not the processing stage: > however, you can't know in general that this will work for dynamically > generated formlets, and personally I would therefore be reluctant to rely > on this, both because it is fragile in terms of changes to your > implementation of matrix-formlet that you might make later. > > If you are using native continuations (i.e. not the stateless #lang > web-server), you can use send/formlet or embed-formlet to deal with this > correctly and easily. (At a lower level, you could also do something like > >> (define-values (xexpr-forest bindings->result i) >> ((matrix-formlet "3") 0)) > > to access the low-level rendering, processing function, and next allowable > input integer directly.) > > It is a little more tricky in #lang web-server, because your formlet and > it's generated processing function are plain functions, and thus not > serializable (see this old thread for discussion: https://groups. > google.com/d/topic/racket-users/lMYWjodgpmo/discussion). The easy > work-around is to keep the arguments you need to reproduce your formlet > around as part of the closure, e.g. by rewriting your matrix-submission > like this: > >> (define (matrix-submission req id) >> (define-values (number-of-ones user-id) >> (formlet-process (matrix-formlet id) req)) >> ...) > > The alternative is to either recompile the formlet library using #lang > web-server/base or to implement your formlet at a low level using > serial-lambda and not use any of the library tools. > > In this specific case, though, if you are using this hidden input field > just to remember the user id from one request to the next (i.e. you > wouldn't ever change it on the client side with JavaScript or something), > the best solution is not to put it in the form at all, but have it be part > of the continuation. This could also have security advantages, if you > cryptographically sign your continuations or store them on the server. If > that works for your use case, you could simply write: > (define matrix-formlet > input-string) > display it with > >> (matrix-submission >> (send/suspend >> (λ (k-url) >> (response/xexpr >> `(html (body (form >> ((action "/the-matrix-submitted") >> (method "post")) >> ,@(formlet-display matrix-formlet) >> (input ([type "submit"])))))))) >> user-id) > > and then define matrix-submission as > >> (define (matrix-submission req user-id) >> (define number-of-ones >> (formlet-process matrix-formlet req)) >> ...) > > > -Philip > > On Tue, Mar 14, 2017 at 12:01 PM, Marc Kaufmann <marc.kaufman...@gmail.com > > wrote: > >> Hi, >> >> I have created a formlet like so: >> >> (define (matrix-formlet id) >> (formlet >> (#%# ,{input-string . => . ones} >> ,{(to-string (required (hidden id))) . => . user-id} >> ) >> (values ones user-id))) >> >> I display this as follows: >> >> `(form >> ((action "/the-matrix-submitted") >> (method "post")) >> ,@(formlet-display (matrix-formlet user-id)) >> (input ([type "submit"])))) >> >> and process it like this: >> >> (define (matrix-submission req) >> (define-values (number-of-ones user-id) >> (formlet-process (matrix-formlet "3") req)) >> ...) >> >> While this works, the formlet I use to display and process are not really >> the same, since they are created on the spot, and created with different >> parameters. So far it seems to work, but I am worried that this will break >> down, and clearly this isn't the right way to go about creating >> parametrizable formlets. >> >> The questions are (i) is the above going to work despite being a hack, >> and (ii) is there a correct and easy way to do the same thing? >> >> Cheers, >> Marc >> >> -- >> You received this message because you are subscribed to the Google Groups >> "Racket Users" group. >> To unsubscribe from this group and stop receiving emails from it, send an >> email to racket-users+unsubscr...@googlegroups.com. >> For more options, visit https://groups.google.com/d/optout. >> > > -- You received this message because you are subscribed to the Google Groups "Racket Users" group. To unsubscribe from this group and stop receiving emails from it, send an email to racket-users+unsubscr...@googlegroups.com. For more options, visit https://groups.google.com/d/optout.
