[Ur] callbacks from C FFI (again)

[Sergey Mironov]

I just remember that it's not always possible to query server
port/host because application may use external web server or proxy, so
it is not always possible to visit callback url. What I actually want
is to mimic page request from inside the server without actually
opening HTTP session, parsing headers, etc. I guess it is not that
simple currently. Correct me if I'm wrong..

2013/12/16 Sergey Mironov <grrwlf at gmail.com>:
> Hi. I'd like to re-open the topic [1] regarding UrWeb callbacks from C
> code. Imagine the 'anonymous downloader' application, where
> 1. user logs in and enters the link to download
> 2. server starts the download task, task runs for hours
> 3. server sends an email to user when the download finishes
>
> Also, imagine that I have two separate UrWeb,library: one for starting
> downloader threads and one for sending emails. I'd like to glue them
> together but in order to do so I have to provide the thread with a
> callback to call UrWeb handler which eventually calls email sender.
> AFAIK, there is no pre-designed way callback an UrWeb code, so here
> are my ideas and questions:
>
> 1)
> A simple solution I see is to provide the downloader with an url to
> trigger completion actions. the code may looks like
>
> fun handler (clientId: int) =
>   email <- queryEmail clientId;
>   Emailer.sendTo email "Download completed"
>
> fun download (u:string) = do
>   clientId <- newClient;
>   runThreadWithCallback ("wget " ^ u) (url (handler clientId))
>
> Looks like no problems here except a low-level one: C FFI code needs
> to know not url alone but also the current hostname (127.0.0.1 may
> work) and the port the server is running on. Is there a safe way to
> query this information from the runtime?
>
> 2)
> A cool solution: Invent some kind of 'detached thread monad'. the new
> code would be like
>
> fun handler (clientId: int) =
>   email <- queryEmail clientId;
>   Emailer.sendTo email "Download completed"
>
> fun download (u:string) = do
>   clientId <- newClient;
>   runInSeparateThread (fn thread_id =>
>       u <- execute ("wget " ^ u);
>       (* other logic in Ur/Web language *)
>       visit (url (handler clientId))
>       )
>
> where
>
> val execute : string -> detached_transaction int
> val visit: url -> detached_transaction unit
>
> are both FFI functions declared in detached_transaction monad. But
> AFAIK only `transaction' or pure functions are allowed for C FFI.. And
> worse, this snippet would require runInSeparateThread to call pure
> anonymous Ur/Web function. So we have to
> a) allow arbitrary monads in C FFI code
> b) add support for native [val runInSeparateThread : (int ->
> detached_transaction unit) -> transaction unit]   I think this would
> be awesome, is it hard to implement?
>
> Regards,
> Sergey
>
> [1] - http://www.impredicative.com/pipermail/ur/2012-September/001227.html