[Haskell-cafe] Need comments on a libusb asynchronous select/poll loop

[Johan Tibell]

On Tue, Apr 19, 2011 at 10:36 PM, Bas van Dijk <v.dijk.bas at gmail.com> wrote:
> On 19 April 2011 15:06, John Obbele <john.obbele at gmail.com> wrote:
>        -- Step 3 is the most important step. Submitting the transfer:
>        handleUSBException $ c'libusb_submit_transfer transPtr
>
>        -- TODO: Now we need to do the complicated stuff described in:
>        -- http://libusb.sourceforge.net/api-1.0/group__poll.html
>        --
>        -- First we need the function:
>        -- getPollFds ∷ Ctx → IO [C'libusb_pollfd]
>        --
>        -- A C'libusb_pollfd:
>        -- http://libusb.sourceforge.net/api-1.0/structlibusb__pollfd.html
>        -- is a structure containing a file descriptor which should be
>        -- polled by the GHC event manager and an abstract integer
>        -- describing the event flags to be polled.
>        --
>        -- The idea is to call getPollFds and register the returned
>        -- file descriptors and associated events with the GHC event
>        -- manager using registerFd:
>        -- http://hackage.haskell.org/packages/archive/base/4.3.1.0/doc/html/System-Event.html#v:registerFd
>        --
>        -- As the callback we use libusb_handle_events_timeout.
>        --
>        -- But here we run into a problem: We need to turn our
>        -- concrete event integer into a value of the _abstract_ type
>        -- Event. But the only way to create Events is by evtRead or
>        -- evtWrite!
>        --
>        -- I would really like a solution for this.
>        -- Bryan, Johan any ideas?

Could you do something like:

toEvent :: Int -> Event
toEvent flag
    | flag `xor` (#const POLLIN) = evtRead

etc?

Not that evtRead and evtWrite maps to different things on different platforms.

Johan