I would probably go (untested):

    ...
    usbDevs <- ...
    matches <- findM (isTarget foo) $ V.toList usbDevs
    ...
    where
      findM :: Monad m => (a -> m Boolean) -> [a] -> m (Maybe a)
      findM _ [] = return Nothing
      findM f (x:xs) = do
        b <- f x
        return $ if b
          then Just x
          else findM f xs

I can almost guarantee you there is a findM already out there somewhere to use, but hayoo is down right now so I can't search for it.

On Sun, Mar 3, 2013 at 1:28 PM, emacstheviking <objitsu@gmail.com> wrote:

I now have a working USB application that can find, locate and switch LED-s on and off on my Hexwax board, for which I thank all those that helped me out last week.

I am now trying to "Haskell-ise" my code i.e. make it less amateurish with respect to some of its inner workings in a constant drive for inner cleanliness and warm fuzziness etc.

When attempting to find the device, I use the System.USB.getDevices function which returns me IO (Vector Device), a list of everything that's currently plugged in and available and I then use Data.Vector.filterM like so:

handleFor ctx (cVendor, cProd) = do
    usbDevs <- getDevices ctx
    matches <- V.filterM (isTarget (cVendor, cProd)) usbDevs
    case V.null matches of
      True  -> return Nothing
      False -> return $ Just $ matches!

isTarget :: (Word16, Word16) -> Device -> IO Bool
isTarget (vid, pid) dev = do
  getDeviceDesc dev >>= \info ->
    return $ (deviceVendorId info, deviceProductId info) == (vid, pid)


but... that is not as efficient as it could be because I could have N devices and then I just throw all but the first. Tut tut. Could do better. If I knew how... well I kind of do but I can't figure it out by myself yet!

In the Data.Vector there is "Data.Vector.find" which is *exactly* what I want with small dent in the bodywork, the predicate function is pure:

find :: (a -> Bool) -> Vector a -> Maybe a

So my question is, how do I make it work? I know (but don't yet feel comfortable with) about liftM and all that but in this case I can't see how and where it would work. I "know" (like Spiderman knows there is danger) that it's crying out for something and the return type is perfect too as it would just match.

SO...how can I do it chaps?

And as usual... .any comments, style notes, idiomatic pointers(!) etc. are always welcome.

Thanks,
Sean Charles.

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