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
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 -> Boolhttp://hackage.haskell.org/packages/archive/base/4.6.0.0/doc/html/Data-Bool....) -> Vectorhttp://hackage.haskell.org/packages/archive/vector/0.10.0.1/doc/html/Data-Ve...a -> Maybehttp://hackage.haskell.org/packages/archive/base/4.6.0.0/doc/html/Data-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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