[Haskell-beginners] Help with some heavy lifting...

emacstheviking objitsu at gmail.com
Sun Mar 3 19:28:16 CET 2013

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

*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<http://hackage.haskell.org/packages/archive/base/>)
-> Vector<http://hackage.haskell.org/packages/archive/vector/>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.

Sean Charles.
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://www.haskell.org/pipermail/beginners/attachments/20130303/7e43cfc4/attachment.htm>

More information about the Beginners mailing list