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

[emacstheviking]

David,

Hoogle doesn't appear to have any matches for "findM" that I could find.
Your code is pretty close to what I came up with this morning except yours
is clever with monads and mine was just boring recursing through the list
till I hit a match. This solution of yours looks like it is in the spirit
of what I think I saw in my mind so I am going to study it very hard and
understand it!

Sometimes you just have to grind it out!
Thanks.


On 3 March 2013 19:46, David McBride <toad3k at gmail.com> wrote:

> 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 at 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<http://hackage.haskell.org/packages/archive/base/4.6.0.0/doc/html/Data-Bool.html#t:Bool>)
>> -> Vector<http://hackage.haskell.org/packages/archive/vector/0.10.0.1/doc/html/Data-Vector.html#t:Vector>a ->
>> Maybe<http://hackage.haskell.org/packages/archive/base/4.6.0.0/doc/html/Data-Maybe.html#t: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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>
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