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

David McBride toad3k at gmail.com
Mon Mar 4 03:04:33 CET 2013

Hoogle only indexes some small subset of libraries, while hayoo indexes
everything on hackage.  Unfortunately it seems like peoples' first
inclination is to visit hoogle because hey, it's like google right?  But in
fact hayoo is far better unless you know what you are looking for something
in the core libraries.

I think there is some technical merit behind hoogle, like being able to
search for function prototypes, and I do know you can make your own hoogle
command line searcher to search libraries installed on your machine.  But
usually you are searching for something you don't already have.

An actual google search comes up with
actually has findM proving my point that others have felt its absense
in the standard library.

On Sun, Mar 3, 2013 at 5:24 PM, emacstheviking <objitsu at gmail.com> wrote:

> 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/>)
>>> -> Vector<http://hackage.haskell.org/packages/archive/vector/>a ->
>>> Maybe<http://hackage.haskell.org/packages/archive/base/>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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>>> Beginners at haskell.org
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