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

emacstheviking objitsu at gmail.com
Mon Mar 4 11:08:44 CET 2013 

For the record... by using Control.Monad.MissingM I have achieved two
things...

  * more concise code
  * yet more stuff to learn!

Here is what i ended up with and I am happy that for me at my current
level, that this is clean enough whilst still readable!

handleFor :: Ctx -> (Word16,Word16) -> IO (Maybe Device)
handleFor ctx (cVendor, cProd) = do
    getDevices ctx >>= \devs -> M.findM (isTarget (cVendor, cProd)) $
V.toList devs

handlesFor :: Ctx -> (Word16, Word16) -> IO (Vector Device)
handlesFor ctx (vendorId, productId) = do
  getDevices ctx >>= \devs -> V.filterM (isTarget (vendorId, productId))
devs

isTarget :: (Word16, Word16) -> Device -> IO Bool
isTarget (vid, pid) dev = do
  info <- getDeviceDesc dev
  return $ (deviceVendorId info, deviceProductId info) == (vid, pid)*
*
If I feel braver later I may try to convert some of it to "point-free"
style just to see if I can but to be honest I find that hard to read in all
but the simplest and most obvious compositions.

Once again a big thank-you to everybody for their time.
Sean.



On 4 March 2013 02:04, David McBride <toad3k at gmail.com> wrote:

> 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
> http://hackage.haskell.org/packages/archive/MissingM/0.0.4/doc/html/Control-Monad-MissingM.htmlwhich 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/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.
>>>>
>>>>
>>>> _______________________________________________
>>>> Beginners mailing list
>>>> Beginners at haskell.org
>>>> http://www.haskell.org/mailman/listinfo/beginners
>>>>
>>>>
>>>
>>> _____________________________
>>>
>>>
>>>
>>> -- | Internal helper to simply check that the product and
>>> vendor
>>> -- values in the current Device handle match those in (vid,
>>> pid).
>>>
>>> __________________
>>> Beginners mailing list
>>> Beginners at haskell.org
>>> http://www.haskell.org/mailman/listinfo/beginners
>>>
>>>
>>
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>
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