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Re: [Haskell-beginners] Help with some heavy lifting...
by emacstheviking 04 Mar '13

04 Mar '13

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(a)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… 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(a)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(a)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(a)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…>) >>>> -> Vector<http://hackage.haskell.org/packages/archive/vector/0.10.0.1/doc/html/Data-V…>a -> >>>> Maybe<http://hackage.haskell.org/packages/archive/base/4.6.0.0/doc/html/Data-Mayb…>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(a)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(a)haskell.org >>> http://www.haskell.org/mailman/listinfo/beginners >>> >>> >> >> _______________________________________________ >> Beginners mailing list >> Beginners(a)haskell.org >> http://www.haskell.org/mailman/listinfo/beginners >> >> > > _______________________________________________ > Beginners mailing list > Beginners(a)haskell.org > http://www.haskell.org/mailman/listinfo/beginners > >

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Parsec lexeme parsers and whitespace/eol characters
by Sean Cormican 02 Mar '13

02 Mar '13

Hi, I'm working with expressions in which I use parsec libraries lexeme parsers: identifier, integer, whiteSpace etc. I am attempting to parse haskell declarations and I am facing some difficulty when trying to delimit the declarations which span multiple lines using eol character for some reason ( I am led to believe is due to the removal of whitespace with lexemes ) data Express = Seq [Express] | ID String | Num Integer | BoolConst Bool | EmptyList String seqOfExpr8 = do list <- (sepBy1 expr8 eol) return $ if length list == 1 then head list else Seq list where Seq list is a data declaration for a list of expression8's The parser works correctly unless trying to parse a lexeme parser token such as an identifier. Upon reading a lexeme token the parser will stop and output all tokens up until and including that token but no tokens past it. Also when trying to separate declarations that span multiple lines, does anyone have an idea on how to achieve this? I have been thinking that the parser should ignore any end line characters that are followed by whitespace ( as whitespace is used in Haskell layout rules ) Any thoughts on how I might solve this would be greatly appreciated! Seán

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unix-2.6.0.1 fails to build on the current haskell platform on the mac
by Miguel Negrao 27 Feb '13

27 Feb '13

Hi I’m trying to install iCalendar which depends on some package which depends on unix-2.6.0.1. unix-2.6.0.1 does not build on the mac with ghc 7.4. The issue has been reported at http://hackage.haskell.org/trac/ghc/ticket/7359, but I don’t think it’s fixed yet. It appears unix-2.6.0.1 works fine with ghc 7.6.1 but the current haskell platform comes with 7.4. What are my options ? Should I install ghc 7.6 ? If I do that what else do I have to install/update to make everything functional again ? best, Miguel http://www.friendlyvirus.org/miguelnegrao/

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System.USB.writeInterrupt -- confused by error message from type system
by emacstheviking 27 Feb '13

27 Feb '13

Hello, I am building a USB gadget with a HexWax ExpandIO-USB chip, here: http://hexwax.com/Products/expandIO-USB/ The device is working fine (lsusb shows it) and so I thought that in the name of continued training and doing things the hard way, that I would use Haskell to talk to it! I have only ever done USB with 'C' in the past and I am now stuck fighting the type checker. Here is my full code so far, it works in so far as it finds the device, opens and closes a connection and prints out some stuff. I then wanted to send four NUL bytes which elicits the same as a response according to the documentation but I cannot get it to compile. Apologies for it in advance, it's the journey you know, not the arrival, anyway, here it is... module Main where import Data.ByteString.Char8 as BS hiding (putStrLn) import Data.Word (Word16) import Data.Vector as V (filterM, null, (!)) --as V hiding ((++)) import System.USB import System.Environment -- hexwax expandIO-USB default code: "0b40:0132" main :: IO () main = do findHexWax (0xb40,0x132) >>= \hw -> case hw of Just dev -> hexwaxGo dev Nothing -> putStrLn "HexWax device not found" findHexWax :: (Word16,Word16) -> IO (Maybe Device) findHexWax (cVendor, cProd) = do usbCtx <- newCtx usbDevs <- getDevices usbCtx setDebug usbCtx PrintWarnings boards <- V.filterM (isTarget (cVendor, cProd)) usbDevs case V.null boards of True -> return Nothing False -> return $ Just $ boards!0 where isTarget :: (Word16, Word16) -> Device -> IO Bool isTarget (cVendor, cProd) dev = do info <- getDeviceDesc dev let vid = deviceVendorId info let vpr = deviceProductId info return $ ((vid, vpr) == (cVendor, cProd)) hexwaxGo :: Device -> IO () hexwaxGo dev = withDeviceHandle dev (testBoard dev) testBoard :: Device -> DeviceHandle -> IO () testBoard dev handle = do putStrLn $ "Inspecting device: \"" ++ (show dev) ++ "\"\n" -- write 0x00 0x00 0x00 0x00, get back same... let payload = pack "\x00\x00\x00\x00" let endPoint = EndpointAddress 0 Out action <- writeInterrupt handle endPoint (size, status) <- action payload 1000 return () And the error message: Prelude> :r [1 of 1] Compiling Main ( usb1.hs, interpreted ) usb1.hs:60:13: Couldn't match expected type `IO t0' with actual type `WriteAction' In the return type of a call of `writeInterrupt' Probable cause: `writeInterrupt' is applied to too few arguments In a stmt of a 'do' block: action <- writeInterrupt handle endPoint In the expression: do { putStrLn $ "Inspecting device: \"" ++ (show dev) ++ "\""; let payload = pack "\NUL\NUL\NUL\NUL"; let endPoint = EndpointAddress 0 Out; action <- writeInterrupt handle endPoint; .... } Failed, modules loaded: none. I have spent a few hours trying this and that, my Haskell is improving but not much good if I can't compile my program now is it! So, can anybody point out the error of my ways here please as I just can't see it! Thanks, Sean Charles. PS: Code style comments, alternative ways of making it more concise (but still readable and understandable mind you) are also welcome.

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Use a list as data for a data type?
by Bryce Verdier 24 Feb '13

24 Feb '13

Lets say I have a data type like so: data IpAddress = IpAddress {$ o1 :: !Int,$ o2 :: !Int,$ o3 :: !Int,$ o4 :: !Int$ } deriving (Show)$ which you can create by doing: IpAddress 192 168 1 1 Is there a way to do the same thing with the input data in a list? Like: IpAddress [192,168,1,1] or using some operator (or combination of operators):Like (I'm guessing (<????>) isn't in use): IpAddress <????> [192,168,1,1] Thanks in advance, Bryce

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Re: [Haskell-beginners] Beginners Digest, Vol 56, Issue 33
by xiao Ling 24 Feb '13

24 Feb '13

Hi All: How do you define a function of signature h :: M Int -> M Int -> M Int so that h ( M x ) ( M y ) = M ( x + y ), but without unwrapping the value from the monad? This question is from the article "Trivial Monad" found at http://blog.sigfpe.com/2007/04/trivial-monad.html. The provided answer is h x y = x >>= (\x -> g x y) or equivalently ( in context of the article ) h :: M Int -> M Int -> M Int h x y = bind ( \x-> g x y ) x where g is g :: Int -> W Int -> W Int g x y = y >>= (return . (+x)) for the monad: data M a = M a deriving Show Now I am a little confused, how can you put x in g if it takes an Int as first parameter but x is M Int? Thanks!

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How to extend fuinctions?
by Heinrich Ody 22 Feb '13

22 Feb '13

Hi, I have a function Delta and two sets S and A. Delta maps some elements from (S x A) to S. How can I extend Delta to some Delta' such that it assigns to every pair in (S x A) that Delta is not defined for a value not occuring in S? Something like: Delta' = if (Delta s a) /= _Undefined_ then Delta s a else if () n where n is the new value. However Delta' should not assign values to pairs not in (S x A). Note: (S x A) is meant to be the set-crossprroduct. Background: I want to learn Haskell by building a small library for Finite State Automata, Büchi Automata etc. I need this extension of Delta for making automata complete. Regards and thanks for reading, Heinrich

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How do I declare a set?
by Heinrich Ody 22 Feb '13

22 Feb '13

Hi, with [1,2] I can declare a list. Is there a similar notation for sets? Currently the only way I know is 'Set.fromList [1,2]' which is unhandy... Regards, Heinrich

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Is my understanding of ">>=" correct?
by Martin Drautzburg 22 Feb '13

22 Feb '13

Hello all, It took me quite some time to understand >>=. What confused me, was that the second operand is a function, not just a Monad. I believe I am beginning to see the light. First there is no way to sequence operations by just writing one after the other. However we can chain operations by passing the output of one operation to the next. But this does not not allow capturing the intermediate results. With >>= the output of one operation gets magically bound to the parameter of the function and remains in scope all the way through. I was confused by the signature m a -> (a -> mb) -> mb because it looks like there is (as second parameter) a function which returns a Monad. But I doubt I ever saw such a function, in the sense that it does something intelligent. It is typically a function that does not do a whole lot with its parameter, but just returns another Monad, such that its parameter is bound to the result of the previous expression. But it could be done: do x <- SomeMonad y <- someCleverlyConstructedMonad x If I want to see the function of the >>= signature I have to read the do block right->left (following the arrow) and then diagonally to the lower right. The diagonal steps reveal the function. In the above example we have \x -> someCleverlyConstructedMonad but many times I just see do x <- SomeMonad y <- someOtherMonad and the function does not do anything intelligent but just binds x and enforces the sequence of operations. is this about right? -- Martin

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Stack overflow folding a map
by tcollin371 21 Feb '13

21 Feb '13

I've been playing around with Haskell on and off for a while, and am getting a "Stack space overflow" that I can't seem to correct. I have a map containing about 2 million items. Actually, it's a map keyed off of an Int where the element is another map keyed off of a string that has three Ints as its data. The number of top level maps is pretty small (< 50), and the maps held by them hold a lot of entries. I want to walk through all of the entries and gather some statistics, but I keep getting the stack overflow. I worked down my test to an inner and outer fold that just counts the entries, and I still get the stack overflow. I added 'seq' calls to make sure that the count isn't building up a bunch of undone operations, and I'm using foldlWithKey, and have also tried foldlWithKey' with the same result. I get the count in my main function and immediately print it out. Here is the relevant code: outputIfNotFilteredCount1 :: Int -> KeyMap -> PuzzleMapRecord -> Int outputIfNotFilteredCount1 inCount key entry = seq inCount (inCount + 1) outputDatabaseCount1 :: Int -> Int -> InnerDB -> Int outputDatabaseCount1 inCount smndCount innerDB = seq inCount (inCount + outCount) where outCount = Map.foldlWithKey' outputIfNotFilteredCount1 0 innerDB main = do ... let count = Map.foldlWithKey' outputDatabaseCount1 0 resultDB putStrLn $ "Database entries:" ++ (show count) Any thoughts on why this is running out of stack space? -Truman

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