November 2009 - Beginners

FFI: FinalizerPtr and freeHaskellFunPtr
by Patrick LeBoutillier 26 Nov '09

26 Nov '09

Hi, I'm trying to write a binding to a C library, and so far here is the code I have: {-# LANGUAGE ForeignFunctionInterface #-} import Foreign.Ptr import Foreign.ForeignPtr newtype C_mlp_context = C_mlp_context (Ptr C_mlp_context) data MLPContext = MLPContext !(ForeignPtr C_mlp_context) deriving (Show) foreign import ccall "wrapper" makeFinalizer :: (Ptr a -> IO ()) -> IO (FinalizerPtr a) foreign import ccall unsafe "mlp.h mlp_context_new" c_new :: IO (Ptr C_mlp_context) new :: IO MLPContext new = do mlp_context <- c_new fin <- mlp_context_finalizer fctx <- newForeignPtr fin mlp_context return $ MLPContext fctx mlp_context_finalizer :: IO (FinalizerPtr C_mlp_context) mlp_context_finalizer = do makeFinalizer $ \ctx -> do c_delete ctx foreign import ccall unsafe "mlp.h mlp_context_delete" c_delete :: Ptr C_mlp_context -> IO () main = do ctx <- new putStrLn $ show ctx This seems to work as expected, but I read that I'm supposed to call freeHaskellFunPtr on the finalizer when I'm done with it. However I don't know how I can do this since it is called by the GC... Can anyone offer any advice? Thanks, Patrick -- ===================== Patrick LeBoutillier Rosemère, Québec, Canada

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Easier alternatives to existential types?
by Emile Melnicov 25 Nov '09

25 Nov '09

Hello everyone. I'm trying to write RIFF parser and can't decide what to do with "container" type. "Container" chunk should hold a header (two Word32's), a "type" (Word32 like "WAVE" etc) and a "list" of other chunks. Of course lists can't be used because we have far more than one chunk types. What can I do with this? I discovered existentials for myself recently, but those could be too hard for me yet. It feels like I'm missing something very obvious here. Thanks for attention. -- Emile.

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combinatorial
by Michael P. Mossey 24 Nov '09

24 Nov '09

I'm trying to write a combinatorial search algorithm with evaluation, and kind of stuck. Not sure how to do this. I'm constructing a musical phrase, which is a list of MidiPitch: [MidiPitch] I have an evaluation function that determines the fitness of any given phrase: eval :: [MidiPitch] -> Maybe Float This returns Nothing if the phrase is completely unacceptable. The idea is to build up a phrase one midi pitch at a time, choosing all possible next pitches (notes) from a range: next pitch comes from: [10..90] Most of the pitches will result in a phrase that evaluates to Nothing, so the combinatoral "explosion" will be limited. I'd like to write a function that constructs a phrase of length n, and in fact will have to return a list of all phrases that have equal scores of the maximum. -- <length of output phrase> -> <first pitch> -> <eval func> -> -- <all tied phrases of best score> coolFunc :: Int -> MidiPitch -> ([MidiPitch] -> Maybe Float) -> [[MidiPitch]] I am stuck on how to write this. thanks, Mike

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how to install "import Foreign.C.Types( CFloat )" in Ubuntu
by Zsolt Ero 24 Nov '09

24 Nov '09

I am trying to run a file, which works well on the university system, but I don't know what do I need to do to make it work on my home Ubuntu system. The particular line which makes the error is: import Foreign.C.Types( CFloat ) And the error is: ----------------------------------------------------------- LSystem.hs:130:4: No instance for (ColorComponent CFloat) arising from a use of `color' at LSystem.hs:130:4-26 Possible fix: add an instance declaration for (ColorComponent CFloat) ..... ------------------------------------------------------------- If I write "import Foreign.C.Types" at the Prelude it says nothing. If I write "import Foreign.C.Types( CFloat )" at the Prelude, it says <interactive>:1:0: parse error on input `import' What can it be? Cheers, Zsolt

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questions about quickCheck
by Zsolt Ero 24 Nov '09

24 Nov '09

Hi, I have just started learning Haskell this semester, and I have some questions of things which puzzles me: 1. I would like to ask that how is it possible to set quickCheck test numbers to something else then the default 100 passes? I have found an article which says: deepCheck p = check (defaultConfig { configMaxTest = 10000}) p which I tried and it works at a home Ubuntu install but when I tried it at my university network, it says: Not in scope: `check' Not in scope: `defaultConfig' Not in scope: `configMaxTest' Isn't there a way of making quickCheck use a different number by using '==>' functions for example? Is quickCheck p = check (defaultConfig) p? 2. How could I print the result of a complex quickCheck generator? I mean if there is an error is says it, but what if I would like to generate just random expressions which quickCheck can do? Zsolt

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Retrieving the decimal portion of a Double
by Dominic Sim 23 Nov '09

23 Nov '09

Hi all, I am trying to retrieve the decimal portion of a Double. My code is: getDecimal x = x - floor(x) I would expect the following: Main> getDecimal 1.23 0.23 :: [Double] but instead, I get: Main> getDecimal 1.23 ERROR - Unresolved overloading *** Type : (RealFrac a, Integral a) => a *** Expression : getDecimal 1.23 I have tried adding "getDecimal :: Double -> Double" but it only causes the following message to appear: ERROR file:C:\Documents and Settings\User\file.hs:60 - Instance of Integral Double required for definition of getDecimal How should I define my function so that it'll work? Thanks! Regards, Dom

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FFI and Cabal
by Jeffrey Drake 23 Nov '09

23 Nov '09

I am trying to use FFI to access some windows console functions via two simple C functions I created. Essentially my module looks like is this: {-# CFILES mouse.c #-} {-# LANGUAGE ForeignFunctionInterface #-} module Mouse where -- import Foreign import Foreign.C.Types foreign import ccall unsafe "mouse.h SetupConsole" c_setupConsole :: IO CInt foreign import ccall unsafe "mouse.h MouseEvent" c_mouseEvent :: IO CInt setupConsole :: IO Bool setupConsole = do c <- c_setupConsole return $ if c == 0 then True else False mouseEvent :: IO (Maybe (Int, Int)) mouseEvent = do c <- c_mouseEvent let d = toInteger c let e = (fromInteger d)::Int return $ if e < 0 then Nothing else Just (e `mod` 256, e `div` 256) It seems to compile fine. The problem comes when I need to link. Linking dist\build\BoardGames\BoardGames.exe ... dist\build\BoardGames\BoardGames-tmp\Mouse.o:fake:(.text+0x15): undefined reference to `SetupConsole' dist\build\BoardGames\BoardGames-tmp\Mouse.o:fake:(.text+0x11d): undefined reference to `MouseEvent' collect2: ld returned 1 exit status The problem seems obvious - it isn't compiling OR linking mouse.c which is this: #include <windows.h> int SetupConsole() { HANDLE hStdin; DWORD fdwMode, fdwSaveOldMode; int counter=0; hStdin = GetStdHandle(((DWORD)-10)); if (hStdin == ((HANDLE)(LONG_PTR)-1)) return -3; if (! GetConsoleMode(hStdin, &fdwSaveOldMode) ) return -2; fdwMode = 0x0010; if (! SetConsoleMode(hStdin, fdwMode) ) return -1; return 0; } int MouseEvent() { HANDLE hStdin; INPUT_RECORD irInBuf[128]; short x, y; unsigned i; DWORD cNumRead; static int ignoreNext = 0; hStdin = GetStdHandle(((DWORD)-10)); if (hStdin == ((HANDLE)(LONG_PTR)-1)) return -2; if (!ReadConsoleInputW( hStdin, irInBuf, 128, &cNumRead) ) return -1; for (i = 0; i < cNumRead; i++) { if (irInBuf[i].EventType == 0x0002 && irInBuf[i].Event.MouseEvent.dwEventFlags == 0) { if (ignoreNext == 1) { ignoreNext = 0; return -1; } x = irInBuf[i].Event.MouseEvent.dwMousePosition.X; y = irInBuf[i].Event.MouseEvent.dwMousePosition.Y; ignoreNext = 1; return x + (y << 8); } else { continue; } } return -1; } So far, I have only tested this with MSVC (it works), but not with haskell platform's mingw (which includes windows.h - so I would have to wait for this...) In my cabal file I have: c-sources: mouse.c So that basically sums up what I have, I just don't know how to do this. The cabal documentation says for this directive: c-sources: filename list A list of C source files to be compiled and linked with the Haskell files. Which is what I expected it to do - but to no avail. I should note that when I try to comment out with -- the c-sources line I get this: Linking dist\build\BoardGames\BoardGames.exe ... dist\build\BoardGames\BoardGames-tmp\Main.o:fake:(.text+0x27d): undefined reference to `__stginit_Mouse_' dist\build\BoardGames\BoardGames-tmp\Main.o:fake:(.text+0xb4): undefined reference to `Mouse_a1_info' dist\build\BoardGames\BoardGames-tmp\Main.o:fake:(.text+0xcc): undefined reference to `Mouse_czusetupConsole_info' dist\build\BoardGames\BoardGames-tmp\Main.o:fake:(.text+0xf8): undefined reference to `Mouse_a1_info' dist\build\BoardGames\BoardGames-tmp\Main.o:fake:(.text+0x17a): undefined reference to `Mouse_czusetupConsole_info' dist\build\BoardGames\BoardGames-tmp\Main.o:fake:(.data+0x14): undefined reference to `Mouse_a1_closure' collect2: ld returned 1 exit status Any help that can be offered would be appreciated. I would tell you where to find the project, but patch-tag is being unavailable for me at this time. It is the 'BoardGames' project, under iaefai. Thank you again, iaefai.

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Important coding guideline and library proposal
by Daniel Fischer 22 Nov '09

22 Nov '09

Guideline: NEVER, *never ever*, use 'length' ***unless you really want to know the exact length of a list*** Proposal: rename 'length' to 'yesIReallyWantToKnowTheExactLengthOfThisListSoPleaseCalculateItForMe' to reduce performance bugs caused by naive uses of length.

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Fwd: [Haskell-beginners] combinatorial
by Patrick LeBoutillier 22 Nov '09

22 Nov '09

Forgot to forward my response to the list... ---------- Forwarded message ---------- From: Patrick LeBoutillier <patrick.leboutillier(a)gmail.com> Date: Sun, Nov 22, 2009 at 5:03 PM Subject: Re: [Haskell-beginners] combinatorial To: "Michael P. Mossey" <mpm(a)alumni.caltech.edu> Michael, Here's how I would go about this: type MidiPitch = Int range :: [MidiPitch] range = [10..90] -- Just a dummy eval function, allows up or down one unit eval ps | length ps <= 1 = Just 1.0 eval ps | length ps > 1 = let a = last ps b = last . init $ ps in if abs (a - b) == 1 then Just 1.0 else Nothing coolFunc :: Int -> MidiPitch -> ([MidiPitch] -> Maybe Float) -> [[MidiPitch]] coolFunc n p f = generate n f [[p]] -- Generate MidiPitch lists up to a given length, making sure each step -- satisfies the eval function generate :: Int -> ([MidiPitch] -> Maybe Float) -> [[MidiPitch]] -> [[MidiPitch]] generate n f cs | n == 1 = cs generate n f cs = generate (n-1) f $ concat . map (\p -> try p cs) $ range -- Tries to add a MidiPitch to each of the MidiPitch lists, keep -- only the sequences that pass the eval. try :: MidiPitch -> [[MidiPitch]] -> [[MidiPitch]] try p = filter (not . null) . map (test p) where test p ps = let ps' = ps ++ [p] in case eval ps' of Nothing -> [] otherwise -> ps' It's seems to work, although I'm sure many bits sould be made more elegant... Note: After that another pass would be required over the result set to find the sequences that have the highest scores. Patrick On Sun, Nov 22, 2009 at 5:25 AM, Michael P. Mossey <mpm(a)alumni.caltech.edu> wrote: > I'm trying to write a combinatorial search algorithm with evaluation, and > kind of stuck. Not sure how to do this. > > I'm constructing a musical phrase, which is a list of MidiPitch: > > [MidiPitch] > > I have an evaluation function that determines the fitness of any given > phrase: > > eval :: [MidiPitch] -> Maybe Float > > This returns Nothing if the phrase is completely unacceptable. > > The idea is to build up a phrase one midi pitch at a time, choosing all > possible next pitches (notes) from a range: > > next pitch comes from: [10..90] > > Most of the pitches will result in a phrase that evaluates to Nothing, so > the combinatoral "explosion" will be limited. > > I'd like to write a function that constructs a phrase of length n, and in > fact will have to return a list of all phrases that have equal scores of the > maximum. > > -- <length of output phrase> -> <first pitch> -> <eval func> -> > -- <all tied phrases of best score> > coolFunc :: Int -> MidiPitch -> ([MidiPitch] -> Maybe Float) -> > [[MidiPitch]] > > I am stuck on how to write this. > > thanks, > Mike > > > _______________________________________________ > Beginners mailing list > Beginners(a)haskell.org > http://www.haskell.org/mailman/listinfo/beginners > -- ===================== Patrick LeBoutillier Rosemère, Québec, Canada -- ===================== Patrick LeBoutillier Rosemère, Québec, Canada

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Performance
by Philip Scott 22 Nov '09

22 Nov '09

Hi again folks, I am still at it with my time-series problem, for those who haven't been following; I have a list of (time stamp, value) pairs and I need to do various bits and bobs with them. I have got arithmetic down pat now, thanks to the kind help of various members of the list - now I am looking at functions that look at some historical data in the time-series and do some work on that to give me an answer for a particular day. I have chosen to represent my time series in reverse date order, since non of the operations will ever want to look into the future, but often they would like to look in to the past. A function I would like to write is 'avg'. For a particular day, it computes the average of the values last 'n' points; if there are not n points to fetch, thee is no answer. I then combine those to make a new time series. e.g. If my input time series was [(5,10),(4,20),(3,30),(2,40), (1,50)] (Where 5, 4, 3, 2, 1 are timestamps and 10, 20, 30, 50, 50 are values) I would like the answer [(5,20), (4,30), (3,40)] (e.g. 20 = (10+20+30)/3 etc.. I can't get an answer for timestamps 2 and 1 because there isn't enough historical data) So I have written some code to do this, and it works nicely enough; but it is _slow_. To do 1000 averages of different lengths on a series with only 3000 points takes about 200 seconds on my (not overly shabby) laptop. The equivalent C program takes under a second. I am entirely sure that this is due to some failing on my part. I have been mucking around with the profiler all afternoon lazifying and delazifying various bits and bobs with no dramatic success so I thought I might put it to y'all if you don't mind! So here's some code. I've kept it quite general because there are a lot of functions I would like to implement that do similar things with bits of historical data. General comments on the Haskellyness/goodness of my code are welcomed as well, I'm still very much a beginner at this! --------- SNIP -------------- -- Take n elements from a list if at least n exist takeMaybe n l | length l < n = Nothing | otherwise = Just $! (take n l) -- Little utility function, take a function f and apply it to the whole list, -- then the tail etc... lMap _ [] = [] lMap f (x:xs) = (f (x:xs)):(lMap f xs) -- Little utility function to take a list containing Maybes and delete them -- Returning a list with the values inside the Just maybeListToList [] = [] maybeListToList (x:xs) = maybe (maybeListToList xs) (\y -> y:(maybeListToList xs)) x -- Return a list of lists, where each sublist is a list of the next n values histMaybe x = lMap (takeMaybe x) hist n x = maybeListToList $ histMaybe n x -- Take a function which works on a list of things and apply it only to a -- list of the second elements in a list of tuples 'l'. applyToValues f l = let (ts,vs) = unzip l in zip ts $ f vs -- Create a timeseries with the cumulative sum of the last n values cumL n l = map sum (hist n l) cum = applyToValues . cumL -- Creates a timeseries with the average of the last n values avgL n l = map ((*) (1/fromIntegral(n))) $ cumL n l avg = applyToValues . avgL --------- SNIP -------------- According to the profiler (log attached), the vast majority of the time is spent in takeMaybe, presumably allocating and deallocating enormous amounts of memory for each of my little temporary sublists. I have tried liberally sprinkling $! and 'seq' about, thinking that might help but I am clearly not doing it right. Perhaps list is the wrong basic data structure for what I am doing? I hope I didn't bore you with that rather long email, I will leave it at that. If it would be useful, I could give you the complete program with a data set if anyone is keen enough to try for themselves. Thanks, Philip

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