- Beginners - Haskell.org

Re: [Haskell-beginners] fmap versus bind
by Sean Perry 03 May '11

03 May '11

On Tue May 3 8:50 , Felipe Almeida Lessa sent: >On Tue, May 3, 2011 at 12:32 PM, Patrick LeBoutillier >patrick.leboutillier(a)gmail.com> wrote: >> I'm trying to understand this example and I can't get the types to line up. >> Can you provide the "real" types for ds and (applyOp op x y)? >> >> This is what I'm working out: >> >> fmap ((:) ds) (applyOp op x y) >> == fmap (ds :) (applyOp op x y) >> == fmap (\dss -> ds : dss) (applyOp op x y) >> >> (applyOp op x y) >>= (flip (:) ds) >> == (applyOp op x y) >>= (: ds) >> == (applyOp op x y) >>= (\d -> d : ds) >> >> To my untrained eyes is doesn't even look like the code is doing the >> same thing... >> What am I missing here (or more probably where is my mistake)? > >I think that OP forgot a return, i.e., he meant 'applyOp op x y >>= >return . flip (:) ds'. > You are correct, I did forget the return. Sorry for the confusion. This particular bit of code was pulled out of the middle of a RPN calculator. So 'ds' is a list of doubles. applyOp takes 2 numbers and emits a 'm double' which we then want to extract and add to the top of the list. The 'm' is there because I do my calculation in the Either monad to support unknown operators, invalid numbers, etc. A left value in the bind call stops the execution. To my still novice eyes 'foo >>= handleResult' is more readable than 'fmap handleResult foo'. When I look at the fmap call it looks like handlResult is the more important computation.

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Trying to convert from planar NV12 to RGB
by John Obbele 02 May '11

02 May '11

I'm playing with some raw video streams. These raw data provide YUV (or YCrCb) components interleaved between each other and it is sometimes quite cumbersome to find a triplet of matching YUV bytes before converting them to RGB. For "packed" formats such as YUY2, the operation seems pretty straight forward in Haskell: >-- | Converting from packed YUY2 4:2:2 pixels to RGB ones. >-- YUY2: Y0 U0 Y1 V0 < -- 2 macro pixels --> (Y0:U0:V0)+(Y1:U0:V0) >yuy2ToRGB :: B.ByteString -> B.ByteString >yuy2ToRGB bs = f [] bs > where > f acc xs | B.null xs = B.pack $ reverse acc -- > | otherwise = let [y0, u0, y1, v0] = B.unpack (B.take 4 xs) > (r0,g0,b0) = yuv2rgb (y0,u0,v0) > (r1,g1,b1) = yuv2rgb (y1,u0,v0) > in f (b1:g1:r1:b0:g0:r0:acc) (B.drop 4 xs) But on the other hand, I'm having some headaches converting from planar NV12 to RGB. For example, A width=4 and height=4 NV12 frame is stored in a 1-dimensional array of (4 * 4 * 3/2 = 24) elements: | Y00 | Y01 | Y02 | Y03 | Y04 | Y05 | Y06 | Y07 | Y08 | Y09 | Y10 | Y11 | Y12 | Y13 | Y14 | Y15 | U00 | V00 | U01 | V01 | U02 | V02 | U03 | V03 | This array could be unfold to the following matrix representation, and you see that the luminance (Y) bytes are provided first, followed by a sequence of (U,V) pairs. | Y00 | Y01 | Y02 | Y03 | | Y04 | Y05 | Y06 | Y07 | | Y08 | Y09 | Y10 | Y11 | | Y12 | Y13 | Y14 | Y15 | |-----------------------| | U00 | V00 | U01 | V01 | | U02 | V02 | U03 | V03 | Now, if you want to get the RGB value for every pixel, you have to fetch the corresponding (U,V) components at the end of the frame. pixel00 will need (Y00,U00,V00) pixel02 will need (Y02,U01,V01) ... pixel15 will need (Y15,U03,V03) So far, I have this Haskell function to convert NV12 to RGB. >nv12ToRGB w h bs = > B.pack $ concatMap f [0.. w * h - 1] > where > f i = let m = (i `div` w) `div` 2 -- line index modulo 2 > n = (i `rem` w) `div` 2 -- column index modulo 2 > iuv = m * w `div` 2 + n > (r,g,b) = yuv2rgb (pixelY i, pixelU iuv, pixelV iuv) > in [r,g,b] > > pixelY i = B.index bs i > pixelU i = B.index bs (w*h+2*i) > pixelV i = B.index bs (w*h+2*i+1) I don't know if converting ByteString to list, using map with a not-so-simple f function, then concatenating and repacking the ByteString is the best solution one could find in Haskell and I would like to know if by chance some "senior" haskellers would know some tricks (like list comprehension or ByteString.unfoldWhatever) to improve the previous code. regards, /john

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Looking for more information about Pattern Matching
by Graham Hopper 02 May '11

02 May '11

Thanks to all who replied. This is a lot clearer now. Thanks, Graham

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All about monads, exercise 4
by Nathan Hüsken 01 May '11

01 May '11

Hey, I am currently reading "All about monads". Unfortanlty I have only a pdf without the solutions to the exercises. in the previous exercises I defined: data Sheep = Sheep (Maybe Sheep) (Maybe Sheep) father :: Sheep -> Maybe Sheep father (Sheep f m) = f mother :: Sheep -> Maybe Sheep mother (Sheep f m) = m parent :: Sheep -> Maybe Sheep parent s = (mother s) `mplus` (father s) parentList :: Sheep -> [Sheep] parentList s = (maybeToList (mother s)) `mplus` (maybeToList (father s)) Now I am supposed to combine "parent" and "parentList" to a function parent :: MonadPlus m => Sheep -> m Sheep Problem, my parent function relies on "mother" and "father", and they give a Maybe monad. So I need: mother :: MonadPlus m => Sheep -> m Sheep do I not? But I have no Idea of how the body of this function would look. Anyone can give me a hint? Thanks! Nathan but what I really need, is a function

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Longest common prefix of a list of lists
by philipp siegmantel 29 Apr '11

29 Apr '11

Hi, For one of my toy project I had to find the longest common prefix of a list of strings. I figured, that this function should only work on list-structure, meaning it should have type > longestCommonPrefix :: [[a]] -> [a] Sadly the best I could come up with was using explicit recursion. It works as far as I tested it, but I have the feeling that there's a better way to do it. Here is the code: > longestCommonPrefix :: Eq a => [[a]] -> [a] > longestCommonPrefix [] = [] > longestCommonPrefix xss | any null xss = [] > | otherwise = loop xss [] > where loop ::Eq b => [[b]] -> [b] -> [b] > loop xss acc = > let xs = concatMap (take 1) xss > in if any (\x -> x /= head xs) (tail xs) > then reverse acc > else loop (map tail xss) (head xs : acc)

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is there a better way to ask for a hex formatted number?
by Sean Perry 29 Apr '11

29 Apr '11

I am using this currently: makeHex n = "0x" ++ map toUpper (showHex n "") -- gives me 0xFF for example I know i can use printf but I am not sending this to output when I am doing the formatting.

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Looking for more information about Pattern Matching
by Graham Hopper 29 Apr '11

29 Apr '11

Hi, I am very much a newbie to Haskell and currently working through http://learnyouahaskell.com and trying some of the http://www.haskell.org/haskellwiki/99_questions. I am struggling to get my head around pattern matching. I think I can understand the individual patterns presented in LYAH, but can't seem to find any information about generalising the formats. Is there a good resource that describes the patterns that are available or provides more details about how to create them. For example the format of [x] and (x:xs) seem very different and I cant quite get a handle on the syntax(?) behind them. I can see how patterns work when shown an example, but wouldn't know how to create one from scratch. I hope you can understand what I am trying to say - its a little difficult to explain. Thanks Graham

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Antw.: Question about CFloat and Float
by ch.gosch＠googlemail.com 29 Apr '11

29 Apr '11

Hi Alberto, thanks for the hint and the link. Do you have any information on whether that will continue to work in the future? I somehow have a bad feeling about it because the C* versions of the types would then be unnecessary. Thanks again, Christian ----- Reply message ----- Von: "Alberto Ruiz" <aruiz(a)um.es> An: "C Gosch" <ch.gosch(a)googlemail.com> Cc: <beginners(a)haskell.org> Betreff: [Haskell-beginners] Question about CFloat and Float Datum: Do., Apr. 28, 2011 15:27 Hi Christian, On 04/28/2011 02:27 PM, C Gosch wrote: > Dear list members, > I have the following problem: I have wrapped a C library using the FFI > and c2hs. > The C functions all take a bunch of pointers to C arrays of floats, > doubles, or ints as arguments. > Naturally, the Haskell representation of these is e.g. > Ptr CFloat > or, for single values, CFloat. > My question is: if I have an array, e.g. allocated with mallocArray, to > Haskell Float values (Ptr Float), > is the representation of Float exactly the same as for CFloat? Can I > safely use Ptr Float where the C function > expects the C type float* ? > I assume that I can not, and in that case I would like to find a way to > have Float on the Haskell side and CFloat on the C side. > Any hint on how I would do that without copying data and without too > large performance penalties? > > Thanks a lot, > Christian > I think that in ghc you can assume the same representation for CFloat and CDouble, they are newtypes of Float and Double: http://www.haskell.org/ghc/docs/7.0.3/html/libraries/base-4.3.1.0/Foreign-C… (I directly use arrays of Float and Double with C libraries without any problem). But Int may be different from CInt depending on the architecture, so we need explicit conversions. Alberto

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how to save the GHC errormessages in a file on Windows
by Kees Bleijenberg 28 Apr '11

28 Apr '11

When I try in Windows in cmd: ghc --make myprogram.hs > test.txt I only get the first lines in test.txt. The errormessages itself are on the screen, not in test.txt. In test.txt: Resolving dependencies... Configuring ftphs-1.0.7... Preprocessing library ftphs-1.0.7... Preprocessing executables for ftphs-1.0.7... Building ftphs-1.0.7... The errormessages: Could not find module 'Network.FTP.Client'.... and a lot more. How do I 'intercept' these messages? Kees

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Question about CFloat and Float
by C Gosch 28 Apr '11

28 Apr '11

Dear list members, I have the following problem: I have wrapped a C library using the FFI and c2hs. The C functions all take a bunch of pointers to C arrays of floats, doubles, or ints as arguments. Naturally, the Haskell representation of these is e.g. Ptr CFloat or, for single values, CFloat. My question is: if I have an array, e.g. allocated with mallocArray, to Haskell Float values (Ptr Float), is the representation of Float exactly the same as for CFloat? Can I safely use Ptr Float where the C function expects the C type float* ? I assume that I can not, and in that case I would like to find a way to have Float on the Haskell side and CFloat on the C side. Any hint on how I would do that without copying data and without too large performance penalties? Thanks a lot, Christian

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