January 2012 - Beginners

cmath Install troubles
by Tom Murphy 20 Apr '12

20 Apr '12

Any advice? If you can't even install a package with no dependencies except base, I think there's trouble: $ cabal install cmath Resolving dependencies... Configuring cmath-0.3... Preprocessing library cmath-0.3... Building cmath-0.3... [1 of 1] Compiling Foreign.C.Math.Double ( dist/build/Foreign/C/Math/Double.hs, dist/build/Foreign/C/Math/Double.o ) /var/folders/+p/+pUVycVKFDWsYP8YiYnM3k+++TM/-Tmp-/ghc70799_0/ghc70799_0.hc: In function ‘s2sB_ret’: /var/folders/+p/+pUVycVKFDWsYP8YiYnM3k+++TM/-Tmp-/ghc70799_0/ghc70799_0.hc:2850:0: error: ‘gamma’ undeclared (first use in this function) /var/folders/+p/+pUVycVKFDWsYP8YiYnM3k+++TM/-Tmp-/ghc70799_0/ghc70799_0.hc:2850:0: error: (Each undeclared identifier is reported only once /var/folders/+p/+pUVycVKFDWsYP8YiYnM3k+++TM/-Tmp-/ghc70799_0/ghc70799_0.hc:2850:0: error: for each function it appears in.) /var/folders/+p/+pUVycVKFDWsYP8YiYnM3k+++TM/-Tmp-/ghc70799_0/ghc70799_0.hc: In function ‘s2t0_ret’: /var/folders/+p/+pUVycVKFDWsYP8YiYnM3k+++TM/-Tmp-/ghc70799_0/ghc70799_0.hc:2896:0: error: ‘gamma’ undeclared (first use in this function) /var/folders/+p/+pUVycVKFDWsYP8YiYnM3k+++TM/-Tmp-/ghc70799_0/ghc70799_0.hc: In function ‘s2uH_ret’: /var/folders/+p/+pUVycVKFDWsYP8YiYnM3k+++TM/-Tmp-/ghc70799_0/ghc70799_0.hc:3088:0: error: ‘isinf’ undeclared (first use in this function) /var/folders/+p/+pUVycVKFDWsYP8YiYnM3k+++TM/-Tmp-/ghc70799_0/ghc70799_0.hc: In function ‘s2v6_ret’: /var/folders/+p/+pUVycVKFDWsYP8YiYnM3k+++TM/-Tmp-/ghc70799_0/ghc70799_0.hc:3134:0: error: ‘isinf’ undeclared (first use in this function) /var/folders/+p/+pUVycVKFDWsYP8YiYnM3k+++TM/-Tmp-/ghc70799_0/ghc70799_0.hc: In function ‘s2vv_ret’: /var/folders/+p/+pUVycVKFDWsYP8YiYnM3k+++TM/-Tmp-/ghc70799_0/ghc70799_0.hc:3180:0: error: ‘isnan’ undeclared (first use in this function) /var/folders/+p/+pUVycVKFDWsYP8YiYnM3k+++TM/-Tmp-/ghc70799_0/ghc70799_0.hc: In function ‘s2vU_ret’: /var/folders/+p/+pUVycVKFDWsYP8YiYnM3k+++TM/-Tmp-/ghc70799_0/ghc70799_0.hc:3226:0: error: ‘isnan’ undeclared (first use in this function) /var/folders/+p/+pUVycVKFDWsYP8YiYnM3k+++TM/-Tmp-/ghc70799_0/ghc70799_0.hc: In function ‘s2wj_ret’: /var/folders/+p/+pUVycVKFDWsYP8YiYnM3k+++TM/-Tmp-/ghc70799_0/ghc70799_0.hc:3272:0: error: ‘finite’ undeclared (first use in this function) /var/folders/+p/+pUVycVKFDWsYP8YiYnM3k+++TM/-Tmp-/ghc70799_0/ghc70799_0.hc: In function ‘s2wI_ret’: /var/folders/+p/+pUVycVKFDWsYP8YiYnM3k+++TM/-Tmp-/ghc70799_0/ghc70799_0.hc:3318:0: error: ‘finite’ undeclared (first use in this function) /var/folders/+p/+pUVycVKFDWsYP8YiYnM3k+++TM/-Tmp-/ghc70799_0/ghc70799_0.hc: In function ‘s2Kh_ret’: /var/folders/+p/+pUVycVKFDWsYP8YiYnM3k+++TM/-Tmp-/ghc70799_0/ghc70799_0.hc:4856:0: error: ‘significand’ undeclared (first use in this function) /var/folders/+p/+pUVycVKFDWsYP8YiYnM3k+++TM/-Tmp-/ghc70799_0/ghc70799_0.hc: In function ‘s2KG_ret’: /var/folders/+p/+pUVycVKFDWsYP8YiYnM3k+++TM/-Tmp-/ghc70799_0/ghc70799_0.hc:4902:0: error: ‘significand’ undeclared (first use in this function) cabal: Error: some packages failed to install: cmath-0.3 failed during the building phase. The exception was: ExitFailure 1 Thanks for any help! amindfv / Tom

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Haskell wants the type, but I only know the class.
by Amy de Buitléir 26 Mar '12

26 Mar '12

I'm trying to read something from a file, do something with it, and then write it out again. I don't know, or care about, the type of the object I'm reading, but I do know its class. Here's my code: ---------- import Data.Binary ( Binary, encode, decode ) import Data.ByteString.Lazy as B ( readFile, writeFile ) import Codec.Compression.GZip ( compress, decompress ) class ( Binary a, Show a, Eq a ) => Thing a where doSomething :: a -> IO a readThing :: ( Thing a ) => FilePath -> IO a readThing f = return . decode . decompress =<< B.readFile f writeThing :: ( Thing a ) => FilePath -> a -> IO () writeThing f = B.writeFile f . compress . encode main = do a <- readThing "file1.txt" a' <- doSomething a writeThing "file2.txt" a' ---------- And here's the compiler error I get. Amy.hs:18:3: Ambiguous type variable `a0' in the constraint: (Thing a0) arising from a use of `writeThing' Probable fix: add a type signature that fixes these type variable(s) In the expression: writeThing "file2.txt" a' In the expression: do { a <- readThing "file1.txt"; a' <- doSomething a; writeThing "file2.txt" a' } In an equation for `main': main = do { a <- readThing "file1.txt"; a' <- doSomething a; writeThing "file2.txt" a' } Is there a way to fix my code? I tried adding a type signature, e.g., a <- readThing "file1.txt" :: (Thing t) => IO t a' <- doSomething a :: (Thing t) => IO t but then it says... Couldn't match type `t0' with `t' because type variable `t' would escape its scope This (rigid, skolem) type variable is bound by an expression type signature: Thing t => IO t The following variables have types that mention t0 a :: t0 (bound at Amy.hs:16:3) In a stmt of a 'do' expression: a' <- doSomething a :: Thing t => IO t In the expression: do { a <- readThing "file1.txt" :: Thing t => IO t; a' <- doSomething a :: Thing t => IO t; writeThing "file2.txt" a' } In an equation for `main': main = do { a <- readThing "file1.txt" :: Thing t => IO t; a' <- doSomething a :: Thing t => IO t; writeThing "file2.txt" a' }

5 16

gui pkgs
by Andy 18 Mar '12

18 Mar '12

Hi I'm trying to create a calculator in Haskell, something like the one in the enclosed image. To program the gui , I thought i might use a toolkit such as wxHaskell to create the buttons and frame etc.. I followed all the instructions on the wxHaskell homepage and everything seemed to install properly. Next I tried to use it with ghci, but to no avail. It seems you have to register wxHaskell with ghci by using a command like <ghci -package wx> but that crashes . Every tutorial I read didn't work. Does anyone have a clear set of instructions,that does work? Also, is there a better toolkit for doing guis than wxHaskell? I tried Glade, but it's so bug-ridden I couldn't get anything done. I need help.

2 1

Performance of Prime Generator
by Zhi-Qiang Lei 05 Feb '12

05 Feb '12

Hi, I have bungled the Prime Generator on SPOJ for many times. Although the program is faster and faster on my machine, but it keeps "time limited exceeded" on SPOJ (6 seconds limit). Now my approach is for every number in the range, check if it is prime by checking if can be divided by all the primes smaller than or equal its square root. For the numbers between 999900000 and 1000000000, it take 0.64 seconds on my laptop. Could anyone give me some hints to enhance it? Thanks. === Test Data === 1 999900000 1000000000 === Test Data === === Code === {-# OPTIONS_GHC -O2 -fno-cse #-} import System.IO data Wheel a = Wheel a [a] roll :: Integral a => Wheel a -> [a] roll (Wheel n rs) = [n*k+r | k <- [0..], r <- rs] nextSize :: Integral a => Wheel a -> a -> Wheel a nextSize (Wheel n rs) p = Wheel (p*n) [r' | k <- [0..(p-1)], r <- rs, let r' = n*k+r, r' `mod` p /= 0] mkWheel :: Integral a => [a] -> Wheel a mkWheel ds = foldl nextSize (Wheel 1 [1]) ds primes :: Integral a => [a] primes = small ++ large where 1:p:candidates = roll $ mkWheel small small = [2,3,5,7] large = p : filter isPrime candidates isPrime n = all (not . divides n) $ takeWhile (\p -> p*p <= n) large divides :: Integral a => a -> a -> Bool divides n p = n `mod` p == 0 sqrt' :: Integral a => a -> a sqrt' = floor . sqrt . fromIntegral primesFromTo :: [Int] -> [Int] primesFromTo [x, y] = filter isPrime [x..y] isPrime :: Integral a => a -> Bool isPrime 1 = False isPrime 2 = True isPrime n = all (not . divides n) $ takeWhile (<= sqrt' n) primes main :: IO () main = do count <- fmap read getLine inputLines <- fmap (take count . lines) getContents let answers = map (primesFromTo . map read . words) inputLines putStr . unlines . map (unlines . map show) $ answers === Code === Best regards, Zhi-Qiang Lei zhiqiang.lei(a)gmail.com

5 12

Immutable refs for the functional code to get emulation of o.o. inner/nested classes
by Gabriel Riba 31 Jan '12

31 Jan '12

I have worked out a solution with System.IO.Unsafe.unsafePerformIO / unsafeDupablePerformIO. I would like any criticism in order to get a good solution. The idea is to have immutable refs readable safely from the functional code. Possible use: -------------------------- import Data.ImmIORef (ImmIORef, newImmIORef, readImmIORef) data Framework = Framework {prop :: Int} deriving (Eq, Show) -- ''Inner'' element with ref. to the framework data FrameworkElement = FrameworkElement {dta::Int, frameworkRef :: (ImmIORef Framework)} deriving (Eq, Show) getElement'sFramework_Prop :: FrameworkElement -> Int getElement'sFramework_Prop elem = k where k = prop $ readImmIORef $ frameworkRef elem -------------------------- -- Data.ImmIORef adapted from GHC.IORef -- module Data.ImmIORef ( ImmIORef, newImmIORef, readImmIORef ) where import GHC.Base import GHC.STRef import GHC.IO import Text.Show (Show, show) import System.IO.Unsafe -- |An immutable variable in the 'IO' monad newtype ImmIORef a = ImmIORef (STRef RealWorld a) -- explicit instance instance Eq (ImmIORef a) where ImmIORef x == ImmIORef y = x == y -- |Build a new 'ImmIORef' newImmIORef :: a -> IO (ImmIORef a) newImmIORef v = stToIO (newSTRef v) >>= \ var -> return (ImmIORef var) -- |Read the value of an 'ImmIORef' via unsafePerformIO {-# NOINLINE readImmIORef #-} -- recommended in System.IO.Unsafe doc readImmIORef :: ImmIORef a -> a #if __GLASGOW_HASKELL__>=721 readImmIORef (ImmIORef var) = unsafeDupablePerformIO $ stToIO (readSTRef var) #else readImmIORef (ImmIORef var) = unsafePerformIO $ stToIO (readSTRef var) #endif instance (Show a) => Show (ImmIORef a) where show xRef = "ImmIORef->(" ++ show (readImmIORef xRef) ++ ")"

2 2

arrays
by Dennis Raddle 30 Jan '12

30 Jan '12

I've got a problem that is linear time with mutable arrays, and something god-awful complicated (not sure of time complexity) without them. What's the easiest way to use and understand mutable arrays? is it Data.Array that I want? Any good tutorials? Thanks, Dennis

3 2

Iterating a monadic action with memoization
by Tim Cowlishaw 29 Jan '12

29 Jan '12

Hi all, I was wondering if any of you knew whether the following was possible, (I discussed this a little on #haskell at the weekend but didn't quite get to the bottom of it): Say I have a monadic action: > f :: a -> m a and an initial value of type a. I'd like to iterate this action and collect the results of each iteration in a list, as with 'iterate' on normal functions: > iterate (>>= f) . return $ a However, I would also like to memoize the intermediate results of the iteration, such that the entire iteration runs in O(n) time rather than O(n^2). This also implies a slight semantic difference, as side-effecting or non-deterministic actions will not be repeated in each iteration, while maintaining the laziness of the iteration function. Is it possible to do this? Many thanks, Tim

4 4

data at different stages
by Dennis Raddle 28 Jan '12

28 Jan '12

I'm working on an application for reading Music XML and manipulating it, including translating it into midi events. Storing music data can take place from different perspectives or requirements. - a literal translation of the MusicXML would store each note and marking on the page - another representation might relate to higher concepts: for instance, instead of storing individual dynamic markings and indications for swelling and ebbing of dynamics, it might create a single data structure which is a dynamic profile over time. Instead of storing all the markings over the notes individually (staccato, accents, slur marks, etc.) it might examine all the markings at once and compute concrete performance instructions note by note. There are about four distinct stages in processing the music, in which things become either more abstract (grouping together details into higher concepts) or more concrete (such as more literal performance instructions). One of the stages, by the way, tries to represent notes in a way such that they can be moved in time or "sliced out" without disturbing the continuity of the performance instructions. At first I created several different data structures, one for each useful stage. Many of the fields are copied literally. There are similar fields which need distinct names. This is starting to seem like a confusing mess. Another approach is to create one data structure which contains some "Maybe" type fields. When the processing reaches a certain stage, they are set to "Just <value>" if applicable to that stage. Otherwise they are set to Nothing. I didn't want to do this at first because it's too much like a sloppy script language. The type system of the compiler would no longer assist me in verifying the validity of the data at each stage. But it's turning into a mess- and the key reason it's a mess, I think, is that I DON'T have a clear model of the stages. It keeps changing as I expand the requirements of my program. What seemed like a useful representation last month needs to be changed today. Any ideas welcome. Dennis

3 3

Testing IO based monads and functions
by David Hinkes 27 Jan '12

27 Jan '12

Hey Haskellers, I had a design question: I'm using Haskell for the first time for something other than a simple toy application. I notice I'm coming up with a lot of IO based APIs that looks something like this: type UserInfo = (String, String) -- (name, password) getMagicNumberFromHTTPServer :: UserInfo -> IO (Maybe Int) getMagicNumberFromHTTPServer user = do let curlHTTPRequest = ... --Setup curl HTTP request httpRepsponse <- ... -- Make curl HTTP request return $ tryGetMagicNumberFromResponse httpResponse I like this API, because it's easy to understand. However, I find it hard to test. In OO land, I'd create a mock CurlHTTPRequest object and pass that into the magic number logic and test for the correct usage of the curl library. Is there a FP way of achieving the same goals? I suppose given the example above I could break down the above into several smaller pure functions and test them. I don't know if this strategy will hold-up when dealing with more complex monads. Thanks for your time, Dave

2 1

HDBC-ODBC and Windows-1251 encoding
by Sergiy Nazarenko 26 Jan '12

26 Jan '12

Hi, guys! I use HDBC-ODBC module to work with windows data base and I got following problem: when I try to do INSERT > import Codec.Text.IConv (convert) > import qualified Data.ByteString.Lazy as BSL > ... > let name = convert "UTF-8" "WINDOWS-1251" $ BSL.pack "bla" > quickQuery "INSERT INTO Mytable (name,descript) VALUES (?, ?)" [toSql name), toSql name] and this also not worked: > quickQuery "INSERT INTO Mytable (name,descript) VALUES (?, ?)" [toSql ( BS.pack "bla "), toSql ( BS.pack "blabla" )] MSSQL server response me error: ... [ODBC SQL Server Driver][SQL Server]Invalid character value for cast specification It seems to be different encoding then HDBC-ODBC module can work with. I read documentation for HDBC: > To accomplish this, whenever a ByteString must be converted to or from a String, the ByteString is assumed to be in UTF-8 encoding, and will be decoded or encoded as appropriate. > Database drivers will generally present text or string data they have received from the database as a SqlValue holding a ByteString, which fromSql will automatically convert to a String, and thus automatically decode UTF-8, when you need it. In the other direction, database drivers will generally convert a SqlString to a ByteString in UTF-8 encoding before passing it to the database engine. How can I work with HDBC-ODBC module with non UTF-8 encoding? Cheers, Sergiy

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