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  • Point-free in Haskell

    - by wwrob
    I have this code that I want to make point-free; (\k t -> chr $ a + flip mod 26 (ord k + ord t -2*a)) How do I do that? Also are there some general rules for point free style other than "think about this amd come up with something"?

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  • Haskell search an element on a List

    - by user1887556
    I want a function that changes (1 to 0) on a list, when number of 1's isn't even. I have done these functions: 1) Sees if the lines in a list are even or not: parityLine :: [[Int]] -> [Bool] parityLine [] =[] parityLine (x:xs) |sum(x) `mod` 2 == 0 = True:(parityLine(xs)) |otherwise = False:(parityLine(xs)) 2) Sum the corresponding elements on a list of lists: sumPositions :: [[Int]] -> [Int] sumPositions [] = [] sumPositions (x:xs) = foldl (zipWith (+)) (repeat 0) (x:xs) 3) Sees if the columns in a list are even or not: parityColumn :: [Int] -> [Bool] parityColumn [] = [] parityColumn (x:xs) |head(x:xs) `mod` 2 == 0 = True:parityColumn(xs) |otherwise = False:parityColumn(xs) 4) Does the operation or with two boolean lists: bol :: [Bool] -> [Bool] -> [[Bool]] bol [] _ = [] bol (x:xs) (y:ys)= (map (||x) (y:ys)):(bol xs (y:ys)) 5) Correct List: correct :: [[Int]] -> [[Bool]] correct [] = [] correct (x:xs)=(bol(parityLine (x:xs))(parityColumn(sumPositions(x:xs)))) So what I want is to alter the function correct to [[Int]]-[[Int]] that does this: My Int list(x:xs) With my correct function applied [[0,0,1,1], [[True,True,False,True], [1,0,1,1], [True,True,True,True], [0,1,0,1], [True,True,True,True] [1,1,1,1]] [True,True,True,True]] Now I can see that in the first line third column, False, so I have to correct that number 1 to have a number of 1's even. Result I want that function correct does: [[0,0,0,1], [1,0,1,1], [0,1,0,1], [1,1,1,1]] Thanks.

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  • best way to write a-> ..->[a] recursive functions in haskell

    - by Roman A. Taycher
    So I keep having this small problem where I have something like func :: a -> b -> [a] -- or basically any a-> ...-> [a] where ... is any types -> func x y = func' [x] y -- as long as they are used to generate a list of [a] from x func' :: [a] -> b -> [a] func = undefined --situation dependant generates a list from each element and returns it as one long list should I keep it like this? should I use func' hidden by a where? should I only use the [a] - b - [a] version and leave the responsibility of passing [variable] to the callee? I might well need to compose these functions and might want to mess around with the order so I'm leaning towards option 3. What do you think?

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  • Haskell lists difference

    - by user559354
    I'm trying make a lists difference. Found directly prelude operator \\\\ that makes lists difference. But errors Not in scope: '\\\\' occurs. Here is my simple from command line interpreter: Prelude> ([1,2,3] ++ [5,6]) -- works like expected [1,2,3,4,5,6] prelude> ([1,2,3] \\\\ [1,2]) -- erros occurs <interactive>:1:11: Not in scope: "\\\\" Thanks for explanation where I make a mistake.

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  • find if list 1 is a sequence of list 2 in haskell

    - by Isaak Wahb
    im trying to check if a given list is a subsequence of another list: here are example of lists which gives true: subseq "" "w" subseq "w" "w" subseq "ab" "cab" subseq "cb" "cab" subseq "aa" "xaxa" not (subseq "aa" "xax") not (subseq "ab" "ba") i just come to this but in some cases it gives a wrong result subseq :: Eq a => [a] -> [a] -> Bool subseq [] [] = True subseq [] ys = True subseq xs [] = False subseq (x:xs) (y:ys) = x == y || subseq xs ( 1 `drop` ys )

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  • haskell. words into binary numbers

    - by Katja
    I need to convert words into binary numbers. With a bit help of yours I got this blCo::String -> Integer blCo x = num2bin(lett2num x) blCo (x:xs) | x:xs = num2bin(lett2num x):blCo xs num2lett :: Int -> Char num2lett n | (n <= ord 'A') && (n <= ord 'Z') = chr(ord 'A'+ n - 1) | (n <= ord 'a') && (n <= ord 'Z') = chr(ord 'A'+ n - 1) num2bin :: Integer -> String num2bin n | n >= 0 = concatMap show (reverse ( n2b n)) | otherwise = error "num2bin: negative number" where n2b 0 = [] n2b n = n `mod` 2 : n2b (n `div` 2) he tells me a mistake.I dont undertsand it mistake: Couldn't match expected type `Char' against inferred type `String' In the first argument of `lett2num', namely `x' In the first argument of `num2bin', namely `(lett2num x)' In the expression: num2bin (lett2num x)

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  • Haskell Input & Passing Values to Functions

    - by Pradeep
    putStrLn "Enter the Artist Name" art <- getLine putStrLn "Enter the Number of CD's" num <- getLine let test= buyItem currentStockBase art num printListIO (showcurrentList test) the values i have to pass for buyItem is buyItem currentStockBase "Akon" 20 but i want to send "Akon" to art and for 20 i want to send num it gives me this error ERROR file:.\Project2.hs:126 - Type error in application *** Expression : buyItem currentStockBase art num *** Term : num *** Type : [Char] *** Does not match : Int please help me

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  • How do you solve this Haskell problem?

    - by Linda Cohen
    I want to define a function replicate to replicate a list of numbers by its value using only list comprehension, for example: replicate [5,1,3,2,8,1,2] output: [5,5,5,5,5,1,3,3,3,2,2,8,8,8,8,8,8,8,8,1,2,2] I know this would be easy to use the 'replicate' built in function but only list comprehension is allow, how can I do this? THANKS!

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  • Haskell type signature with multiple type somethings (predicates?, for example Eq a =>)

    - by Andrew
    I'm not sure if type predicates is the right term, in fact I've never learned the word for this, so an edit to correct would be helpful - I'm referring to when you give the tipe of function f :: a -> b and you want to say a is a Eq and you say f :: Eq a => a -> b, the name for Eq a => - this is the thing i called a type predicate. My question, though, is how to have multiple of these, so if A is an Eq and B is a Num, I could say either f :: Eq a => a -> b or f :: Num b => a -> b. So, how can I have Eq a => and Num b => at the same time? f :: Eq a => Num b => a -> b, f :: Eq a -> Num b => a -> b, and f :: Eq a, Num b => a -> b all didn't do what I wanted.

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  • Simple haskell splitlist

    - by js7354
    I have the following function which takes a list and returns two sublists split at a given element n. However, I only need to split it in half, with odd length lists having a larger first sublist splitlist :: [a] -> Int -> ([a],[a]) splitlist [] = ([],[]) splitlist l@(x : xs) n | n > 0 = (x : ys, zs) | otherwise = (l, []) where (ys,zs) = splitlist xs (n - 1) I know I need to change the signature to [a] - ([a],[a]), but where in the code should I put something like length(xs) so that I don't break recursion? Thank you.

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  • Haskell: list of elements with class restriction

    - by user1760586
    here's my question: this works perfectly: type Asdf = [Integer] type ListOfAsdf = [Asdf] Now I want to do the same but with the Integral class restriction: type Asdf2 a = (Integral a) => [a] type ListOfAsdf2 = (Integral a) => [Asdf2 a] I got this error: Illegal polymorphic or qualified type: Asdf2 a Perhaps you intended to use -XImpredicativeTypes In the type synonym declaration for `ListOfAsdf2' I have tried a lot of things but I am still not able to create a type with a class restriction as described above. Thanks in advance!!! =) Dak

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  • Nested/Sub data types in haskell

    - by Tom Carstens
    So what would be nice is if you could do something like the following (not necessarily with this format, just the general idea): data Minor = MinorA | MinorB data Major = Minor | MajorB isMinor :: Major -> Bool isMinor Minor = True isMinor _ = False So isMinor MinorA would report True (instead of an error.) At the moment you might do something like: data Major = MinorA | MinorB | MajorB isMinor :: Major -> Bool isMinor MinorA = True isMinor MinorB = True isMinor _ = False It's not terrible or anything, but it doesn't expand nicely (as in if Minor when up to MinorZ this would be terribly clunky). To avoid that problem you can wrap Minor: data Minor = MinorA | MinorB data Major = MajorA Minor | MajorB isMinor :: Major -> Bool isMinor (MajorA _) = True isMinor _ = False But now you have to make sure to wrap your Minors to use them as a Major... again not terrible; just doesn't really express the semantics I'd like very well (i.e. Major can be any Minor or MajorB). The first (legal) example is "Major can be MinorA..." but doesn't have any knowledge of Minor and the second is "Major can be MajorA that takes a Minor..." p.s. No, this isn't really about anything concrete.

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  • Deploying Socket.IO App to Windows Azure Web Site with Azure CLI

    - by shiju
    In this blog post, I will demonstrate how to deploy Socket.IO app to Windows Azure Website using Windows Azure Cross-Platform Command-Line Interface, which leverages the Windows Azure Website’s new support for Web Sockets. Recently Windows Azure has announced lot of enhancements including the support for Web Sockets in Windows Azure Websites, which lets the Node.js developers deploy Socket.IO apps to Windows Azure Websites. In this blog post, I am using  Windows Azure CLI for create and deploy Windows Azure Website. Install  Windows Azure CLI The Windows Azure CLI available as a NPM module so that you can install Windows Azure CLI using  NPM as shown in the below command. After installing the azure-cli, just enter the command “azure” which will show the useful commands provided by Azure CLI. Import Windows Azure Subscription Account In order to import our Azure subscription account, we need to download the Windows Azure subscription profile. The Azure CLI command “account download” lets you download the  Windows Azure subscription profile as shown in the below command. The command redirect you login to Windows Azure portal and allow you to download the Windows Azure publish settings file. The account import command lets you import the downloaded publish settings file so that you can create and manage Websites, Cloud Services, Virtual Machines and Mobile Services in Windows Azure. Create Windows Azure Website and Enable Web Sockets In this post, we are going to deploy Socket.IO app to Windows Azure Website by using the Web Socket support provided by Windows Azure. Let’s create a Website named “socketiochatapp” using the Azure CLI. The above command will create a Windows Azure Website that will also initialize a Git repository with a remote named Azure. We can see the newly created Website from Azure portal. By default, the Web Sockets will be disabled. So let’s enable it by navigating to the Configure tab of the Website, and select “ON” in Web Sockets option and save the configuration changes. Deploy a Node.js Socket.IO App to Windows Azure Now, our Windows Azure Website supports Web Sockets so that we can easily deploy Socket.IO app to Windows Azure Website. Let’s add Node.js chat app which leverages Socket.IO module. Please note that you have to add npm module dependencies in the package.json file so that Windows Azure can install the dependencies when deploying the app. Let’s add the Node.js app and add the files to git repository. Let’s commit the changes to git repository. We have committed the changes to git local repository. Let’s push the changes to Windows Azure production environment. The successful deployment can see from the Windows Azure portal by navigating to the deployments tab of the selected Windows Azure Website. The screen shot below shows that our chat app is running successfully.   You can follow me on Twitter @shijucv

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  • SQL SERVER Improve Performance by Reducing IO Creating Covered Index

    This blog post is in the response of the T-SQL Tuesday #004: IO by Mike Walsh. The subject of this month is IO. Here is my quick blog post on how Cover Index can Improve Performance by Reducing IO.Let us kick off this post with disclaimers about Index. Index is a very complex subject and [...]...Did you know that DotNetSlackers also publishes .net articles written by top known .net Authors? We already have over 80 articles in several categories including Silverlight. Take a look: here.

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  • How do I know if my disks are being hit with too much IO reads or writes or both?

    - by Mark F
    Hi All, So I know a bit about disk I/O and bottlenecks relating to this especially when relating to databases. But how do I really know what the max IO numbers will be for my disks? What metric might be available to me for working out roughly (but needs to be a good approximation) of how much capacity (if you will) have I got left available in I/O. I've seen it before where things are bubbling along nicely and then all of a sudden, everything screams to a halt, and it ends up being an IO bound problem. Is there a better way to predict when IO is reaching its limits? This article was interesting but not giving the answer I desire. "http://serverfault.com/questions/61510/linux-how-can-i-see-whats-waiting-for-disk-io". So is my best bet surrounding just looking at 'CPU IO WAIT'? There must be a more reactive method for this? Best, M

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  • Java File IO Compendium

    - by Warren Taylor
    I've worked in and around Java for nigh on a decade, but have managed to ever avoid doing serious work with files. Mostly I've written database driven applications, but occasionally, even those require some file io. Since I do it so rarely, I end up googling around for quite some time to figure out the exact incantation that Java requires to read a file into a byte[], char[], String or whatever I need at the time. For a 'once and for all' list, I'd like to see all of the usual ways to get data from a file into Java, or vice versa. There will be a fair bit of overlap, but the point is to define all of the subtle different variants that are out there. For example: Read/Write a text file from/to a single String. Read a text file line by line. Read/Write a binary file from/to a single byte[]. Read a binary file into a byte[] of size x, one chunk at a time. The goal is to show concise ways to do each of these. Samples do not need to handle missing files or other errors, as that is generally domain specific. Feel free to suggest more IO tasks that are somewhat common and I have neglected to mention.

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  • java.io.StreamCorruptedException: invalid stream header: 7371007E

    - by Alex
    Hello, this is pprobably a simple question . I got a client Server application which communicate using objects. when I send only one object from the client to server all works well. when I attempt to send several objects one after another on the same stream I get StreamCorruptedException. can some one direct me to the cause of this error . Thanks client write method private SecMessage[] send(SecMessage[] msgs) { SecMessage result[]=new SecMessage[msgs.length]; Socket s=null; ObjectOutputStream objOut =null; ObjectInputStream objIn=null; try { s=new Socket("localhost",12345); objOut=new ObjectOutputStream( s.getOutputStream()); for (SecMessage msg : msgs) { objOut.writeObject(msg); } objOut.flush(); objIn=new ObjectInputStream(s.getInputStream()); for (int i=0;i<result.length;i++) result[i]=(SecMessage)objIn.readObject(); } catch(java.io.IOException e) { alert(IO_ERROR_MSG+"\n"+e.getMessage()); } catch (ClassNotFoundException e) { alert(INTERNAL_ERROR+"\n"+e.getMessage()); } finally { try {objIn.close();} catch (IOException e) {} try {objOut.close();} catch (IOException e) {} } return result; } server read method //in is an inputStream Defined in the server SecMessage rcvdMsgObj; rcvdMsgObj=(SecMessage)new ObjectInputStream(in).readObject(); return rcvdMsgObj; and the SecMessage Class is public class SecMessage implements java.io.Serializable { private static final long serialVersionUID = 3940341617988134707L; private String cmd; //... nothing interesting here , just a bunch of fields , getter and setters }

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  • Sending multiline message via sockets without closing the connection

    - by Yasir Arsanukaev
    Hello folks. Currently I have this code of my client-side Haskell application: import Network.Socket import Network.BSD import System.IO hiding (hPutStr, hPutStrLn, hGetLine, hGetContents) import System.IO.UTF8 connectserver :: HostName -- ^ Remote hostname, or localhost -> String -- ^ Port number or name -> IO Handle connectserver hostname port = withSocketsDo $ do -- withSocketsDo is required on Windows -- Look up the hostname and port. Either raises an exception -- or returns a nonempty list. First element in that list -- is supposed to be the best option. addrinfos <- getAddrInfo Nothing (Just hostname) (Just port) let serveraddr = head addrinfos -- Establish a socket for communication sock <- socket (addrFamily serveraddr) Stream defaultProtocol -- Mark the socket for keep-alive handling since it may be idle -- for long periods of time setSocketOption sock KeepAlive 1 -- Connect to server connect sock (addrAddress serveraddr) -- Make a Handle out of it for convenience h <- socketToHandle sock ReadWriteMode -- Were going to set buffering to LineBuffering and then -- explicitly call hFlush after each message, below, so that -- messages get logged immediately hSetBuffering h LineBuffering return h sendid :: Handle -> String -> IO String sendid h id = do hPutStr h id -- Make sure that we send data immediately hFlush h -- Retrieve results hGetLine h The code portions in connectserver are from this chapter of Real World Haskell book where they say: When dealing with TCP data, it's often convenient to convert a socket into a Haskell Handle. We do so here, and explicitly set the buffering – an important point for TCP communication. Next, we set up lazy reading from the socket's Handle. For each incoming line, we pass it to handle. After there is no more data – because the remote end has closed the socket – we output a message about that. Since hGetContents blocks until the server closes the socket on the other side, I used hGetLine instead. It satisfied me before I decided to implement multiline output to client. I wouldn't like the server to close a socket every time it finishes sending multiline text. The only simple idea I have at the moment is to count the number of linefeeds and stop reading lines after two subsequent linefeeds. Do you have any better suggestions? Thanks.

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  • printing menu in terminal and choosing an option, how to?

    - by carlos
    I'm a haskell beginner. I'm trying to make a program that shows a menu through terminal and ask user to introduce an option. Here is the code: main :: IO () main = do putStrLn "0 <- quit" putStrLn "1 <- Hello" putStr "Choose an option: " c <- getChar case c of '0' -> return () '1' -> putChar '\n' >> putStrLn "Hello World" >> main When I use this module in the ghci interpreter everything works like it's suposed to do. But if i compile this with: ghc hello.hs and run it in the terminal, it doesn't display the line "Choose an option:" before ask for a char to be introduced. I think this may be caused because of haskell lazy nature and I don't know how to fix it. Any ideas?

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