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• Haskell Weird Kinds

  - by SHiNKiROU

  When I was experimenting with Haskell kinds, and trying to get the kind of ->, and this showed up: $ ghci ... Prelude> :k (->) (->) :: ?? -> ? -> * Prelude> Instead of the expected * -> * -> *. What are the ?? and ? things? Do they mean concrete types or "kind variables"? Or something else?

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• Haskell graph data type representation

  - by John Retallack

  I want to represent a graph in Haskell in the following manner: For each node I want to store it's value and a list of adjacent nodes,the problem which i'm having difficulties with is that I want the adjacent nodes to be stored as references to other nodes. For example: I want node ny to be stored as („NY“ (l p)) where l and p are adjacent nodes,and not as („NY“ („London“ „Paris“)). I tried something like this : data Node a = Node { value :: a , neighbors :: [Node a] }deriving (Show) let n1 = Node {value=1, neighbors=[n2]} let n2 = Node {value=1, neighbors=[n1 n3]} let n3 = Node {value=1, neighbors=[n2]} But i get en error in let,What am I doing wrong ?

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• Haskell - generating all paths between nodes

  - by user1460863

  I need to build a function, which return all paths between certain nodes. connect :: Int -> Int-> [[(Int,Int)]] Data.Graph library gives me usefull function 'buildG' which builds graph for me. If I call let g = buildG (1,5) [(1,2),(2,3),(3,4),(4,5),(2,5)], I will get an array where every node is mapped to his neighbours. An example: g!1 = [2] g!2 = [3,5] .. g!5 = [] I was trying to do it using list comprehensions, but I am not very good in haskell and I am getting typing error which I can't repair. connect x y g | x == y = [] | otherwise = [(x,z) | z <- (g!x), connect z y g] I don't need to worry at this moment about cycles. Here is what I want to get: connect 1 5 g = [[(1,2),(2,3),(3,4),(4,5)],[(1,2),(2,5)]]

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• why Haskell can deduce [] type in this function

  - by Sili

  rho x = map (((flip mod) x).(\a -> a^2-1)) (rho x) This function will generate an infinite list. And I tested in GHCi, the function type is *Main> :t rho rho :: Integral b => b -> [b] If I define a function like this fun x = ((flip mod) x).(\a -> a^2-1) The type is *Main> :t fun fun :: Integral c => c -> c -> c My question is, how can Haskell deduce the function type to b - [b]? We don't have any [] type data in this function. Thanks!

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• Function Composition in Haskell

  - by Watts

  I have a function that takes 3 functions and switches the types and combine to make a new function. For example a test case call would be : (chain init tail reverse ) "Haskell!" the output should be lleksa I've tried to do this problem a few different ways including using the map function but I kept getting association problems. so i did chain :: Ord a => [a] -> a chain f g h x = f.g.h$x my error is Couldn't match expected type[t0->t1->t2->a0] When I type the problem directly into prelude like replacing f, g, h, x with the values it comes out right Is there even a way to do three functions, I've only seen two in examples

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• Is there any working implementation of reverse mode automatic differentiation for Haskell?

  - by Ian Fiske

  The closest-related implementation in Haskell I have seen is the forward mode at http://hackage.haskell.org/packages/archive/fad/1.0/doc/html/Numeric-FAD.html. The closest related related research appears to be reverse mode for another functional language related to Scheme at http://www.bcl.hamilton.ie/~qobi/stalingrad/. I see reverse mode in Haskell as kind of a holy grail for a lot of tasks, with the hopes that it could use Haskell's nested data parallelism to gain a nice speedup in heavy numerical optimization.

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• strange error in haskell about indentation of if-then-else

  - by Drakosha

  I have the following code: foo :: Int -> [String] -> [(FilePath, Integer)] -> IO Int foo _ [] _ = return 4 foo _ _ [] = return 5 foo n nameREs pretendentFilesWithSizes = do result <- (bar n (head nameREs) pretendentFilesWithSizes) if result == 0 then return 0 -- <========================================== here is the error else foo n (tail nameREs) pretendentFilesWithSizes I get an error on the line with the comment above, the error is: aaa.hs:56:2: parse error (possibly incorrect indentation) I'm working with emacs, there's no spaces, and i do not understand what did i do wrong.

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• Collecting IO outputs into list

  - by sisif

  how can i do multiple calls to SDL.pollEvent :: IO Event until the output is SDL.NoEvent and collect all the results into a list? in imperative terms something like this: events = [] event = SDL.pollEvent; while( event != SDL.NoEvent ) events.add( event ) event = SDL.pollEvent

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• What are possible reasons for java.io.IOException: "The filename, directory name, or volume label sy

  - by Turismo

  I am trying to copy a file using the following code: File targetFile = new File(targetPath + File.separator + filename); ... targetFile.createNewFile(); fileInputStream = new FileInputStream(fileToCopy); fileOutputStream = new FileOutputStream(targetFile); byte[] buffer = new byte[64*1024]; int i = 0; while((i = fileInputStream.read(buffer)) != -1) { fileOutputStream.write(buffer, 0, i); } For some users the targetFile.createNewFile results in this exception: java.io.IOException: The filename, directory name, or volume label syntax is incorrect at java.io.WinNTFileSystem.createFileExclusively(Native Method) at java.io.File.createNewFile(File.java:850) Filename and directory name seem to be correct. The directory targetPath is even checked for existence before the copy code is executed and the filename looks like this: AB_timestamp.xml The user has write permissions to the targetPath and can copy the file without problems using the OS. As I don't have access to a machine this happens on yet and can't reproduce the problem on my own machine I turn to you for hints on the reason for this exception.

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• A non-blocking server with java.io

  - by Jon

  Everybody knows that java IO is blocking, and java NIO is non-blocking. In IO you will have to use the thread per client pattern, in NIO you can use one thread for all clients. Now my question follows: is it possible to make a non-blocking design using only the Java IO api. (not NIO) I was thinking about a pattern like this (obviously very simplified); List<Socket> li; for (Socket s : li) { InputStream in = s.getInputStream(); byte[] data = in.available(); in.read(data); // processData(data); (decoding packets, encoding outgoing packets } Also note that the client will always be ready for reading data. What are your opinions on this? Will this be suitable for a server that should at least hold a few hundred of clients without major performance issues?

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• Use Shakespeare-text and external file

  - by Adam

  How can I convert the below example to use an external file instead of the embedded lazy text quasi quotes? {-# LANGUAGE QuasiQuotes, OverloadedStrings #-} import Text.Shakespeare.Text import qualified Data.Text.Lazy.IO as TLIO import Data.Text (Text) import Control.Monad (forM_) data Item = Item { itemName :: Text , itemQty :: Int } items :: [Item] items = [ Item "apples" 5 , Item "bananas" 10 ] main :: IO () main = forM_ items $ \item -> TLIO.putStrLn [lt|You have #{show $ itemQty item} #{itemName item}.|] This is from the yesod online book.

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• Haskell: 'No instance for' arising from a trivial usage of Regex library

  - by artemave

  Following the (accepted) answer from this question, I am expecting the following to work: Prelude Text.Regex.Posix Text.Regex.Base.RegexLike Text.Regex.Posix.String> makeRegex ".*" (makeRegex is a shortcut for makeRegexOpts with predefined options) However, it doesn't: <interactive>:1:0: No instance for (RegexMaker regex compOpt execOpt [Char]) arising from a use of `makeRegex' at <interactive>:1:0-13 Possible fix: add an instance declaration for (RegexMaker regex compOpt execOpt [Char]) In the expression: makeRegex ".*" In the definition of `it': it = makeRegex ".*" Prelude Text.Regex.Posix Text.Regex.Base.RegexLike Text.Regex.Posix.String> make Regex ".*"::Regex <interactive>:1:0: No instance for (RegexMaker Regex compOpt execOpt [Char]) arising from a use of `makeRegex' at <interactive>:1:0-13 Possible fix: add an instance declaration for (RegexMaker Regex compOpt execOpt [Char]) In the expression: makeRegex ".*" :: Regex In the definition of `it': it = makeRegex ".*" :: Regex And I really don't understand why. EDIT Haskell Platform 2009.02.02 (GHC 6.10.4) on Windows EDIT2 Prelude Text.Regex.Base.RegexLike Text.Regex.Posix.String> :i RegexMaker class (RegexOptions regex compOpt execOpt) => RegexMaker regex compOpt execOpt source | regex -> compOpt execOpt, compOpt -> regex execOpt, execOpt -> regex compOpt where makeRegex :: source -> regex makeRegexOpts :: compOpt -> execOpt -> source -> regex makeRegexM :: (Monad m) => source -> m regex makeRegexOptsM :: (Monad m) => compOpt -> execOpt -> source -> m regex -- Defined in Text.Regex.Base.RegexLike

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• How do i convert String to Integer/Float in Haskell

  - by Ranhiru

  data GroceryItem = CartItem ItemName Price Quantity | StockItem ItemName Price Quantity makeGroceryItem :: String -> Float -> Int -> GroceryItem makeGroceryItem name price quantity = CartItem name price quantity I want to create a GroceryItem when using a String or [String] createGroceryItem :: [String] -> GroceryItem createGroceryItem (a:b:c) = makeGroceryItem a b c The input will be in the format ["Apple","15.00","5"] which i broke up using words function in haskell. I get this error which i think is because the makeGroceryItem accepts a Float and an Int. But how do i make b and c Float and Int respectively? *Type error in application *** Expression : makeGroceryItem a read b read c *** Term : makeGroceryItem *** Type : String -> Float -> Int -> GroceryItem *** Does not match : a -> b -> c -> d -> e -> f* Thanx a lot in advance :)

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• "Programming In Haskell" error in sat function

  - by Matt Ellen

  I'm in chapter 8 of Graham Hutton's Programming in Haskell and I'm copying the code and testing it in GHC. See the slides here: http://www.cis.syr.edu/~sueo/cis352/chapter8.pdf in particular slide 15 The relevant code I've copied so far is: type Parser a = String -> [(a, String)] pih_return :: a -> Parser a pih_return v = \inp -> [(v, inp)] failure :: Parser a failure = \inp -> [] item :: Parser Char item = \inp -> case inp of [] -> [] (x:xs) -> [(x,xs)] parse :: Parser a -> String -> [(a, String)] parse p inp = p inp sat :: (Char -> Bool) -> Parser Char sat p = do x <- item if p x then pih_return x else failure I have changed the name of the return function from the book to pih_return so that it doesn't clash with the Prelude return function. The errors are in the last function sat. I have copied this directly from the book. As you can probably see p is a function from Char to Bool (e.g. isDigit) and x is of type [(Char, String)], so that's the first error. Then pih_return takes a value v and returns [(v, inp)] where inp is a String. This causes an error in sat because the v being passed is x which is not a Char. I have come up with this solution, by explicitly including inp into sat sat :: (Char -> Bool) -> Parser Char sat p inp = do x <- item inp if p (fst x) then pih_return (fst x) inp else failure inp Is this the best way to solve the issue?

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• Monads and custom traversal functions in Haskell

  - by Bill

  Given the following simple BST definition: data Tree x = Empty | Leaf x | Node x (Tree x) (Tree x) deriving (Show, Eq) inOrder :: Tree x -> [x] inOrder Empty = [] inOrder (Leaf x) = [x] inOrder (Node root left right) = inOrder left ++ [root] ++ inOrder right I'd like to write an in-order function that can have side effects. I achieved that with: inOrderM :: (Show x, Monad m) => (x -> m a) -> Tree x -> m () inOrderM f (Empty) = return () inOrderM f (Leaf y) = f y >> return () inOrderM f (Node root left right) = inOrderM f left >> f root >> inOrderM f right -- print tree in order to stdout inOrderM print tree This works fine, but it seems repetitive - the same logic is already present in inOrder and my experience with Haskell leads me to believe that I'm probably doing something wrong if I'm writing a similar thing twice. Is there any way that I can write a single function inOrder that can take either pure or monadic functions?

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• Zipping with padding in Haskell

  - by Travis Brown

  A couple of times I've found myself wanting a zip in Haskell that adds padding to the shorter list instead of truncating the longer one. This is easy enough to write. (Monoid works for me here, but you could also just pass in the elements that you want to use for padding.) zipPad :: (Monoid a, Monoid b) => [a] -> [b] -> [(a, b)] zipPad xs [] = zip xs (repeat mempty) zipPad [] ys = zip (repeat mempty) ys zipPad (x:xs) (y:ys) = (x, y) : zipPad xs ys This approach gets ugly when trying to define zipPad3. I typed up the following and then realized that of course it doesn't work: zipPad3 :: (Monoid a, Monoid b, Monoid c) => [a] -> [b] -> [c] -> [(a, b, c)] zipPad3 xs [] [] = zip3 xs (repeat mempty) (repeat mempty) zipPad3 [] ys [] = zip3 (repeat mempty) ys (repeat mempty) zipPad3 [] [] zs = zip3 (repeat mempty) (repeat mempty) zs zipPad3 xs ys [] = zip3 xs ys (repeat mempty) zipPad3 xs [] zs = zip3 xs (repeat mempty) zs zipPad3 [] ys zs = zip3 (repeat mempty) ys zs zipPad3 (x:xs) (y:ys) (z:zs) = (x, y, z) : zipPad3 xs ys zs At this point I cheated and just used length to pick the longest list and pad the others. Am I overlooking a more elegant way to do this, or is something like zipPad3 already defined somewhere?

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• Compiled Haskell libraries with FFI imports are invalid when imported into GHCI

  - by John Millikin

  I am using GHC 6.12.1, in Ubuntu 10.04 When I try to use the FFI syntax for static storage, only modules running in interpreted mode (ie GHCI) work properly. Compiled modules have invalid pointers, and do not work. I'd like to know whether anybody can reproduce the problem, whether this an error in my code or GHC, and (if the latter) whether it's a known issue. I'm using sys_siglist because it's present in a standard library on my system, but I don't believe the actual storage used matters (I discovered this while writing a binding to libidn). If it helps, sys_siglist is defined in <signal.h> as: extern __const char *__const sys_siglist[_NSIG]; I thought this type might be the problem, so I also tried wrapping it in a plain C procedure: #include<stdio.h> const char **test_ffi_import() { printf("C think sys_siglist = %X\n", sys_siglist); return sys_siglist; } However, importing that doesn't change the result, and the printf() call prints the same pointer value as show siglist_a. My suspicion is that it's something to do with static and dynamic library loading. Update: somebody in #haskell suggested this might be 64-bit specific; if anybody tries to reproduce it, can you mention your architecture and whether it worked in a comment? Code as follows: -- A.hs {-# LANGUAGE ForeignFunctionInterface #-} module A where import Foreign import Foreign.C foreign import ccall "&sys_siglist" siglist_a :: Ptr CString -- -- B.hs {-# LANGUAGE ForeignFunctionInterface #-} module B where import Foreign import Foreign.C foreign import ccall "&sys_siglist" siglist_b :: Ptr CString -- -- Main.hs {-# LANGUAGE ForeignFunctionInterface #-} module Main where import Foreign import Foreign.C import A import B foreign import ccall "&sys_siglist" siglist_main :: Ptr CString main = do putStrLn $ "siglist_a = " ++ show siglist_a putStrLn $ "siglist_b = " ++ show siglist_b putStrLn $ "siglist_main = " ++ show siglist_main peekSiglist "a " siglist_a peekSiglist "b " siglist_b peekSiglist "main" siglist_main peekSiglist name siglist = do ptr <- peekElemOff siglist 2 str <- maybePeek peekCString ptr putStrLn $ "siglist_" ++ name ++ "[2] = " ++ show str I would expect something like this output, where all pointer values identical and valid: $ runhaskell Main.hs siglist_a = 0x00007f53a948fe00 siglist_b = 0x00007f53a948fe00 siglist_main = 0x00007f53a948fe00 siglist_a [2] = Just "Interrupt" siglist_b [2] = Just "Interrupt" siglist_main[2] = Just "Interrupt" However, if I compile A.hs (with ghc -c A.hs), then the output changes to: $ runhaskell Main.hs siglist_a = 0x0000000040378918 siglist_b = 0x00007fe7c029ce00 siglist_main = 0x00007fe7c029ce00 siglist_a [2] = Nothing siglist_b [2] = Just "Interrupt" siglist_main[2] = Just "Interrupt"

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• Printing a field with additional dots in haskell

  - by Frank Kluyt

  I'm writing a function called printField. This function takes an int and a string as arguments and then then prints a field like this "Derp..." with this: printField 7 "Derp". When the field consists of digits the output should be "...3456". The function I wrote looks like this: printField :: Int -> String -> String printField x y = if isDigit y then concat(replicate n ".") ++ y else y ++ concat(replicate n ".") where n = x - length y This obviously isn't working. The error I get from GHC is: Couldn't match type `[Char]' with `Char' Expected type: Char Actual type: String In the first argument of `isDigit', namely `y' In the expression: isDigit y In the expression: if isDigit y then concat (replicate n ".") ++ y else y ++ concat (replicate n ".") I can't get it to work :(. Can anyone help me out? Please keep in mind that I'm new to Haskell and functional programming in general.

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• Project euler problem 3 in haskell

  - by shk

  I'm new in Haskell and try to solve 3 problem from http://projecteuler.net/. The prime factors of 13195 are 5, 7, 13 and 29. What is the largest prime factor of the number 600851475143 ? My solution: import Data.List getD :: Int -> Int getD x = -- find deviders let deriveList = filter (\y -> (x `mod` y) == 0) [1 .. x] filteredList = filter isSimpleNumber deriveList in maximum filteredList -- Check is nmber simple isSimpleNumber :: Int -> Bool isSimpleNumber x = let deriveList = map (\y -> (x `mod` y)) [1 .. x] filterLength = length ( filter (\z -> z == 0) deriveList) in case filterLength of 2 -> True _ -> False I try to run for example: getD 13195 > 29 But when i try: getD 600851475143 I get error Exception: Prelude.maximum: empty list Why? Thank you @Barry Brown, I think i must use: getD :: Integer -> Integer But i get error: Couldn't match expected type `Int' with actual type `Integer' Expected type: [Int] Actual type: [Integer] In the second argument of `filter', namely `deriveList' In the expression: filter isSimpleNumber deriveList Thank you.

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• Haskell quiz: a simple function

  - by levy

  I'm not a Haskell programmer, but I'm curious about the following questions. Informal function specification: Let MapProduct be a function that takes a function called F and multiple lists. It returns a list containing the results of calling F with one argument from each list in each possible combination. Example: Call MapProduct with F being a function that simply returns a list of its arguments, and two lists. One of the lists contains the integers 1 and 2, the other one contains the strings "a" and "b". It should return a list that contains the lists: 1 and "a", 1 and "b", 2 and "a", 2 and "b". Questions: How is MapProduct implemented? What is the function's type? What is F's type? Can one guess what the function does just by looking at its type? Can you handle inhomogeneous lists as input? (e.g. 1 and "a" in one of the input lists) What extra limitation (if any) do you need to introduce to implement MapProduct?

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• Types in Haskell

  - by Linda Cohen

  I'm kind of new in Haskell and I have difficulty understanding how inferred types and such works. map :: (a -> b) -> [a] -> [b] (.) :: (a -> b) -> (c -> a) -> c -> b What EXACTLY does that mean? foldr :: (a -> b -> b) -> b -> [a] -> b foldl :: (a -> b -> a) -> a -> [b] -> a foldl1 :: (a -> a -> a) -> [a] -> a What are the differences between these? And how would I define the inferred type of something like foldr map THANKS!

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• Body Mass Index program in haskell

  - by user364996

  Hi there. I'm trying to write a simple program in Haskell that can determine someone's body mass index. Here's what I have written: type Height = Float type Weight = Float type PeopleStats = [(String, Height, Weight)] and... bmi :: Height -> Weight -> Float bmi heightCm weightKg = weightKg/(heightCm)^2 healthy :: Height -> Weight -> Bool healthy heightCm weightKg | 25 > index && 18 < index = True | otherwise = False where index = bmi heightCm weightKg So far, the function "healthy" can calculate someone's BMI, and the function "healthyPeople" returns a boolean statement determining if the person's BMI falls within the limits which is considered normal for a healthy person. I want to write a function called "healthyPeople". healthyPeople :: PeopleStats -> [String] This function needs to take a list of PeopleStats and returns a list of names (Strings) of people who are deemed to be "healthy" from the "healthy" function. For example: If I input [("Lee", 65, 185), ("Wang", 170, 100), ("Tsu", 160, 120)] I will get a list of the names of the people whose BMI returns true form the boolean function in "healthy". Please help !!!!

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• "io: postinst-must-call-ldconfig" when creating a package

  - by egarcia

  I'm trying to create an ubuntu .deb package for the (pretty awesome) Io Language. I am not the developer of that language, so I'm not familiar with its sourcecode yet. This is my first attempt at creating a .deb file. In order to create the .deb, I'm following these instructions: http://www.webupd8.org/2010/01/how-to-create-deb-package-ubuntu-debian.html So far I've been able to create a .deb file (io_2010.06.01-1_amd64.deb) and a changes file (io_201.06.01-1_amd64.changes). I'm using lintian to check the changes file, and it reports an issue I don't know how to resolve: $ lintian -Ivi io_2010.06.01-1_amd64.changes ... (lots of messages) I: io: no-symbols-control-file usr/lib/libiovmall.so I: io: no-symbols-control-file usr/lib/libgarbagecollector.so I: io: no-symbols-control-file usr/lib/libbasekit.so E: io: postinst-must-call-ldconfig usr/lib/libiovmall.so N: N: The package installs shared libraries in a directory controlled by the N: dynamic library loader. Therefore, the package must call "ldconfig" in N: its postinst script. N: N: Refer to Debian Policy Manual section 8.1.1 (ldconfig) for details. N: N: Severity: serious, Certainty: certain N: N: Removing /tmp/OYuNShEHYz ... I've read the debian manual 8.8 section. I think I understand what the problem is (I need to make sure that ldconfig is invoked "somewhere", possibly on a place called "posinst") but I don't know how to resolve it (i.e. where this "posinsts" file is and how should I change it). The current way of installing Io in Ubuntu is basically running sudo make install and then sudo ldconfig. Maybe the makefile should be modified so ldconfig is called from it? I don't know. Thanks a lot.

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• Haskell Parsec Numeration

  - by Martin

  I'm using Text.ParserCombinators.Parsec and Text.XHtml to parse an input like this: - First type A\n -- First type B\n - Second type A\n -- First type B\n --Second type B\n And my output should be: <h11 First type A\n</h1 <h21.1 First type B\n</h2 <h12 Second type A\n</h2 <h22.1 First type B\n</h2 <h22.2 Second type B\n</h2 I have come to this part, but I cannot get any further: title1= do{ ;(count 1 (char '-')) ;s <- many1 anyChar newline ;return (h1 << s) } title2= do{ ;(count 2 (char '--')) ;s <- many1 anyChar newline ;return (h1 << s) } text=do { ;many (choice [try(title1),try(title2)]) } main :: IO () main = do t putStr "Error: " print err Right x - putStrLn $ prettyHtml x This is ok, but it does not include the numbering. Any ideas? Thanks!

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• Better data stream reading in Haskell

  - by Tim Perry

  I am trying to parse an input stream where the first line tells me how many lines of data there are. I'm ending up with the following code, and it works, but I think there is a better way. Is there? main = do numCases <- getLine proc $ read numCases proc :: Integer -> IO () proc numCases | numCases == 0 = return () | otherwise = do str <- getLine putStrLn $ findNextPalin str proc (numCases - 1) Note: The code solves the Sphere problem https://www.spoj.pl/problems/PALIN/ but I didn't think posting the rest of the code would impact the discussion of what to do here.

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