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• Haskell - mapping the odd placed values and the even placed values differently

  - by Abstract

  Hey guys, is there an easy way. To take a list of numbers, say 123456. Then multiply the odd placed by three and the even placed by 1. i.e. (1 * 3) + (2 * 1) + (3 * 3) + (4*1) + (5*3) + (6*1) i was thinking the map function somewhere along the lines. But i don't know how to map *3 to just the odd placed values. Oh and if you could give me the version not in prelude that would be great like the actual function or functions, as if its being imported from an external haskell file Thanks for the help

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• simple putStrLn in Haskell/Yampa with arrows syntax

  - by sisif

  i'm using Haskell with the Yampa FRP library which uses the arrows language extension. how can i do a simple putStrLn in a SF? mySF = proc x -> do y <- identity -< x*x putStrLn "Hello World!" ++ show y returnA -< y the arrow syntax complains about the expression not bein an arrow (of course), but even with arrows i get no output output <- identity -< putStrLn "Hello World!"

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• Haskell: reading multiple command line arguments

  - by Survot

  Hi all, Okay, so I am making a program in Haskell that needs to change certain words based on two command line arguments. I have made the replace function and everything works great, but I am stumped getting it to work with command line arguments. Here is the main code: (replace function not included) main = do text <- getContents (command1:command2:_) <- getArgs putStrLn (replace (read command1) (read command2) text) So for intstance in the terminal I want to be able to type something like: "--- cat textfile.txt | ./replace oldword newword" I know this code is close since I have seen others do it this way. O_o Thanks for any help

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• list permutations in haskell

  - by turingcomplete

  So I'm new to haskell and I've been playing with it for a while now. I want to get my function that outputs all list permutations to work. I have written 2 implementations, one works well, the other is giving me an error. Any help would be awesome. This is the first (working) implementation: permute [] = [[]] permute xs = [y| x <- xs, y <- map (x:) $ permute $ delete x xs] This one is giving me an error: permute [] = [[]] permute xs = map (\x -> map (x:) $ permute $ delete x xs) xs and here's the error message: Occurs check: cannot construct the infinite type: t0 = [t0] Expected type: [t0] Actual type: [[t0]] In the expression: map (x :) $ permute $ delete x xs In the first argument of `map', namely `(\ x -> map (x :) $ permute $ delete x xs)' I'd appreciate if someone could explain why I'm getting this error. Thanks

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• Haskell UI framework?

  - by Lance May

  Is there, by chance, and emerging Haskell UI framework for Windows? I recently took up looking over the language, and from what I see, it would be great little "one-off" applications (elaborate scripts). However, without a good UI framework I can't see it getting in under the smoke and mirrors of the more obvious contenders. I've read that there are many frameworks, but none are full-featured. I'm just wondering if this is something that's on the rise, or is it simply too difficult to get enough developers going in the same direction with one?

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• problems with Haskell's Number Types

  - by mindeavor.

  I have the following haskell code: fac n = product [1..n] taylor3s w0 f f' f'' t h = w1 : taylor3s w1 f f' f'' (t+h) h where hp i = h^i / fac i w1 = w0 + (hp 1) * f t w0 + (hp 2) * f' t w0 + (hp 3) * f'' t w0 taylor_results = take 4 $ taylor3s 1 f f' f'' 1 0.25 where f t x = t^4 - 4*x/t f' t x = 4*t^3 - 4*(f t x)/t + 4*x/t^2 f'' t x = 12*t^2 - 4*(f' t x)/t + 8*(f t x)/t^2 - 8*x/t^3 taylor_results is supposed to be a use case of taylor3s. However, there is something wrong with the number type inferencing. When I try to compile, this is the error I get: practice.hs:93:26: Ambiguous type variable `a' in the constraints: `Integral a' arising from a use of `taylor3s' at practice.hs:93:26-51 `Fractional a' arising from a use of `f' at practice.hs:93:37 Possible cause: the monomorphism restriction applied to the following: taylor_results :: [a] (bound at practice.hs:93:0) Probable fix: give these definition(s) an explicit type signature or use -XNoMonomorphismRestriction Can someone help me with understanding what the problem is?

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• Haskell, list of natural number

  - by Hellnar

  Hello, I am an absolute newbie in Haskell yet trying to understand how it works. I want to write my own lazy list of integers such as [1,2,3,4,5...]. For list of ones I have written ones = 1 : ones and when tried, works fine: *Main> take 10 ones [1,1,1,1,1,1,1,1,1,1] How can I do the same for increasing integers ? I have tried this but it indeed fails: int = 1 : head[ int + 1] And after that how can I make a method that multiplies two streams? such as: mulstream s1 s2 = head[s1] * head[s2] : mulstream [tail s1] [tail s2]

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• dealing with IO vs pure code in haskell

  - by Drakosha

  I'm writing a shell script (my 1st non-example in haskell) which is supposed to list a directory, get every file size, do some string manipulation (pure code) and then rename some files. I'm not sure what i'm doing wrong, so 2 questions: How should i arrange the code in such program? I have a specific issue, i get the following error, what am i doing wrong? error: Couldn't match expected type [FilePath]' against inferred typeIO [FilePath]' In the second argument of mapM', namelyfileNames' In a stmt of a 'do' expression: files <- (mapM getFileNameAndSize fileNames) In the expression: do { fileNames <- getDirectoryContents; files <- (mapM getFileNameAndSize fileNames); sortBy cmpFilesBySize files } code: getFileNameAndSize fname = do (fname, (withFile fname ReadMode hFileSize)) getFilesWithSizes = do fileNames <- getDirectoryContents files <- (mapM getFileNameAndSize fileNames) sortBy cmpFilesBySize files

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• Haskell: foldl' accumulator parameter

  - by Clinton

  I've been asking a few questions about strictness, but I think I've missed the mark before. Hopefully this is more precise. Lets say we have: n = 1000000 f z = foldl' (\(x1, x2) y -> (x1 + y, y - x2)) z [1..n] Without changing f, what should I set z = ... So that f z does not overflow the stack? (i.e. runs in constant space regardless of the size of n) Its okay if the answer requires GHC extensions. My first thought is to define: g (a1, a2) = (!a1, !a2) and then z = g (0, 0) But I don't think g is valid Haskell.

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• Couldn't match expected type - Haskell Code

  - by wvyar

  I'm trying to learn Haskell, but the small bit of sample code I tried to write is running into a fairly large amount of "Couldn't match expected type" errors. Can anyone give me some guidance as to what I'm doing wrong/how I should go about this? These are the errors, but I'm not really sure how I should be writing my code. toDoSchedulerSimple.hs:6:14: Couldn't match expected type `[t0]' with actual type `IO String' In the return type of a call of `readFile' In a stmt of a 'do' block: f <- readFile inFile In the expression: do { f <- readFile inFile; lines f } toDoSchedulerSimple.hs:27:9: Couldn't match expected type `[a0]' with actual type `IO ()' In the return type of a call of `putStr' In a stmt of a 'do' block: putStr "Enter task name: " In the expression: do { putStr "Enter task name: "; task <- getLine; return inFileArray : task } toDoSchedulerSimple.hs:34:9: Couldn't match expected type `IO ()' with actual type `[a0]' In a stmt of a 'do' block: putStrLn "Your task is: " ++ (inFileArray !! i) In the expression: do { i <- randomRIO (0, (length inFileArray - 1)); putStrLn "Your task is: " ++ (inFileArray !! i) } In an equation for `getTask': getTask inFileArray = do { i <- randomRIO (0, (length inFileArray - 1)); putStrLn "Your task is: " ++ (inFileArray !! i) } toDoSchedulerSimple.hs:41:9: Couldn't match expected type `[a0]' with actual type `IO ()' In the return type of a call of `putStr' In a stmt of a 'do' block: putStr "Enter the task you would like to end: " In the expression: do { putStr "Enter the task you would like to end: "; task <- getLine; filter (endTaskCheck task) inFileArray } toDoSchedulerSimple.hs:60:53: Couldn't match expected type `IO ()' with actual type `[String] -> IO ()' In a stmt of a 'do' block: schedulerSimpleMain In the expression: do { (getTask inFileArray); schedulerSimpleMain } In a case alternative: "get-task" -> do { (getTask inFileArray); schedulerSimpleMain } This is the code itself. I think it's fairly straightforward, but the idea is to run a loop, take input, and perform actions based off of it by calling other functions. import System.Random (randomRIO) import Data.List (lines) initializeFile :: [char] -> [String] initializeFile inFile = do f <- readFile inFile let parsedFile = lines f return parsedFile displayHelp :: IO() displayHelp = do putStrLn "Welcome to To Do Scheduler Simple, written in Haskell." putStrLn "Here are some commands you might find useful:" putStrLn " 'help' : Display this menu." putStrLn " 'quit' : Exit the program." putStrLn " 'new-task' : Create a new task." putStrLn " 'get-task' : Randomly select a task." putStrLn " 'end-task' : Mark a task as finished." putStrLn " 'view-tasks' : View all of your tasks." quit :: IO() quit = do putStrLn "We're very sad to see you go...:(" putStrLn "Come back soon!" createTask :: [String] -> [String] createTask inFileArray = do putStr "Enter task name: " task <- getLine return inFileArray:task getTask :: [String] -> IO() getTask inFileArray = do i <- randomRIO (0, (length inFileArray - 1)) putStrLn "Your task is: " ++ (inFileArray !! i) endTaskCheck :: String -> String -> Bool endTaskCheck str1 str2 = str1 /= str2 endTask :: [String] -> [String] endTask inFileArray = do putStr "Enter the task you would like to end: " task <- getLine return filter (endTaskCheck task) inFileArray viewTasks :: [String] -> IO() viewTasks inFileArray = case inFileArray of [] -> do putStrLn "\nEnd of tasks." _ -> do putStrLn (head inFileArray) viewTasks (tail inFileArray) schedulerSimpleMain :: [String] -> IO() schedulerSimpleMain inFileArray = do putStr "SchedulerSimple> " input <- getLine case input of "help" -> displayHelp "quit" -> quit "new-task" -> schedulerSimpleMain (createTask inFileArray) "get-task" -> do (getTask inFileArray); schedulerSimpleMain "end-task" -> schedulerSimpleMain (endTask inFileArray) "view-tasks" -> do (viewTasks inFileArray); schedulerSimpleMain _ -> do putStrLn "Invalid input."; schedulerSimpleMain main :: IO() main = do putStr "What is the name of the schedule? " sName <- getLine schedulerSimpleMain (initializeFile sName) Thanks, and apologies if this isn't the correct place to be asking such a question.

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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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• 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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