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• Haskell -> After parsing how to work with strings

  - by bito08

  Hello after doing the parsing with a script in Haskell I got a file with the 'appearance' of lists of strings. However when I call the file content with the function getContents or hGetContents, ie, reading the contents I get something like: String with lines (schematically what I want is: "[" aaa "," bbb "" ccc "]" - ["aaa", "bbb" "ccc"]). I have tried with the read function but without results. I need to work with these lists of strings to concatenating them all in a single list. Thanks.

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• Haskell "Source reduction"

  - by Martin

  I'm revising for an upcoming Haskell exam and I don't understand one of the questions on a past paper. Google turns up nothing useful fst(x, y) = x square i = i * i i) Source reduce, using Haskells lazy evaluation, the expression: fst(square(3+4), square 8) ii) Source reduce, using strict evaluation, the same expression iii) State one advantage of lazy evaluation and one advantage of strict evaluation

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• Does F# have an equivalent to Haskell's take?

  - by McMuttons

  In Haskell, there is a function "take n list" which returns the first n elements from a list. For example "sum (take 3 xs)" sums up the first three elements in the list xs. Does F# have an equivalent? I expected it to be one of the List-functions, but I can't spot anything that seems to match.

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• Clojure vs Haskell for web applications?

  - by user43737

  I want to learn a functional language that will be good for building web applications in the future. I am choosing between Clojure and Haskell. Which one is a better choice for my purpose?

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• haskell check for user input errors

  - by snorlaks

  Hello. Im writing app in haskell and I would like to know that is the best way to check if users input is ok for example is it int when Im asking for int or if its date when asking for well formed date ? Thanks for help

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• pointers in haskell???

  - by curioComp

  hi, do you know if are there pointers in haskell? -If yes, how do you use them? Are there any problems with them? And why aren't they popular? -If no, is there any reason for it? Please help us!! :) Thank you so much!!

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• haskell list to tuple

  - by Alex

  How can I best convert a list to a tuple in Haskell: [1,2,3,4,5,6] -> (1,2,3,4,5,6)

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• adding other files to haskell

  - by snorlaks

  Hello, Im writing app in haskell and I would like to export some functions and datatypes to other files and then be able to use them in my main file. How to do this ? thanks for help

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• Look up or insert new element to string list in Haskell

  - by nightscream

  So I want to have a function that takes a String and a list as an argument, and checks if that element is already on the list, if it is, returns the same list, if it isnt, adds it to the list and returns it, 'im a begginer with haskell so heres what I have tried with no sucess: check:: String ->[String] ->[String] check x [] = []++[x] check x (y:xs) | x==y = (y:xs) | otherwise = check x xs Can someone point me the way ? thks

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• Haskell: monadic takeWhile?

  - by Mark Rushakoff

  I have some functions written in C that I call from Haskell. These functions return IO (CInt). Sometimes I want to run all of the functions regardless of what any of them return, and this is easy. For sake of example code, this is the general idea of what's happening currently: Prelude> let f x = print x >> return x Prelude> mapM_ f [0..5] 0 1 2 3 4 5 Prelude> I get my desired side effects, and I don't care about the results. But now I need to stop execution immediately after the first item that doesn't return my desired result. Let's say a return value of 4 or higher requires execution to stop - then what I want to do is this: Prelude> takeWhile (<4) $ mapM f [0..5] Which gives me this error: <interactive:1:22: Couldn't match expected type `[b]' against inferred type `IO a' In the first argument of `mapM', namely `f' In the second argument of `($)', namely `mapM f ([0 .. 5])' In the expression: takeWhile (< 4) $ mapM f ([0 .. 5]) And that makes sense to me - the result is still contained in the IO monad, and I can't just compare two values contained in the IO monad. I know this is precisely the purpose of monads -- chaining results together and discarding operations when a certain condition is met -- but is there an easy way to "wrap up" the IO monad in this case to stop executing the chain upon a condition of my choosing, without writing an instance of MonadPlus? Can I just "unlift" the values from f, for the purposes of the takeWhile? Is this a solution where functors fit? Functors haven't "clicked" with me yet, but I sort of have the impression that this might be a good situation to use them. Update: @sth has the closest answer to what I want - in fact, that's almost exactly what I was going for, but I'd still like to see whether there is a standard solution that isn't explicitly recursive -- this is Haskell, after all! Looking back on how I worded my question, now I can see that I wasn't clear enough about my desired behavior. The f function I used above for an example was merely an example. The real functions are written in C and used exclusively for their side effects. I can't use @Tom's suggestion of mapM_ f (takeWhile (&lt;4) [0..5]) because I have no idea whether any input will really result in success or failure until executed. I don't actually care about the returned list, either -- I just want to call the C functions until either the list is exhausted or the first C function returns a failure code. In C-style pseudocode, my behavior would be: do { result = function_with_side_effects(input_list[index++]); } while (result == success && index < max_index); So again, @sth's answer performs the exact behavior that I want, except that the results may (should?) be discarded. A dropWhileM_ function would be equivalent for my purposes. Why isn't there a function like that or takeWhileM_ in Control.Monad? I see that there was a similar discussion on a mailing list, but it appears that nothing has come of that.

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• Haskell Monad bind currying

  - by Chime

  I am currently in need of a bit of brain training and I found this article on Haskell and Monads I'm having trouble with exercise 7 re. Randomised function bind. To make the problem even simpler to experiment, I replaced the StdGen type with an unspecified type. So instead of... bind :: (a -> StdGen -> (b,StdGen)) -> (StdGen -> (a,StdGen)) -> (StdGen -> (b,StdGen)) I used... bind :: (a -> c -> (b,c)) -> (c -> (a,c)) -> (c -> (b,c)) and for the actual function impelemtation (just straight from the exercise) bind f x seed = let (x',seed') = x seed in f x' seed' and also 2 randomised functions to trial with: rndf1 :: (Num a, Num b) => a -> b -> (a,b) rndf1 a s = (a+1,s+1) rndf2 :: (Num a, Num b) => a -> b -> (a,b) rndf2 a s = (a+8,s+2) So with this in a Haskell compiler (ghci), I get... :t bind rndf2 bind rndf2 :: (Num a, Num c) => (c -> (a, c)) -> c -> (a, c) This matches the bind curried with rndf2 as the first parameter. But the thing I don't understand is how... :t bind rndf2 . rndf1 Suddenly gives bind rndf2 . rndf1 :: (Num a, Num c) => a -> c -> (a, c) This is the correct type of the composition that we are trying to produce because bind rndf2 . rndf1 Is a function that: takes the same parameter type(s) as rndf1 AND takes the return from rndf1 and pipes it as an input of rndf2 to return the same type as rndf2 rndf1 can take 2 parameters a -> c and rndf2 returns (a, c) so it matches that a composition of these function should have type: bind rndf2 . rndf1 :: (Num a, Num c) => a -> c -> (a, c) This does not match the naive type that I initially came up with for bind bind f :: (a -> b -> (c, d)) -> (c, d) -> (e, f) Here bind mythically takes a function that takes two parameters and produces a function that takes a tuple in order that the output from rndf1 can be fed into rndf2 why the bind function needs to be coded as it is Why the bind function does not have the naive type

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• How are lists implemented in Haskell (GHC)?

  - by eman

  I was just curious about some exact implementation details of lists in Haskell (GHC-specific answers are fine)--are they naive linked lists, or do they have any special optimizations? More specifically: Do length and (!!) (for instance) have to iterate through the list? If so, are their values cached in any way (i.e., if I call length twice, will it have to iterate both times)? Does access to the back of the list involve iterating through the whole list? Are infinite lists and list comprehensions memoized? (i.e., for fib = 1:1:zipWith (+) fib (tail fib), will each value be computed recursively, or will it rely on the previous computed value?) Any other interesting implementation details would be much appreciated. Thanks in advance!

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• Haskell Console IO in notepad++

  - by IVlad

  I've recently started to learn Haskell. I have this code module Main where import IO main = do hSetBuffering stdin LineBuffering putStrLn "Please enter your name: " name <- getLine putStrLn ("Hello, " ++ name ++ ", how are you?") I'm using the GHC compiler together with the notepad++ editor. The problem is the interaction goes like this: Process started Vlad Please enter your name: Hello, Vlad, how are you? <<< Process finished. As you can see, output is only written after I input something. This was a bit unexpected, as I was sure the program would first ask for my name, then I'd get to enter it and then it would say hello. Well, that's exactly what happens if I run the exe manually, yet not if I run it with notepad++ and use its console wrapper... How can I make notepad++ display the output when it should, and not all of it just before the program terminates? Is this even possible?

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• ByteStrings in Haskell

  - by Jon

  So i am trying to write a program that can read in a java class file as bytecode. For this i am using Data.Binary and Data.ByteStream. The problem i am having is because im pretty new to Haskell i am having trouble actually using these tools. module Main where import Data.Binary.Get import Data.Word import qualified Data.ByteString.Lazy as S getBinary :: Get Word8 getBinary = do a <- getWord8 return (a) main :: IO () main = do contents <- S.getContents print (getBinary contents) This is what i have come up with so far and i fear that its not really even on the right track. Although i know this question is very general i would appreciate some help with what i should be doing with the reading.

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• Haskell typeclass

  - by Geoff

  I have a Haskell typeclass question. I can't munge the syntax to get this (seemingly reasonable) program to compile under GHC. import Control.Concurrent.MVar blah1 :: [a] -> IO ([a]) blah1 = return blah2 :: [a] -> IO (MVar [a]) blah2 = newMVar class Blah b where blah :: [a] -> IO (b a) instance Blah [] where blah = blah1 -- BOOM instance Blah (MVar []) where blah = blah2 main :: IO () main = do putStrLn "Ok" I get the following error message, which kind of makes sense, but I don't know how to fix it: `[]' is not applied to enough type arguments Expected kind `*', but `[]' has kind `* -> *' In the type `MVar []' In the instance declaration for `Blah (MVar [])'

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• Haskell: type inference and function composition

  - by Pillsy

  This question was inspired by this answer to another question, indicating that you can remove every occurrence of an element from a list using a function defined as: removeall = filter . (/=) Working it out with pencil and paper from the types of filter, (/=) and (.), the function has a type of removeall :: (Eq a) => a -> [a] -> [a] which is exactly what you'd expect based on its contract. However, with GHCi 6.6, I get gchi> :t removeall removeall :: Integer -> [Integer] -> [Integer] unless I specify the type explicitly (in which case it works fine). Why is Haskell inferring such a specific type for the function?

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• Haskell simple compilation bug

  - by fmsf

  I'm trying to run this code: let coins = [50, 25, 10, 5, 2,1] let candidate = 11 calculate :: [Int] calculate = [ calculate (x+candidate) | x <- coins, x > candidate] I've read some tutorials, and it worked out ok. I'm trying to solve some small problems to give-me a feel of the language. But I'm stuck at this. test.hs:3:0: parse error (possibly incorrect indentation) Can anyone tell me why? I've started with haskell today so please go easy on the explanations. I've tried to run it like: runghc test.hs ghc test.hs but with: ghci < test.hs it gives this one: <interactive>:1:10: parse error on input `=' Thanks

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• Invoke Haskell function with heterogeneous arguments?

  - by thurn

  I'm currently working on a Haskell project which automatically tests some functions based on an XML specification. The XML specification gives the arguments to each function and the expected result that the function will provide (the arguments are of many different types). I know how to extract the function arguments from the XML and parse them using the read function, but I haven't figured out how to invoke the function using the arguments I get out. What I basically want is to read and store the arguments in a heterogeneous list (my current thinking is to use a list of type Data.Dynamic) and then invoke the function, passing this heterogeneous list as its argument list. Is this possible? Modifying the functions under test is not an option.

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• Haskell FlatMap

  - by mvid

  I am a beginner interested in Haskell, and I have been trying to implement the flatmap (=) on my own to better understand it. Currently I have flatmap :: (t -> a) -> [t] -> [a] flatmap _ [] = [] flatmap f (x:xs) = f x : flatmap f xs which implements the "map" part but not the "flat". Most of the modifications I make result in the disheartening and fairly informationless Occurs check: cannot construct the infinite type: a = [a] When generalising the type(s) for `flatmap' error. What am I missing?

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• Haskell IO Passes to Another Function

  - by peterwkc

  This question here is related to http://stackoverflow.com/questions/3066956/haskell-input-return-tuple I wonder how we can passes the input from monad IO to another function in order to do some computation. Actually what i want is something like -- First Example test = savefile investinput -- Second Example maxinvest :: a maxinvest = liftM maximuminvest maxinvestinput maxinvestinput :: IO() maxinvestinput = do str <- readFile "C:\\Invest.txt" let cont = words str let mytuple = converttuple cont let myint = getint mytuple putStrLn "" -- Convert to Tuple converttuple :: [String] -> [(String, Integer)] converttuple [] = [] converttuple (x:y:z) = (x, read y):converttuple z -- Get Integer getint :: [(String, Integer)] -> [Integer] getint [] = [] getint (x:xs) = snd (x) : getint xs -- Search Maximum Invest maximuminvest :: (Ord a) => [a] -> a maximuminvest [] = error "Empty Invest Amount List" maximuminvest [x] = x maximuminvest (x:xs) | x > maxTail = x | otherwise = maxTail where maxTail = maximuminvest xs In the second example, the maxinvestinput is read from file and convert the data to the type maximuminvest expected. Please help. Thanks.

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• Haskell Syntax: Parse Error On Input

  - by NuNu

  As part of a mini-haskell compiler that I'm writing, I have a function named app. What I want this function to do is take in these arguments epp (App e1 e2). The first step would be to evaluate e1 recursively (epp e1) and check if the output would be an error. If not then evaluate e2 and then call another function eppVals to evaluate the outputs of the calls on e1 and e2 which I defined as v1 and v2 respectively. epp (App e1 e2) | epp e1 /= Error = eppVals v1 v2 | otherwise = Error where v1 = epp e1 v2 = epp e2 <- parse error on input `=' Logically I believe what I have written so far works but I'm getting a parse error on input = where I stated above. Any idea why? My second try epp :: Exp -> Error Val epp (App e1 e2) = (eppVals v1 v2) where v1 = (epp e1) v2 = (epp e2) But now throws Couldn't match expected type Val with actual type Error Val

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• Improve a haskell script

  - by Hector Villalobos

  I'm a newbie in Haskell and I'd like some opinions about improving this script. This is a code generator and requires a command line argument to generate the sql script. ./GenCode "people name:string age:integer" Code: import Data.List import System.Environment (getArgs) create_table :: String -> String create_table str = "CREATE TABLE " ++ h (words str) where h (x:xs) = let cab = x final = xs in x ++ "( " ++ create_fields xs ++ ")" create_fields (x:xs) = takeWhile (/=':') x ++ type x ++ sig where sig | length xs > 0 = "," ++ create_fields xs | otherwise = " " ++ create_fields xs create_fields [] = "" type x | isInfixOf "string" x = " CHARACTER VARYING" | isInfixOf "integer" x = " INTEGER" | isInfixOf "date" x = " DATE" | isInfixOf "serial" x = " SERIAL" | otherwise = "" main = mainWith where mainWith = do args <- getArgs case args of [] -> putStrLn $ "You need one argument" (x:xs) -> putStrLn $ (create_table x)

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• Splitting lists inside list haskell

  - by user3713267

  Hi I need to split list by an argument in Haskell. I found function like this group :: Int -> [a] -> [[a]] group _ [] = [] group n l | n > 0 = (take n l) : (group n (drop n l)) | otherwise = error "Negative n" But what if lists that I want to divide are contained by another list? For example group 3 [[1,2,3,4,5,6],[2,4,6,8,10,12]] should return [[[1,2,3],[4,5,6]],[[2,4,6],[8,10,12]]] Is there any way to do that ?

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• Parallel Haskell in order to find the divisors of a huge number

  - by Dragno

  I have written the following program using Parallel Haskell to find the divisors of 1 billion. import Control.Parallel parfindDivisors :: Integer->[Integer] parfindDivisors n = f1 `par` (f2 `par` (f1 ++ f2)) where f1=filter g [1..(quot n 4)] f2=filter g [(quot n 4)+1..(quot n 2)] g z = n `rem` z == 0 main = print (parfindDivisors 1000000000) I've compiled the program with ghc -rtsopts -threaded findDivisors.hs and I run it with: findDivisors.exe +RTS -s -N2 -RTS I have found a 50% speedup compared to the simple version which is this: findDivisors :: Integer->[Integer] findDivisors n = filter g [1..(quot n 2)] where g z = n `rem` z == 0 My processor is a dual core 2 duo from Intel. I was wondering if there can be any improvement in above code. Because in the statistics that program prints says: Parallel GC work balance: 1.01 (16940708 / 16772868, ideal 2) and SPARKS: 2 (1 converted, 0 overflowed, 0 dud, 0 GC'd, 1 fizzled) What are these converted , overflowed , dud, GC'd, fizzled and how can help to improve the time.

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• How to extract terms of specific data constructor from a list in Haskell

  - by finnsson

  A common problem I got in Haskell is to extract all terms in a list belonging to a specific data constructor and I'm wondering if there are any better ways than the way I'm doing it at the moment. Let's say you got data Foo = Bar | Goo , the list foos = [Bar, Goo, Bar, Bar, Goo] and wish to extract all Goos from foos. At the moment I usually do something like goos = [Goo | Goo <- foos] and all is well. The problem is when Goo got a bunch of fields and I'm forced to write something like goos = [Goo a b c d e f | Goo a b c d e f <- foos] which is far from ideal. How you do usually handle this problem?

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