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  • [Haskell]Curious about the Hash Table problem

    - by astamatto
    I read that hash tables in haskell are crippled ( citation: http://flyingfrogblog.blogspot.com/2009/04/more-on-haskells-hash-table-problems.html ) and since i like haskell it worried me. Since the blog-post one year has passed and im curious, The hash table problem in haskell was "fixed" in the traditional compilers? (like ghc) ps: I love stack overflow, im a long time visitor but only today i decided to try to post a question.

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  • Beginning Haskell: "not in scope" Unprecedented error

    - by user1071838
    So I just started learning Haskell, and this (http://learnyouahaskell.com) nifty book is giving a lot of help. So yesterday I wrote in a text file doubleMe x = x + x and saved it as double.hs. So after saving that I open up my command prompt, CD to the right folder, type in "ghci" to get haskell started, and then type in >doubleMe 5 10 and everything seems to work. Now today, I do the same thing and this happens (actual copy paste from command line) . . . C:\Users\myName\haskell>ghci GHCi, version 7.0.3: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer-gmp ... linking ... done. Loading package base ... linking ... done. Loading package ffi-1.0 ... linking ... done. Prelude> :l double.hs [1 of 1] Compiling Main ( double.hs, interpreted ) Ok, modules loaded: Main. *Main> doubleMe 5 <interactive>:1:1: Not in scope: `doubleMe' So basically, everything was working fine, but now haskell can't find the function I wrote in double.hs. Can anyone tell what is going on? I'm pretty lost and confused. This is just a guess but does it have to do with *Main at all? Thanks for the help.

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  • Implementing simple business model in Haskell

    - by elmes
    Supose we have a very simple model: Station has at least one Train Train has at least two Stations The model has to allow to check what stations any given train visits and to check what trains visit a particular station. How to model it in Haskell? I am a Haskell newbie, so please correct me: once an object is created, you cannot modify it - you can only make a new object based on that one (~immutability). Am I right? If so, I'll have to create a lot of temporary variables with semi-initialized objects (during deserialization or even in unit tests). Basically what I need is an example of modeling domain classes in Haskell - after reading "Learn you a haskell.." I still have no idea how to use this language.

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  • Literate Haskell (.lhs) and Haddock

    - by finnsson
    At the moment I'm only using Haddock but after seeing some really interesting examples (e.g. this gist ) of literate Haskell I'm interested in trying it out in a project. The questions I got are: What do you write as Haddock comments and what do you write in the literate part? How do you scale literate programming to multiple files? Can anyone point me to an example where literate programming is used in a package with multiple modules? What is your experience of using literate programming in larger packages? Which flavour (markdown, latex, ...) of literate Haskell is preferred? Why are you programming in literate Haskell or plain vanilla Haskell? Are you programming in both styles and if so why? Do you prefer block-style (\begin{code}) or Bird-style (>)? Why?

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  • Using perfectly formatted input as list in Haskell

    - by RankWeis
    I'm doing a program in Haskell (on the Haskell platform), and I know I'm getting perfectly formatted inputs, so the input may look like [ ['a'], ['b'], ['c'] ] I want Haskell to be able to take this and use it as a list of it's own. And, I'd like this list to be over multiple lines, i.e., I want this to also work: [ ['a'], ['b'], ['c'] ] I can parse this input, but I've been told there's a way to do this easily - this is supposed to be the 'trivial' part of the assignment, but I don't understand it.

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  • What is the most compatible, widely used production language to export knowledge and skills gained from Haskell?

    - by World Engineer
    I like Haskell, plain and simple. While Haskell is used in production software, it's not especially widely deployed from what I've seen. What is the most similar and still widely used language in regards to production projects so that I might have a snowball's chance of using something similarly awesome in industry? Also is the same language from the first part available on large numbers of platforms? If not, what is the best alternative that has wide platform deployment? I'd like a single language to put on my to-do list rather than a massive swarm or family. Hard evidence would be a plus.

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  • Haskell math performance

    - by Travis Brown
    I'm in the middle of porting David Blei's original C implementation of Latent Dirichlet Allocation to Haskell, and I'm trying to decide whether to leave some of the low-level stuff in C. The following function is one example—it's an approximation of the second derivative of lgamma: double trigamma(double x) { double p; int i; x=x+6; p=1/(x*x); p=(((((0.075757575757576*p-0.033333333333333)*p+0.0238095238095238) *p-0.033333333333333)*p+0.166666666666667)*p+1)/x+0.5*p; for (i=0; i<6 ;i++) { x=x-1; p=1/(x*x)+p; } return(p); } I've translated this into more or less idiomatic Haskell as follows: trigamma :: Double -> Double trigamma x = snd $ last $ take 7 $ iterate next (x' - 1, p') where x' = x + 6 p = 1 / x' ^ 2 p' = p / 2 + c / x' c = foldr1 (\a b -> (a + b * p)) [1, 1/6, -1/30, 1/42, -1/30, 5/66] next (x, p) = (x - 1, 1 / x ^ 2 + p) The problem is that when I run both through Criterion, my Haskell version is six or seven times slower (I'm compiling with -O2 on GHC 6.12.1). Some similar functions are even worse. I know practically nothing about Haskell performance, and I'm not terribly interested in digging through Core or anything like that, since I can always just call the handful of math-intensive C functions through FFI. But I'm curious about whether there's low-hanging fruit that I'm missing—some kind of extension or library or annotation that I could use to speed up this numeric stuff without making it too ugly.

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  • Web development: Haskell or Scheme

    - by Robert E. Lester
    I would like to to choose one of these languages for building web applications. I'm not interested in framework per say, but have the following needs: Rapid development. Easy to scale. Strong community for the web. Quick and easy to deploy. I'm very familiar with Haskell, and have some familiarity with scheme (in particular PLT). Scheme appeals to me as good candidate for web development due to it's simple syntax which is homogenous across libraries. I state this despite my subjective opinion that Haskell is a 'cleaner' language. Haskell web apps seem to require learning and building a patchwork of different combinator libraries. On the plus side, I realise this can be quite expressive, although I'd prefer to eliminate impedance mismatches where possible. While scheme-plt looks to be a good fit, I can find but one example of it being used in the "real world". Haskell doesn't seem to fair too much better here, but there seems to be a bigger community behind the web side. Please help me make up my mind. For the most part I'm interested in real-world use cases.

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  • Learning Haskell maps, folds, loops and recursion

    - by Darknight
    I've only just dipped my toe in the world of Haskell as part of my journey of programming enlightenment (moving on from, procedural to OOP to concurrent to now functional). I've been trying an online Haskell Evaluator. However I'm now stuck on a problem: Create a simple function that gives the total sum of an array of numbers. In a procedural language this for me is easy enough (using recursion) (c#) : private int sum(ArrayList x, int i) { if (!(x.Count < i + 1)) { int t = 0; t = x.Item(i); t = sum(x, i + 1) + t; return t; } } All very fine however my failed attempt at Haskell was thus: let sum x = x+sum in map sum [1..10] this resulted in the following error (from that above mentioned website): Occurs check: cannot construct the infinite type: a = a -> t Please bear in mind I've only used Haskell for the last 30 minutes! I'm not looking simply for an answer but a more explanation of it. Thanks in advanced.

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  • Haskell lazy I/O and closing files

    - by Jesse
    I've written a small Haskell program to print the MD5 checksums of all files in the current directory (searched recursively). Basically a Haskell version of md5deep. All is fine and dandy except if the current directory has a very large number of files, in which case I get an error like: <program>: <currentFile>: openBinaryFile: resource exhausted (Too many open files) It seems Haskell's laziness is causing it not to close files, even after its corresponding line of output has been completed. The relevant code is below. The function of interest is getList. import qualified Data.ByteString.Lazy as BS main :: IO () main = putStr . unlines =<< getList "." getList :: FilePath -> IO [String] getList p = let getFileLine path = liftM (\c -> (hex $ hash $ BS.unpack c) ++ " " ++ path) (BS.readFile path) in mapM getFileLine =<< getRecursiveContents p hex :: [Word8] -> String hex = concatMap (\x -> printf "%0.2x" (toInteger x)) getRecursiveContents :: FilePath -> IO [FilePath] -- ^ Just gets the paths to all the files in the given directory. Are there any ideas on how I could solve this problem? The entire program is available here: http://haskell.pastebin.com/PAZm0Dcb

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  • Help! I'm a Haskell Newbie

    - by Darknight
    I've only just dipped my toe in the world of Haskell as part of my journey of programming enlightenment (moving on from, procedural to OOP to concurrent to now functional). I've been trying an online Haskell Evaluator. However I'm now stuck on a problem: Create a simple function that gives the total sum of an array of numbers. In a procedural language this for me is easy enough (using recursion) (c#) : private int sum(ArrayList x, int i) { if (!(x.Count < i + 1)) { int t = 0; t = x.Item(i); t = sum(x, i + 1) + t; return t; } } All very fine however my failed attempt at Haskell was thus: let sum x = x+sum in map sum [1..10] this resulted in the following error (from that above mentioned website): Occurs check: cannot construct the infinite type: a = a -> t Please bear in mind I've only used Haskell for the last 30 minutes! I'm not looking simply for an answer but a more explanation of it. Thanks in advanced.

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  • Haskell as REST server

    - by Dev er dev
    I would like to try Haskell on a smallish project which should be well suited to it. I would like to use it as a backend to a small ajax application. Haskell backend should be able to do authentication (basic, form, whatever, ...), keep track of user session (not much data there except for username) and to dispatch request to handlers based on uri and request type. It should also be able to serialize response to both xml and json format, depending on request parameter. I suppose the handlers are ideally suited for Haskell, since the service is basically stateless, but I don't know where to start for the rest of the story. Searching hackage didn't give me much hints.

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  • How do I do automatic data serialization of data objects in Haskell

    - by Adam Gent
    One of the huge benefits in languages that have some sort of reflection/introspecition is that objects can be automatically constructed from a variety of sources. For example in Java I can use the same objects for persisting to a db (with Hibernate) serializing to XML (with JAXB) or serializing to JSON (json-lib). You can do the same in Ruby and Python also usually following some simple rules for properties or annotations for Java. Thus I don't need lots "Domain Transfer Objects". I can concentrate on the domain I am working in. It seems in very strict FP like Haskell and Ocaml this is not possible. Particularly Haskell. The only thing I have seen is doing some sort of preprocessing or meta-programming (ocaml). Is it just accepted that you have to do all the transformations from the bottom upwards? In other words you have to do lot of boring work to turn a data type in haskell into JSON/XML/DB Row object and back again into a data object.

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  • How to learn Haskell

    - by anderstornvig
    For a few days I've tried to wrap my head around the functional programming paradigm in Haskell. I've done this by reading tutorials and watching screencasts, but nothing really seems to stick. Now, in learning various imperative/OO languages (like C, Java, PHP), excercises have been a good way for me to go. But since I don't really know what Haskell is capable of and because there are many new concepts to utilize, I haven't known where to start. So, how did you learn Haskell? What made you really "break the ice"? Also, any good ideas for beginning excercises?

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  • haskell sorting

    - by snorlaks
    Hello, How can it be done in most simply way to write (or maybe there is something embedded in haskell) function which takse as arguments list of tuples (String, Int) and Int x and return top x tuples as list according to x value. I wonder if its possible to write a function which also takes 3 argument which is the name of (or index) of filed in tuple according to which sorting has to be done. What are best solutions to make it quite generic, Im new to haskell I would do somethink like that in imperative languages without any problems but I want to know how to write it in quite good way in haskell, thanks for help

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  • Simulators for thread scheduling on multicore

    - by shijie xu
    I am seeking a simulator for thread scheduling at multi-core architecture, that is mapping threads to the cores at runtime. During runtime, simulator collects overall cache and IPC statistics. I checked below simulators, but seems there are not sufficient for me: Simplescalar: A simulator only for single core. SESC: multiprocessor simulator with detailed power, thermal, and performance models, QSim: provides instruction-level control of the emulated environment and detailed information about the executing instruction stream. It seems both SESC and QSim supports instructions scheduling instead of thread scheduling on the cores? Anyone can help provide some clues or share experience for this part?

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  • In Haskell, I want to read a file and then write to it. Do I need strictness annotation?

    - by Steve
    Hi, Still quite new to Haskell.. I want to read the contents of a file, do something with it possibly involving IO (using putStrLn for now) and then write new contents to the same file. I came up with: doit :: String -> IO () doit file = do contents <- withFile tagfile ReadMode $ \h -> hGetContents h putStrLn contents withFile tagfile WriteMode $ \h -> hPutStrLn h "new content" However this doesn't work due to laziness. The file contents are not printed. I found this post which explains it well. The solution proposed there is to include putStrLn within the withFile: doit :: String -> IO () doit file = do withFile tagfile ReadMode $ \h -> do contents <- hGetContents h putStrLn contents withFile tagfile WriteMode $ \h -> hPutStrLn h "new content" This works, but it's not what I want to do. The operation in I will eventually replace putStrLn might be long, I don't want to keep the file open the whole time. In general I just want to be able to get the file content out and then close it before working with that content. The solution I came up with is the following: doit :: String -> IO () doit file = do c <- newIORef "" withFile tagfile ReadMode $ \h -> do a <- hGetContents h writeIORef c $! a d <- readIORef c putStrLn d withFile tagfile WriteMode $ \h -> hPutStrLn h "Test" However, I find this long and a bit obfuscated. I don't think I should need an IORef just to get a value out, but I needed "place" to put the file contents. Also, it still didn't work without the strictness annotation $! for writeIORef. I guess IORefs are not strict by nature? Can anyone recommend a better, shorter way to do this while keeping my desired semantics? Thanks!

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  • Evaluation of Haskell Statements/Expressions using GHC API

    - by Cetin Sert
    For a tool I'm writing ( http://hackage.haskell.org/package/explore ) I need a way to read haskell function definitions at run-time, apply them to values from my tool and retrieve the results of their application. Can anyone give me a very basic example using GHC (6.10.4 or 6.12.1) API? example function definition to be read from a file at run-time: f x = 10**(((4/1102)*x)-1) expected program output --mapM_ print $ map f [428, 410, 389] 3.577165388142748 3.077536885227335 2.5821307011665815

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  • Haskell newbie on types

    - by garulfo
    I'm completely new to Haskell (and more generally to functional programming), so forgive me if this is really basic stuff. To get more than a taste, I try to implement in Haskell some algorithmic stuff I'm working on. I have a simple module Interval that implements intervals on the line. It contains the type data Interval t = Interval t t the helper function makeInterval :: (Ord t) => t -> t -> Interval t makeInterval l r | l <= r = Interval l r | otherwise = error "bad interval" and some utility functions about intervals. Here, my interest lies in multidimensional intervals (d-intervals), those objects that are composed of d intervals. I want to separately consider d-intervals that are the union of d disjoint intervals on the line (multiple interval) from those that are the union of d interval on d separate lines (track interval). With distinct algorithmic treatments in mind, I think it would be nice to have two distinct types (even if both are lists of intervals here) such as import qualified Interval as I -- Multilple interval newtype MInterval t = MInterval [I.Interval t] -- Track interval newtype TInterval t = TInterval [I.Interval t] to allow for distinct sanity checks, e.g. makeMInterval :: (Ord t) => [I.Interval t] -> MInterval t makeMInterval is = if foldr (&&) True [I.precedes i i' | (i, i') <- zip is (tail is)] then (MInterval is) else error "bad multiple interval" makeTInterval :: (Ord t) => [I.Interval t] -> TInterval t makeTInterval = TInterval I now get to the point, at last! But some functions are naturally concerned with both multiple intervals and track intervals. For example, a function order would return the number of intervals in a multiple interval or a track interval. What can I do? Adding -- Dimensional interval data DInterval t = MIntervalStuff (MInterval t) | TIntervalStuff (TInterval t) does not help much, since, if I understand well (correct me if I'm wrong), I would have to write order :: DInterval t -> Int order (MIntervalStuff (MInterval is)) = length is order (TIntervalStuff (TInterval is)) = length is and call order as order (MIntervalStuff is) or order (TIntervalStuff is) when is is a MInterval or a TInterval. Not that great, it looks odd. Neither I want to duplicate the function (I have many functions that are concerned with both multiple and track intevals, and some other d-interval definitions such as equal length multiple and track intervals). I'm left with the feeling that I'm completely wrong and have missed some important point about types in Haskell (and/or can't forget enough here about OO programming). So, quite a newbie question, what would be the best way in Haskell to deal with such a situation? Do I have to forget about introducing MInterval and TInterval and go with one type only? Thanks a lot for your help, Garulfo

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