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  • Ur/Web new purely functional language for web programming?

    - by Phuc Nguyen
    I came across the Ur/Web project during my search for web frameworks for Haskell-like languages. It looks like a very interesting project done by one person. Basically, it is a domain-specific purely functional language for web programming, taking the best of ML and Haskell. The syntax is ML, but there are type classes and monad from Haskell, and it's strictly evaluated. Server-side is compiled to native code, client to Javascript. See the slides and FAQ page for other advertised advantages. Looking at the demos and their source code, I think the project is very promising. The latest version is something 20110123, so it seems to be under active development at this time. My question: Has anybody here had any further experience with it? Are there problems/annoyances compared to Haskell, apart from ML's slightly more verbose syntax? Even if it's not well known yet, I hope more people will know of it. OMG this looks very cool to me. I don't want this project to die!!

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  • OS choice for functional developing

    - by Carsten König
    I'm mainly a .NET developer so I normaly use Windows/VisualStudio (that means: I'm spoiled) but I'm enjoying Haskell and other (mostly functional) languagues in my spare time. Now for Haskell the windows-support is ok (you can get the Haskell-Platform) but latley I tried to get a basic Clojure/Scheme environment set up and it's just a pain on windows. So I'm thinking about trying out another OS for better tooling and languague support. Of course that leaves me with MacOS or some Linux distribution. I never used MacOS before and of course Linux would be cheaper (free) and I don't think I can parallel-boot MacOS on your normal PC-Hardware (can you?). PLUS: I don't have a clue about the tools you can use on those (to me) forign OSs. To make it short: I want to explore more Haskell, Clojure, Scala, Scheme and of course need at least good tooling for JavaScript/HTML5/Css. Support for .NET/Mono/F# would be great but for this I will still have my Win7 boot. So I like to know: - what is your prefered OS, Distribution (is Ubuntu viable?) - what Editor/IDE are you using Thank you for your help! PS: I'm not sure if this is the right place for this question but I surely hope so - if not please let me know where I should move this to (StackOverflow don't seem to be the right place IMHO)

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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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  • Type classes or implicit parameters? What do you prefer and why? [closed]

    - by Petr Pudlák
    I was playing a bit with Scalaz and I realized that Haskell's type classes are very similar to Scala's implicit parameters. While Haskell passes the methods defined by a type class using hidden dictionaries, Scala allows a similar thing using implicit parameters. For example, in Haskell, one could write: incInside :: (Functor f) => f Int -> f Int incInside = fmap (+ 1) and the same function using Scalaz: import scalaz._; import Scalaz._; def incInside[F[_]](x: F[Int])(implicit fn: Functor[F]): F[Int] = fn.fmap(x, (_:Int) + 1); I wonder: If you could choose (i.e. your favorite language would offer both), what would you pick - implicits or type classes? And what are your pros/cons?

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  • what is the purpose of arrows?

    - by Simon
    I am learning functionnal programming with Haskell, and I try to grab concepts by first understanding why do I need them. I would like to know the goal of arrows in functional programming languages. What problem do they solve? I checked http://en.wikibooks.org/wiki/Haskell/Understanding_arrows and http://www.cse.chalmers.se/~rjmh/afp-arrows.pdf. All I understand is that they are used to describe graphs for computations, and that they allow easier point free style coding. The article assume that point free style is generally easier to understand and to write. This seems quite subjective to me. In another article (http://en.wikibooks.org/wiki/Haskell/StephensArrowTutorial#Hangman:_Main_program), a hangman game is implemented, but I cannot see how arrows makes this implementation natural. I could find a lot of papers describing the concept, but nothing about the motivation. What I am missing?

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  • Installing EclipseFP on Mac OS X

    - by Dom Kennedy
    I am trying to install EclipseFP. I'm running OS X Mavericks. I've tried following both the official installation instructions and the advice in this answer on SU, but I'm still having the same problem. I can get the plugin itself installed painlessly using Help -> Install New Software..., Bbut when I restart and switch to the Haskell perspective, things start to go wrong. The installation instructions tells me that I should receive a prompt to install BuildWrapper and Scion Browser. I do not receive this prompt. Furthermore, if I create a new Haskell project, my code has no syntax highlighting, and the Hoogle search feature does not appear to do anything. It's clear that the plugin is not set up correctly yet. I've tried running cabal update in Terminal, but this does not change anything. After several attempts going round in circles with this on Eclipse Juno, I uninstalled Eclispe and the Haskell Platform and performed a clean install of Eclipse Luna and the latest Haskell Platform. However, the problems are persisting. I've tried going into Preferences to see if I could sort any of this out manually. I should initially point out that my GHC installation seems to be correctly references under Preferences -> Haskell Implementations Under Haskell -> Helper executables, there are areas for configuring the options of both BuildWrapper and Scion Browser. At present, both are blank. I tried clicking the Install from Hackage... button beside each of them with no success; I receive an error message saying Expected executable <workspace>/.metadata/.plugins/net.sf.eclipsefp.haskell.ui/sandbox/.cabal-sandbox/bin/buildwrapper not found!` (replace buildwrapper for scion-browser and the message is the same) The Eclipse console displays the following exception after doing the above with BuildWrapper: src/Language/Haskell/BuildWrapper/GHCStorage.hs:313:32: Not in scope: data constructor ‘MatchGroup’ cabal.real: Error: some packages failed to install: buildwrapper-0.7.4 failed during the building phase. The exception was: ExitFailure 1 and after doing it for Scion-Browser: zip-archive-0.2.3.4 (reinstall) changes: text-1.1.0.0 -> 0.11.3.1 pandoc-1.12.3.3 (latest: 1.13) -http-conduit (new version) Graphalyze-0.14.1.0 (reinstall) changes: pandoc-1.12.4.2 -> 1.12.3.3, text-1.1.0.0 -> 0.11.3.1 cabal.real: The following packages are likely to be broken by the reinstalls: pandoc-1.12.4.2 unordered-containers-0.2.4.0 aeson-0.7.0.4 scientific-0.2.0.2 case-insensitive-1.1.0.3 HTTP-4000.2.10 Use --force-reinstalls if you want to install anyway. After receiving similar results as the above on previous attempts, I've tried using force-reinstalls and ended up at more dead ends. I am at a loss as to what is wrong and how to solve this. I should point out that my GHC installation appears to be correctly configured under Preferences -> Haskell -> Haskell Implementations. Apologies if any of this information is irrelevant, I'm just not really sure what is important and what isn't at this point. Any help anyone could provide me with would be greatly appreciated.

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  • Why is it good not to rely on changing state?

    - by Slomojo
    This question arises out of the question Is Haskell worth learning? Generally a few often repeated statements are made, about how Haskell improves your coding skills in other languages, and furthermore, this is because Haskell is stateless, and that's a good thing. Why? I've seen someone compare this to only typing with the left hand, or perhaps closing your eyes for a day and just relying on touch. Surely there is more to it than that? Does it relate to hardware memory access, or something else which is a big performance gain?

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  • Are indivisible operations still indivisible on multiprocessor and multicore systems?

    - by Steve314
    As per the title, plus what are the limitations and gotchas. For example, on x86 processors, alignment for most data types is optional - an optimisation rather than a requirement. That means that a pointer may be stored at an unaligned address, which in turn means that pointer might be split over a cache page boundary. Obviously this could be done if you work hard enough on any processor (picking out particular bytes etc), but not in a way where you'd still expect the write operation to be indivisible. I seriously doubt that a multicore processor can ensure that other cores can guarantee a consistent all-before or all-after view of a written pointer in this unaligned-write-crossing-a-page-boundary situation. Am I right? And are there any similar gotchas I haven't thought of?

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  • Should I learn Haskell or F# if I already know OCaml?

    - by Unknown
    I am wondering if I should continue to learn OCaml or switch to F# or Haskell. Here are the criteria I am most interested in: Longevity Which language will last longer? I don't want to learn something that might be abandoned in a couple years by users and developers. Will Inria, Microsoft, University of Glasgow continue to support their respective compilers for the long run? Practicality Articles like this make me afraid to use Haskell. A hash table is the best structure for fast retrieval. Haskell proponents in there suggest using Data.Map which is a binary tree. I don't like being tied to a bulky .NET framework unless the benefits are large. I want to be able to develop more than just parsers and math programs. Well Designed I like my languages to be consistent. Please support your opinion with logical arguments and citations from articles. Thank you.

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  • Create My own language with "Functional Programming Language"

    - by esehara
    I prefer Haskell. I already know How to create my own language with Procedural Language (for example: C, Java, Python, etc). But, I know How to create my own language with Functional Language (for example Haskell, Clojure and Scala). I've already read: Internet Resources Write Yourself a Scheme in 48 Hours Real World Haskell - Chapter 16.Using Persec Writing A Lisp Interpreter In Haskell Parsec, a fast combinator parser Implementing functional languages: a tutorial Books Introduction Functional Programming Using Haskell 2nd Edition -- Haskell StackOverflow (but with procedural language) Learning to write a compiler create my own programming language Source Libraries and tools/HJS -- Haskell Are there any other good sources? I wants to get more links,or sources.

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  • How do I code a tree of objects in Haskell with pointers to parent and children?

    - by axilmar
    I've got the following problem: I have a tree of objects of different classes where an action in the child class invalidates the parent. In imperative languages, it is trivial to do. For example, in Java: public class A { private List<B> m_children = new LinkedList<B>(); private boolean m_valid = true; public void invalidate() { m_valid = false; } public void addChild(B child) { m_children.add(child); child.m_parent = this; } } public class B { public A m_parent = null; private int m_data = 0; public void setData(int data) { m_data = 0; m_parent.invalidate(); } } public class Main { public static void main(String[] args) { A a = new A(); B b = new B(); b.setData(0); //invalidates A } } How do I do the above in Haskell? I cannot wrap my mind around this, since once I construct an object in Haskell, it cannot be changed. I would be much obliged if the relevant Haskell code is posted.

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  • Misconceptions about purely functional languages?

    - by Giorgio
    I often encounter the following statements / arguments: Pure functional programming languages do not allow side effects (and are therefore of little use in practice because any useful program does have side effects, e.g. when it interacts with the external world). Pure functional programming languages do not allow to write a program that maintains state (which makes programming very awkward because in many application you do need state). I am not an expert in functional languages but here is what I have understood about these topics until now. Regarding point 1, you can interact with the environment in purely functional languages but you have to explicitly mark the code (functions) that introduces them (e.g. in Haskell by means of monadic types). Also, AFAIK computing by side effects (destructively updating data) should also be possible (using monadic types?) but is not the preferred way of working. Regarding point 2, AFAIK you can represent state by threading values through several computation steps (in Haskell, again, using monadic types) but I have no practical experience doing this and my understanding is rather vague. So, are the two statements above correct in any sense or are they just misconceptions about purely functional languages? If they are misconceptions, how did they come about? Could you write a (possibly small) code snippet illustrating the Haskell idiomatic way to (1) implement side effects and (2) implement a computation with state?

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  • What's the proper term for a function inverse to a constructor? Deconstructor, destructor, or something else?

    - by Petr Pudlák
    Edit: I'm rephrasing the question a bit. Apparently I caused some confusion because I didn't realize that the term destructor is used in OOP for something quite different - it's a function invoked when an object is being destroyed. In functional programming we (try to) avoid mutable state so there is no such equivalent to it. (I added the proper tag to the question.) Instead, I've seen that the record field for unwrapping a value (especially for single-valued data types such as newtypes) is sometimes called destructor or perhaps deconstructor. For example, let's have (in Haskell): newtype Wrap = Wrap { unwrap :: Int } Here Wrap is the constructor and unwrap is what? I've seen both, for example: ... Most often, one supplies smart constructors and destructors for these to ease working with them. ... at Haskell wiki, or ... The general theme here is to fuse constructor - deconstructor pairs like ... at Haskell wikibook (here it's probably meant in a bit more general sense). The questions are: How do we call unwrap in functional programming? Deconstructor? Destructor? Or by some other term? And to clarify, is this terminology applicable to other functional languages, or is it used just in the Has

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  • F# vs Haskell vs Lisp - which language to learn?

    - by empi
    I've heard a lot about functional programming languages and I'm willing to learn one. I guess it will be mostly for fun, however, I hope it will improve my programming skills. I have mostly C#/.NET background, so my first choice is to learn F# (because of .NET and familiarity with Visual Studio). On the on other hand, I wonder if F# has features like Lisp macros or Haskell higher order functions. Could you compare F#, Haskell and Lisp? Which one will be the language of your choice?

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  • Finding the shortest path between two points on a grid, using Haskell.

    - by esperantist
    This is a problem that I can easily enough solve in a non-functional manner. But solving it in Haskell is giving me big problems. Me being inexperienced when it comes to functional programming is surely a reason. The problem: I have a 2D field divided into rectangles of equal size. A simple grid. Some rectangles are empty space (and can be passed through) while others are impassable. Given a starting rectangle A and a destination rectangle B, how would I calculate the shortest path between the two? Movement is possible only vertically and horizontally, in steps a single rectangle large. How would I go about accomplishing this in Haskell? Code snippets certainly welcome, but also certainly not neccessary. And links to further resources also very welcome! Thanks!

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  • Are there tools that would be suitable for maintaining a changelog for a Cabal Haskell package?

    - by Norman Ramsey
    I'm working fast and furiously on a new Haskell package for compiler writers. I'm going through many minor version numbers daily, and the Haskell packaging system, Cabal, doesn't seem to offer any tools for updating version numbers or for maintaining a change log. (Logs are going into git but that's not visible to anyone using the package.) I would kill for something equivalent to Debian's uupdate or dch/debchange tools. Does anyone know of general-purpose tools that could be used to increment version numbers automatically and add an entry to a change log?

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  • EE Major : Should I learn Ruby on Rails or Haskell?

    - by Vivek
    Hi,I've just completed my freshman year in college and am majoring in EE (with a lot of interest in CS as well) . I know some Python,C/C++ and Java and also a little bit of Actionscript . I am planning to learn either Haskell or Ruby on Rails. Haskell because it is a functional programming language, and I've been really impressed by this paradigm and Ruby on Rails , as I don't know any 'web' programming language and have heard that you can develop apps in RoR very easily and quickly . Which one should I learn ? and please suggest some links / books for starting off .

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  • How do Scala parser combinators compare to Haskell's Parsec?

    - by artif
    I have read that Haskell parser combinators (in Parsec) can parse context sensitive grammars. Is this also true for Scala parser combinators? If so, is this what the "into" (aka "") function is for? What are some strengths/weaknesses of Scala's implementation of parser combinators, vs Haskell's? Do they accept the same class of grammars? Is it easier to generate error messages or do other miscellaneous useful things with one or the other? How does packrat parsing (introduced in Scala 2.8) fit into this picture? Is there a webpage or some other resource that shows how different operators/functions/DSL-sugar from one language's implementation maps onto the other's?

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  • How do I get the sums of the digits of a large number in Haskell?

    - by Tim
    I'm a C++ Programmer trying to teach myself Haskell and it's proving to be challenging grasping the basics of using functions as a type of loop. I have a large number, 50!, and I need to add the sum of its digits. It's a relatively easy loop in C++ but I want to learn how to do it in Haskell. I've read some introductory guides and am able to get 50! with sum50fac.hs:: fac 0 = 1 fac n = n * fac n - 1 x = fac 50 main = print x Unfortunately at this point I'm not entirely sure how to approach the problem. Is it possible to write a function that adds (mod) x 10 to a value and then calls the same function again on x / 10 until x / 10 is less than 10? If that's not possible how should I approach this problem? Thanks!

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