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  • Image drawing library for Haskell?

    - by absz
    I'm working on a Haskell program for playing spatial games: I have a graph of a bunch of "individuals" playing the Prisoner's Dilemma, but only with their immediate neighbors, and copying the strategies of the people who do best. I've reached a point where I need to draw an image of the world, and this is where I've hit problems. Two of the possible geometries are easy: if people have four or eight neighbors each, then I represent each one as a filled square (with color corresponding to strategy) and tile the plane with these. However, I also have a situation where people have six neighbors (hexagons) or three neighbors (triangles). My question, then, is: what's a good Haskell library for creating images and drawing shapes on them? I'd prefer that it create PNGs, but I'm not incredibly picky. I was originally using Graphics.GD, but it only exports bindings to functions for drawing points, lines, arcs, ellipses, and non-rotated rectangles, which is not sufficient for my purposes (unless I want to draw hexagons pixel by pixel*). I looked into using foreign import, but it's proving a bit of a hassle (partly because the polygon-drawing function requires an array of gdPoint structs), and given that my requirements may grow, it would be nice to use an in-Haskell solution and not have to muck about with the FFI (though if push comes to shove, I'm willing to do that). Any suggestions? * That is also an option, actually; any tips on how to do that would also be appreciated, though I think a library would be easier.

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  • Haskell Cons Operator (:)

    - by Carson Myers
    I am really new to Haskell (Actually I saw "Real World Haskell" from O'Reilly and thought "hmm, I think I'll learn functional programming" yesterday) and I am wondering: I can use the construct operator to add an item to the beginning of a list: 1 : [2,3] [1,2,3] I tried making an example data type I found in the book and then playing with it: --in a file data BillingInfo = CreditCard Int String String | CashOnDelivery | Invoice Int deriving (Show) --in ghci $ let order_list = [Invoice 2345] $ order_list [Invoice 2345] $ let order_list = CashOnDelivery : order_list $ order_list [CashOnDelivery, CashOnDelivery, CashOnDelivery, CashOnDelivery, CashOnDelivery, CashOnDelivery, CashOnDelivery, CashOnDelivery, CashOnDelivery, CashOnDelivery, CashOnDelivery, CashOnDelivery, CashOnDelivery, CashOnDelivery, ...- etc... it just repeats forever, is this because it uses lazy evaluation? -- EDIT -- okay, so it is being pounded into my head that let order_list = CashOnDelivery:order_list doesn't add CashOnDelivery to the original order_list and then set the result to order_list, but instead is recursive and creates an infinite list, forever adding CashOnDelivery to the beginning of itself. Of course now I remember that Haskell is a functional language and I can't change the value of the original order_list, so what should I do for a simple "tack this on to the end (or beginning, whatever) of this list?" Make a function which takes a list and BillingInfo as arguments, and then return a list? -- EDIT 2 -- well, based on all the answers I'm getting and the lack of being able to pass an object by reference and mutate variables (such as I'm used to)... I think that I have just asked this question prematurely and that I really need to delve further into the functional paradigm before I can expect to really understand the answers to my questions... I guess what i was looking for was how to write a function or something, taking a list and an item, and returning a list under the same name so the function could be called more than once, without changing the name every time (as if it was actually a program which would add actual orders to an order list, and the user wouldn't have to think of a new name for the list each time, but rather append an item to the same list).

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  • SOAP web services in haskell?

    - by Dave
    I have to write a bunch of small web services. They must be defined by a WSDL and work via SOAP-RPC, in order to work with an existing workflow engine and service registry framework. I can, however, serve them on a service stack/platform of my choice. I'm presently writing them in Java, and it's not too bad. But I'm thinking my life might be easier if I was able to write these services in Haskell. Searching on Google, it looks like, once upon a time, someone else had the same idea and started a project called "HAIFA". However, it looks like HAIFA hasn't been maintained for some years, and I couldn't find any other frameworks supporting serving up services written in Haskell as SOAP web services. Does anyone know of any other frameworks that will allow me to easily write SOAP-based web services using Haskell? If not, has anyone done this manually (i.e., use XML libraries from hackage to process the incoming soap-rpc requests, and create soap-rpc compliant replies)? Was it difficult to do? Any gotchas? Was it worth the effort? Thanks in advance!

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  • Haskell: Dealing With Types And Exceptions

    - by Douglas Brunner
    I'd like to know the "Haskell way" to catch and handle exceptions. As shown below, I understand the basic syntax, but I'm not sure how to deal with the type system in this situation. The below code attempts to return the value of the requested environment variable. Obviously if that variable isn't there I want to catch the exception and return Nothing. getEnvVar x = do { var <- getEnv x; Just var; } `catch` \ex -> do { Nothing } Here is the error: Couldn't match expected type `IO a' against inferred type `Maybe String' In the expression: Just var In the first argument of `catch', namely `do { var <- getEnv x; Just var }' In the expression: do { var <- getEnv x; Just var } `catch` \ ex -> do { Nothing } I could return string values: getRequestURI x = do { requestURI <- getEnv x; return requestURI; } `catch` \ex -> do { return "" } however, this doesn't feel like the Haskell way. What is the Haskell way?

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  • TFS API-Process Template currently applied to the Team Project

    - by Tarun Arora
    Download Demo Solution - here In this blog post I’ll show you how to use the TFS API to get the name of the Process Template that is currently applied to the Team Project. You can also download the demo solution attached, I’ve tested this solution against TFS 2010 and TFS 2011.    1. Connecting to TFS Programmatically I have a blog post that shows you from where to download the VS 2010 SP1 SDK and how to connect to TFS programmatically. private TfsTeamProjectCollection _tfs; private string _selectedTeamProject;   TeamProjectPicker tfsPP = new TeamProjectPicker(TeamProjectPickerMode.SingleProject, false); tfsPP.ShowDialog(); this._tfs = tfsPP.SelectedTeamProjectCollection; this._selectedTeamProject = tfsPP.SelectedProjects[0].Name; 2. Programmatically get the Process Template details of the selected Team Project I’ll be making use of the VersionControlServer service to get the Team Project details and the ICommonStructureService to get the Project Properties. private ProjectProperty[] GetProcessTemplateDetailsForTheSelectedProject() { var vcs = _tfs.GetService<VersionControlServer>(); var ics = _tfs.GetService<ICommonStructureService>(); ProjectProperty[] ProjectProperties = null; var p = vcs.GetTeamProject(_selectedTeamProject); string ProjectName = string.Empty; string ProjectState = String.Empty; int templateId = 0; ProjectProperties = null; ics.GetProjectProperties(p.ArtifactUri.AbsoluteUri, out ProjectName, out ProjectState, out templateId, out ProjectProperties); return ProjectProperties; } 3. What’s the catch? The ProjectProperties will contain a property “Process Template” which as a value has the name of the process template. So, you will be able to use the below line of code to get the name of the process template. var processTemplateName = processTemplateDetails.Where(pt => pt.Name == "Process Template").Select(pt => pt.Value).FirstOrDefault();   However, if the process template does not contain the property “Process Template” then you will need to add it. So, the question becomes how do i add the Name property to the Process Template. Download the Process Template from the Process Template Manager on your local        Once you have downloaded the Process Template to your local machine, navigate to the Classification folder with in the template       From the classification folder open Classification.xml        Add a new property <property name=”Process Template” value=”MSF for CMMI Process Improvement v5.0” />           4. Putting it all together… using System; using System.Collections.Generic; using System.ComponentModel; using System.Data; using System.Drawing; using System.Linq; using System.Text; using System.Windows.Forms; using Microsoft.TeamFoundation.Client; using Microsoft.TeamFoundation.VersionControl.Client; using Microsoft.TeamFoundation.Server; using System.Diagnostics; using Microsoft.TeamFoundation.WorkItemTracking.Client; namespace TfsAPIDemoProcessTemplate { public partial class Form1 : Form { public Form1() { InitializeComponent(); } private TfsTeamProjectCollection _tfs; private string _selectedTeamProject; private void btnConnect_Click(object sender, EventArgs e) { TeamProjectPicker tfsPP = new TeamProjectPicker(TeamProjectPickerMode.SingleProject, false); tfsPP.ShowDialog(); this._tfs = tfsPP.SelectedTeamProjectCollection; this._selectedTeamProject = tfsPP.SelectedProjects[0].Name; var processTemplateDetails = GetProcessTemplateDetailsForTheSelectedProject(); listBox1.Items.Clear(); listBox1.Items.Add(String.Format("Team Project Selected => '{0}'", _selectedTeamProject)); listBox1.Items.Add(Environment.NewLine); var processTemplateName = processTemplateDetails.Where(pt => pt.Name == "Process Template") .Select(pt => pt.Value).FirstOrDefault(); if (!string.IsNullOrEmpty(processTemplateName)) { listBox1.Items.Add(Environment.NewLine); listBox1.Items.Add(String.Format("Process Template Name: {0}", processTemplateName)); } else { listBox1.Items.Add(String.Format("The Process Template does not have the 'Name' property set up")); listBox1.Items.Add(String.Format("***TIP: Download the Process Template and in Classification.xml add a new property Name, update the template then you will be able to see the Process Template Name***")); listBox1.Items.Add(String.Format(" - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -")); } } private ProjectProperty[] GetProcessTemplateDetailsForTheSelectedProject() { var vcs = _tfs.GetService<VersionControlServer>(); var ics = _tfs.GetService<ICommonStructureService>(); ProjectProperty[] ProjectProperties = null; var p = vcs.GetTeamProject(_selectedTeamProject); string ProjectName = string.Empty; string ProjectState = String.Empty; int templateId = 0; ProjectProperties = null; ics.GetProjectProperties(p.ArtifactUri.AbsoluteUri, out ProjectName, out ProjectState, out templateId, out ProjectProperties); return ProjectProperties; } } } Thank you for taking the time out and reading this blog post. If you enjoyed the post, remember to subscribe to http://feeds.feedburner.com/TarunArora. Have you come across a better way of doing this, please share your experience here. Questions/Feedback/Suggestions, etc please leave a comment. Thank You! Share this post : CodeProject

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  • Types in Lisp and Scheme

    - by user2054900
    I see now that Racket has types. At first glance it seems to be almost identical to Haskell typing. But is Lisp's CLOS covering some of the space Haskell types cover? Creating a very strict Haskell type and an object in any OO language seems vaguely similar. It's just that I've drunk some of the Haskell kool-aid and I'm totally paranoid that if I go down the Lisp road, I'll be screwed due to dynamic typing.

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  • How to use GD binding in Haskell?

    - by milosz
    I've installed the GD package (http://hackage.haskell.org/package/gd-3000.4.0) using cabal. When I try to compile the program which uses GD I receive an error: [mp262554@students:~/jpp/haskell]$ ghc gd.hs compilation IS NOT required gd.o: In function `sV8_info': (.text+0x1bf): undefined reference to `gdzm3000zi4zi0_GraphicsziGD_saveJpegFile_closure' gd.o: In function `sUY_info': (.text+0x3be): undefined reference to `gdzm3000zi4zi0_GraphicsziGD_drawLine_closure' gd.o: In function `sUI_info': (.text+0x51a): undefined reference to `gdzm3000zi4zi0_GraphicsziGD_newImage_closure' gd.o: In function `sUI_info': (.text+0x6a5): undefined reference to `__stginit_gdzm3000zi4zi0_GraphicsziGD_' gd.o: In function `sVc_srt': (.data+0x34): undefined reference to `gdzm3000zi4zi0_GraphicsziGD_drawLine_closure' gd.o: In function `sVc_srt': (.data+0x38): undefined reference to `gdzm3000zi4zi0_GraphicsziGD_saveJpegFile_closure' gd.o: In function `sUI_srt': (.data+0x4c): undefined reference to `gdzm3000zi4zi0_GraphicsziGD_newImage_closure' collect2: ld returned 1 exit status

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  • Haskell Convert Byte String To UTC Time

    - by Steve
    I have been trying to make a function in Haskell to take a ByteString which is a datetime and convert it to UTC time taking into account the time zone from the original encoding. I am very new to Haskell so I may be making a really basic mistake. convertStringToUtc s = do estTimeZone <- hoursToTimeZone -5 time <- read $ B.unpack(s) localTimeToUTC estTimeZone time The error I get is: Couldn't match expected type `Int -> b' against inferred type `UTCTime' In the expression: localTimeToUTC estTimeZone time In the expression: do { estTimeZone <- hoursToTimeZone - 5; time <- read $ B.unpack (s); localTimeToUTC estTimeZone time }

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  • Haskell Lazy Evaluation and Reuse

    - by Jonathan Sternberg
    I know that if I were to compute a list of squares in Haskell, I could do this: squares = [ x ** 2 | x <- [1 ..] ] Then when I call squares like this: print $ take 4 squares And it would print out [1.0, 4.0, 9.0, 16.0]. This gets evaluated as [ 1 ** 2, 2 ** 2, 3 ** 2, 4 ** 2 ]. Now since Haskell is functional and the result would be the same each time, if I were to call squares again somewhere else, would it re-evaluate the answers it's already computed? If I were to re-use squares after I had already called the previous line, would it re-calculate the first 4 values? print $ take 5 squares Would it evaluate [1.0, 4.0, 9.0, 16.0, 5 ** 2]?

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  • Haskell: how to get through 'no instance for' ?

    - by artemave
    I am learning Haskell. I am on the 8th chapter of this book. The main thing I've learned so far is that Haskell is very unfriendly to me and it bites my ass where possible. Moreover... Heck! Enough mourning, to business. Here is the code: module GlobRegex ( globToRegex, matchesGlob ) where import Text.Regex.Posix import Text.Regex.Posix.String import Text.Regex.Base.RegexLike data CaseOpt = Case | NoCase deriving (Eq) matchesGlob :: String -> String -> CaseOpt -> Bool matchesGlob name pat caseopt = match regex name where regex = case caseopt of NoCase -> makeRegexOpts (defaultCompOpt + compIgnoreCase) defaultExecOpt (globToRegex pat) Case -> makeRegex (globToRegex pat) globToRegex :: String -> String ... And here is how it fails to compile: Prelude Text.Regex.Posix Text.Regex.Base.RegexLike> :load globtoregex\GlobRegex. hs [1 of 1] Compiling GlobRegex ( globtoregex\GlobRegex.hs, interpreted ) globtoregex\GlobRegex.hs:14:31: No instance for (RegexLike regex [Char]) arising from a use of `match' at globtoregex\GlobRegex.hs:14:31-46 Possible fix: add an instance declaration for (RegexLike regex [Char]) In the expression: match regex name In the definition of `matchesGlob': matchesGlob name pat caseopt = match regex name where regex = case caseopt of { NoCase -> makeRegexOpts (defaultCompOpt + compIgnoreCase) defaultExecOpt (globToRegex pat) Case -> makeRegex (globToRegex pat) } globtoregex\GlobRegex.hs:17:23: No instance for (RegexMaker regex CompOption execOpt String) arising from a use of `makeRegex' at globtoregex\GlobRegex.hs:17:23-49 Possible fix: add an instance declaration for (RegexMaker regex CompOption execOpt String) In the expression: makeRegex (globToRegex pat) In a case alternative: Case -> makeRegex (globToRegex pat) In the expression: case caseopt of { NoCase -> makeRegexOpts (defaultCompOpt + compIgnoreCase) defaultExecOpt (globToRegex p at) Case -> makeRegex (globToRegex pat) } Failed, modules loaded: none. To my best understanding, Text.Regex.Posix.String provides instances for RegexLike Regex String and RegexMaker Regex CompOption ExecOption String, so it should work. On the other hand, I can see that regex in the error message is type variable, not a concrete type, so, perhaps not... Anyway, this is where I am stuck. May be there is a common pattern for resolving no instance for type of problems? Or, in Haskell terms, instance of SmartGuess typeclass for no instance for?

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  • Stack overflow in OCaml and F# but not in Haskell

    - by Fernand Pajot
    I've been comparing for fun different languages for speed in execution of the following program: for i from 1 to 1000000 sum the product i*(sqrt i) One of my implementations (not the only one) is constructing a list [1..1000000] and then folding with a specific funtion. The program works fine and fast in Haskell (even when using foldl and not foldl') but stack overflows in OCaml and F#. Here is the Haskell code: test = foldl (\ a b -> a + b * (sqrt b)) 0 create 0 = [] create n = n:(create (n-1)) main = print (test (create 1000000)) And here is the OCaml one: let test = List.fold_left (fun a b -> a +. (float_of_int b) *. (sqrt (float_of_int b))) 0. ;; let rec create = function | 0 -> [] | n -> n::(create (n-1)) ;; print_float (test (create 1000000));; Why does the OCaml/F# implementation stack overflows?

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  • CPP extension and multiline literals in Haskell

    - by jetxee
    Is it possible to use CPP extension on Haskell code which contains multiline string literals? Are there other conditional compilation techniques for Haskell? For example, let's take this code: -- If the next line is uncommented, the program does not compile. -- {-# LANGUAGE CPP #-} msg = "Hello\ \ Wor\ \ld!" main = putStrLn msg If I uncomment {-# LANGUAGE CPP #-}, then GHC refutes this code with a lexical error: [1 of 1] Compiling Main ( cpp-multiline.hs, cpp-multiline.o ) cpp-multiline.hs:4:17: lexical error in string/character literal at character 'o' Using GHC 6.12.1, cpphs is available. I confirm that using cpphs.compat wrapper and -pgmP cpphs.compat option helps, but I'd like to have a solution which does not depend on custom shell scripts. -pgmP cpphs does not work. P.S. I need to use different code for GHC < 6.12 and GHC = 6.12, is it possible without preprocessor?

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  • C++: Declaration of template class member specialization

    - by Ziv
    When I specialize a (static) member function/constant in a template class, I'm confused as to where the declaration is meant to go. Here's an example of what I what to do - yoinked directly from IBM's reference on template specialization: ===IBM Member Specialization Example=== template<class T> class X { public: static T v; static void f(T); }; template<class T> T X<T>::v = 0; template<class T> void X<T>::f(T arg) { v = arg; } template<> char* X<char*>::v = "Hello"; template<> void X<float>::f(float arg) { v = arg * 2; } int main() { X<char*> a, b; X<float> c; c.f(10); // X<float>::v now set to 20 } The question is, how do I divide this into header/cpp files? The generic implementation is obviously in the header, but what about the specialization? It can't go in the header file, because it's concrete, leading to multiple definition. But if it goes into the .cpp file, is code which calls X::f() aware of the specialization, or might it rely on the generic X::f()? So far I've got the specialization in the .cpp only, with no declaration in the header. I'm not having trouble compiling or even running my code (on gcc, don't remember the version at the moment), and it behaves as expected - recognizing the specialization. But A) I'm not sure this is correct, and I'd like to know what is, and B) my Doxygen documentation comes out wonky and very misleading (more on that in a moment a later question). What seems most natural to me would be something like this, declaring the specialization in the header and defining it in the .cpp: ===XClass.hpp=== #ifndef XCLASS_HPP #define XCLASS_HPP template<class T> class X { public: static T v; static void f(T); }; template<class T> T X<T>::v = 0; template<class T> void X<T>::f(T arg) { v = arg; } /* declaration of specialized functions */ template<> char* X<char*>::v; template<> void X<float>::f(float arg); #endif ===XClass.cpp=== #include <XClass.hpp> /* concrete implementation of specialized functions */ template<> char* X<char*>::v = "Hello"; template<> void X<float>::f(float arg) { v = arg * 2; } ...but I have no idea if this is correct. Any ideas? Thanks much, Ziv

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  • die $template->error() produces no line number

    - by Kinopiko
    In the following short program: use Template; my $template = Template->new (INCLUDE_PATH => "."); $template->process ("non-existent-file") or die $template->error (); why does "die" not produce a line number and newline? Output looks like this: $ perl template.pl file error - non-existent-file: not found ~ 503 $

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  • Issue with class template partial specialization

    - by DeadMG
    I've been trying to implement a function that needs partial template specializations and fallen back to the static struct technique, and I'm having a number of problems. template<typename T> struct PushImpl<const T&> { typedef T* result_type; typedef const T& argument_type; template<int StackSize> static result_type Push(IStack<StackSize>* sptr, argument_type ref) { // Code if the template is T& } }; template<typename T> struct PushImpl<const T*> { typedef T* result_type; typedef const T* argument_type; template<int StackSize> static result_type Push(IStack<StackSize>* sptr, argument_type ptr) { return PushImpl<const T&>::Push(sptr, *ptr); } }; template<typename T> struct PushImpl { typedef T* result_type; typedef const T& argument_type; template<int StackSize> static result_type Push(IStack<StackSize>* sptr, argument_type ref) { // Code if the template is neither T* nor T& } }; template<typename T> typename PushImpl<T>::result_type Push(typename PushImpl<T>::argument_type ref) { return PushImpl<T>::Push(this, ref); } First: The struct is nested inside another class (the one that offers Push as a member func), but it can't access the template parameter (StackSize), even though my other nested classes all could. I've worked around it, but it would be cleaner if they could just access StackSize like a normal class. Second: The compiler complains that it doesn't use or can't deduce T. Really? Thirdly: The compiler complains that it can't specialize a template in the current scope (class scope). I can't see what the problem is. Have I accidentally invoked some bad syntax?

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  • SFINAE failing with enum template parameter

    - by zeroes00
    Can someone explain the following behaviour (I'm using Visual Studio 2010). header: #pragma once #include <boost\utility\enable_if.hpp> using boost::enable_if_c; enum WeekDay {MONDAY, TUESDAY, WEDNESDAY, THURSDAY, FRIDAY, SATURDAY, SUNDAY}; template<WeekDay DAY> typename enable_if_c< DAY==SUNDAY, bool >::type goToWork() {return false;} template<WeekDay DAY> typename enable_if_c< DAY!=SUNDAY, bool >::type goToWork() {return true;} source: bool b = goToWork<MONDAY>(); compiler this gives error C2770: invalid explicit template argument(s) for 'enable_if_c<DAY!=6,bool>::type goToWork(void)' and error C2770: invalid explicit template argument(s) for 'enable_if_c<DAY==6,bool>::type goToWork(void)' But if I change the function template parameter from the enum type WeekDay to int, it compiles fine: template<int DAY> typename enable_if_c< DAY==SUNDAY, bool >::type goToWork() {return false;} template<int DAY> typename enable_if_c< DAY!=SUNDAY, bool >::type goToWork() {return true;} Also the normal function template specialization works fine, no surprises there: template<WeekDay DAY> bool goToWork() {return true;} template<> bool goToWork<SUNDAY>() {return false;} To make things even weirder, if I change the source file to use any other WeekDay than MONDAY or TUESDAY, i.e. bool b = goToWork<THURSDAY>(); the error changes to this: error C2440: 'specialization' : cannot convert from 'int' to 'const WeekDay' Conversion to enumeration type requires an explicit cast (static_cast, C-style cast or function-style cast)

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  • Parsing Indentation-based syntaxes in Haskell's Parsec

    - by pavpanchekha
    I'm trying to parse an indentation-based language (think Python, Haskell itself, Boo, YAML) in Haskell using Parsec. I've seen the IndentParser library, and it looks like it's the perfect match, but what I can't figure out is how to make my TokenParser into an indentation parser. Here's the code I have so far: import qualified Text.ParserCombinators.Parsec.Token as T import qualified Text.ParserCombinators.Parsec.IndentParser.Token as IT lexer = T.makeTokenParser mylangDef ident = IT.identifier lexer This throws the error: parser2.hs:29:28: Couldn't match expected type `IT.TokenParser st' against inferred type `T.GenTokenParser s u m' In the first argument of `IT.identifier', namely `lexer' In the expression: IT.identifier lexer In the definition of `ident': ident = IT.identifier lexer What am I doing wrong? How should I create an IT.TokenParser? Or is IndentParser broken and to be avoided?

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  • Machine learning in OCaml or Haskell?

    - by griffin
    I'm hoping to use either Haskell or OCaml on a new project because R is too slow. I need to be able to use support vectory machines, ideally separating out each execution to run in parallel. I want to use a functional language and I have the feeling that these two are the best so far as performance and elegance are concerned (I like Clojure, but it wasn't as fast in a short test). I am leaning towards OCaml because there appears to be more support for integration with other languages so it could be a better fit in the long run (e.g. OCaml-R). Does anyone know of a good tutorial for this kind of analysis, or a code example, in either Haskell or OCaml?

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  • Improve my Haskell implementation of Filter

    - by mvid
    I have recently been teaching myself Haskell, and one of my exercises was to re-implement the filter function. However, of all the exercises I have performed, my answer for this one seems to me the most ugly and long. How could I improve it? Are there any Haskell tricks I don't yet know? myfilter :: (a -> Bool) -> [a] -> [a] myfilter f (x:xs) = if f x then x : myfilter f xs else myfilter f xs myfilter _ [] = [] Thank You

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  • Type patterns in Haskell

    - by finnsson
    I'm trying to compile a simple example of generic classes / type patterns (see http://www.haskell.org/ghc/docs/latest/html/users_guide/generic-classes.html) in Haskell but it won't compile. Any ideas about what's wrong with the code would be helpful. According to the documentation there should be a module Generics with the data types Unit, :*:, and :+: but ghc (6.12.1) complaints about Not in scope: data constructor 'Unit' etc. It seems like there's a package instant-generics with the data types :*:, :+: and U but when I import that module (instead of Generics) I get the error Illegal type pattern in the generic bindings {myPrint _ = ""} The complete source code is import Generics.Instant class MyPrint a where myPrint :: a -> String myPrint {| U |} _ = "" myPrint {| a :*: b |} (x :*: y) = "" (show x) ++ ":*:" ++ (show y) myPrint {| a :+: b |} _ = "" data Foo = Foo String instance MyPrint a => MyPrint a main = myPrint $ Foo "hi" and I compile it using ghc --make Foo.hs -fglasgow-exts -XGenerics -XUndecidableInstances P.S. The module Generics export no data types, only the functions: canDoGenerics mkGenericRhs mkTyConGenericBinds validGenericInstanceType validGenericMethodType

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  • File processing-Haskell

    - by Martinas Maria
    How can I implement in haskell the following: I receive an input file from the command line. This input file contains words separated with tabs,new lines and spaces.I have two replace this elements(tabs,new lines and spaces) with comma(,) .Observation:more newlines,tabs,spaces will be replaced with a single comma.The result has to be write in a new file(output.txt). Please help me with this.My haskell skills are very scarse. This is what I have so far: processFile::String->String processFile [] =[] processFile input =input process :: String -> IO String process fileName = do text <- readFile fileName return (processFile text) main :: IO () main = do n <- process "input.txt" print n In processFile function I should process the text from the input file. I'm stuck..Please help.

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  • how to translate Haskell into Scalaz?

    - by TOB
    One of my high school students and I are going to try to do a port of Haskell's Parsec parser combinator library into Scala. (It has the advantage over Scala's built-in parsing library that you can pass state around fairly easily because all the parsers are monads.) The first hitch I've come across is trying to figure out how Functor works in scalaz. Can someone explain how to convert this Haskell code: data Reply s u a = Ok a !(State s u) ParseError | Error ParseError instance Functor (Reply s u) where fmap f (Ok x s e) = Ok (f x) s e fmap _ (Error e) = Error e -- XXX into Scala (using Scalaz, I assume). I got as far as sealed abstract class Reply[S, U, A] case class Ok[S, U, A](a: A, state: State[S, U], error: ParseError) extends Reply[S, U, A] case class Error[S, U, A](error: ParseError) extends Reply[S, U, A] and know that I should make Reply extend the scalaz.Functor trait, but I can't figure out how to do that. (Mostly I'm having trouble figuring out what the F[_] parameter does.) Any help appreciated! Thanks, Todd

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