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  • Call an haskell function in .net

    - by SuperBloup
    Hi, I want to use an Haskell function with the following type :: string -> string from a C# programm. I want to use hs-dotnet to bridge both worlds, the author claim that it's possible, but provide no sample of this case. The only samples provided are the one to use .net from haskell. Does anyone got a sample of this use, or got an idea how to use it? (I used reflector on the bridging assembly but didn't understood a thing)

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  • Generalized Bottom up Parser Combinators in Haskell

    - by Panini Sai
    I am wondered why there is no generalized parser combinators for Bottom-up parsing in Haskell like a Parsec combinators for top down parsing. ( I could find some research work went during 2004 but nothing after https://haskell-functional-parsing.googlecode.com/files/Ljunglof-2002a.pdf http://www.di.ubi.pt/~jpf/Site/Publications_files/technicalReport.pdf ) Is there any specific reason for not achieving it?

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  • using haskell for "business applications"

    - by snorlaks
    Hello, I would like to know if there is any posiibility that I can use Haskell with small database like sql server compact so that client wont have to install any server on his desktop. Is there any api providing sql statements and so on ... What is the best solution to achieve small database application using haskell. thanks for help

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  • Haskell mutability in compiled state?

    - by pile of junk
    I do not know much about Haskell, but from what I have read about the mutability of computations (e.g: functions returning functions, complex monads and functions, etc.) it seems like you can do a lot of meta-programming, even at runtime. How can Haskell, if everything like functions and monads are so complex, compile to machine code and retain all this?

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  • Haskell Linear Algebra Matrix Library for Arbitrary Element Types

    - by Johannes Weiß
    I'm looking for a Haskell linear algebra library that has the following features: Matrix multiplication Matrix addition Matrix transposition Rank calculation Matrix inversion is a plus and has the following properties: arbitrary element (scalar) types (in particular element types that are not Storable instances). My elements are an instance of Num, additionally the multiplicative inverse can be calculated. The elements mathematically form a finite field (??2256). That should be enough to implement the features mentioned above. arbitrary matrix sizes (I'll probably need something like 100x100, but the matrix sizes will depend on the user's input so it should not be limited by anything else but the memory or the computational power available) as fast as possible, but I'm aware that a library for arbitrary elements will probably not perform like a C/Fortran library that does the work (interfaced via FFI) because of the indirection of arbitrary (non Int, Double or similar) types. At least one pointer gets dereferenced when an element is touched (written in Haskell, this is not a real requirement for me, but since my elements are no Storable instances the library has to be written in Haskell) I already tried very hard and evaluated everything that looked promising (most of the libraries on Hackage directly state that they wont work for me). In particular I wrote test code using: hmatrix, assumes Storable elements Vec, but the documentation states: Low Dimension : Although the dimensionality is limited only by what GHC will handle, the library is meant for 2,3 and 4 dimensions. For general linear algebra, check out the excellent hmatrix library and blas bindings I looked into the code and the documentation of many more libraries but nothing seems to suit my needs :-(. Update Since there seems to be nothing, I started a project on GitHub which aims to develop such a library. The current state is very minimalistic, not optimized for speed at all and only the most basic functions have tests and therefore should work. But should you be interested in using or helping out developing it: Contact me (you'll find my mail address on my web site) or send pull requests.

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  • Type-conditional controls in Haskell

    - by estanford
    I'm going through the 99 Haskell problems to build my proficiency with the language. On problem 7 ("Flatten a nested list structure"), I found myself wanting to define a conditional behavior based on the type of argument passed to a function. That is, since *Main> :t 1 1 :: (Num t) => t *Main> :t [1,2] [1,2] :: (Num t) => [t] *Main> :t [[1],[2]] [[1],[2]] :: (Num t) => [[t]] (i.e. lists nested at different levels have different data types) it seems like I should be able to write a function that can read the type of the argument, and then behave accordingly. My first attempt was along these lines: listflatten l = do if (:t l) /= ((Num t) => [t]) then listflatten (foldl (++) [] l) else id l But when I try to do that, Haskell returns a parse error. Is Haskell flexible enough to allow this sort of type manipulation, do I need to find another way?

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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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  • Yet another Haskell vs. Scala question

    - by Travis Brown
    I've been using Haskell for several months, and I love it—it's gradually become my tool of choice for everything from one-off file renaming scripts to larger XML processing programs. I'm definitely still a beginner, but I'm starting to feel comfortable with the language and the basics of the theory behind it. I'm a lowly graduate student in the humanities, so I'm not under a lot of institutional or administrative pressure to use specific tools for my work. It would be convenient for me in many ways, however, to switch to Scala (or Clojure). Most of the NLP and machine learning libraries that I work with on a daily basis (and that I've written in the past) are Java-based, and the primary project I'm working for uses a Java application server. I've been mostly disappointed by my initial interactions with Scala. Many aspects of the syntax (partial application, for example) still feel clunky to me compared to Haskell, and I miss libraries like Parsec and HXT and QuickCheck. I'm familiar with the advantages of the JVM platform, so practical questions like this one don't really help me. What I'm looking for is a motivational argument for moving to Scala. What does it do (that Haskell doesn't) that's really cool? What makes it fun or challenging or life-changing? Why should I get excited about writing it?

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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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  • 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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  • 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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