Search Results

Search found 18249 results on 730 pages for 'real world haskell'.

facebook twitter digg google delicious technorati stumbleupon myspace wordpress linkedin gmail igoogle windows live tumblr viadeo yahoo buzz yahoo mail yahoo bookmarks favorites email print

Page 5/730 | < Previous Page | 1 2 3 4 5 6 7 8 9 10 11 12  | Next Page >

  • 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).

    Read the article

  • 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!

    Read the article

  • 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?

    Read the article

  • 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.

    Read the article

  • 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

    Read the article

  • 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 }

    Read the article

  • 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]?

    Read the article

  • 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?

    Read the article

  • 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?

    Read the article

  • 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?

    Read the article

  • 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?

    Read the article

  • 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?

    Read the article

  • 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

    Read the article

  • 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

    Read the article

  • 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.

    Read the article

  • 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

    Read the article

  • Translate imperative control flow with break-s/continue-s to haskell

    - by dorserg
    Consider the following imperative code which finds the largest palindrome among products of 3-digit numbers (yes, it's the one of the first tasks from "Project of [outstanding mathematician of 18th century]" site): curmax = 0 for i in range(999,100): for j in range(999,100): if ((i*j) < curmax): break if (pal(i*j)): curmax = i*j break print curmax As I'm learning Haskell currently, my question is, how do you translate this (and basically any imperative construct that contains something more complex than just plain iteration, e.g. breaks, continues, temporary variables and all this) to Haskell? My version is maxpal i curmax | i < 100 = curmax | otherwise = maxpal (i-1) (innerloop 999) where innerloop j | (j < 100) || (p < curmax) = curmax | pal p = p | otherwise = innerloop (j-1) where p = i*j main = print $ maxpal 999 0 but this looks like we're still in imperative uglytown. So what could you advise, what are the approaches of dealing with such cases FP-style?

    Read the article

  • Strategy for desugaring Haskell

    - by luqui
    I'm developing a virtual machine for purely functional programs, and I would like to be able to test and use the the wide variety of Haskell modules already available. The VM takes as input essentially terms in the untyped lambda calculus. I'm wondering what would be a good way to extract such a representation from modern Haskell modules (eg. with MPTC's, pattern guards, etc.). I did a little research and there doesn't seem to be a tool that does this already (I would be delighted to be mistaken), and that's okay. I'm looking for an approach. GHC Core seems too operationally focused, especially since one of the things the VM does is to change the evaluation order significantly. Are there any accessible intermediate representations that correspond more closely to the lambda calculus?

    Read the article

  • Fibonacci Numbers in Haskell

    - by boraer
    Hi everbody I need to change my F# code to Haskell code but I am so new in Haskell and I can not this My code simply read data from keyboard if data not an integer return an error message then calculate the n fibonacci number then writes to a list after that writes the list into a txt file Here is my code open System let rec fib n = match n with |0->0 |1->1 |2->1 |n->fib(n-1)+fib(n-2);; let printFibonacci list = for i=0 to (List.length list)-1 do printf "%d " (list.Item(i));; let writeToFile list = let file = System.IO.File.Create("C:\out2.txt") let mutable s ="" let writer = new System.IO.StreamWriter(file) try for i=0 to (List.length list)-1 do s <- list.Item(i).ToString() writer.Write(s+" ") finally writer.Close() file.Dispose() printfn "Writed To File" let mutable control = true let mutable num = 0 while control do try printfn "Enter a Number:" num <- Convert.ToInt32(stdin.ReadLine()) let listFibonacci = [for i in 0 .. num-1->fib(i)] printFibonacci(listFibonacci) printfn "\n%A"(listFibonacci) writeToFile(listFibonacci) control<-false with | :? System.FormatException->printfn "Number Format Exception"; Console.ReadKey true|>ignore

    Read the article

  • Fork to shell script and terminate original process with Haskell

    - by Neth
    I am currently writing a Haskell program that does some initialization work and then calls ncmpcpp. What I am trying to do is start ncmpcpp and terminate the Haskell program, so that only ncmpcpp is left (optionally, the program can keep running in the background, as long as it's unintrusive) However, even though I am able to start ncmpcpp, I cannot interact with it. I see its output, but input appears to be impossible. What I am currently doing is: import System.Process (createProcess, proc) ... spawnCurses :: [String] -> IO () spawnCurses params = do _ <- createProcess (proc "ncmpcpp" params) return () What am I doing wrong/What should I do differently?

    Read the article

  • Why is there "data" and "newtype" in Haskell?

    - by martingw
    To me it seems that a newtype definition is just a data definition that obeys some restrictions (only one constructor and such), and that due to these restrictions the runtime system can handle newtypes more efficiently. Ok, and the handling of pattern matching for undefined values is slightly different. But suppose Haskell would only knew data definitions, no newtypes: Couldn't the compiler find out for himself whether a given data definition obeys these restrictions, and automatically treat it more efficiently? I'm sure I'm missing out on something, these Haskell designers are so clever, there must be some deeper reason for this...

    Read the article

  • Projection matrix + world plane ~> Homography from image plane to world plane

    - by B3ret
    I think I have my wires crossed on this, it should be quite easy. I have a projection matrix from world coordinates to image coordinates (4D homogeneous to 3D homgeneous), and therefore I also have the inverse projection matrix from image coordinates to world "rays". I want to project points of the image back onto a plane within the world (which is given of course as 4D homogeneous vector). The needed homography should be uniquely identified, yet I can not figure out how to compute it. Of course I could also intersect the back-projected rays with the world plane, but this seems not a good way, knowing that there MUST be a homography doing this for me. Thanks in advance, Ben

    Read the article

facebook twitter digg google delicious technorati stumbleupon myspace wordpress linkedin gmail igoogle windows live tumblr viadeo yahoo buzz yahoo mail yahoo bookmarks favorites email print

< Previous Page | 1 2 3 4 5 6 7 8 9 10 11 12  | Next Page >