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• Implementing simple business model in Haskell

  - by elmes

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

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• Literate Haskell (.lhs) and Haddock

  - by finnsson

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

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• Using perfectly formatted input as list in Haskell

  - by RankWeis

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

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• Haskell: read input character from console immediately, not after newline.

  - by Steves

  I've tried this: main = do hSetBuffering stdin NoBuffering c <- getChar but it waits until the enter is pressed, which is not what I want. I want to read the character immediately after user presses it. I am using ghc v6.12.1 on Windows 7.

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• Haskell math performance

  - by Travis Brown

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

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• Web development: Haskell or Scheme

  - by Robert E. Lester

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

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• Learning Haskell maps, folds, loops and recursion

  - by Darknight

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

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• Haskell lazy I/O and closing files

  - by Jesse

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

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• Help! I'm a Haskell Newbie

  - by Darknight

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

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• Haskell as REST server

  - by Dev er dev

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

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

  - by Adam Gent

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

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• How to learn Haskell

  - by anderstornvig

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

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• haskell sorting

  - by snorlaks

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

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

  - by Steve

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

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• Evaluation of Haskell Statements/Expressions using GHC API

  - by Cetin Sert

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

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• Haskell newbie on types

  - by garulfo

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

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• Haskell composition (.) vs F#'s pipe forward operator (|>)

  - by Ben Lings

  In F#, use of the the pipe-forward operator (|>) is pretty common. However, in Haskell I've only ever seen function composition (.) being used. I understand that they are related, but is there a language reason that pipe-forward isn't used in Haskell or is it something else

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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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