- Beginners - Haskell.org

Interrupting a thread
by Floptical Logic 31 Dec '09

31 Dec '09

Hi, I am new to concurrency in Haskell and I am having trouble implementing the notion of interrupting a thread. In a new thread, call it waitNotify, I am trying to do the following: pop a number from a stack, wait some number of seconds based on the number popped from the stack, perform some notification, and repeat until there are no more numbers in the stack at which point we wait for a new number. These numbers will be supplied interactively by the user from main. When the user supplies a new number, I want to interrupt whatever waiting is happening in waitNotify, insert the number in the proper position in the current stack, and resume waitNotify using the updated stack. Note, here "stack" is just a generalization; it will likely just be a list. What is the most idiomatic way to capture this sort of behavior in Haskell? My two challenges are the notion of interrupting a thread, and sharing and updating this stack between threads (main and waitNotify). Thank you

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I need advice on design
by Amy de Buitléir 30 Dec '09

30 Dec '09

I'm building a library of components for artificial neural networks. I'm used to object-oriented languages, so I'm struggling a bit to figure out how to do a good design in a functional programming language like Haskell. Q1: I've come up with two designs, and would appreciate any advice on improvements and what approach to take. ===== Design #1 ===== class Neuron n where activate :: [Double] -> n -> Double train :: [Double] -> Double -> n -> n ...and then I would have instances of this typeclass. For example: data Perceptron = Perceptron { weights :: [Double], threshold :: Double, learningRate :: Double } deriving (Show) instance Neuron Perceptron where activate inputs perceptron = ... train inputs target perceptron = ... The disadvantage of this approach is that I need to define and name each instance of neuron before I can use it. I'd rather create a neuron on-the-fly by calling a general-purpose constructor and telling it what functions to use for activation and training. I think that would make it easier to re-use activation and training functions in all sorts of different combinations. So I came up with... ===== Design #2 ===== data Neuron = Neuron { weights :: [Double], activate :: [Double] -> Double, train :: [Double] -> Double -> Neuron } Q2: I thought there might be some way to define a function type, but the following doesn't work. Is there something along these lines that would work? type activationFunction = [Double] -> Double

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Advice on what to install
by Chris Saunders 29 Dec '09

29 Dec '09

I noticed the availability of Haskell Platform 2009.2.0.2 and did install this and when I run GHCi it says that its version is 6.10.4. Now on the GHC site I notice that the current version is 6.12. Should I have installed this more recent version or stick with what I have? Warning: many more dumb questions to come from this guy. Regards Chris Saunders

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Ghci & wxhaskell
by legajid 29 Dec '09

29 Dec '09

Hello, i've downloaded and installed wshaskell (GUI for windows). All messages are ok. Running an example program, the dll cannot be found. Below are the messages when i type : ghci -package wx : GHCi, version 6.10.4: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer ... linking ... done. Loading package base ... linking ... done. Loading package syb ... linking ... done. Loading package array-0.2.0.0 ... linking ... done. Loading package containers-0.2.0.1 ... linking ... done. Loading package bytestring-0.9.1.4 ... linking ... done. Loading package Win32-2.2.0.0 ... linking ... done. Loading package filepath-1.1.0.2 ... linking ... done. Loading package old-locale-1.0.0.1 ... linking ... done. Loading package old-time-1.0.0.2 ... linking ... done. Loading package directory-1.0.0.3 ... linking ... done. Loading package process-1.0.1.1 ... linking ... done. Loading package random-1.0.0.1 ... linking ... done. Loading package haskell98 ... linking ... done. Loading package stm-2.1.1.2 ... linking ... done. Loading package wxcore-0.11.1.2 ... <command line>: can't load .so/.DLL for: wxc-msw2.8.10-0.11.1.2 (addDLL: could not load DLL) : wxc-msw2.8.10-0.11.1.2: Cette application n'a pas pu démarrer car la configuration de l'application est incorrecte. Réinstaller l'application pourrait résoudre ce problème. (application couldn' start because configuration is incorrect. Reinstall could solve this problem) wxc-msw2.8.10-0.11.1.2.dll has been copied to windows\system32 (windows XP) Can someone help for this installation issue? Thanks, Didier

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Problems with EclipseFP
by Maciej Piechotka 28 Dec '09

28 Dec '09

Did anyone used eclipsefp? I installed version 2 but I didn't change anything in eclipse (I haven't notice a difference - it does not recognise .hs file, nothing about haskell in help/new project/new file etc.). Regards

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Performance of parallel mergesort
by Jon Harrop 28 Dec '09

28 Dec '09

I took the parallel merge sort described here: http://martin.sulzmann.googlepages.com/AMP-Spring2008-intro.pdf and added a simple test: main :: IO () main = do putStrLn $ show $ sum $ mergesort $ [sin x | x <- [1.0 .. 1000000.0]] Compiled it with GHC 6.8.2 from Debian testing and ran it with various +RTS -N<n> arguments to measure its performance and obtained the following results on an 8 core: $ ghc --make -O2 -threaded mergesort.hs -o mergesort [1 of 1] Compiling Main ( mergesort.hs, mergesort.o ) Linking mergesort ... $ time ./mergesort +RTS -N1 -0.117109518058233 real 0m9.723s user 0m9.461s sys 0m0.232s $ time ./mergesort +RTS -N2 -0.117109518058233 real 0m13.574s user 0m15.225s sys 0m0.140s $ time ./mergesort +RTS -N3 -0.117109518058233 real 0m13.185s user 0m15.529s sys 0m0.184s $ time ./mergesort +RTS -N4 -0.117109518058233 real 0m13.251s user 0m15.829s sys 0m0.336s $ time ./mergesort +RTS -N5 -0.117109518058233 real 0m13.093s user 0m16.929s sys 0m0.164s $ time ./mergesort +RTS -N6 Segmentation fault real 0m5.711s user 0m7.408s sys 0m0.136s That segfault must be due to a bug in GHC so I thought perhaps a newer version of GHC might fix the segfault but I installed GHC 6.10.4 from Sid and now I get: $ ghc --make -O2 -threaded mergesort.hs -o mergesort Binary: Int64 truncated to fit in 32 bit Int ghc: panic! (the 'impossible' happened) (GHC version 6.10.4 for i386-unknown-linux): Prelude.chr: bad argument Please report this as a GHC bug: http://www.haskell.org/ghc/reportabug I'll try the haskell-platform package next. Are these known problems? -- Dr Jon Harrop, Flying Frog Consultancy Ltd. http://www.ffconsultancy.com/?e

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Enumerated types
by legajid 28 Dec '09

28 Dec '09

Hello, I have some trouble evaluating Enum class. Following is my source: data Authors= Buzzati | Werber | Verne | Ray | Asimov | Voltaire deriving (Enum, Show) auths=[Buzzati .. Verne] ++ enumFrom Ray pref=take 3 auths -- 1 disp_pref=mapM_ (putStrLn) pref auth1 = Buzzati auth2 = succ auth1 -- Werber auth3 = pred Asimov -- Ray num_auth4=fromEnum Verne -- 2 -- 2 toEnum main= display Buzzati display x = do putStrLn (show x) display (succ x) -- 3 end of enum 1. could'nt match expected type [Char] against inferred type Authors I would like to display each data constructor name on one line , giving : Buzzati Werber Verne How can i translate data from one type to another (Authors to [Char])? 2. Like fromEnum gives data constructor index, is it possible with toEnum (or other way) to get the nth constructor of the given type ? eg : 2 applied to Authors (not in scope ?) would give Verne 3. tried to take 'succ' of last tag in enumeration How to detect the end of an enumeration ? Can i get the maxbound index ? Thanks, Didier.

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Maybe and IO
by legajid 28 Dec '09

28 Dec '09

Hi, My purpose is to get data from the keyboard and return a maybe value, saying "nothing" if the data entered is invalid. I get a message "could'nt match expected IO Integer against inferred type maybe integer" on line x<- getdata Why should x be Maybe ? getdatanum is, but getdata is not. Is it possible to write only one function rather than 2 distinct ones ? Thanks for your help, Didier. Here's my code getdata :: IO Integer getdata=do x<-getLine let xn=read x ::Integer return xn getdatanum :: Maybe Integer getdatanum = do x <- getdata {- if x < 5 then do return (Just x) else do return Nothing -} return (Just x)

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parser for expressions
by John Moore 28 Dec '09

28 Dec '09

Hi, What I'm trying to do is create a parser so as when I enter say 1+1 it will return Add(Val 1)(Val 1). A couple of questions do I have to state three basic parser. As in item, fail, synbol and then combine them with below. And by the way the one below seems to be all wrong. parse :: String -> expr parse expr = [(expr,string)] expr :: Parser value expr = do t <- term do char '+' e <- expr return Add (Val t)(Val e) +++ return t (+++) :: Parser a -> Parser a -> Parser a t +++ w inp = case t inp of [] -> w inp [(v,out)] -> [(v,out)] This is what I mean by item fail and symbol type Parser s a = [s] -> [(a,[s])] item [] = [] item (c:cs) = [(c,cs)] pFail :: Parser s a pFail = \cs -> [] pSymbol :: Eq s => s -> Parser s s pSymbol a (b:bs) |a == b = [(b,bs)] |otherwise = [] I know this is a bit of a mess, but could someone explain where I should start and if I should use any of the code above. Also to point out I want to write the complete parser without using prelude and other built in functions as I will be changing as I go. John

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Design Question: fmap with two lists
by jonathanGfischoff＠gmail.com 28 Dec '09

28 Dec '09

I have a class that is basically: data TwoLists = TL {doubleList :: [Double], intList :: [Int]} Almost all of the operations of this type map, fold, etc on one of the lists or the other, and then return a new instance of the type. So I am implementing to functions dFmap, and iFmap. I actually would like all of the list functions, but I don't want to reimplement them. So my question is, what is the haskell way of handling a type like this. Is there some sort of TwoTypeTraversable I should be deriving from? Essential is there a Haskell idiom for this situation. -Jonathan

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