December 2009 - Beginners

Circular Linked Lists
by Matthew J. Williams 04 Feb '10

04 Feb '10

Dear All How would one mimic, in Haskell, a C++ circular linked list i.e., where the last element precedes (points to) the first? Sincerely Matthew J. Williams

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writing many1Till combinator for Parsec
by Vlad Skvortsov 01 Jan '10

01 Jan '10

Hi, I'm writing a parser where I find myself in need to use manyTill combinator with an additional constraint that there should be at least one meaningful element before the trailer (say, a word ended with a period: 'abc.'). Here is what I have: many1Till :: Parser a -> Parser end -> Parser [a] many1Till p end = do p1 <- p ps <- manyTill p end return (p1:ps) The problem here is that I want to catch and report a case when 'p1' could be actually parsed by 'end' ('..' is not a word ended by a period). Generally 'p' and 'end' can parse the same subset of strings. Another version a had was: many1Till :: Parser a -> Parser end -> Parser [a] many1Till p end = do try (end >> (unexpected "sequence terminator")) <|> (do { p1 <- p; ps <- manyTill p end; return (p1:ps) }) Here there are two disadvantages: 1) I don't like hardcoding "sequence terminator" here; 2) the error output should say that only 'p' parser is expected, while it says (technically correct) that either 'p' or 'end' is expected: Prelude Main Text.ParserCombinators.Parsec> parseTest (many1Till letter (char '.')) "1" parse error at (line 1, column 1): unexpected "1" expecting "." or letter (What I want here is to say "expecting letter") Any suggestions? Thanks! -- Vlad Skvortsov, vss(a)73rus.com, http://vss.73rus.com

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Global variable question
by Joe Van Dyk 31 Dec '09

31 Dec '09

What's the appropriate way to write this in Haskell? # Ruby code, ignore the thread-unsafety for now. class Ticker @@tick = 0 def self.update! @@tick += 1 end def self.current return @@tick end end Ticker.update! # 1 Ticker.update! # 2 Ticker.current # 2 -- Joe Van Dyk http://fixieconsulting.com

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Is there a (^) function for exponents other than integral?
by Ken Overton 31 Dec '09

31 Dec '09

Sorry if this is from a faq list somewhere, I've been looking through the more common locations/libs and have been unable to find a way to raise numbers to a fractional exponent. Can someone help me to find such a function in haskell? Thanks, -- kov

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Data.Map, types and debug
by legajid 31 Dec '09

31 Dec '09

Hi, studying IO with files, i got this tutorial program. By the way, i discover the Data.Map module. Several questions then come to my mind : q1: why, in mapM_, make (sort (keys grades)) ? keys is already sorted, isn't it? q2: why, in mapM_, have grades twice (in draw then keys). I wonder if writing only draw grades, one could then extract the s and g parts in the draw function (via keys and elems). From this, i have problems with the type of grades; which is it? The insert function, using insertWith should give Map k a, but foldr seems to change this (couldn't match expected type [Map k a] against inferred type Map [String] a1 on the mapM_ line . q3: To solve the types problem, I tried to debug : i can get the types for s, marks and avg but for g, it says not in scope. How can i get this information ? q4: I also tried to type the parameter in draw (draw (x::Map k a) = ...) but i got an error : Illegal signature in pattern use -XScopedTypeVariables to permit it Since this parameter is visibly not set by default in ghci, is it a good idea to set it ? Thanks and happy new year. Didier import Data.Char <http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#t:Char> import Data.Maybe <http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#t:Maybe> import Data.List import Data.Map hiding (map <http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#v:map>) import Text.Printf main = do src <- readFile <http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#v:readF…> "grades" let pairs = map <http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#v:map> (split.words <http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#v:words>) (lines <http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#v:lines> src) let grades = foldr <http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#v:foldr> insert empty pairs mapM_ <http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#v:mapM_> (draw grades) (sort (keys grades)) where insert (s, g) = insertWith (++ <http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#v:.>) s [g] split [name,mark] = (name, read <http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#v:read> mark) draw g s = printf "%s\t%s\tAverage: %f\n" s (show <http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#v:show> marks) avg where marks = findWithDefault (error <http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#v:error> "No such student") s g avg = sum <http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#v:sum> marks / fromIntegral <http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#v:fromI…> (length <http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#v:length> marks) :: Double <http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#t:Double>

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Re: [Haskell-beginners] Interrupting a thread
by Floptical Logic 31 Dec '09

31 Dec '09

On Thu, Dec 31, 2009 at 1:21 AM, Dean Herington <heringtonlacey(a)mindspring.com> wrote: > At 6:08 PM -0600 12/30/09, Floptical Logic wrote: >> >> On Sun, Dec 27, 2009 at 1:47 PM, Stephen Blackheath [to >> Haskell-Beginners] <mutilating.cauliflowers.stephen(a)blacksapphire.com> >> wrote: >>> >>> MVars are the lowest-level operation for this kind of thing in Haskell, >>> and they're very fast. Anything can be done with MVars but in some cases >>> you need extra worker threads (cheap in Haskell), and you may even need >>> to kill threads (which is a safe operation in Haskell). CHP is higher >>> level and designed for this sort of complexity, so you might want to >>> look at that. I'll give you the answer I know, which is a low-level >>> MVar answer. >>> >>> I have *not* tried compiling this code. >>> >>> >>> import Control.Concurrent >>> import Control.Concurrent.MVar >>> import Data.Int >>> import System.Time >>> >>> type Microseconds = Int64 >>> >>> getSystemTime :: IO Microseconds >>> getSystemTime = do >>> (TOD sec pico) <- getClockTime >>> return $! >>> (fromIntegral sec::Int64) * 1000000 + >>> (fromIntegral pico::Int64) `div` 1000000 >>> >>> type Stack a = [a] -- or whatever type you want >>> >>> isEmpty :: Stack a -> Bool >>> isEmpty [] = True >>> isEmpty _ = False >>> >>> pop :: Stack a -> (a, Stack a) >>> >>> data ScheduleInput = ModifyStack (Stack -> Stack) | WaitFor Microseconds >>> | Timeout >>> >>> never = maxBound :: Microseconds >>> >>> schedule :: MVar ScheduleInput -> MVar a -> Stack a -> IO () >>> schedule inpVar wnVar stack = schedule_ never stack >>> where >>> schedule_ :: Microseconds -> Stack -> IO () >>> schedule_ timeout stack = do >>> now <- getSystemTime >>> let tillTimeout = 0 `max` (timeout - now) >>> if tillTimeout == 0 && not (isEmpty stack) then do >>> let (val, stack') = pop stack >>> putMVar wnVar (PopValue val) >>> schedule never stack' >>> else do >>> inp <- takeMVarWithTimeout (fromIntegral tillTimeout) inpVar >>> case inp of >>> ModifyStack f -> schedule_ timeout (f stack) >>> WaitFor t -> do >>> now <- getSystemTime >>> schedule (t+now) stack >>> Timeout -> schedule timeout stack >>> >>> readMVarWithTimeout :: Int -> MVar ScheduleInput -> IO ScheduleInput >>> readMVar timeoutUS inpVar = do >>> tid <- forkIO $ do >>> threadDelay timeoutUS >>> putMVar inpVar Timeout >>> inp <- takeMVar inpVar >>> killThread tid >>> return inp >>> >>> waitNotify :: MVar ScheduleInput -> MVar Int -> IO () >>> waitNotify schInp wnInp = do >>> val <- takeMVar wnInp >>> ...notify... >>> let t = .... >>> putMVar schInp $ WaitFor t -- block input for the specified period >>> >>> main = do >>> schVar <- newEmptyMVar >>> wnVar <- newEmptyMVar >>> forkIO $ schedule schVar wnVar [] >>> forkIO $ waitNotify wnVar schVar >>> ... >>> -- Modify stack according to user input inside your main IO loop >>> putMVar schVar $ ModifyStack $ \stack -> ... >>> >>> >>> I'm sure this is not exactly what you want, but at least it illustrates >>> how you can achieve anything you like by using MVars + extra worker >>> threads + killing threads (useful for implementing timeouts). >>> >>> >>> Steve >>> >>> Floptical Logic wrote: >>>> >>>> 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 >>>> _______________________________________________ >>>> Beginners mailing list >>>> Beginners(a)haskell.org >>>> http://www.haskell.org/mailman/listinfo/beginners >>>> >>> _______________________________________________ >>> Beginners mailing list >>> Beginners(a)haskell.org >>> http://www.haskell.org/mailman/listinfo/beginners >>> >> >> Thanks Stephen. That worked and things are starting to make more >> sense. The crux of it all was the readMVarWithTimeout function which >> does exactly what I want. >> >> Now I want to extend this a bit further to use StateT. I've modified >> the schedule function quite a bit, and now it has type schedule :: >> StateT MyState IO. I keep the MVar actVar in MyState as well the list >> (stack) of wait times. However, the type of forkIO is forkIO :: IO () >> -> IO ThreadId. How do I fork a new thread for schedule even though >> it is wrapped in the State monad? >> >> Thanks again. > > You'll want something like this: > > myFork :: StateT MyState IO () -> MyState -> StateT MyState IO ThreadId > myFork action initialState = liftIO (forkIO (evalStateT action > initialState)) > > Dean > That's not quite what I'm looking for. I don't want to create a new thread in which to run the monad but rather to be able to create threads from inside the monad (as a result of running the monad).

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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 29 Dec '09

29 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 28 Dec '09

28 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 28 Dec '09

28 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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