- Beginners - Haskell.org

Random animals
by Amy de Buitléir 17 Nov '10

17 Nov '10

I have this function: -- | Choose an element at random from a list and return the element and its index -- Ex: runState (randomListSelection ['a', 'b', 'c', 'd', 'e']) (mkStdGen 1) randomListSelection :: (RandomGen s) => [a] -> State s (Int, a) randomListSelection xs = do i <- State $ randomR (0,length xs - 1) return (i, xs !! i) I want to repeatedly select random elements from a list. I know the code below is totally wrong, but it's the closest I've gotten so far. When I run it in ghci, I get the SAME random element each time, until I reload the module. I guess that's because g is always the same. I'm still struggling with monads, and would appreciate any advice on how to fix this. chooseCreatur = do g <- getStdGen return (evalState (randomListSelection ["cat", "dog", "lion", "mouse"]) g) FWIW, I'll be calling it from a loop within the IO monad. The real list will be growing and shrinking in size.

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Heterogeneous Lists
by John Smith 17 Nov '10

17 Nov '10

My program needs a list such as [IO 3, IO "f", IO 'z'] :: Show s => [IO s]. Is there any way to achieve this without wrapping all the values in existentially quantified types?

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Simple Haskell daemon
by Amy de Buitléir 17 Nov '10

17 Nov '10

I've written a very simple daemon, and it's working just fine. But I'd appreciate it if someone could take a look at it and tell me if any of the code I've written is... stupid. (I'm trying to get more comfortable with this monad stuff.) Thank you in advance. Daemon.hs --------- module Daemon where import System.Exit import System.IO.Unsafe import System.Posix.Signals import Control.Concurrent class DaemonState a where initialise :: IO a work :: a -> IO a finalise :: a -> IO () termReceived = unsafePerformIO (newMVar False) handleTERM :: IO () handleTERM = swapMVar termReceived True >> return () loop :: (DaemonState a) => a -> IO (Maybe a) loop d = do timeToStop <- readMVar termReceived if timeToStop then finalise d >> return Nothing else work d >>= loop start :: (DaemonState a) => IO (Maybe a) start = installHandler sigTERM (Catch handleTERM) Nothing >> initialise >>= loop ----- Here's a simple example of how I use it ----- Main.hs ------- module Main where import Daemon instance DaemonState Int where initialise = do putStrLn "Starting up" return 0 work i = do putStrLn (show i) return (i+1) finalise i = do putStrLn "Shutting down" return () main = start :: IO (Maybe Int)

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Re: Apparent bug
by Russ Abbott 16 Nov '10

16 Nov '10

I can understand "while(true) { }" producing an endless loop. Can you explain how the code I entered is interpreted to mean an unending cycle? Furthermore most systems can be interrupted, even when in a tight while(true) { } loop. In the example I gave, it seems that GHCi cannot be interrupted. (When I try it on some machines I get an out-of-memory error, but it still(!) can't be interrupted.) I think GHCi is wonderful software. But I doubt that anyone would claim that it's bug-free. I'm surprised that there is so much resistance to acknowledging what appears to be a bug. Why not just fix it? To review, how should this be interpreted? data Test = Test instance Show Test -- The problem occurs even without "where" > Test What interpretation justifies GHCi going into an uninterruptable state at this point? * -- Russ * On Tue, Nov 16, 2010 at 9:00 AM, <beginners-request(a)haskell.org> wrote: > On 16 November 2010 12:17, Thomas Davie <tom.davie(a)gmail.com> wrote: > > > GHCi's job is to run the code you enter. > > > Likewise, in many other languages when you write the following you'll get > an > infinite rule. And it just compiles (or gets interpreted) fine. > > while(true) { } > > Just another perspective. > > Ozgur > >

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Re: [Haskell-beginners] GHCi bug?
by Ozgur Akgun 16 Nov '10

16 Nov '10

Unfortunately, yes. On 16 November 2010 11:39, Russ Abbott <russ.abbott(a)gmail.com> wrote: > I'm not sure what you are saying here. Are you saying that it's not a bug > that GHCi went into a coma when I file was loaded and a prompt entered? > * > -- Russ* > *** > * > ** > > > On Tue, Nov 16, 2010 at 3:34 AM, Ozgur Akgun <ozgurakgun(a)gmail.com> wrote: > >> >> On 16 November 2010 05:12, Russ Abbott <russ.abbott(a)gmail.com> wrote: >> >>> I know the code isn't correct. My point is that the compiler didn't >>> complain when the code was loaded, and the interpreter died when it was >>> executed. That shouldn't happen. >>> * >>> -- Russ* >>> >> >> It would be *cool* for GHC to make an analysis about the instance methods >> of a type class, and their default implementations to find the minimal >> subset of methods you have to implement to get a valid (for some definition >> of valid) instance declaration. But it doesn't do such a thing. And it is >> not an easy task. >> >> GHC only warns you if you do not implement a method which doesn't have a >> default implementation. In your example, this is not the case. >> >> Best, >> >> -- >> Ozgur Akgun >> > > -- Ozgur Akgun

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GHCi bug?
by Russ Abbott 16 Nov '10

16 Nov '10

This looks like a GHCi bug. Load a file with these two lines into GHCi data Test = Test instance Show Test where -- No body to the instance statement * * Then enter Test at the prompt. * * > Test * * This seems to bomb GHCi. It isn't even interruptible but must be shut down and restarted. * * * -- Russ *

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Re: Beginners Digest, Vol 29, Issue 18
by Russ Abbott 16 Nov '10

16 Nov '10

This looks like a GHCi bug. Load data Test = Test instance Show Test where (The instance statement has no body.) Then enter Test at the prompt > Test This bombs GHCi. It can't even be interrupted. It must be shut down and restarted. * -- Russ* On Mon, Nov 15, 2010 at 9:00 AM, <beginners-request(a)haskell.org> wrote: > Send Beginners mailing list submissions to > beginners(a)haskell.org > > To subscribe or unsubscribe via the World Wide Web, visit > http://www.haskell.org/mailman/listinfo/beginners > or, via email, send a message with subject or body 'help' to > beginners-request(a)haskell.org > > You can reach the person managing the list at > beginners-owner(a)haskell.org > > When replying, please edit your Subject line so it is more specific > than "Re: Contents of Beginners digest..." > > > Today's Topics: > > 1. Re: Why the length function I wrote has such a type > signature? (Tom Murphy) > 2. Re: Why the length function I wrote has such a type > signature? (Daniel Fischer) > 3. Rewriting using State and/or Reader? (Paul Sargent) > 4. Re: Rewriting using State and/or Reader? (Brent Yorgey) > 5. (no subject) (David Schonberger) > 6. Ralf Laemmel's riddle on surviving without the monad > transformation library (C K Kashyap) > > > ---------------------------------------------------------------------- > > Message: 1 > Date: Fri, 12 Nov 2010 14:14:25 -0500 > From: Tom Murphy <amindfv(a)gmail.com> > Subject: Re: [Haskell-beginners] Why the length function I wrote has > such a type signature? > Cc: beginners(a)haskell.org > Message-ID: > <AANLkTimjA0auRvt8z35BStt604on374Nk4YXUfUVCxzy(a)mail.gmail.com> > Content-Type: text/plain; charset="iso-8859-1" > > > > > > >> > > And the type signature given by ghci is > > > >> myLength :: (Num t1) => [t] -> t1 > >> > > > > > But why is the signature not written as: > myLength :: [t] -> (Num t1) => t1 > or something similar? > > I thought that the Num typeclass being first implied that it was the > function's first argument. >

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Lazy list Binary instance
by uu1101＠gmail.com 16 Nov '10

16 Nov '10

Dear haskellers, I need to collect data to a file through a long running computation (the population of each generation in an genetic algorithm) and afterwards read the file and compute some statistics in a single pass. I'm using a Writer monad with Data.DList as its Monoid, telling it 'DList.singleton population', at each generation and 'DList.toList . execWriter ...' at the end. Do you think this is the right approach? As the amount of data is very large I'd like to Data.Binary.serialize and Codec.ZLib.compress when writing to the file, and the opposite when reading. As it happens the default list Binary instance does not serve my purpose, as it needs to retain the whole list prior to writing the file. I tried implementing a different Binary instance for the list wrapping it in a newtype, but without success so far. This issue has been discussed before so I tried using Felipe's code [0] and some variations. newtype List a = List [a] split255 :: [a] -> (Word8, [a], [a]) split255 = s 0 where s 255 xs = (255, [], xs) s n (x:xs) = let (n', ys, zs) = s (n+1) xs in (n', x:ys, zs) s n [] = (n, [], []) instance Binary a => Binary (List a) where put (List l) = let (n, xs, ys) = split255 l in do putWord8 n mapM_ put xs when (n == 255) (put $ List ys) get = do n <- getWord8 xs <- replicateM (fromEnum n) get if n == 255 then get >>= \(List ys) -> return (List $ xs ++ ys) else return (List xs) But in this case the Binary.decode needed to read the whole file prior to accessing the first element. Why does this happen? How can I both write and read the file lazily? My guess is that in the line: then get >>= \(List ys) -> return (List $ xs ++ ys) the pattern in the lambda forces get to retrieve the next chunk, which in turn forces the next... until the end is reached. Could this be solved by making it a irrefutable pattern? (I'm sorry, I tried different variations this afternoon and only now that I can't try it this occurs to me). Best regards, Adrian. [0] http://www.mail-archive.com/haskell-cafe@haskell.org/msg30761.html

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Ralf Laemmel's riddle on surviving without the monad transformation library
by C K Kashyap 15 Nov '10

15 Nov '10

Hi, Can someone provide me the solution to the following riddle that Ralf asked in his lecture at http://channel9.msdn.com/Shows/Going+Deep/C9-Lectures-Dr-Ralf-Lmmel-Advance… Riddle: define a custom made monad (only involving (->) and Either String) to survive without the monad transformation library. -- Regards, Kashyap

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(no subject)
by David Schonberger 13 Nov '10

13 Nov '10

http://ungoalperlavita.org/mainlink.html

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