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Re: [Haskell-cafe] Random number example
by michael rice 23 Apr '09

23 Apr '09

Hi Ross, Thanks for going the extra mile. A lot of what you did I haven't seen before, so it's going to take me some time to go through it. But I'll be back. Michael --- On Thu, 4/23/09, Ross Mellgren <rmm-haskell(a)z.odi.ac> wrote: From: Ross Mellgren <rmm-haskell(a)z.odi.ac> Subject: Re: [Haskell-cafe] Random number example To: "michael rice" <nowgate(a)yahoo.com> Cc: haskell-cafe(a)haskell.org Date: Thursday, April 23, 2009, 11:51 AM So there are a couple problems. First is you are trying to rebind prelude functions, when instead you should be creating an instance of Monad. This requires a bit of shuffling because without language extensions you can't instance Monad Random for your type of Random, as it is a type synonym. So, changing the type synonym to a newtype and instancing monad, you get: module Rand9b where import Control.Applicative (Applicative(..), (<$>), (<*>))import Control.Monad (ap, liftM) type Seed = Intnewtype Random a = Rand { unRand :: (Seed -> (a, Seed)) } randomNext :: Seed -> SeedrandomNext rand = if newRand > 0 then newRand else newRand + 2147483647 where newRand = 16807 * lo - 2836 * hi (hi,lo) = rand `divMod` 127773 rollDie :: Random IntrollDie = Rand $ \ seed -> ((seed `mod` 6) + 1, randomNext seed) instance Monad Random where (>>=) = randomBind return = randomReturn instance Functor Random where fmap = liftM instance Applicative Random where pure = return (<*>) = ap randomBind :: Random a -> (a -> Random b) -> Random bm `randomBind` g = Rand $ \seed0 -> let (result1, seed1) = unRand m $ seed0 (result2, seed2) = unRand (g result1) $ seed1 in (result2, seed2) randomReturn :: a -> Random arandomReturn x = Rand $ \ seed0 -> (x, seed0) sumTwoDice :: Random IntsumTwoDice = (+) <$> rollDie <*> rollDie I also threw in instances of Functor and Applicative, so that I could simplify sumTwoDice using applicative form (much nicer, no? Applicative is totally rockin') Now you need one more thing, a way to convert a series of Random actions into a pure function: runRandom :: Seed -> Random a -> arunRandom s f = fst . unRand f $ s which now makes what you want to do in GHCi easy and well wrapped: Prelude> :reload[1 of 1] Compiling Rand9b ( rand9b.hs, interpreted )Ok, modules loaded: Rand9b.*Rand9b> runRandom 0 sumTwoDice3 Hope this helps,-Ross On Apr 23, 2009, at 11:28 AM, michael rice wrote: I pretty much followed the sequence of steps that led to this final code (see below), but will be looking it over for a while to make sure it sinks in. In the meantime, I get this when I try to use it (sumTwoDice) at the command line: [michael@localhost ~]$ ghci rand9 GHCi, version 6.10.1: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer ... linking ... done. Loading package base ... linking ... done. [1 of 1] Compiling Main ( rand9.hs, interpreted ) Ok, modules loaded: Main. *Main> sumTwoDice <interactive>:1:0: No instance for (Show (Seed -> (Int, Seed))) arising from a use of `print' at <interactive>:1:0-9 Possible fix: add an instance declaration for (Show (Seed -> (Int, Seed))) In a stmt of a 'do' expression: print it *Main> Can I employ a 'do' expression from the command line? Also, can I now use functions (>>) (>>=) and 'return' defined in the Prelude and still have this code work? Michael ================== {-# LANGUAGE NoImplicitPrelude #-} import Prelude hiding ((>>), (>>=), return) type Seed = Int type Random a = Seed -> (a, Seed) randomNext :: Seed -> Seed randomNext rand = if newRand > 0 then newRand else newRand + 2147483647 where newRand = 16807 * lo - 2836 * hi (hi,lo) = rand `divMod` 127773 rollDie :: Random Int rollDie seed = ((seed `mod` 6) + 1, randomNext seed) (>>) :: Random a -> Random b -> Random b (>>) m n = \seed0 -> let (result1, seed1) = m seed0 (result2, seed2) = n seed1 in (result2, seed2) (>>=) :: Random a -> (a -> Random b) -> Random b (>>=) m g = \seed0 -> let (result1, seed1) = m seed0 (result2, seed2) = (g result1) seed1 in (result2, seed2) return :: a -> Random a return x = \seed0 -> (x, seed0) sumTwoDice :: Random Int sumTwoDice = rollDie >>= (\die1 -> rollDie >>= (\die2 -> return (die1 + die2))) _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe(a)haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

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ANNOUNCE: HPong-0.1.2: A simple OpenGL Pong game based on GLFW
by Niemeijer, R.A. 23 Apr '09

23 Apr '09

About two days ago the beta version of the Haskell Platform was released. Since it comes with an OpenGL library, it is now trivially easy to start making OpenGL programs with Haskell. To test this, I decided to make a simple Pong clone for two reasons: * To experiment with making an OpenGL game * I chose to use the GLFW instead of the GLUT library because GLUT doesn’t respond to closing a window with the close button very well. However, I could only find one piece of sample code for GLFW. Because of that, I decided to make a small game that others can use as a starting point for their own games. The controls are very simple: The up and down arrow keys move your paddle and Esc quits the game. This is my first upload to Hackage, so any comments or suggestions are welcome.

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a question about *** Exception: stack overflow ..
by Luke Palmer 23 Apr '09

23 Apr '09

On Thu, Apr 23, 2009 at 7:02 AM, Mozhgan Kabiri <mozhgan_kch(a)hotmail.com>wrote: > Hi Luck , > > I got you email from the Haskell Cafe list. Hope you don't mind. > Recently I was running a simple program in Haskell and keep getting *** > Exception: stack overflow error ! > > I don't know how to solve it or handle it ! I tried to find the same > problem on the web in order to get some guide but it wasn't successful.Hope > you can give me some clue. > In the future, why don't you ask this kind of question on haskell-cafe or haskell-beginners? > > > Here is the code : > > import Control.Monad > import Text.Printf > > sumit :: Int -> Int > sumit n = sumCal 0 n 0 where > sumCal i n sum | i < n = sumCal (i+1) n (sum+i) > | otherwise = sum > This is probably your problem. If you do tail recursion, it needs to be strict. If you don't strictly tail recurse, you end up returning a massive thunk (i.e. a lazy expression like 0+1+2+3+4+5+6+7+8+9+10+11+12+...) which needs to be evaluated all in one go, which will overflow your stack. It is a little tricky, I know... sumCal i n sum | i < n = sumCal (i+1) n $! sum+i | otherwise = sum However, there is a function "sum" in the prelude, so you can do this much more simply: sumit :: Int -> Int sumit n = sum [1..n] :-) Luke > > loopOut :: Int -> IO() > loopOut n | n <= 1000000 > = do > loopIn 0 (1000000000*n) n 0 > loopOut (n*10) > | otherwise = printf "Done!" > > > loopIn :: Int -> Int -> Int -> Int -> IO() > loopIn i ub n sum | i < ub > = do > loopIn (i+1) ub n (sumit n) > printf "n=%10d sum=%15d" n sum > | otherwise = printf "\n" > > main :: IO () > main > =do > printf "Started .." > loopOut 1000 > > Actually the next step it to parallelize the code with STM.But even at this > stage it doesn't work ! So, if you can help me with this,I'd be grateful. > > Thanks, > Mozhgan > > ------------------------------ > " Upgrade to Internet Explorer 8 Optimised for MSN. " Download Now<http://extras.uk.msn.com/internet-explorer-8/?ocid=T010MSN07A0716U> >

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GADT on the wiki: I'm lost
by Peter Verswyvelen 23 Apr '09

23 Apr '09

I was reading the explanation of GADTs on the wiki<http://en.wikibooks.org/wiki/Haskell/GADT> , and but can't make any sense of the examples. Sure I understand what a GADT is, but I'm looking for practical examples, and the ones on the wiki seem to show what you *cannot* do with them... For example, the article gives an alternative approach to the safeHead function (see code below) But now that does not work either, since Cons x y never evaluates to MarkedList x Safe, so safeHead (Cons 1 Nil) will give a type error... Am I missing something or is this wikibook just confusing? Does anybody have good links to examples of GADTs? Yampa surely seems a good example, but it's a bit too advanced. data NotSafe data Safe data MarkedList :: * -> * -> * where Nil :: MarkedList t NotSafe Cons :: t -> MarkedList t y -> MarkedList t z safeHead :: MarkedList x Safe -> x safeHead (Cons x _) = x silly 0 = Nil silly 1 = Cons () Nil silly n = Cons () $ silly (n-1)

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Base version on Hackage?
by Magnus Therning 23 Apr '09

23 Apr '09

I'm not sure why building of my recently uploaded version of dataenc fails to build on Hackage[1]. Where can I find out what version of base is available on Hackage? /M [1]: http://hackage.haskell.org/packages/archive/dataenc/0.12.1.0/logs/failure/g… -- Magnus Therning (OpenPGP: 0xAB4DFBA4) magnus＠therning．org Jabber: magnus＠therning．org http://therning.org/magnus identi.ca|twitter: magthe Haskell is an even 'redder' pill than Lisp or Scheme. -- PaulPotts

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ANN: Elerea, another FRP library
by Patai Gergely 23 Apr '09

23 Apr '09

Hi everyone, I'm pleased to announce Elerea, aka "Eventless reactivity", a minimalistic FRP implementation that - comes with a convenient applicative interface (similar to Reactive) - supports recursive definition of signals - supports signals fed from outside by IO actions - supports dynamism in the signal structure (I think ;) - seems to play nice with resources, especially memory - is based on some unsafePerformIO dark magic (that might easily break depending on many factors) - might have some parallelisation potential - has absolutely no formal foundations, it's just the result of some furious hacking over the last few days! There are working examples to show off the current capabilities of the library, found in the separate elerea-examples package. Have fun playing with it! Gergely -- http://www.fastmail.fm - A no graphics, no pop-ups email service

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Random number example
by michael rice 23 Apr '09

23 Apr '09

I pretty much followed the sequence of steps that led to this final code (see below), but will be looking it over for a while to make sure it sinks in. In the meantime, I get this when I try to use it (sumTwoDice) at the command line: [michael@localhost ~]$ ghci rand9 GHCi, version 6.10.1: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer ... linking ... done. Loading package base ... linking ... done. [1 of 1] Compiling Main ( rand9.hs, interpreted ) Ok, modules loaded: Main. *Main> sumTwoDice <interactive>:1:0: No instance for (Show (Seed -> (Int, Seed))) arising from a use of `print' at <interactive>:1:0-9 Possible fix: add an instance declaration for (Show (Seed -> (Int, Seed))) In a stmt of a 'do' expression: print it *Main> Can I employ a 'do' expression from the command line? Also, can I now use functions (>>) (>>=) and 'return' defined in the Prelude and still have this code work? Michael ================== {-# LANGUAGE NoImplicitPrelude #-} import Prelude hiding ((>>), (>>=), return) type Seed = Int type Random a = Seed -> (a, Seed) randomNext :: Seed -> Seed randomNext rand = if newRand > 0 then newRand else newRand + 2147483647 where newRand = 16807 * lo - 2836 * hi (hi,lo) = rand `divMod` 127773 rollDie :: Random Int rollDie seed = ((seed `mod` 6) + 1, randomNext seed) (>>) :: Random a -> Random b -> Random b (>>) m n = \seed0 -> let (result1, seed1) = m seed0 (result2, seed2) = n seed1 in (result2, seed2) (>>=) :: Random a -> (a -> Random b) -> Random b (>>=) m g = \seed0 -> let (result1, seed1) = m seed0 (result2, seed2) = (g result1) seed1 in (result2, seed2) return :: a -> Random a return x = \seed0 -> (x, seed0) sumTwoDice :: Random Int sumTwoDice = rollDie >>= (\die1 -> rollDie >>= (\die2 -> return (die1 + die2)))

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Can't find space leak in external sort with tournament trees (test case attached)
by Denis Bueno 23 Apr '09

23 Apr '09

Hello haskell-cafe, I'm running into what I think is a space leak. I've modified external-sort-0.2 [0] to use tournament trees for merging, and although the block sorting works in constant space, merging seems to produce a space leak. The external sort works by lazily consuming an input list, chopping it up into fixed size blocks, sorting each block in memory and writing each sorted block out to disk. At this point we have a big, on-disk array of k sorted blocks. To produce the final, sorted output list, we lazily read back each block into a tournament tree (a heap of sorts), which should only compare the first element of block to figure out the tree root. The merging algorithm in full is in `tourMerge' [1]. I've confirmed by profiling (with -hc) that the merge step gobbles up memory. I've attached a self-contained program (two haskell source files). It requires binary, mersenne-random*, and ArrayRef from hackage. $ tar xvzf leak.tar.gz leak.hs Data/TournamentTree.hs $ ghc -O --make leak.hs $ ./leak 5000000 # generate 5 million entries in ./entries.tmp, sort them, and output to ./leak.out Writing 5000000 entries to './entries.tmp' Done. Henceforth, if you omit the number argument to leak it will sort './entries.tmp'. Sorting 5000000 entries from './entries.tmp' External block sort using 1000 blocks: -- 5000 entries per block -- 240000 bytes per block 1000--blocks sorted. Merging blocks and writing sorted entries to './leak.out' Depending on how much RAM you have, you may need to tweak the argument to leak and/or numBlocks (at the top of leak.hs) -- a block needs to be able to fit in RAM. If the argument to leak is too low, the sort will succeed and the leak won't be apparent. Otherwise, the OS will kill the process because it allocated too much. Anyone have an idea how I can fix the leak? Denis *Only required to generate the initial input file to sort; it's much faster than System.Random. [0] http://hackage.haskell.org/cgi-bin/hackage-scripts/package/external-sort-0.2 [1] tourMerge is in leak.hs, but I've also inlined it here for convenience. TT is the Data.TournamentTree module, included in the tarball I've attached. tourMerge :: (Ord a) => [[a]] -> [a] tourMerge [] = [] tourMerge blks = let t = {-SCC "TMfromList" #-} TT.fromList blks in tM (TT.minElem t) (TT.deleteMinF t) where tM :: (Ord a) => [a] -> TT.Forest [a] -> [a] tM l f | null f = l | otherwise = let t = TT.tournament f next = TT.minElem t (x, xs) = {-# SCC "TTspan" #-} span (<= head next) l in x ++ (tM next (if null xs then TT.deleteMinF t else (TT.singleton xs : TT.deleteMinF t)))

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Overriding a Prelude function?
by michael rice 23 Apr '09

23 Apr '09

I've been working through this example from: http://en.wikibooks.org/wiki/Haskell/Understanding_monads I understand what they're doing all the way up to the definition of (>>), which duplicates Prelude function (>>). To continue following the example, I need to know how to override the Prelude (>>) with the (>>) definition in my file rand.hs. Michael ============== [michael@localhost ~]$ cat rand.hs import System.Random type Seed = Int randomNext :: Seed -> Seed randomNext rand = if newRand > 0 then newRand else newRand + 2147483647 where newRand = 16807 * lo - 2836 * hi (hi,lo) = rand `divMod` 127773 toDieRoll :: Seed -> Int toDieRoll seed = (seed `mod` 6) + 1 rollDie :: Seed -> (Int, Seed) rollDie seed = ((seed `mod` 6) + 1, randomNext seed) sumTwoDice :: Seed -> (Int, Seed) sumTwoDice seed0 = let (die1, seed1) = rollDie seed0 (die2, seed2) = rollDie seed1 in (die1 + die2, seed2) (>>) m n = \seed0 -> let (result1, seed1) = m seed0 (result2, seed2) = n seed1 in (result2, seed2) [michael@localhost ~]$

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Hac5 roundup
by Martijn van Steenbergen 23 Apr '09

23 Apr '09

Dear Haskell Hackers, Many thanks to all those who attended Hac5! We had a spectacular number of participants: over 50 hackers showed up, representing several countries. Many thanks also to the organising committee and sponsors! Those bagels on Sunday were delicious. :-) Please check out the updated wiki page [1] for: * a nice group photo of most attendants taken by Thomas Davie; * a list of blog posts/galleries/videos of the event; * a list of projects worked on and the progress that was made. Additionally, if you covered Hac5 in some way or contributed to any of the projects, please add your links or progress summary to the page. Thanks, Martijn. [1] http://www.haskell.org/haskellwiki/Hac5

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