April 2009 - Haskell-Cafe

Functor and Haskell
by Daryoush Mehrtash 24 Apr '09

24 Apr '09

In category theory functors are defined between two category of C and D where every object and morphism from C is mapped to D. I am trying to make sense of the above definition with functor class in Haskell. Let say I am dealing with List type. When I define List to be a instance of a functor I am saying the source category (C) is Haskell types and the destination category is List (D). In this the "fmap" is implementation of the mapping between every morphism in my Haskell Categroy (C) to morphism in List cataegory (D). With type constructor I also have the mapping of types (objects in Haskell Category, or my source cataegroy C) to List category (D). So my functor in the catarogy sense is actually the fmap and type constructor. Am I remotely correct? If this is correct.... With this example, then can you then help me understand the transformation between functors and natural transformations? Specifically, what does it means to have two different functors between Haskell cateogry and List category? Thanks, Daryoush

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can I query if package is in Hackage?
by Vasili I. Galchin 24 Apr '09

24 Apr '09

Hello, Hoogle allows me to query about Haskell functionality. But is there a mechanism for querying about a package, e.g. Swish? Kind regards, Vasili

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Re: [Haskell-cafe] HPong-0.1.2 fails to compile in Debian ghc 6.10.1
by trentbuck＠gmail.com 23 Apr '09

23 Apr '09

"Ahn, Ki Yung" <kyagrd(a)gmail.com> writes: >>> I think this is because the filename of the OpenGL shared library is >>> /usr/lib/libGL.so.1 rather than libGL.so. >> >> No. Do not manually create the symlink. It sounds like you don't >> have the -dev packages for OpenGL installed. > > I do already have the dev package libgl1-mesa-dev installed. [...] > It's just seems to the bug of libgl1-mesa-dev 7.4-2 packaging. It > doesn't seem to make the symbolic link which it claims to make. Duh The file exists in that package, so it seems to me the problem is on your workstation, rather than in the package. I suggest you just reinstall the package in question. You may also want to use cruft(8) to find out if any other files have gone missing, and run a fsck over your root partition. $ aptitude download -t unstable libgl1-mesa-dev [...] Get:1 http://mirror.internode.on.net unstable/main libgl1-mesa-dev 7.4-2 $ dpkg -x libgl1-mesa-dev_7.4-2_all.deb . $ find -name \*.so -ls 13003 0 lrwxrwxrwx 1 twb twb 10 Apr 24 10:41 ./usr/lib/libGL.so -> libGL.so.1 $

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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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