May 2009 - Haskell-Cafe

gcd
by Steve 04 May '09

04 May '09

[Question moved over from Haskell-Beginners] I had a look at the gcd definition in GHC 6.10.1 ghc-6.10.1/libraries/base/GHC/Real.lhs -- | @'gcd' x y@ is the greatest (positive) integer that divides both @x@ -- and @y@; for example @'gcd' (-3) 6@ = @3@, @'gcd' (-3) (-6)@ = @3@, -- @'gcd' 0 4@ = @4@. @'gcd' 0 0@ raises a runtime error. gcd :: (Integral a) => a -> a -> a gcd 0 0 = error "Prelude.gcd: gcd 0 0 is undefined" gcd x y = gcd' (abs x) (abs y) where gcd' a 0 = a gcd' a b = gcd' b (a `rem` b) Why is gcd 0 0 undefined? http://en.wikipedia.org/wiki/Greatest_common_divisor says: "It is useful to define gcd(0, 0) = 0 and lcm(0, 0) = 0 because then the natural numbers become a complete distributive lattice with gcd as meet and lcm as join operation. This extension of the definition is also compatible with the generalization for commutative rings given below." An added advantage, for haskell, of defining gcd 0 0 = 0 is that gcd would change from being a partial function to a total function. Regards, Steve

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Research in functional programming
by Louis Wasserman 04 May '09

04 May '09

Where might I find or submit a paper on functional data structures? Examples I've found so far include ICFP <http://www.icfpconference.org/> and the JFP <http://journals.cambridge.org/action/displayJournal?jid=JFP>, but Google hasn't found me anything else. Louis Wasserman wasserman.louis(a)gmail.com

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*system*-time timeout?
by brian＠lorf.org 04 May '09

04 May '09

System.Timeout.timeout is OK for wall clock time, but can I limit the amount of system time an IO action can take?

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converting IOException to Either in ErrorT
by brian＠lorf.org 04 May '09

04 May '09

I wrote this to make it a little nicer to catch IO exceptions and convert them to ErrorT failure: onExceptionThrowError :: (Error ce) => IO a -> (String -> ce) -> ErrorT ce IO a onExceptionThrowError a ce = liftIO (try a) >>= either (\(e :: IOException) -> throwError (ce (show e))) return So now I can do, e.g., writeFile fp s `onExceptionThrowError` SpecificErrorConstructor ... It works, but seems bad. Please let me know if you see ways to improve it. Thanks.

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ANN: Silkworm game
by Duane Johnson 03 May '09

03 May '09

Reprinted from my blog post [1]: === The semester is over, my final project was a success (at least in that I passed the class) and it’s time now to release the game I made for Graphics 455: Silkworm! This is my first full application in Haskell. The process has been an enlarging experience–I’ve come to really enjoy the mental work that goes into thinking about a program in a functional way. I highly recommend the challenge to other software engineers. Silkworm combines the Hipmunk binding to Chipmunk 2D Game Dynamics with OpenGL, and GLFW (an alternative to GLUT). It’s built to work on Mac OS X, but it uses cross-platform libraries so it should be fairly easy to port to other platforms. The source code is here [2] and below are some screenshots [1] -- Duane Johnson (canadaduane) === [1] http://blog.inquirylabs.com/2009/05/02/silkworm-game-written-in-haskell/ [2] http://inquirylabs.com/downloads/Silkworm.tgz

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Combining computations
by michael rice 03 May '09

03 May '09

I posted something similar about an hour ago but it seems to have gotten lost. Very strange. I've read that Monads can combine computations. Can a Maybe monad be combined with a List monad such that Nothing `mplus` [] ==> [] Just 1 `mplus` [] ==> [1] If not, can someone supply a simple example of combining computations? Michael

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Haskell vs Clean (speed comparison)
by Daniel Carrera 03 May '09

03 May '09

Hi, I think the mail server may have been acting up earlier. I sent this to Haskell-beginners, but it more properly belongs here. I found something interesting. "General wisdom" is that Clean (or OCaml) is faster than Haskell. The claim is often followed by a link to the Debian shootout. But on closer inspection, I question this conclusion. The Debian shoot out actually has four sets of benchmarks: 1) Intel 32-bit one core. 2) Intel 32-bit quad-core. 3) Intel 64-bit one core. 4) Intel 64-bit quad-core. It turns out that Clean is only faster for (1). For the others, Haskell is faster. Here I compare Haskell, Clean, OCaml, Lisp SBCL, C# Mono and Fortran because they are all in the same ball mark: 32-bit sing core [1]: Lisp, Fortran, Clean, Haskell, C# Mono. 32-bit quad-core [2]: Haskell, C# Mono, Lisp, Clean, Fortran. 64-bit sing core [3]: Fortran, OCaml, Haskell, Clean, C# Mono, Lisp. 64-bit quad-core [4]: Haskell, OCaml, Lisp, C# Mono, Fortran, Clean. Notes: * The order is "fast language first". * There are no results for OCaml for 32-bit. * "Lisp" is "List SCBL" whatever that is. Tentative conclusions: 1) Haskell makes very good use of multiple cores. It smokes Clean. 2) For single core, they are neck and neck. Whether Clean is faster depends non the architecture. What do you think? Daniel. P.S. [1]http://shootout.alioth.debian.org/u32/benchmark.php?test=all&lang=all&d=d… [2]http://shootout.alioth.debian.org/u32q/benchmark.php?test=all&lang=all&d=… [3]http://shootout.alioth.debian.org/u64/benchmark.php?test=all&lang=all&d=d… [4]http://shootout.alioth.debian.org/u64q/benchmark.php?test=all&lang=all&d=…

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using haskell for a project
by Nicolas Martyanoff 03 May '09

03 May '09

Hi, I don't think I already presented myself; I'm Nicolas, a 23y french student, trying to learn and use haskell. I've been using C for years, for all sort of tasks, and am quite comfortable with it. I'm also using it 40h a week in my internship for network systems, so I kind of know how to use it. I discovered Haskell some monthes ago, bought `Real World Haskell', quickly read, and enjoyed it. So now I'd want to use it for a small project of mine, a simple multiplayer roguelike based on telnet. I wrote a minimal server in C, and it took me a few hours. Now I'm thinking about doing the same in Haskell, and I'm in trouble. I don't really know how to map my ideas in haskell code. For example, a character can cast spells, so I'd have something like this in C: struct hashtable spells; struct character { int n_spells; struct spell **spells; }; I thought I could do something like this in haskell: spells = Data.Map.Map Int Spell data Character = Character { charSpells :: [Int] } But now I don't know how to dynamically add new spells (new spells can be created in my gameplay). Since I can't assign a new value to the `spells' variable (Data.Map.insert returns a new map), I just don't know where to go. I have the same problem for a bout every problem. I love writing pure functions in haskell, but as soon as I try to write some code involving states or side effects, I can't write a line. I just wanted a 2d array to store a zone, for example, dead simple in C, but this kind of link http://greenokapi.net/blog/2009/03/10/rough-grids-in-haskell make me shiver. Point is, I'd like to use haskell, but I don't know how, it seems totally alien. How did you manage to change the way you map ideas to code, from imperative to pure functional ? Thank you. Regards, -- Nicolas Martyanoff http://codemore.org khaelin(a)gmail.com

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Array Binary IO & molecular simulation
by Grigory Sarnitskiy 03 May '09

03 May '09

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Re: [Haskell-cafe] Combining computations
by michael rice 03 May '09

03 May '09

Thanks for all the help, everyone. I think this stuff is starting to come together. Michael --- On Sun, 5/3/09, Tillmann Rendel <rendel(a)cs.au.dk> wrote: From: Tillmann Rendel <rendel(a)cs.au.dk> Subject: Re: [Haskell-cafe] Combining computations To: "michael rice" <nowgate(a)yahoo.com> Cc: haskell-cafe(a)haskell.org Date: Sunday, May 3, 2009, 7:33 AM Hi, normally, one uses monads to express and combine computations in the same monad. However, you can convert between some monads, e.g. from Maybe to List: import Data.Maybe (maybeToList) > let m1 = Nothing > let m2 = [1] > let m3 = maybeToList m1 `mplus` m2 > let m1 = Just 1 > let m2 = [] > let m3 = maybeToList m1 `mplus` m2 In fact, you can convert from Maybe to any MonadPlus. maybeToMonadPlus Nothing = mzero maybeToMonadPlus (Just x) = return x And you can convert from List to any MonadPlus: listToMonadPlus Nothing = [] listToMonadPlus (x : xs) = return x `mplus` listToMonadPlus xs Now you should be able to do: m1 = maybeToMonadPlus (Just 1) m2 = listtoMonadPlus [2, 3] m3 = m1 `mplus` m2 :: Just Int -- Just 1 m4 = m1 `mplus` m2 :: [Int] -- [1, 2, 3] The reason this is possible is that Maybe and List do not support additional effects beyond what is common to all MonadPlus instances. Another option is to never specify which monad your computations are in in the first place. Instead, only specify which computational effects the monad should support. m1 = mzero :: MonadPlus m => m a m2 = return 1 :: (Monad m, Num a) => m a m3 = m1 `mplus` m2 `mplus` Just 2 -- Just 1 m4 = m1 `mplus` m2 `mplus` [2, 3] -- [1, 2, 3] In this version, m1 and m2 are polymorphic computations, which can be used together with List computations, Maybe computations, or any other MonadPlus instances. m1 needs MonadPlus, while m2 is happy with any Monad instance. This fact is encoded in their type. Tillmann

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