- Haskell-Cafe - Haskell.org

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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Combining computations
by michael rice 03 May '09

03 May '09

If you look at this stuff long enough it almost begins to make sense. Maybe. ;-) I've been messing around with MonadPlus and I understand its usage with the Maybe and List monads. Since one use of Monads is combining computations, how can I combine a Maybe with a List? let m1 = Nothing let m2 = [1] let m3 = m1 `mplus` m2 ==> [1] --if the Maybe is Nothing, do nothing let m1 = Just 1 let m2 = [] let m3 = m1 `mplus` m2 ==> [1] --if the Maybe is not Nothing, add it to the list Or am I misunderstanding combining computations? Michael

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Re: [Haskell-cafe] Combining computations
by Kalman Noel 03 May '09

03 May '09

michael rice schrieb: > let m1 = Just 1 > let m2 = [] > let m3 = m1 `mplus` m2 ==> [1] --if the Maybe is not Nothing, add it to the list > > Or am I misunderstanding combining computations? You just got the type of mplus wrong: mplus :: (MonadPlus m) => m a -> m a -> m a Note that it takes two values of the same type (m a), but you're giving it values of different types. That is, combining computations of different types is not within the scope of MonadPlus. In this case, it makes sense to convert (Just 1) to [1] via Data.Maybe.maybeToList, thus: m1 = Just 1 m2 = [2,3] m3 = maybeToList m1 `mplus` m2 -- [1,2,3] Note also that, in this example, Monoid (mappend) instead of MonadPlus (mplus) would be sufficient. Actually MonadPlus becomes useful only when you are concerned about the extra properties that its instances are expected to satisfy. Kalman

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Dynamically altering sort order
by Denis Bueno 03 May '09

03 May '09

Hi all, Suppose I have the following interface to a sorting function: sort :: (Ord a) => [a] -> IO [a] -- sort large, on-disk array of records but I don't have a sortBy where you can simply pass a compare function. Wrapped around this is a command-line program which should allow the user to specify different orderings on Records. For example, if the Record has three fields, the user should be able to say "sort only on the first two". Is there an Ord instance that can be dynamically changed in this way? My first idea is something like this: data CompareRecord = CR{ rCompare :: Record -> Record -> Ordering, unCR :: Record } instance Ord CompareRecord where compare (CR cmp x) (CR _ y) = cmp x y where the rCompare field would be a function that is based on the flags passed to the command-line problem. But this has an ugly asymmetry. Does anyone have any other ideas? Thanks. Denis

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Runge-Kutta and architectural style
by Richard O'Keefe 02 May '09

02 May '09

I was interested to see a Runge-Kutta package posted to this list recently, particularly as I have a fairly simple-minded non-adaptive RK "generator": an AWK script that takes a table and some optional stuff and spits out C. The Haskell package is, of course, a lot prettier than my AWK program, as well as offering some adaptive methods, which is important. We can imagine a spectrum of RK packages: (1) Higher order function taking some runtime parameters. That's what we got. (2) The same specialised for a known table at compile time. Doable using the code that we were given plus SPECIALIZE pragmas. I don't how well that works across modules. (3) A generator that writes Haskell source. (4) Template Haskell. (5) A generator that generates native code to be called through FFI. The question is, how do you decide what's the appropriate approach?

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Decoupling OpenAL/ALUT packages from OpenGL
by Sven Panne 02 May '09

02 May '09

I'd like to get some feedback from the Haskell community about some packaging issues, so here is my problem: As a medium-term goal, I'd like to decouple the OpenAL/ALUT packages from the OpenGL package, because there are very sensible use cases where you might need some sound, but not OpenGL. The current coupling has mostly historic reasons. The OpenAL package depends on the OpenGL package in 3 areas: * OpenAL uses OpenGL's notion of StateVars all over the place. * OpenAL's Buffer and Source are instances of OpenGL's ObjectName class. * OpenAL's sources and listeners have some properties like velocity, orientation, direction and position which are modeled by OpenGL's Vector3 and Vertex3 data types. The ALUT package depends on the OpenGL package because of GettableStateVars. The packages are supposed to fit nicely together, so using the same types is a must, but the actual packaging is not nice. So the obvious idea is to introduce 3 new packages which lift out functionality from the OpenGL package: * a small "StateVar" package, consisting only of OpenGL's StateVar module (in a better place in the name hierarchy, of course, perhaps "Data.StateVar"?) * a tiny "ObjectName" package, consisting only of OpenGL's ObjectName class (In "Data.ObjectName"? I'm not very sure about a good place in the hierarchy here.) * a package containing most of the data types/newtypes in OpenGL's VertexSpec module (Vertex{2,3,4}, Color{3,4}, etc.) plus instances for the base classes like Eq, Ord, Show, etc. I really don't know a good name for this package and what a good place in the hierarchy would be (probably something like "Data.Foo", but what is Foo?) The point is: The first two package would be very small. Would this be OK? Does anybody see other alternatives? What would be good names for those packages and where in the naming hierarchy should they really go? Cheers, S.

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