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Anyone here from New Zealand?
by Tim Matthews 01 Aug '10

01 Aug '10

Any of the haskellers here from NZ? Are you using haskell in production, internally within your company or just outside of work in your own time? In Aus they've got hackathons<http://www.haskell.org/haskellwiki/AusHac2010>and user groups <http://www.haskell.org/haskellwiki/User_groups#Australia>. Anything even remotely similar here in New Zealand? Thanks Tim Matthews

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Problem loading hxt
by David Place 01 Aug '10

01 Aug '10

Hello: I am trying to load hxt into my Haskell Platform 2010.2.0.0 on OSX. I get the following bizarre comment: David-Places-Mac-Mini:dev2 davidplace$ cabal install hxt Resolving dependencies... cabal: dependencies conflict: ghc-6.12.3 requires directory ==1.0.1.1 however directory-1.0.1.1 was excluded because ghc-6.12.3 requires directory ==1.0.1.2 I see that I have the older directory in my global package list and the newer one in my local list. Help? Thanks. David-Places-Mac-Mini:dev2 davidplace$ ghc-pkg list /Library/Frameworks/GHC.framework/Versions/612/usr/lib/ghc-6.12.3/package.conf.d Cabal-1.8.0.6 GLUT-2.1.2.1 HTTP-4000.0.9 HUnit-1.2.2.1 OpenGL-2.2.3.0 QuickCheck-2.1.1.1 array-0.3.0.1 base-3.0.3.2 base-4.2.0.2 bin-package-db-0.0.0.0 bytestring-0.9.1.7 cgi-3001.1.7.3 containers-0.3.0.0 deepseq-1.1.0.0 directory-1.0.1.1 dph-base-0.4.0 dph-par-0.4.0 dph-prim-interface-0.4.0 dph-prim-par-0.4.0 dph-prim-seq-0.4.0 dph-seq-0.4.0 extensible-exceptions-0.1.1.1 ffi-1.0 fgl-5.4.2.3 filepath-1.1.0.4 ghc-6.12.3 ghc-binary-0.5.0.2 ghc-prim-0.2.0.0 haskell-src-1.0.1.3 haskell98-1.0.1.1 hpc-0.5.0.5 html-1.0.1.2 integer-gmp-0.2.0.1 mtl-1.1.0.2 network-2.2.1.7 old-locale-1.0.0.2 old-time-1.0.0.5 parallel-2.2.0.1 parsec-2.1.0.1 pretty-1.0.1.1 process-1.0.1.3 random-1.0.0.2 regex-base-0.93.2 regex-compat-0.93.1 regex-posix-0.94.2 rts-1.0 stm-2.1.2.1 syb-0.1.0.2 template-haskell-2.4.0.1 time-1.1.4 unix-2.4.0.2 xhtml-3000.2.0.1 zlib-0.5.2.0 /Users/davidplace/.ghc/i386-darwin-6.12.3/package.conf.d Cabal-1.8.0.6 binary-0.5.0.2 directory-1.0.1.2 fingertree-0.0.1.0 haskell98-1.0.1.1 process-1.0.1.3 random-1.0.0.2 time-1.2.0.3 uu-parsinglib-2.4.2 uu-parsinglib-2.4.4 uu-parsinglib-2.4.5 uu-parsinglib-2.5.0 uulib-0.9.12

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minimizing jitter while maximizing throughput
by Claude Heiland-Allen 01 Aug '10

01 Aug '10

Greeting, Jitter vs Throughput ==================== Scenario -------- I have the following scenario: CPU with [C] cores concurrent program the 1 main thread uses OpenGL for animated visual output [W] worker threads uses FFI to lengthy numerical computations with the following desires : (J) minimize jitter : the 1 main thread needs to be responsive (T) maximize throughput: idle CPU time is a waste of time The problem is, I want both (J) and (T)! Benchmarks ---------- Some rough benchmarks from my 'production' implementation[1] where: jitter = stddev of actual period (target period = 40ms) idle = C - (real time) / (user+sys time) (cores used elsewhere) (but for example Xorg will use some time for the OpenGL output, etc) C W jitter idle 2 0 1.5ms 168% threaded RTS 2 1 1.9ms 65% " " 2 2 9.4ms 46% " " 2 3 13.3ms 37% " " For comparison, a (very) minimal GLUT program gives: 0.3ms threaded RTS 0.5ms non-threaded RTS Picking (J) over (T), then setting W=1 when C=2 gives best jitter Picking (T) over (J), then setting W=2 when C=2 gives best throughput Question -------- What is the best way to re-structure the program to have both low jitter and high throughput? Options I am considering: 1. worker threads estimate the time needed to complete each job, and don't start a job if it is likely to break the deadline (bonus points if just those worker Haskell threads running on the same OS thread as the main Haskell thread need to pause) 2. change the foreign code to break jobs into smaller pieces, for example perhaps something like: worker :: IO (IO Bool) -> IO () worker getJob = forever $ do job <- getJob whileM_ job yield -- [2] instead of worker :: IO (IO ()) -> IO () worker = forever . join 3. re-implement the foreign code in Haskell instead of C and hope that GHC makes the Haskell go as fast as GCC makes the C go 5. wait (for a long time) for a new RTS with: full pre-emption (including interrupting/resuming foreign code) user settable thread priorities (1) is "a fun challenge" (there may be PhDs awarded for less, I imagine) (2) isn't quite trivial, some scope for tuning subjob chunk size (3) is boring translation but could lead to interesting benchmarks even (especially?) if it fails to be as fast as C Which would you pick? Links ----- [1] http://hackage.haskell.org/package/mandulia [2] http://hackage.haskell.org/packages/archive/monad-loops/latest/doc/html/Con… Thanks for any insight and advice, Claude -- http://claudiusmaximus.goto10.org

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dear traversable
by Ben 01 Aug '10

01 Aug '10

dear traversable geniuses -- i am looking for "better" implementations of unzipMap :: M.Map a (b, c) -> (M.Map a b, M.Map a c) unzipMap m = (M.map fst m, M.map snd m) unliftMap :: (Ord a) => M.Map a (b -> c) -> M.Map a b -> M.Map a c unliftMap mf ma = M.mapWithKey (\k v -> mf M.! k $ v) ma the first is obviously inefficient as it traverses the map twice. the second just seems like it is some kind of fmap. any ideas? ben

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Type Families: deleting from HList.
by Serguey Zefirov 31 Jul '10

31 Jul '10

Is it possible to delete an element from heterogenous list using type families alone? I can do it using multiparameter type classes: class Del a list result instance Del a (a,list) list instance Del a list list' => Del a (a',list) list' instance Del a () () I tried to express the same using type families: type family Del a list type instance Del a () = () type instance Del a (a,list) = list type instance Del a (a',list) = (a',Del a list) to no avail. I got "conflicting family instance declarations" error. I tried to express that with associated type synonyms that contains both multiparameter type classes and type families: class HDel a list where type Del a list instance HDel a () where type Del a () = () instance HDel a (a,list) where type Del a (a,list) = list instance HDel a list => HDel a (a',list) where type Del a (a',list) = (a',Del a list) And I once again got "conflicting family instance declaration". It looks like right now there is no way to express type equality or inequality with type families, even when they are combined with multiparameter type classes. What is the right way to express something like that? Only MPTC?

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Constructor question
by michael rice 31 Jul '10

31 Jul '10

From: Data.Complex data (RealFloat a) => Complex a = !a :+ !a What's the purpose of the exclamation marks? Michael

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Monad transformers, design
by Tony Morris 31 Jul '10

31 Jul '10

Hello I have a question regarding monad transformers and how to design an API with a transformer. I have a narrowed code example of the question. Please see the questions in the comments below. import Data.Monoid import Control.Monad -- Suppose some data type newtype Inter a = Inter (Int -> a) -- and a monad transformer for that data type. newtype InterT m a = InterT (m (Inter a)) -- It's easy to implement this type-class instance (Monoid a) => Monoid (Inter a) where mempty = Inter (const mempty) Inter a `mappend` Inter b = Inter (a `mappend` b) -- and for the transformer too by lifting into the monad instance (Monad m, Monoid a) => Monoid (InterT m a) where mempty = InterT (return mempty) InterT a `mappend` InterT b = InterT (liftM2 mappend a b) -- But what about this type-class? class Ints a where ints :: a -> Int -> Int -- Seems easy enough instance (Integral a) => Ints (Inter a) where ints (Inter a) n = fromIntegral (a n) -- OH NO! {- instance (Monad m, Integral a) => Ints (InterT m a) where ints (InterT a) n = error "OH NO!" -} -- We could try this class Copointed f where copoint :: f a -> a -- but it seems rather impractical instance (Copointed m, Integral a) => Ints (InterT m a) where ints (InterT a) = ints (copoint a) {- So it seems that for some type-classes it is possible to implement for both the data type and the transformer, but not all type-classes. Is there a general approach to this problem? -} -- Tony Morris http://tmorris.net/

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using the orc library
by Günther Schmidt 31 Jul '10

31 Jul '10

Hello, I'd like to download 1,000 web pages with up to 6 six concurrent downloads at a time. How can I express such a thread limit within the orc EDSL? Günther

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Re: Re: Can we come out of a monad?
by Jason Catena 31 Jul '10

31 Jul '10

On Jul 30, 11:17 am, Anton van Straaten wrote: > Prelude> :m Control.Monad.State > Prelude Control.Monad.State> let addToState :: Int -> State Int (); > addToState x = do s <- get; put (s+x) > Prelude Control.Monad.State> let mAdd4 = addToState 4 > Prelude Control.Monad.State> :t mAdd4 > m :: State Int () > Prelude Control.Monad.State> let s = execState mAdd4 2 > Prelude Control.Monad.State> :t s > s :: Int > Prelude Control.Monad.State> s > 6 By this example State doesn't seem to give you anything more than a closure would, since it doesn't act like much of an accumulator (by, for example, storing 6 as its new internal value). Could you use State for something like storing the latest two values of a Fibonacci series? For example, each time you call it, it generates the next term, discards the oldest term, and stores the newly-generated term? And could you then use this Fibonacci State monad in a lazy computation, to grab for example the first twenty even Fibonacci numbers, without computing and storing the series beyond what the filter asks for? We can generate Fibonacci series double-recursively in a lazy computation. Would it be more or less efficient to use a Fibonacci State monad instead? Would the State implementation provide a larger range before it blew the stack (which tail-recursion should prevent), or became too slow for impatient people? Would Haskell memoize already-generated values in either case? Could we write a general memoizer across both the recursive and State implementations, or must we write a specific one to each case? Jason Catena

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Announce type-level-natural-number-1.0: Simple, Haskell 2010-compatible type level natural numbers
by Gregory Crosswhite 31 Jul '10

31 Jul '10

Hey everyone, I am pleased to announce the release of the "type-level-natural-number" package, which implements natural numbers at the type level. Although there are many packages on Hackage that implement type level natural numbers, this package is distinguished by its simplicity: it does not offer any operations other than predecessor, successor, and conversion to Int, and so the only Haskell extension it needs is EmptyDataDecls. Thus, this package is compatible with Haskell 2010. The envisioned use case is for annotating types with natural numbers, so that extra functionality such as addition and subtraction of natural numbers is unnecessary. Nothing is stopping someone from implementing this functionality, and in fact I may do so myself; however, since such functionality is not needed merely for using these numbers, I decided to leave it out in order to avoid having to use non-Haskell 2010 extensions (especially UndecidableInstances). In particular, a major motivation behind designing the package in this way is to make it more appealing as a standard means for representing natural number annotations in order to promote interoperability between libraries. I welcome any feedback that the community has to offer. Cheers, Gregory Crosswhite

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