March 2011 - Haskell-Cafe

ANNOUNCE: Haskell Platform 2011.2 released!
by Don Stewart 10 Mar '11

10 Mar '11

We're pleased to announce the 2011.2 release of the Haskell Platform: a single, standard Haskell distribution for everyone. Download the Haskell Platform 2011.2.0.0: http://haskell.org/platform/ The specification, along with installers (including Windows, Apple and Unix installers for a full Haskell environment) are available. The Haskell Platform is a single, standard Haskell distribution for every system, in the form of a blessed library and tool suite for Haskell distilled from the thousands of libraries on Hackage, along with installers for a wide variety of systems. It saves developers work picking and choosing the best Haskell libraries and tools to use for a task. When you install the Haskell Platform, you get the latest stable compiler, an expanded set of core libraries, additional development tools, and cabal-install – so you can download anything else you need from Hackage. This release ships with GHC 7. What you get is specified here: http://hackage.haskell.org/platform/contents.html Thanks! -- The Platform Infrastructure Team P.S. a big thanks to Mark Lentczner and Mikhail Glushenkov who built the Mac and Windows installers!

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Re: [Haskell-cafe] Parallel Haskell stories
by John Lato 10 Mar '11

10 Mar '11

From: Simon Peyton-Jones <simonpj(a)microsoft.com> > > Friends > > I'm giving a talk at a developer conference in London on Friday 18th, about > parallel programming in Haskell. > http://skillsmatter.com/event/scala/functionalpx-2011/ad-1382 > > I know that some of you have been using Haskell for parallel or concurrent > programming quite a bit, so this email is just to ask > > could you contribute a little vignette or story > about using Haskell in a *parallel/concurrent* application > that I could use to illustrate my talk? > I have a physical-model-based audio synthesis application I wanted to convert from an off-line renderer into a real-time engine. I decided upon a concurrent architecture with an interface thread, an engine thread, and an output thread, using STM for communication. It only took a couple hours to make the conversion, most of which was spent modifying the synthesis to work in blocks instead of a single run. I'm particularly happy with this snippet, where the engine collects input from the interface thread: readStrikeVar = do ct <- getCPUTime atomically $ rsv' ct `orElse` return [] -- if the read fails, return an empty list because we can't afford to wait. -- the strikes will eventually catch up... rsv' ct = do strikes <- readTVar tstrikes let carry = map (updateCarry blockdur) $ filter carryOver strikes writeTVar tstrikes carry writeTVar tsync ct return (reverse strikes) This code is the most complicated part of the communication scheme, nearly everything else is at the level of "readTVarIO". A nice feature is that the engine doesn't block waiting for the interface. If the transaction fails because the interface is updating the TVar, the engine moves ahead and can catch up later. On my dual-core macbook, performance is perhaps slightly disappointing compared to the offline renderer which is single-threaded and not parallelized. With one simulation the offline renderer will generate 5 seconds of audio in about 3.3seconds at 100% CPU usage, giving me the equivalent of 66% CPU over 5 seconds. The realtime renderer consistently occupies about 171% CPU for the same simulation. A different simulation has a runtime of 4.4 seconds and realtime usage of 191%. Of course there's going to be some overhead, but I hope to improve this by changing the way data is fed to the engine to a TChan, and also running threadscope to make sure threads yield at the proper times. Speaking of which, threadscope is an awesome tool and deserves a mention. John L.

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ANN: Updates in the monadic regions family
by Bas van Dijk 10 Mar '11

10 Mar '11

Dear all, I released new versions of some of my packages. Here are the changelogs: http://hackage.haskell.org/package/regions-0.9 http://hackage.haskell.org/package/regions-mtl-0.3.1.5 http://hackage.haskell.org/package/regions-monadstf-0.3.1.5 * Switch from monad-peel to monad-control. * Removed Control.Monad.Trans.Region.Concurrent. The fork functions contained bugs which could not be fixed. I'm working on a way to copy handles from one thread to another but the design has not settled yet. * Removed Data.RegionRef. I always considered this module a bit of a wart. * Added support for local regions. Primarily needed for the alloca functions from the regional-pointers package. More on that below. http://hackage.haskell.org/package/regional-pointers-0.6 * Switch from monad-peel to monad-control. * Major API change: Use the new local regions. This means that instead of: alloca ∷ (Storable α, MonadPeelIO pr) ⇒ (∀ s. RegionalPtr α (RegionT s pr) → RegionT s pr β) → pr β I now have: alloca ∷ (Storable α, MonadControlIO pr) ⇒ (∀ sl. LocalPtr α (LocalRegion sl s) → RegionT (Local s) pr β) → RegionT s pr β This allows my alloca to use the native alloca which is more efficient than doing a manual malloc and free as I did before. The type also allows to open resources in the continuation and use them outside the continuation and visa versa. * Have separate types for the three different pointers: - RegionalPtr: for pointers created using malloc. - LocalPtr: for pointers created using alloca. - NullPtr: for the nullPtr. * Add the type classes Pointer and AllocatedPointer to classify them. http://hackage.haskell.org/package/safer-file-handles-0.10 http://hackage.haskell.org/package/safer-file-handles-bytestring-0.2 http://hackage.haskell.org/package/safer-file-handles-text-0.2 * Switch from monad-peel to monad-control. * Support regional-pointers-0.6 and use its overloaded pointers. * Add separate types for standard handles: StdFileHandle and normal handles: RegionalFileHandle. * Add the type class FileHandle to group them. * Overload the filehandle in operations. http://hackage.haskell.org/package/usb-0.8 * getStrDesc and getStrDescFirstLang now return a Text instead of a String. This is a more efficient and cleaner interface. http://hackage.haskell.org/package/usb-enumerator-0.3 * Switch from monad-peel to monad-control. * Support iteratee-0.8.*. * Support usb-0.9. http://hackage.haskell.org/package/usb-safe-0.12 * Switched from monad-peel to monad-control. * Exported the ReadEndpoint, WriteEndpoint and EnumReadEndpoint type classes so you can refer to them in type signatures. * Support usb-0.8: getStrDesc and getStrDescFirstLang now return a Text instead of a String. * Support regions-0.9. * Support usb-enumerator-0.3. * Support iteratee-0.8.*. Regards, Bas

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Re: [Haskell-cafe] Haskell mail server fail?
by wren ng thornton 10 Mar '11

10 Mar '11

Like Kenneth Hoste, I haven't been receiving mails from haskell-cafe@ nor libraries@ for a few days to a week now. What is the status of the mailing lists? (Please CC me off-list, for obvious reasons) -- Live well, ~wren

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Re: [Haskell-cafe] Tagless interpreter, expression problem and zipper
by oleg＠okmij.org 10 Mar '11

10 Mar '11

The Tagless final approach can do context-sensitive evaluation, using the well-known trick from the denotational semantics that explicating the context turns context-sensitive semantics to compositional. The trick isn't out of place given how much tagless-final approach is related to denotational semantics. The non-compositional processing has been discussed at length at lectures last year. Please see the explanation and the code at Non-compositionality: Fold-unlike processing http://okmij.org/ftp/tagless-final/course/course.html#non'compositionality Interestingly, one example FlatF.hs mentioned on the above page specifically talks about the context of being the left operand of the addition (the goal was to `flatten' additions, to re-associate them to the right). That is the same context as you are after, it seems. The detailed lecture notes will be eventually available.

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Update Cabal
by Hauschild, Klaus (EXT) 10 Mar '11

10 Mar '11

Hallo, I'm using Haskell Platform 2010.2.0.0 on a Windows XP machine. This haskell platform includes cabal-1.8.0.6. Now I want to update cabal by "cabal install cabal". Installation works well. Call like "runhaskell ./Setup.hs" will use the updatetd cabal-1.10.0.0. But "cabal --version" says still 1.8.0.6 and I have to re-configure. How I update cabal in my current Haskel Platform? Thanks Klaus

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IPv6 issues for (code|community).haskell.org?
by Erlend Hamberg 10 Mar '11

10 Mar '11

Hi, When I wanted to get the newest xmonad code from darcs today¹ it was really, really slow. Running “host” revealed that code.haskell.org is an alias for community.haskell.org and that it has an IPv6 address associated with it. Being on an IPv6 network, and having been burnt by similar problems before, I tried adding IPv4 address for code.haskell.org in /etc/hosts in order to force the connection to happen over IPv4. This fixed the issue and “darcs get” now takes a few minutes. I just wanted to alert people in case the problem is at the server end. (And in case this isn’t just a temporary problem.) Connecting to code.haskell.org in a Web browser also takes minutes, while e.g. http://ipv6.google.com/ works perfectly. [1] With “darcs get http://code.haskell.org/xmonad” -- Erlend Hamberg ehamberg(a)gmail.com

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Custom monad using ST
by Yves Parès 10 Mar '11

10 Mar '11

Hello, I am trying to make a monad that uses ST internally. But even when reducing this to the simplest case I'm still cramped by the 's' phantom type : {-# LANGUAGE Rank2Types #-} newtype MyST a = MyST (forall s. ST s a) -- ^ I cannot use " deriving (Monad) " through GeneralizedNewtypeDeriving runMyST (MyST m) = runST m -- ^ works thanks to declaration of 's' at rank 2 in the definition of MyST -- It refuses to compile if MyST is declared as such: -- data MyST s a = MyST (ST s a) instance Monad MyST where return = MyST . return -- and this does not compile (MyST m) >>= f = MyST $ do x <- m case f x of (MyST m) -> m If you try it, GHC will complain: Simple.hs:13:20: Couldn't match expected type `forall s. ST s a' with actual type `ST s a' Expected type: a -> forall s1. ST s1 a Actual type: a -> ST s a In the second argument of `(.)', namely `(return :: a -> (forall s. ST s a))' In the expression: MyST . (return :: a -> (forall s. ST s a))

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Automated tests with cabal
by Hauschild, Klaus (EXT) 10 Mar '11

10 Mar '11

Hi Haskellers, I read about the cabal features for running test code from Setup.hs with "defaultMainWithHooks". I'm looking for more generic code that allows me to place any haskell in a subdirectory "test" or so and "cabal test" will run this test without any modification of my Setup.hs. Is there a possibility? Thanks Klaus

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Learn You a Haskell for Great Good - a few doubts
by Karthick Gururaj 10 Mar '11

10 Mar '11

Hello, I'm learning Haskell from the extremely well written (and well illustrated as well!) tutorial - http://learnyouahaskell.com/chapters. I have couple of questions from my readings so far. In "typeclasses - 101" (http://learnyouahaskell.com/types-and-typeclasses#typeclasses-101) there is a paragraph that reads: Enum members are sequentially ordered types - they can be enumerated. The main advantage of the Enum typeclass is that we can use its types in list ranges. They also have defined successors and predecesors, which you can get with the succ and pred functions. Types in this class: (), Bool, Char, Ordering, Int, Integer, Float and Double. What is the "()" type? Does it refer to a tuple? How can tuple be ordered, let alone be enum'd? I tried: Prelude> take 10 [(1,1) ..] <interactive>:1:8: No instance for (Enum (t, t1)) arising from the arithmetic sequence `(1, 1) .. ' at <interactive>:1:8-17 Possible fix: add an instance declaration for (Enum (t, t1)) In the second argument of `take', namely `[(1, 1) .. ]' In the expression: take 10 [(1, 1) .. ] In the definition of `it': it = take 10 [(1, 1) .. ] This is expected and is logical. But, surprise: Prelude> (1,1) > (1,2) False Prelude> (2,2) > (1,1) True Prelude> (1,2) > (2,1) False Prelude> (1,2) < (2,1) True So tuples are in "Ord" type class atleast. What is the ordering logic? Another question, on the curried functions - specifically for infix functions. Suppose I need a function that takes an argument and adds five to it. I can do: Prelude> let addFive = (+) 5 Prelude> addFive 4 9 The paragraph: "Infix functions can also be partially applied by using sections. To section an infix function, simply surround it with parentheses and only supply a parameter on one side. That creates a function that takes one parameter and then applies it to the side that's missing an operand": describes a different syntax. I tried that as well: Prelude> let addFive' = (+5) Prelude> addFive' 3 8 Ok. Works. But on a non-commutative operation like division, we get: Prelude> let x = (/) 20.0 Prelude> x 10 2.0 Prelude> let y = (/20.0) Prelude> y 10 0.5 So a curried infix operator fixes the first argument and a "sectioned infix" operator fixes the second argument? Regards, Karthick

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