- Haskell-Cafe - Haskell.org

Software patents covered in GHC?
by Shakthi Kannan 20 Jun '11

20 Jun '11

Hi, I would like to know if there are any software algorithms used in the Glasgow Haskell Compiler that have been patented? If yes, in which countries do they apply? Just curious to know. SK -- Shakthi Kannan http://www.shakthimaan.com

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Text.CSV questions
by Dmitri O.Kondratiev 20 Jun '11

20 Jun '11

Hi, I try to parse csv file with Text.CSV, like this: import Text.CSV import System main = do [inpFileName] <- getArgs putStrLn ("Parsing "++inpFileName++"...") let result = parseCSVFromFile inpFileName print result === As a result I get: No instance for (Show (IO (Either Text.Parsec.Error.ParseError CSV))) arising from a use of `print' Possible fix: add an instance declaration for (Show (IO (Either Text.Parsec.Error.ParseError CSV))) === Question: How to add a Show instance for "(IO (Either Text.Parsec.Error.ParseError CSV)))" ? Thanks!

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Re: [Haskell-cafe] Semantics of ForeignPtr Finalizers
by Antoine Latter 19 Jun '11

19 Jun '11

Sending to the list as well. On Jun 19, 2011 9:58 AM, "Antoine Latter" <aslatter(a)gmail.com> wrote: > On Jun 19, 2011 7:49 AM, "Bas van Dijk" <v.dijk.bas(a)gmail.com> wrote: >> >> On 19 June 2011 14:13, wren ng thornton <wren(a)freegeek.org> wrote: >> > The documentation claims that >> > the finalizers are run only once the ForeignPtr becomes unreachable, > which >> > implies that keeping the ForeignPtr is sufficient to keep the foreign >> > object alive. >> >> Right, that was also my thinking when designing the usb library. What >> made me doubt this was the implementation of storable vectors: >> >> > http://hackage.haskell.org/packages/archive/vector/0.7.1/doc/html/src/Data-… >> >> Note that a storable vector stores both a foreign pointer and a >> pointer (presumably so that you don't need to add an offset to the >> pointer, inside the foreign pointer, each time you use it): >> >> data Vector a = Vector !(Ptr a) !Int !(ForeignPtr a) >> >> Now, if we follow the above semantics we don't need to call >> withForeignPtr when we need to read the memory because the foreign >> pointer is referenced by the vector. >> However the vector library still calls withForeignPtr. For example: >> >> basicUnsafeIndexM (Vector p _ fp) i = return >> . unsafeInlineIO >> $ withForeignPtr fp $ \_ -> >> peekElemOff p i >> >> So is the withForeignPtr redundant so that this function can be >> rewritten to just: >> >> basicUnsafeIndexM (Vector p _ _) i = return >> . unsafeInlineIO >> peekElemOff p i >> > > I'd be afraid of optimizations getting ride of the constructor, and since > I'm not using all of the fields I would no longer have a reference to the > foreign ptr. > > Since withForeignPtr touches the foreign pointer the optimizations won't > make it go away no matter how it transfoms the function. > > Antoine > >> Regards, >> >> Bas >> >> _______________________________________________ >> Haskell-Cafe mailing list >> Haskell-Cafe(a)haskell.org >> http://www.haskell.org/mailman/listinfo/haskell-cafe

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Re: [Haskell-cafe] Semantics of ForeignPtr Finalizers
by wren ng thornton 19 Jun '11

19 Jun '11

On 6/19/11 3:52 AM, Bas van Dijk wrote: > Hello, > I have a question about the semantics of finalizers of ForeignPtrs: If an in scope value has a reference to a ForeignPtr is the foreign object always kept alive, even if that reference is not used? Or do I always need to wrap each computation, which needs to have the foreign object alive, inside withForeignPtr? I know that the Ptr obtained by withForeignPtr will _not_ keep the foreign object alive (though withForeignPtr itself will be sure to keep it alive until the function using the Ptr exits). For instance, this shows up with ByteStrings if you try to save the Ptr somewhere and then use it after withForeignPtr exits. I believe that ForeignPtr itself does promise to keep the foreign object alive, since it's what holds the finalizers. The documentation claims that the finalizers are run only once the ForeignPtr becomes unreachable, which implies that keeping the ForeignPtr is sufficient to keep the foreign object alive. I haven't used ForeignPtr for truly foreign objects, only for pseudo-foreign objects like ByteString, but in my experience just having the ForeignPtr is enough to keep the target alive. -- Live well, ~wren

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Semantics of ForeignPtr Finalizers
by Bas van Dijk 19 Jun '11

19 Jun '11

Hello, I have a question about the semantics of finalizers of ForeignPtrs: If an in scope value has a reference to a ForeignPtr is the foreign object always kept alive, even if that reference is not used? Or do I always need to wrap each computation, which needs to have the foreign object alive, inside withForeignPtr? To make my question more concrete, I will give a simplified example from my usb library (a high-level binding to the libusb C library): https://github.com/basvandijk/usb To use the library you first need to initialize libusb which will yield a context: newtype Ctx = Ctx {getCtxFrgnPtr ∷ (ForeignPtr C'libusb_context)} When you're finished with libusb you need to exit the library. This is done automatically using a ForeignPtr: newCtx ∷ IO Ctx newCtx = mask_ $ do ctxPtr ← libusb_init Ctx <$> newForeignPtr p'libusb_exit ctxPtr where libusb_init ∷ IO (Ptr C'libusb_context) libusb_init = alloca $ \ctxPtrPtr → do handleUSBException $ c'libusb_init ctxPtrPtr peek ctxPtrPtr I provide a handy internal utility function for accessing the pointer to the context: withCtxPtr ∷ Ctx → (Ptr C'libusb_context → IO α) → IO α withCtxPtr = withForeignPtr ∘ getCtxFrgnPtr Because this function uses withForeignPtr it's guaranteed that the context is kep alive during the duration of the given function. For example, in the following, it is guaranteed that c'libusb_set_debug is always given a live context: setDebug ∷ Ctx → Verbosity → IO () setDebug ctx verbosity = withCtxPtr ctx $ \ctxPtr → c'libusb_set_debug ctxPtr $ genFromEnum verbosity There are also types which are derived from a context. For example: getDevices ∷ Ctx → IO [Device] is a function which given a context returns a list of USB devices currently attached to your system. The requirement is that when you need to use a device the context has to be kept alive. Therefor I keep a reference to the context inside the device: data Device = Device { getCtx ∷ !Ctx , getDevFrgnPtr ∷ !(ForeignPtr C'libusb_device) , deviceDesc ∷ !DeviceDesc } The idea is that as long as you have a reference to a device you also keep the context alive because the device references the context. (Note that a device, in turn, also contains a ForeignPtr) getDevices ∷ Ctx → IO [Device] getDevices ctx = ... where mkDev ∷ Ptr C'libusb_device → IO Device mkDev devPtr = liftA2 (Device ctx) (newForeignPtr p'libusb_unref_device devPtr) (getDeviceDesc devPtr) Again I provide a handy internal utility function for accessing the pointer to a device: withDevicePtr ∷ Device → (Ptr C'libusb_device → IO α) → IO α withDevicePtr = withForeignPtr ∘ getDevFrgnPtr And now my question. Is it guaranteed that when the device is in scope the context is always kept alive? For example, is the context kept alive in the following: openDevice ∷ Device → IO DeviceHandle openDevice dev = withDevicePtr dev $ \devPtr → alloca $ \devHndlPtrPtr → do handleUSBException $ c'libusb_open devPtr devHndlPtrPtr DeviceHandle dev <$> peek devHndlPtrPtr Or do I also need to call withCtxPtr inside withDevicePtr as such: withDevicePtr (Device ctx devFP _) f = withCtxPtr ctx $ \_ → withForeignPtr devFP f Thanks, Bas

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Re: [Haskell-cafe] Data Type Inheritance ala OO - Inheritence -- howto best in Haskell ?
by David Barbour 17 Jun '11

17 Jun '11

Look into Oleg's HList (heterogeneous list) and OOHaskell http://homepages.cwi.nl/~ralf/HList/ http://homepages.cwi.nl/~ralf/OOHaskell/ On Thu, Jun 16, 2011 at 5:08 AM, kaffeepause73 <kaffeepause73(a)yahoo.de>wrote: > Dear all, > > I'm created a timeSignal datatype as container around a "Vector Double" > data > type (see simple code below) and subsequently started to instanciate Num & > Eq to be able to perform operations on it. Additionally I want store ifno > like an index, time information and eventually an inheritence log (the log > is not yet in there). > > As I will in the end need up to 10 different datatypes, however using > slightly different content (time signal, single value, distribution, ...) I > ask myself, how I could define a super data-type with sub-data-types to > inherit, but then also overload certain functions (like u would do in OO). > > What is best way in haskell to achieve this ? (I'm unsure wether haskell > classes are what I'm looking for) > > Cheers Phil > > ########## Code below > > import qualified Data.Vector.Unboxed as V > > data TimeSig = TimeSig Int Double (V.Vector Double) -- signal Index > timeStep > data > > getVect :: TimeSig -> (V.Vector Double) > getVect (TimeSig idx dt vect)= vect > > getIdx :: TimeSig -> Int > getIdx (TimeSig idx dt vect) = idx > > getdt :: TimeSig -> Double > getdt (TimeSig idx dt vect) = dt > > pzipWith :: (Double -> Double -> Double) -> TimeSig -> TimeSig -> TimeSig > pzipWith f p1 p2 = TimeSig idx dt vect > where > vect = V.zipWith f (getVect p1) (getVect p2) > idx = getIdx p1 > dt = getdt p1 > > pmap :: (Double -> Double) -> TimeSig -> TimeSig > pmap f p = TimeSig (getIdx p) (getdt p) (V.map f (getVect p)) > > instance Num TimeSig > where > (+) p1 p2 = pzipWith (+) p1 p2 > (-) p1 p2 = pzipWith (-) p1 p2 > negate p1 = pmap negate p1 > abs p1 = pmap abs p1 > (*) p1 p2 = pzipWith (*) p1 p2 > > instance Eq TimeSig where > (==) p1 p2 = (==) (getVect p1) (getVect p2) > > > instance Show TimeSig where > show (TimeSig idx dt vect) = "TimeSignal Nr: " ++ show idx ++ " dt: " ++ > show dt ++ " val:" ++ show vect > > > > main = do > > let p = TimeSig 5 0.1 (V.fromList [0..10::Double]) > putStrLn (show p) > putStrLn (show (p+p)) > > -- > View this message in context: > http://haskell.1045720.n5.nabble.com/Data-Type-Inheritance-ala-OO-Inheriten… > Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com. > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe >

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ANNOUNCE: Groundhog-0.0.1.1, a database connectivity library
by Boris Lykah 17 Jun '11

17 Jun '11

Hi, everyone! I am happy to announce a new persistence library, Groundhog. http://hackage.haskell.org/package/groundhog http://hackage.haskell.org/package/groundhog-sqlite Groundhog does mapping between datatypes and database like Persistent in Haskell or Hibernate in Java and makes dealing with database very easy. The API is inspired by Persistent, a great framework by Michael Snoyman, from which I used some ideas and code. At first I wanted to enhance Persistent, but many useful ideas don't fit into its design. Groundhog offers features I wanted to see in a modern Haskell database library. My intent was to create a package which you can plug into your existing project and it would just work. So it does not require defining datatypes using its own mechanisms (quasiquotation, XML, etc). You can use your own types defined anywhere. The restrictions on type structure are very mild. Groundhog uses data families and GADTs and they did not play nicely together until GHC 7, so GHC 6.12.x and earlier is not supported. Currently there is support only for Sqlite but I hope to add more backends soon. Sqlite backend is based on direct-sqlite package by Dan Knapp. I modified it to improve performance and provide better error messages. Now it is bundled with groundhog-sqlite but I hope to merge it with direct-sqlite. On simple datatypes performance is ~2.5 times higher compared to Persistent. Some of this gain is achieved because of Sqlite specific optimisations, but I expect to see high performance on other backends as well. In fact, it could be even faster. I sacrificed ~30% of backend-independent performance for flexibility when I chose DbPersist to be a monad transformer instead of sticking with IO and replaced direct mentions of DbPersist with Monad m in PersistBackend. Features: * Persists datatypes defined in an ordinary way * Supports fields of user-defined types * Supports polymorphic datatypes and datatypes with several constructors * Basic support for lists and tuples * Type safety * Migration from an empty schema * Powerful expression DSL for use in queries * Execution of arbitrary queries * High performance Plans (in priority order): * Add more backends, particularly PostgreSQL * Allow migration when data definition changes * Add Persistent-like quasiquotation syntax Here is an example from the documentation: {-# LANGUAGE GADTs, TypeFamilies, TemplateHaskell #-} import Control.Monad.IO.Class(liftIO) import Database.Groundhog.TH import Database.Groundhog.Sqlite data Customer a = Customer {customerName :: String, details :: a} deriving Show data Item = ProductItem {productName :: String, quantity :: Int, customer :: Customer String} | ServiceItem {serviceName :: String, deliveryAddress :: String, servicePrice :: Int} deriving Show deriveEntity ''Customer $ Just $ do setConstructor 'Customer $ do setConstraints [("NameConstraint", ["customerName"])] deriveEntity ''Item Nothing main = withSqliteConn ":memory:" $ runSqliteConn $ do -- Customer is also migrated because Item contains it runMigration silentMigrationLogger $ migrate (undefined :: Item) let john = Customer "John Doe" "Phone: 01234567" johnKey <- insert john -- John is inserted only once because of the name constraint insert $ ProductItem "Apples" 5 john insert $ ProductItem "Melon" 2 john insert $ ServiceItem "Taxi" "Elm Street" 50 insert $ ProductItem "Melon" 6 (Customer "Jack Smith" "Don't let him pay by check") -- bonus melon for all large melon orders update [QuantityField =. toArith QuantityField + 1] (ProductNameField ==. "Melon" &&. QuantityField >. (5 :: Int)) productsForJohn <- select (CustomerField ==. johnKey) [] 0 0 liftIO $ putStrLn $ "Products for John: " ++ show productsForJohn -- check bonus melon <- select (ProductNameField ==. "Melon") [Desc QuantityField] 0 0 liftIO $ putStrLn $ "Melon orders: " ++ show melon Currently Hackage cannot build it due to technical issues (as Dons assumed several packages it depends on are not exposed), but you can install it with cabal. It is still very early beta and it may have some bugs. I am very interested to hear your feedback. Thanks, Boris Lykah

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ANNOUNCE: doctest-0.3.0
by Simon Hengel 17 Jun '11

17 Jun '11

I just uploaded a new version of doctest[1] to Hackage. WHAT IS doctest? ================ doctest is a port of Python's doctest[2] to Haskell. It can be used to verify, that examples in Haddock comments[3] do still work. This also provides you with a simple mechanism to write unit test, without the burden of maintaining a dedicated test suite. A basic example of usage is at [4]. WHAT'S NEW IN THIS VERSION? =========================== It is now possible to intersperse comments between a longer, continuing example. All examples within the same comment now share a namespace. The following now works : -- | Calculate Fibonacci number of given 'Num'. -- -- First let's set `n` to ten: -- -- >>> let n = 10 -- -- And now calculate the 10th Fibonacci number: -- -- >>> fib n -- 55 fib :: Integer -> Integer fib 0 = 0 fib 1 = 1 fib n = fib (n - 1) + fib (n - 2) Thanks to Sakari Jokinen for this contribution! In addition I changed the name from DocTest to doctest. I think using all lower-case package names is a good thing. And as we will use doctest as a library in the near future, this was the last chance for this change. Cheers, Simon [1] http://hackage.haskell.org/cgi-bin/hackage-scripts/package/doctest [2] http://docs.python.org/library/doctest.html [3] http://www.haskell.org/haddock/doc/html/ch03s08.html#id566093 [4] http://haskell.org/haskellwiki/DocTest

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Simple CSV parser?
by Dmitri O.Kondratiev 17 Jun '11

17 Jun '11

Hi, This time I am looking for a simple CSV parser that supports commas and quotes. I have no time now to learn Parsec, so I hope to find something simple and easy to use. Thanks!

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type signature of parsec functions and how to warp them up.
by 吴兴博 17 Jun '11

17 Jun '11

I have some different parsers of Parsec to use in a project, and I want to make a warp function to make the testing easy. here is some of my body of parser : they all has type of "parsecT ***" stringSet :: ParsecT String u Identity [String] intSet :: ParsecT String u Identity [Integer] tupleSet :: ParsecT String u Identity [(String, String)] all of the returned type are instance of 'Show'. then I write these warp function: ------------------ import System.IO import Data.Functor.Identity (Identity) import Text.Parsec.Prim (ParsecT, runParserT, parse, Stream) runIOParse :: (Show a) => ParsecT String u Identity a -> String -> IO () runIOParse pa fn = do inh <- openFile fn ReadMode outh <- openFile (fn ++ ".parseout") WriteMode instr <- hGetContents inh let result = show $ parse pa fn instr hPutStr outh result hClose inh hClose outh ------------------- > :l RunParse.hs ------------------- RunParse.hs:12:31: Could not deduce (u ~ ()) from the context (Show a) bound by the type signature for runIOParse :: Show a => ParsecT String u Identity a -> String -> IO () at RunParse.hs:(7,1)-(15,15) `u' is a rigid type variable bound by the type signature for runIOParse :: Show a => ParsecT String u Identity a -> String -> IO () at RunParse.hs:7:1 Expected type: Text.Parsec.Prim.Parsec String () a Actual type: ParsecT String u Identity a In the first argument of `parse', namely `pa' In the second argument of `($)', namely `parse pa fn instr' Failed, modules loaded: none. ------------------- then I modify the type signature of 'runIOParse': runIOParse :: (Show a) => ParsecT String () Identity a -> String -> IO () then load again > :l RunParse.hs ------------------- RunParse.hs:12:25: Could not deduce (Stream String Identity t0) arising from a use of `parse' from the context (Show a) bound by the type signature for runIOParse :: Show a => ParsecT String () Identity a -> String -> IO () at RunParse.hs:(7,1)-(15,15) Possible fix: add (Stream String Identity t0) to the context of the type signature for runIOParse :: Show a => ParsecT String () Identity a -> String -> IO () or add an instance declaration for (Stream String Identity t0) In the second argument of `($)', namely `parse pa fn instr' In the expression: show $ parse pa fn instr In an equation for `result': result = show $ parse pa fn instr Failed, modules loaded: none. ------------------- I also tried some 'possible fix' in the information, but it still failed to pass the compiler. Main Question: **** How can I warp a parsec function interface for do the IO test with different 'ParsecT String u Identity a'? -- ---------------- 吴兴博 Wu Xingbo

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Haskell: The Craft of Functional Programming 3rd ed is out.
by KC 17 Jun '11

17 Jun '11

http://www.haskellcraft.com/craft3e/Home.html -- -- Regards, KC

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Can one drop GHC 7.0.4 into the Haskell Platform or does the platform need to be updated?
by KC 17 Jun '11

17 Jun '11

-- -- Regards, KC

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Haskell on NUMA
by Michael Lesniak 17 Jun '11

17 Jun '11

Hi, what ist the current state of Haskell's parallel performance on NUMA machines? I've (probably) run into some performance problems using a 2x8 cores machine and am unsure how to fix them (if it's possible at all at the moment). I've only found two (helpful?) resources on the web, namely - an old ticket which is closed [1] - a current talk (13. May 2011) about a new runtime system for GHC7 for NUMA machines [2] Are any other informations available? Kind regards, Michael [1] http://hackage.haskell.org/trac/summer-of-code/ticket/1123 [2] http://www.dcs.gla.ac.uk/~jsinger/mmnet11_talks/orczyk.pdf

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8th Ghent Functional Programming Group Meeting on Thursday, the 30th of June, 2011
by Jeroen Janssen 16 Jun '11

16 Jun '11

(apologies if you receive multiple postings) Dear All, We are pleased to announce the 8th Ghent Functional Programming Group (GhentFPG) meeting, which takes place on Thursday, the 30th of June, 2011 at 19:30 in the Technicum building of Ghent University (Sint-Pietersnieuwstraat 41, 9000 Gent). As before, the electronic sliding doors will be locked, but a phone number that you can call to get in will be provided at the doors on the far left of the building. The structure of the meeting is a bit different than before: we begin with two short talks and end with a small problem solving activity where everyone tries to solve a given problem in his favorite functional language. The exact problem will be given at the meeting. The abstracts for the talks are as follows: Pieter Wuille ------------- I'll talk about the monadic constraint programming (MCP) framework, and in particular the Finite Domain layer i've developed for it. As explained by Tom Schrijvers in a previous talk, MCP is a general Haskell framework for Constraint Programming (CP), allowing users to search for assignments to variables that satisfy given constraints, while abstracting from the actual algorithm used to find these solutions. Finite Domain problems are a subset of CP where the variable's initial domains are finite, typically small integers. Problems that can be expressed elegantly in FD include Sudoku and N-Queens. MCP's FD layer provides an embedded Domain Specific Language (DSL) to describe these problems declaratively, some backend solvers, and an intermediate translation layer. I'll introduce the features and possibilities of the system by showing some examples, and explain some implementation details along the way. Jasper Van der Jeugt -------------------- I will give a talk about the techniques and patterns used in the Text library. Text is quickly becoming the de-facto standard type for presenting human-readable text in Haskell. My current work focuses on improving the performance of this library by porting the internally used encoding from UTF-16 to UTF-8. The talk will present a high-level overview of some tools and techniques to study and improve the performance of a Haskell program, including: strictness analysis, pitfalls for benchmarking, fusion frameworks, unboxed type representations, and GHC core. If you would like to receive updates on our activities, you can follow us on twitter (@ghentfpg), google groups (http://groups.google.com/group/ghent-fpg?pli=1) or linked.in (http://t.co/cjXbGyT) Hope to see you there! Andy Georges Jasper Van der Jeugt Jeroen Janssen The GhentFPG Organizing Committee

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Acquiring a random set of a specific size (w/o dups) from a range of Ints
by michael rice 16 Jun '11

16 Jun '11

Is there an (existing) way to select 5 Ints randomly (no duplicates) from a population, say 1-20 (inclusive)? Michael

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Haskell Parallel Digest 3
by Nicolas Wu 16 Jun '11

16 Jun '11

Parallel Haskell Digest ======================= Edition 3 2011-06-16 http://www.well-typed.com/blog/55 Hello Haskellers! Welcome to the third edition of the Parallel Haskell Digest, bringing you news, discussions and tasters of parallelism and concurrency in Haskell. The digest is made possible by the Parallel GHC project. More news about how we're doing below. This digest is brought to you by Eric Kow and Nicolas Wu. Nick will be taking over as maintainer of the Parallel Digest for the next few months. It's tutorial season in the Parallel Haskell world, with Simon Marlow's [Parallel and Concurrent Programming in Haskell][1], and Don Stewart's [Numeric Haskell: A Repa Tutorial][2]. The former gives a broad tour of parallelism and concurrency techniques in Haskell and the latter focuses on the Repa parallel array library. Both are very concrete and focus on real working code. Give them a try! News ---------------------------------------------------------------------- * Haskell in the Economist! The Economist article [Parallel Bars][3] discusses the rise of multicore computing, and the essential obstacle that programs have to be specially written with parallelism in mind. The article gives a tour of some problems (overhead and dependencies between subtasks, programmers being trained for sequential programming, debugging parallel programs) and possible solutions, among which is functional programming: > Meanwhile, a group of obscure programming languages used in > academia seems to be making slow but steady progress, crunching > large amounts of data in industrial applications and behind the > scenes at large websites. Two examples are Erlang and Haskell, > both of which are “functional programming” languages. Are we doomed to succeed? Word of the Month ---------------------------------------------------------------------- The word of the month (well, phrase really!) for this digest is *parallel arrays*. Parallel array manipulation fits nicely into Haskell's arsenal of parallel processing techniques. In fact, you might have seen the Repa library, as mentioned above, in the news a while back. And now there's a new tutorial on the [Haskell Wiki][2]. So what's all the fuss? Parallel arrays manipulation is a particularly nice way of writing parallel programs: it's pure and it has the potential to scale very well to large numbers of CPUs. The main limitation is that not all programs can be written in this style. Parallel arrays are a way of doing *data parallelism* (as opposed to *control parallelism*). Repa (REgular Parallel Arrays) is a Haskell library for high performance, regular, multi-dimensional parallel arrays. All numeric data is stored unboxed and functions written with the Repa combinators are automatically parallel. This means that you can write simple array code and get parallelism for free. As an example, Repa has been used for real time edge detection using a [Canny edge algorithm][12], which has resulted in performance comparable to the standard OpenCV library, an industry standard library of computer vision algorithms written in C and C++ (a single threaded Haskell implementation is about 4 times slower than the OpenCV implementation, but is on par when using 8 threads on large images). While the Canny algorithm written with Repa doesn't quite match the speed of its procedural counterparts, it benefits from all of Haskell's built in support for purity and type safety, and painlessly scales to multiple cores. You can find more details on Ben Lippmeier's [blog][10]. Parallel GHC project news ---------------------------------------------------------------------- The Parallel GHC Project is an MSR-funded project to promote the real-world use of parallel Haskell. Part of this project involves effort by Well-Typed to provide tools for use by the general community. Last week, Well-Typed were excited to [announce][11] that we have capacity to support another partner for the parallel project for at least another 12 months. So, if you have a project that involves scaling up to multiple cores, or that deals with some heavy duty concurrency, then we'd love to hear from you. In return for your time and commitment to the parallel project, we'll unleash our team of expert programmers to help build tools and provide support that will help you and the community towards making parallel Haskell even more accessible. For more information on the Parallel GHC project, on the [Parallel GHC Project wiki page][27]. Blogs, papers and packages ---------------------------------------------------------------------- * [Parallel and Concurrent Programming in Haskell (19 May)][1] Simon Marlow released version 1.1 of his tutorial introducing the main programming models available for concurrent and parallel programming in Haskell. "This tutorial takes a deliberately practical approach: most of the examples are real Haskell programs that you can compile, run, measure, modify and experiment with". The tutorial covers a lot of ground, including use of the new `Par` monad, so if you're still not sure where to start, this might be the place. * [Explicit speculative parallelism for Haskell's Par monad][4] Tomas Petricek builds off the Par monad to provide a way to write speculative computations in Haskell. If there are two different ways to compute something and each of the ways is faster for different kinds of input (but we don't know exactly which), one useful trick is to run both ways in parallel and return the result of the one that finishes first. Tomas' speculative `Par` monad lets you do just that, starting computations in parallel and cancelling unwanted ones. You can check his code out on [GitHub](https://github.com/tpetricek/Haskell.ParMonad) * [Etage-Graph, data-flow based graph algorithms (3 Apr)][5] Mitar announced the Etage-Graph package, which implements graph algorithms on top of his data-flow framework. He invites us to have a look and see how one might implement known control-flow algorithms in a data-flow manner, which has the appeal of being easy to parallelise. * [stm-chans (4 Apr)][6] The stm-chans library offers a collection of channel types, similar to Control.Concurrent.STM.TChan but with additional features. It offers bounded, closable, and bounded closable FIFO channels, along with a partial TChan compatibility layer. * [timeplot 0.3.0, Eugene Kirpichov (30 Apr)][7] Eugene Kirpichov announced the release of timeplot 0.3.0, a tool to help you visualize arbitrary time-dependent data, e.g. log files, into big picture visualisations. Eugene also linked to a presentation with plenty of graphical examples for the tool and its sister "splot", mostly on cluster computing use cases. The tools follow the basic philosophy of depending neither on the log format (the expected workflow is like `cat log | awk foo | plot`), nor on the type of data (Eugene wants to visualise arbitrary signals). You can also find more information in Eugene's [presentation][8]. Mailing list discussions --------------------------------------------------------------------- * [Weird multi-threading runtime behaviour of single-threaded program with GHC-7.0.3 (1 Apr)][13] Herbert Valerio Riedel has a program which parses a roughly 8 MiB JSON file (with the aeson library) without any use of parallelism or concurrency constructs: just a straightforward, demanding, single-threaded computation. He compiled the program with `-threaded` and ran with `+RTS -N12` on a 12 core machine. By rights, only one core should be doing work but when he looks at the output of `top` it seems like all of them consume a substantial amount of CPU cycle. What's up with the remaining 11? * [A parallelization problem (25 Apr)][14] Wren Ng Thornton has some loopy code that he would like to parallelise. He was hoping to use pure parallelism (`par` and friends) instead of explicit concurrency because he's concerned that "lightweight as Haskell's threads are [...] the overhead would swamp the benefits." Wren's code has an outer and inner loop fleshing out a map of maps. Wren wants to speed up his inner loop, which he thinks should be possible because the keys in the inner map are independent of each other. * [Multi agent system (2 Apr)][15] Yves Parès wants to build a simple game with multiple agents that communicate with each other. He asked if it was reasonable to launch one Haskell thread per agent and have them communicate over Chans (he will have something like 200 agents at a time). Felipe Lessa responded that hundreds of agents should be no problem but hundreds of thousands of agents might not work. * [Using DPH (12 Apr)][16] When experimenting with Repa and DPH, Wilfried Kirschenmann encountered strange compiler errors when parallelising his code; in particular, a strange pattern match failure in a do expression. Wilfried supplied a small demonstrator computing the norm of a vector. Ben Lippmeier responded with help and also a reminder that Repa and DPH are separate projects (the latter still being in the "research prototype" stage). Ben's version with more tweaks from Wilfried was 15x faster than his original (non-parallel?) version but still 15x slower than the C version. Parallelising the Repa sum and fold functions should help. * [Service Component Architecture and Distributed Haskell (13 Apr)][17] James Keats noticed the recent talk about distributed Haskell and wished to see "interest from an experienced member of the Haskell community in implementing Haskell components with SCA/[Tuscany][18]", as this would help him to use Haskell in the same project with Java and Scala. * [parallel-haskell mailing list (14 Apr)][19] Eric Kow announced the parallel-haskell mailing list and provided instructions for subscribing to the list. Whereas the Haskell Cafe list and Stack Overflow are the best places to ask your parallelism and concurrency questions, the parallel-haskell list is a great place to follow and discuss the research. * [Questioning seq (14 Apr)][20] Among other questions about `seq`, Andrew Coppin asked how `pseq` was different from it. Austin Seipp responded that while pseq and seq are semantically equivalent, (A) `pseq` is only strict in its first argument whereas `seq` is strict in both and (B) `pseq` guarantees the order of evaluation so that `pseq a b` means `a` will be strictly evaluated before `b` whereas for the purpose of potential optimisations `seq` does not. Albert Y. C. Lai provided a short example showing that "there are non-unique evaluation orders to fulfill the mere strictness promise of `seq`". * [select(2) or poll(2)-like function? (17 Apr)][21] Matthias Kilian wanted to know if the Haskell standard libraries had anything like a select or poll like function. Answering the immediate question, Don Stewart pointed to [Control.Concurrent.threadWaitRead][22]. Meanwhile, the thread opened into a long discussion on using high-level Haskell concurrency constructs vs. low-level polling. For background, `select` (along with variants like poll, epoll, kqueue, etc) is a system-call which is often used for asynchronous IO, concurrently doing some processing work while some IO operation is underway (Matthias' use case is "networking stuff listening on v4 and v6 sockets at the same time"). A typical way of using "select" or its cousins would be to implement an event-driven programming model with a "select loop": sleeping until some condition occurs on a file descriptor and, when woken, executing the appropriate bit of code depending on the file descriptor and the condition. Some programmers opt for this approach because they believe using processes and OS threads would be too inefficient. Ertugrul Soeylemez and others argue that this is unnecessary in Haskell because Haskell threads are so lightweight (epoll under the hood); we can just stick with high-level concurrency constructs and trust the compiler and RTS to be smarter than us. On the other hand, Mike Meyer is sceptical about the scalability and robustness of this solution. The sort of code Mike wants to write is for systems "that run 7x24 for weeks or even months on end [where] [h]ardware hiccups, network failures, bogus input, hung clients, etc. are all just facts of life." Using `select` loops in his experience is not about efficiency, but avoiding the problems that arise from using high-level concurrency constructs in other languages. He favours to achieve concurrency with a "more robust and understandable" approach using separate Unix processes and an event-driven programming model. So far the only high-level approach Mike has found that provides for the sort of scalability is Eiffel's [SCOOP][23] How does Haskell stack up? And what about Haskell for the Cloud? * [Killing threads in foreign calls. (17 Apr)][24] Jason Dusek is building an application using Postgres for storage (with the help of the libpq binding on Hackage). He would like to kill off threads for queries that take too long to run, but trying Control.Concurrent.killThread seems not to work. This turns out to be a fairly non-trivial problem: most C code is not written to gracefully handle things like its underlying OS thread being killed off. The solution is to use whatever graceful cancellation mechanism is supplied by the foreign library; the PGCancel procedure in Jason's case. * [Painless parallelization. (19 Apr)][25] Grigory Sarnitskiy believes that parallel programming is actually easier than sequential programming because it allows one "to avoid sophisticated algorithms that were developed to gain performance on sequential architecture". So what are his options (preferably "release state" for writing pure functional parallel code with a Haskellish level of abstraction? Mats Rauhala pointed Grigory to the use of `par` and `pseq`, and the RWH chapter on parallelism and concurrency. Don Stewart followed up with a list of options: * the "parallel" package Repa (parallel arrays) * DPH (experimental) * using concurrency Grigory's question also prompted Eric Kow to prioritise the [Parallel Haskell portal][26]. Hopefully it will give us a standard resource to point new parallel Haskellers like Grigory to. Stack Overflow ---------------------------------------------------------------------- * [Howto kill a thread in Haskell](http://stackoverflow.com/questions/6185189/howto-kill-a-thread-in-… * [Comparing Haskell threads to kernel threads - is my benchmark viable?](http://stackoverflow.com/questions/5926501/comparing-haskell-threa… * [Fair concurrent `map` function in haskell?](http://stackoverflow.com/questions/5942615/fair-concurrent-map-fu… * [Program crashing when trying to create multiple threads](http://stackoverflow.com/questions/5935852/program-crashing-when-t… * [A way to form a 'select' on MVars without polling.](http://stackoverflow.com/questions/5879128/a-way-to-form-a-select… * [Haskell lightweight threads overhead and use on multicores](http://stackoverflow.com/questions/5847642/haskell-lightweight-… * [Haskell repa - mapping with indices](http://stackoverflow.com/questions/6242442/haskell-repa-mapping-wi… * [Do Accelerate and Repa have different use cases?](http://stackoverflow.com/questions/6260263/do-accelerate-and-repa-h… Help and feedback ---------------------------------------------------------------------- Got something for the digest? Send us an email at <parallel(a)well-typed.com>. We're particularly interested in short parallelism/concurrency puzzles, and cool projects for featured code. Other comments and feedback (criticism and corrections especially!) would be welcome too. [1]: http://community.haskell.org/~simonmar/par-tutorial.pdf [2]: http://www.haskell.org/haskellwiki/Numeric_Haskell:_A_Repa_Tutorial [3]: http://www.economist.com/node/18750706?story_id=18750706 [4]: http://tomasp.net/blog/speculative-par-monad.aspx [5]: http://www.haskell.org/pipermail/haskell-cafe/2011-April/090736.html [6]: http://www.haskell.org/pipermail/haskell-cafe/2011-April/090743.html [7]: http://www.haskell.org/pipermail/haskell-cafe/2011-April/091520.html [8]: http://jkff.info/presentations/two-visualization-tools.pdf [9]: http://hackage.haskell.org/package/repa [10]: http://disciple-devel.blogspot.com/2011/03/real-time-edge-detection-in-hask… [11]: http://www.well-typed.com/blog/54 [12]: http://en.wikipedia.org/wiki/Canny_edge_detector [13]: http://www.haskell.org/pipermail/haskell-cafe/2011-April/090673.html [14]: http://www.haskell.org/pipermail/haskell-cafe/2011-April/091325.html [15]: http://www.haskell.org/pipermail/haskell-cafe/2011-April/090708.html [16]: http://www.haskell.org/pipermail/haskell-cafe/2011-April/090970.html [17]: http://www.haskell.org/pipermail/haskell-cafe/2011-April/091014.html [18]: http://en.wikipedia.org/wiki/Apache_Tuscany [19]: http://www.haskell.org/pipermail/haskell-cafe/2011-April/091039.html [20]: http://www.haskell.org/pipermail/haskell-cafe/2011-April/091040.html [21]: http://www.haskell.org/pipermail/haskell/2011-April/022730.html [22]: http://hackage.haskell.org/packages/archive/base/latest/doc/html/Control-Co… [23]: http://scoop.origo.ethz.ch/ [24]: http://www.haskell.org/pipermail/haskell-cafe/2011-April/091089.html [25]: http://www.haskell.org/pipermail/haskell-cafe/2011-April/091134.html [26]: http://haskell.org/haskellwiki/Parallel [27]: http://www.haskell.org/haskellwiki/Parallel_GHC_Project

1 0

Data.Map: Values to keys and keys to values
by Dmitri O.Kondratiev 16 Jun '11

16 Jun '11

Hi, Data.Map has many great functions, yet I could not find the one that allows from one map create another map where keys are values and values are keys of the first one. Something like: transMap:: (Ord k, Ord a) => Map k a -> Map a k Does such function exist? Thanks!

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Re: [Haskell-cafe] Win32: cabal install --prefix=$HOME --user ?
by Dmitri O.Kondratiev 16 Jun '11

16 Jun '11

On Thu, Jun 16, 2011 at 6:18 PM, Ryan Yates <fryguybob(a)gmail.com> wrote: > There is a nice table in the cabal docs that explains how prefix gets used: > > http://www.haskell.org/ghc/docs/7.0.4/html/Cabal/builders.html#simple-paths > > For Vista and above C:\Documents and settings\myusername\ is > C:\Users\myusername. The equivalent to $HOME on Windows would be > $USERPROFILE but the syntax of expanding an environment variable is > going to depend on the shell you are using: > > cmd window: cabal install --prefix=%USERPROFILE% --user > PowerShell: cabal install --prefix=${env:USERPROFILE} --user > msys window: cabal install --prefix=$USERPROFILE --user > > Note that msys does define $HOME and it is a different physical path > than $USERPROFILE. > > Ryan, thanks! This win32 $HOME story is clear now. And why not just use: "cabal install" instead of: "cabal install --prefix=%USERPROFILE% --user" for win32 platform? Where in this case "cabal install" root will be?

2 1

Win32: cabal install --prefix=$HOME --user ?
by Dmitri O.Kondratiev 16 Jun '11

16 Jun '11

On win32 "cabal install --prefix=$HOME --user" fails: cabal install --prefix=$HOME --user Resolving dependencies... Configuring split-0.1.4... cabal: expected an absolute directory name for --prefix: $HOME cabal: Error: some packages failed to install: split-0.1.4 failed during the configure step. The exception was: ExitFailure 1 What is the right equivalent of "cabal install --prefix=$HOME --user" on Win32?

2 1

Data Type Inheritance ala OO - Inheritence -- howto best in Haskell ?
by kaffeepause73 16 Jun '11

16 Jun '11

Dear all, I'm created a timeSignal datatype as container around a "Vector Double" data type (see simple code below) and subsequently started to instanciate Num & Eq to be able to perform operations on it. Additionally I want store ifno like an index, time information and eventually an inheritence log (the log is not yet in there). As I will in the end need up to 10 different datatypes, however using slightly different content (time signal, single value, distribution, ...) I ask myself, how I could define a super data-type with sub-data-types to inherit, but then also overload certain functions (like u would do in OO). What is best way in haskell to achieve this ? (I'm unsure wether haskell classes are what I'm looking for) Cheers Phil ########## Code below import qualified Data.Vector.Unboxed as V data TimeSig = TimeSig Int Double (V.Vector Double) -- signal Index timeStep data getVect :: TimeSig -> (V.Vector Double) getVect (TimeSig idx dt vect)= vect getIdx :: TimeSig -> Int getIdx (TimeSig idx dt vect) = idx getdt :: TimeSig -> Double getdt (TimeSig idx dt vect) = dt pzipWith :: (Double -> Double -> Double) -> TimeSig -> TimeSig -> TimeSig pzipWith f p1 p2 = TimeSig idx dt vect where vect = V.zipWith f (getVect p1) (getVect p2) idx = getIdx p1 dt = getdt p1 pmap :: (Double -> Double) -> TimeSig -> TimeSig pmap f p = TimeSig (getIdx p) (getdt p) (V.map f (getVect p)) instance Num TimeSig where (+) p1 p2 = pzipWith (+) p1 p2 (-) p1 p2 = pzipWith (-) p1 p2 negate p1 = pmap negate p1 abs p1 = pmap abs p1 (*) p1 p2 = pzipWith (*) p1 p2 instance Eq TimeSig where (==) p1 p2 = (==) (getVect p1) (getVect p2) instance Show TimeSig where show (TimeSig idx dt vect) = "TimeSignal Nr: " ++ show idx ++ " dt: " ++ show dt ++ " val:" ++ show vect main = do let p = TimeSig 5 0.1 (V.fromList [0..10::Double]) putStrLn (show p) putStrLn (show (p+p)) -- View this message in context: http://haskell.1045720.n5.nabble.com/Data-Type-Inheritance-ala-OO-Inheriten… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

1 0

Best platform for development with GHC?
by Dmitri O.Kondratiev 16 Jun '11

16 Jun '11

Which platform - Mac OS X, Linux or Win32 is best for development with GHC today? How are things with Ubuntu? It was quite a while already that I used Haskell on Linux. Today I have to code on Win32 and Mac OS X. Installing extra libraries caused most of the pains for me on Win32 - for example GTK and Gnuplot is not easy to install on Win32. Installing GTK on Max OSX is also a hard work, but no problems with Gnuplot at all. Haskell mode for Emacs works fine on Mac OS X, but only to some extent on Win32. I have not managed to start GHCi in Emacs buffer on Win32. Haskell Win32 Platform also has a weird bug of missing documentation on some widely used modules from Haskell Hierarchical Library. I understand that Linux should be a much better environment for integrating with external libraries (just because most of them were originally developed for Linux). I don't want to start religious wars, yet I wonder on what platform Haskell run-time works best and development has minimum obstacles? Thanks!

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Parsing Haskell in Parsec
by Alex Rozenshteyn 16 Jun '11

16 Jun '11

I'm trying to write a parser for a small functional language in Parsec, as part of a larger project of porting this<http://www.cs.jhu.edu/%7Escott/pl/book/dist/>from ocaml to Haskell. I was wondering if there was a parser for Haskell written in Parsec that I could use as a reference. -- Alex R

4 5

Build failure on Hackage, missing syb
by José Pedro Magalhães 16 Jun '11

16 Jun '11

Hi all, What causes the following build failure on Hackage? *** setup configure > > Configuring instant-generics-0.3.2... > cabal-setup: At least the following dependencies are missing: > syb <0.4 > This is in package instant-generics-0.3.2<http://hackage.haskell.org/packages/archive/instant-generics/0.3.2/logs/fai…>. The syb package built fine on Hackage. Previously, packages depending on syb built fine too (e.g. http://hackage.haskell.org/package/syz) So why is instant-generics-0.3.2 not building? Thanks, Pedro

1 0

Beginners Question: Problem with Data Type Declaration
by kaffeepause73 16 Jun '11

16 Jun '11

I try to create an own data type containing "Vector Double" from the H-Matrix package. The code: ## data PowerSig = PowerSig Int Double Vector Double main = do let p5 = PowerSig 1 0.1 (fromList [1,2,3]) ## When compiling with ghci, I get however the following message: `Vector' is not applied to enough type arguments Expected kind `?', but `Vector' has kind `* -> *' In the type `Vector' In the definition of data constructor `PowerSig' In the data type declaration for `PowerSig' I don't understand the compilers message and couldn't find any wiki-page/other material to help me. The Data Declaration somehow seems not to understand that "Vector Double" belongs together, but brackets don't help either. Cheers Phil -- View this message in context: http://haskell.1045720.n5.nabble.com/Beginners-Question-Problem-with-Data-T… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

2 2

Haskell Weekly News: Issue 186
by Daniel Santa Cruz 16 Jun '11

16 Jun '11

Glad you are reading this issue! This is issue 186, not 180... Seems that the last couple of issues got stuck on 180. Hopefully you will find pointers to interesting things happening in the Haskell community. This issue covers the week of June 05 to 11, 2011. Announcements Hakim Cassimally shared with us slides and screencast from Geekup's first ever "Functional Programming Night." http://goo.gl/1oCO8 Johannes Waldmann sent an invite to a Local Haskell Meeting (Stammtisch) in Leipzig, Germany on June 27. http://goo.gl/UM9ci We are very proud of the two Simons, Simon Peyton Jones and Simon Marlow. Isaac Potoczny-Jones relayed the announcement from ACM SIGPLAN of the SIGPLAN Programming Language Software Award. Many congrats! http://goo.gl/sAuVv Quotes of the Week * augustss: So JHC has bad code. GHC had a lot of that a long time ago, and they have slowly been converting to the pure way. It's the only sensible way. * <geheimdienst> monochrom: i thought it maybe would end executing the current do block ... <monochrom> use "when". <mm_freak_> use "goto" <hpc> use Java * cmccann: programming is a lot easier if you avoid working with dates/times, text, or numbers. I say stick to category theory, it's simpler * xplat: the underlying graph of a category is transitive. transitive graphs have no bridges. this is why trolls always have problems with category theory. Top Reddit Stories * SIGPLAN Programming Languages Software Award, to Simon Peyton Jones and Simon Marlow for GHC Domain: permalink.gmane.org, Score: 115, Comments: 12 On Reddit: http://goo.gl/d1yww Original: http://goo.gl/ym5q9 * Haskell gets a new super-fast hash table library Domain: gregorycollins.net, Score: 80, Comments: 20 On Reddit: http://goo.gl/gNcRa Original: http://goo.gl/O6oc6 * Pinpointing space leaks in big programs : Inside T5 Domain: blog.ezyang.com, Score: 47, Comments: 4 On Reddit: http://goo.gl/T3ZVs Original: http://goo.gl/4rL1y * iPad app that is a lisp interpreter written in haskell by David Pollak, creator of the Scala web framework Lift. Domain: github.com, Score: 37, Comments: 16 On Reddit: http://goo.gl/eXf03 Original: http://goo.gl/c859K * Memory footprints of some common data types Domain: blog.johantibell.com, Score: 36, Comments: 23 On Reddit: http://goo.gl/bjvFC Original: http://goo.gl/QVrIN * Gilad Bracha: Types are Anti-Modular Domain: gbracha.blogspot.com, Score: 22, Comments: 22 On Reddit: http://goo.gl/rnbeF Original: http://goo.gl/44kCd * Biggest / most popular / most trafficked web app written in Haskell? Janrain, TypLAB, or ... ? Domain: reddit.com, Score: 19, Comments: 15 On Reddit: http://goo.gl/eMn8Y Original: http://goo.gl/hUzFc * Three Haskell web frameworks Domain: blogs.linux.ie, Score: 17, Comments: 3 On Reddit: http://goo.gl/5HGd6 Original: http://goo.gl/oR6Zu * Parallel GHC project: new opportunity for an organisation to participate Domain: well-typed.com, Score: 17, Comments: 0 On Reddit: http://goo.gl/rppqC Original: http://goo.gl/CB4ZG * Building Ajax Sites With Snap - Imagining Heist-Async Domain: blog.dbpatterson.com, Score: 17, Comments: 0 On Reddit: http://goo.gl/o8jTF Original: http://goo.gl/j1y2o Top StackOverflow Questions * why write type declarations in Haskell? votes: 11, answers: 4 Read on SO: http://goo.gl/SqM4y * Creating functions over Enumerations votes: 11, answers: 5 Read on SO: http://goo.gl/0lguE * Understanding a recursively defined list (fibs in terms of zipWith) votes: 10, answers: 3 Read on SO: http://goo.gl/kbVtC * Speeding up SHA256 in Haskell votes: 10, answers: 1 Read on SO: http://goo.gl/whGhV * Poor performance with transpose and cumulative sum in Repa votes: 10, answers: 1 Read on SO: http://goo.gl/abxuD * Would you mind to explain the code in the forum ? votes: 9, answers: 1 Read on SO: http://goo.gl/ftoI3 About the Haskell Weekly News To help create new editions of this newsletter, please send stories to dstcruz(a)gmail.com. Until next time, Daniel Santa Cruz

1 0

Fwd: [iteratee] how to do nothing .. properly
by Sergey Mironov 15 Jun '11

15 Jun '11

Forgot reply to all ---------- Forwarded message ---------- From: Sergey Mironov <ierton(a)gmail.com> Date: 2011/6/16 Subject: Re: [Haskell-cafe] [iteratee] how to do nothing .. properly To: John Lato <jwlato(a)gmail.com> Thanks for explanations! /ErrorT String (Iteratee s m) a/ definitely does the work. But wrapping Enumeratee is harder since /ErrorT String (Enumeratee s s' m) a/ doesn't compiles. Maybe most users actually don't need to wrap Enumeratee, will check it. Heh, throwRecoverableErr name tells me "use me, I am all you need":) Well, one just need to grok the interface.. Anyway, iteratees are much deeper and powerful than I expected when switched to it from Data.Binary. I'll better open new threads for asking more questions. Again thanks, Sergey PS sorry for delayed answer, had vacation without email access 2011/6/5 John Lato <jwlato(a)gmail.com>: > Yes, this is expected. 'throwErr' is only meant to be used when the error > should be non-recoverable, and the stream would often be invalid then, so > throwErr doesn't take any steps to preserve it. You could retain the rest > of the stream with getChunk and use throwRecoverableErr though. > > Wrapping an iteratee with ErrorT is fine, and I use this approach often. I > typically would use an explicit Either rather than ErrorT, but the two > approaches are exactly the same. > > Note that wrapping the other way, Iteratee s (ErrorT e m), can sometimes > cause problems, and is best avoided unless you really know what you're > doing. This may change in a future release. > > John > > On Thu, Jun 2, 2011 at 4:27 PM, Sergey Mironov <ierton(a)gmail.com> wrote: >> >> I am glad to help! Looks like upgrading to 0.8.5.0 also fixes initial >> problem that involved me into testing! >> >> I'll take the opportunity and ask another thing about iteratee: Is it >> expected behavior that throwErr consumes all data in current chunk? I >> wish it to stop in place and let after-checkErr code to continue the >> parsing. Well, I already found solution (or workaround?) - I wrap >> Iteratee with ErrorT monad and use ErrorT's raiseError instead of >> throwErr. Is it correct? >> >> Here is example code >> >> instance Exception Int >> >> iter4 = do >> I.dropWhile (/= 3) >> h<-I.head >> throwErr $ toException $ (-4::Int) -- doesn't meter what exactly to >> throw >> return h >> >> -- catch the error with checkErr >> iter5 = do >> (_,b)<-countBytes $ I.checkErr $ iter4 >> s <- I.stream2list >> return (b,s) >> >> print5 = enumPure1Chunk [1..10] (iter5) >>= run >>= print >> >> >> Thanks a lot! >> Sergey >> >> 2011/6/2 John Lato <jwlato(a)gmail.com>: >> > Hi Sergey, >> > >> > I've got an explanation; quite surprisingly it's a bug in >> > enumPure1Chunk. >> > Even though it is an odd case, I'm surprised that it hasn't come up >> > before >> > now since enumPure1Chunk appears frequently. >> > >> > I've just uploaded 0.8.5.0 which has the fix. There's now an additional >> > Monoid constraint on enumPure1Chunk, unfortunately. >> > >> > Thanks very much for reporting this. >> > >> > John L >> > >> > On Thu, Jun 2, 2011 at 10:02 AM, Sergey Mironov <ierton(a)gmail.com> >> > wrote: >> >> >> >> Ok. I've checked iteratee-0.8.3.0 and 0.8.4.0. Results are same. >> >> >> >> Sergey >> >> >> >> 2011/6/2 John Lato <jwlato(a)gmail.com>: >> >> > Hi Sergey, >> >> > I can't explain this; maybe it's a bug in enumWith? I'll look into >> >> > it. >> >> > Thanks, >> >> > John >> >> > >> >> >> >> >> >> Message: 20 >> >> >> >> >> >> Date: Thu, 2 Jun 2011 02:46:32 +0400 >> >> >> From: Sergey Mironov <ierton(a)gmail.com> >> >> >> Subject: [Haskell-cafe] [iteratee] how to do nothing .. properly >> >> >> To: haskell-cafe(a)haskell.org >> >> >> Message-ID: <BANLkTimMFRWgH9Nopt-eua+L7jQcGq+u=g(a)mail.gmail.com> >> >> >> Content-Type: text/plain; charset=ISO-8859-1 >> >> >> >> >> >> Hi. Would anybody explain a situation with iter6 and iter7 below? >> >> >> Strange thing - first one consumes no intput, while second consumes >> >> >> it >> >> >> all, while all the difference is peek which should do no processing >> >> >> (just copy next item in stream and return to user). >> >> >> What I am trying to do - is to write an iteratee consuing no input, >> >> >> but returning a constant I give to it. I thought (return a) should >> >> >> do >> >> >> it, but it seems I was wrong as return actually consumes all >> >> >> unparsed >> >> >> stream. iter6 experience tells me that (peek>>return a) is what I >> >> >> need, but it's completely confusing and not what I expected. >> >> >> >> >> >> Thanks, >> >> >> Sergey >> >> >> >> >> >> import Data.Iteratee as I >> >> >> import Data.Iteratee.IO >> >> >> import Control.Monad >> >> >> import Control.Exception >> >> >> import Data.ByteString >> >> >> import Data.Char >> >> >> import Data.String >> >> >> >> >> >> -- countBytes :: (..., Num b) => Iteratee s m a -> Iteratee s m (a, >> >> >> b) >> >> >> countBytes i = enumWith i I.length >> >> >> >> >> >> iter6 = do >> >> >> h <- countBytes $ (peek >> return 0) >> >> >> s <- I.stream2list >> >> >> return (h,s) >> >> >> >> >> >> iter7 = do >> >> >> h <- countBytes $ (return 0) >> >> >> s <- I.stream2list >> >> >> return (h,s) >> >> >> >> >> >> print6 = enumPure1Chunk [1..10] (iter6) >>= run >>= print >> >> >> print7 = enumPure1Chunk [1..10] (iter7) >>= run >>= print >> >> >> >> >> >> >> >> >> Here is example ghci session >> >> >> >> >> >> *Main> print6 >> >> >> ((0,0),[1,2,3,4,5,6,7,8,9,10]) >> >> >> -- read 0 items, returns 0 >> >> >> *Main> print7 >> >> >> ((0,10),[]) >> >> >> -- read 10 items (???) returns 0 >> >> >> *Main> >> >> >> >> >> >> >> >> >> >> >> >> ------------------------------ >> >> >> >> >> >> _______________________________________________ >> >> >> Haskell-Cafe mailing list >> >> >> Haskell-Cafe(a)haskell.org >> >> >> http://www.haskell.org/mailman/listinfo/haskell-cafe >> >> >> >> >> >> >> >> >> End of Haskell-Cafe Digest, Vol 94, Issue 3 >> >> >> ******************************************* >> >> > >> >> > >> > >> > > >

1 0

Simple string tokenizer?
by Dmitri O.Kondratiev 15 Jun '11

15 Jun '11

Sorry if this question was asked billions of times already, but I can not find a simple string tokenizer. All I need is to split a line in chunks at specified delimiter such as (,), nothing more. Don't want to write it myself for two reasons: 1) have to write lots of other code in very short time 2) don't want undermine idea of code reuse ) Where to get latest Data.String.Utils library that I saw somewhere on the web and that has a nice and simple 'split' function? Thanks!

2 2

wxHaskell for Haskell Platform 2011.2.0.1
by Eric Kow 15 Jun '11

15 Jun '11

Hi all, With green light from maintainer Jeremy O'Donoghue, I've uploaded a trivial update to wxHaskell, relaxing the dependencies so that it builds with the latest Haskell Platform and does not try to install other versions of packages. What is wxHaskell? ---------------------------------------------------------------------- wxHaskell is a Haskell library for writing GUIs that work on multiple platforms. It provides a reasonable compromise between getting a native user interface and saving effort. It uses the wxWidgets library, which in turn wraps around the native GUI toolkit for the target platform and tries to follow platform-specific customs and conventions so that you don't have to worry about them. Here are the sort of results the minor details wxWidgets seems to get right on Mac OS X: - system preferences are under the Preferences menu for the application - file open/save and print dialogues are the actual ones used by Mac applications instead of some half-baked Aqua coloured monstrosity It may not be perfect, but it's not bad considering you don't even have to think about it. wxHaskell is available under a relaxed LGPL license, which basically means you can release both open sourced and closed source with it. About this release ---------------------------------------------------------------------- This is just a minor release bumping some dependencies. Installation comes in two basic steps: 1. Install wxWidgets 2.8 - On MacOS X, Homebrew works well for me (if you update to get patches to build wxWidgets with Unicode enabled). - On Ubuntu/Debian, I use apt-get wxgtk. - I have not personally tried it on Windows, but I've hear reports that it's easy to install there as well. 2. cabal install wx See http://www.haskell.org/haskellwiki/WxHaskell for more information What's next? ---------------------------------------------------------------------- Jeremy has been doing quite a lot of work on wxHaskell. Among other things [1], he's been: - updating it for what will eventually be wxWidgets 3.0 - trying to resolve issues restarting the UI from GHCi - improved Event handling (for Mac users, the upshot is that wxHaskell would work with the wxWidgets that ships with OS X, meaning "cabal install wx" from the HP would "just work") There are also mutterings about pushing for inclusion in the Haskell Platform, but there is still a lot of work of to do before we get there [2]. Help wanted :-) Enjoy! Eric [1] http://thread.gmane.org/gmane.comp.lang.haskell.wxhaskell.devel/612 [2] http://thread.gmane.org/gmane.comp.lang.haskell.wxhaskell.devel/616 -- Eric Kow <http://erickow.com>

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Re: [Haskell-cafe] Simple string tokenizer?
by Thedward Blevins 15 Jun '11

15 Jun '11

On Jun 15, 2011 11:22 AM, "Dmitri O.Kondratiev" <dokondr(a)gmail.com> wrote: > > Sorry if this question was asked billions of times already, but I can not find a simple string tokenizer. > All I need is to split a line in chunks at specified delimiter such as (,), nothing more. Don't want to write it myself for two reasons: 1) have to write lots of other code in very short time 2) don't want undermine idea of code reuse ) > Sounds like you want `Data.List.Split.splitOn` from the 'split' package ( http://hackage.haskell.org/package/split ).

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Please help : Data.Time.Format parseTime
by Dmitri O.Kondratiev 15 Jun '11

15 Jun '11

I am trying to convert data string to time: import Data.Time import Data.Time.Format import Locale ds = "10/11/2009 7:04:28 PM" t = parseTime defaultTimeLocale "%D %H:%M:%S %p" ds :: Maybe UTCTime and get "Nothing". What is wrong? Thanks ! Dmitri.

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GHC 7.0.3 / Win32: Data.Time library?
by Dmitri O.Kondratiev 15 Jun '11

15 Jun '11

It looks like GHC, 7.0.3 "Haskell Hierarchical Libraries" documentation generated for Win32, that goes with Haskell Platform installation package, does not have a section on Data.Time module. How can that be? I need to convert a string of the form ",10/11/2009 7:04:28 PM" to Haskell type that can be used for comparing and abstracting dates. What is the simplest way to day that? Thanks!

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Re: [Haskell-cafe] Haskell-Cafe Digest, Vol 94, Issue 34
by Gregory Guthrie 15 Jun '11

15 Jun '11

------------------------------------------- > Subject: Re: [Haskell-cafe] Best platform for development with GHC? > On Wed, 15 Jun 2011, Dmitri O.Kondratiev wrote: > > Since I maintain the gnuplot binding for Haskell - what are the particular > problems with that package on Windows? > > ******************************************** [Guthrie] I installed gnuplot using cabal on Windows, and get this: :m + Graphics.Gnuplot.Simple plotFunc [] (linearScale 1000 (-10,10)) sin Microsoft Windows [Version 6.1.7601] Copyright (c) 2009 Microsoft Corporation. All rights reserved. C:\Users\guthrie>ghci GHCi, version 7.0.3: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer-gmp ... linking ... done. Loading package base ... linking ... done. Loading package ffi-1.0 ... linking ... done. Prelude> :m + Graphics.Gnuplot.Simple Prelude Graphics.Gnuplot.Simple> plotFunc [] (linearScale 1000 (-10,10)) sin Loading package bytestring-0.9.1.10 ... linking ... done. Loading package Win32-2.2.0.1 ... linking ... done. Loading package old-locale-1.0.0.2 ... linking ... done. Loading package time-1.2.0.3 ... linking ... done. Loading package filepath-1.2.0.0 ... linking ... done. Loading package old-time-1.0.0.6 ... linking ... done. Loading package directory-1.1.0.0 ... linking ... done. Loading package process-1.0.1.5 ... linking ... done. Loading package array-0.3.0.2 ... linking ... done. Loading package containers-0.4.0.0 ... linking ... done. Loading package monoid-transformer-0.0.2 ... linking ... done. Loading package utility-ht-0.0.7 ... linking ... done. Loading package gnuplot-0.4.2 ... linking ... done. *** Exception: gnuplot: createProcess: does not exist (No such file or directory)

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Data import from octave / text file
by kaffeepause73 15 Jun '11

15 Jun '11

Dear all, I want to read numeric data in vector / matrix format generated by octave. As I haven't seen any octave specific libraries yet (only matlab), I'm tyrying the way via ascii - file. The folloing file log.txt contains three signal traces in the three columns (time, signal 1, signal 2) 0 30 9 0.1 30 9 0.2 30 9 0.3 30 9 0.4 30 9 0.5 30 9 0.6000000000000001 30 9 0.7000000000000001 30 9 0.8 30 9 0.9 30 9 1 30 9 I got the following bit of code compiled with ghc: import System.IO main = do text <- readFile "./log.txt" putStrLn text putStrLn "SecondLot" let m = read text :: [[Double]] putStrLn (show m) putStrLn "Finish" Reading text works, but not reading text into m. The Output: 0 30 9 0.1 30 9 0.2 30 9 0.3 30 9 0.4 30 9 0.5 30 9 0.6000000000000001 30 9 0.7000000000000001 30 9 0.8 30 9 0.9 30 9 1 30 9 SecondLot signalImport: Prelude.read: no parse Any suggestions ? - Thanks in advance. -- View this message in context: http://haskell.1045720.n5.nabble.com/Data-import-from-octave-text-file-tp44… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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Lazy lists vs generators
by oleg＠okmij.org 15 Jun '11

15 Jun '11

Yves Pare`s wrote: > I cannot help smiling when I hear Java/C#/Python developers telling about > the "wondeful features" of their languages ;) > (- We have generators! We can _yield_ values! [1] > - Yeah, so nice... We have simple lists... with lazy evaluation) Just for the record, lazy lists per se are sufficient only for simplest generators. Re-writing an accumulating traversal to accommodate suspension by hand is too ungainly and error prone. Still, if we just add exceptions (the Either monad), we can idiomatically write all the generator examples of Python or Icon, for example. If we do not need effects other than yielding or non-determinism, [Either e a], which is ErrorT e [] a, suffices. If we need IO or mutable state, we should replace [] with LogicT (which is the monad of lazy lists with effect). The following web page talks about lazy lists and generators http://okmij.org/ftp/continuations/generators.html in (perhaps too) great detail.

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Re: [Haskell-cafe] Best platform for development with GHC?
by michael rice 15 Jun '11

15 Jun '11

I don't know about the other Linux distros, but Fedora seems to have excellent support for Haskell. Michael

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YesodAuth documentation
by Никита Тимофеев 14 Jun '11

14 Jun '11

Does exist some simple examples using yesod authentication except code of Haskellers site? Does exist some examples using YesodAuthEmail? For example, can i use third-party mail server instead of Sendmail for YesodAuthEmail? Can i disable registration confirmation? Does passwords stored in a database in an unencrypted form? -- Timofeev N.D.

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Re: [Haskell-cafe] Acquiring a random set of a specific size (w/o dups) from a range of Ints
by Patrick Perry 14 Jun '11

14 Jun '11

There are time/space tradeoffs in sampling without replacement. The version in monte-carlo takes space O(n) and time O(k), for all k. I chose this algorithm instead of a streaming algorithm, with takes space O(k) and time O(n). If k is much less than n, you can improve a bit. Here is a version taking space O(k) and time O(k^2), *provided k << n*; when k is large relative to n, the time increases to roughly O(n log n). import Control.Monad( liftM ) import Control.Monad.MC import Data.List( nub ) sampleSmallIntSubset :: MonadMC m => Int -> Int -> m [Int] sampleSmallIntSubset n k = (take k . nub) `liftM` repeatMC (sampleInt n) sampleSmallSubset :: MonadMC m => [a] -> Int -> m [a] sampleSmallSubset xs k = (map (xs !!)) `liftM` sampleSmallIntSubset (length xs) k -- strict version of the above sampleSmallSubset' :: MonadMC m => [a] -> Int -> m [a] sampleSmallSubset' xs k = do ys <- sampleSmallSubset xs k (length ys) `seq` return ys -- by forcing the length, we make sure we are done with the RNG when we return If you're wondering about the strict version, the following example should be instructive: Let's note the first three numbers generated by the RNG with seed 0: ghci> (replicateM 3 $ uniform 0 1) `evalMC` mt19937 0 [0.999741748906672,0.16290987539105117,0.28261780529282987] Now, let's look at the first number we get after a call to sampleSmallSubset, if we don't evaluate the subset: ghci> sampleSmallSubset [1..10000] 2 >> uniform 0 1) `evalMC` mt19937 0 0.999741748906672 Apparently, the call to sampleSmallSubset doesn't consume any random numbers. Now, let's try the strict version: ghci> (sampleSmallSubset' [1..10000] 2 >> uniform 0 1) `evalMC` mt19937 0 0.28261780529282987 This time, the call to sampleSmallSubset' consumes two random numbers, even if we throw away the result. Since you're just starting out, I recommend using the strict versions of the functions. They are less space-efficient than the lazy versions, but they are guaranteed to be safe. > From: Jonas Almström Duregård <jonas.duregard at chalmers.se> > Subject: Re: [Haskell-cafe] Acquiring a random set of a specific size (w/o dups) from a range of Ints > To: "michael rice" < > nowgate at yahoo.com > > > Cc: "Felipe Almeida Lessa" < > felipe.lessa at gmail.com>, haskell-cafe at haskell.org > > Date: Tuesday, June 14, 2011, 5:17 AM > > > > Shuffle [1..20], then take 5? > > > > Yes, so simple, I'm embarrassed I didn't think of it. > > > That works well for small numbers, but I'm guessing it will evaluate the entire list so it should not be used for large inputs. If you have a large interval and use a relatively small part of it, the following function should be significantly faster (it builds a random permutation lazily):

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YesodAuth documentation
by Тимофеев Никита Дмитриевич 14 Jun '11

14 Jun '11

Does exist some simple examples using yesod authentication except code of Haskellers site? Does exist some examples using YesodAuthEmail? For example, can i use third-party mail server instead of Sendmail for YesodAuthEmail? Can i disable registration confirmation? Does passwords stored in a database in an unencrypted form? -- Timofeev N.D.

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Dead else branch does influence runtime?
by Johannes Waldmann 14 Jun '11

14 Jun '11

Dear all, I am very puzzled by a program that contains an "else" branch that is never executed, but still seems to slow down the program. (When I replace it by "undefined", the resulting program runs much faster.) http://hackage.haskell.org/trac/ghc/ticket/5256 I thought it may be a type issue (the type of the else branch forces the type of the "then" branch to be more general, thus some optimization might not fire) but the types of the branches look identical. (They are generic, but the specializer should take care of that.) I am sure GHC headquarters will look at this when they find the time but perhaps there's some additional knowledge on this mailing list that might help. J.W.

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Re: [Haskell-cafe] Acquiring a random set of a specific size (w/o dups) from a range of Ints
by michael rice 14 Jun '11

14 Jun '11

Thanks, all. It seemed like something like this should exist in a prob/stat package, and if so, didn't want to reinvent the wheel. Shuffle [1..20], then take 5? Yes, so simple, I'm embarrassed I didn't think of it. Michael --- On Mon, 6/13/11, Felipe Almeida Lessa <felipe.lessa(a)gmail.com> wrote: From: Felipe Almeida Lessa <felipe.lessa(a)gmail.com> Subject: Re: [Haskell-cafe] Acquiring a random set of a specific size (w/o dups) from a range of Ints To: "michael rice" <nowgate(a)yahoo.com> Cc: haskell-cafe(a)haskell.org Date: Monday, June 13, 2011, 9:38 PM On Mon, Jun 13, 2011 at 8:56 PM, michael rice <nowgate(a)yahoo.com> wrote: > Is there an (existing) way to select 5 Ints randomly (no duplicates) from a population, say 1-20 (inclusive)? Yes, already implemented in the monte-carlo package as sampleSubset [1], sampleSubset :: MonadMC m => [a] -> Int -> m [a] Complete example code for your example: evalMC (sampleSubset [1..20] 5) (mt19937 0) Cheers! [1] http://hackage.haskell.org/packages/archive/monte-carlo/0.4.1/doc/html/Cont… -- Felipe.

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GHC7 build problem
by Scott Lawrence 14 Jun '11

14 Jun '11

Trying to compile GHC7 from source (as the ubuntu repository is still on GHC6), I came across the following error in the final phase: libraries/base/GHC/ST.lhs:78:1: You cannot SPECIALISE `forever' because its definition has no INLINE/INLINABLE pragma (or its defining module `Control.Monad' was compiled without -O) This was after: git clone [...] ./sync-all get perl boot ./configure make Is there some problem with trying to build GHC7 on a machine with GHC6, or did I just get the build procedure wrong (the last line of the error looks like a hint that I did). -- Scott Lawrence

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Re: [Haskell-cafe] Haskell-Cafe Digest, Vol 93, Issue 58
by Gregory Guthrie 14 Jun '11

14 Jun '11

An earlier note on students reactions to the imperative style forced on them by some Haskell libraries ("do ...") is interesting, and seems similar to an observation in a project I was developing for students; making a version of a simple lab from previous SML assignment. It uses a dictionary to do some statistics for a text analysis, but since the dictionary is read at a leaf node of the analysis algorithm, that function must be IO(), and thus the analysis using it also, ... etc. all the way up to the top. So the implication of the rules: 1) all IO must start from the top level, and there is only one IO 2) you cannot extract anything from an IO Seems to be that the whole program structure becomes a series of do... blocks, which is basically a sequential imperative looking style. The general advice of "Strive to keep as much of the program pure as possible" thus seems difficult. An option I suppose would be to read the dictionary at the top level, and then pass it all the way down to the analysis routine that uses it, but that exposes the details of how the analysis is done, and couples the top and bottom levels of the previously modular functions. While this is all logical and understandable, it is quite different than how I did this in SML; where I could encapsulate the IO in the analysis function. It was a local secret of the analysis what data it needed, and where it came from. Note that (of course...) if the dictionary was static and an internal data structure, then this would all go away. It is interesting to me (and curious at first) that one could not somehow treat a "constant" data definition file resource in a more encapsulated manner, and not have it ripple all the way up through the program because it was "impure" once converted to an external definition (=IO). My first impulse was to read the dictionary in a do... and then try to extract it and go merrily on, but that won't work - by design of course! Considering something like a properties file in Java, it thus seems like every part of a program wanting to use these, must either be passed some global definition, or become a leaf on a hierarchy of do.. blocks if it does its own IO to read the properties. Anyway, while the more precise treatment and isolation of IO in Haskell seems valuable, it also seems to have a notable impact on the lack of ability to encapsulate and decouple parts of the program, and keep things pure between them. I suppose this is because that by definition, once you have touched IO at any leaf of a program, the whole thing is impure all the way up the functional tree. I rather had the feeling expressed by Robert Harper: " Once you're in the IO monad, you're stuck there forever, and are reduced to Algol-style imperative programming." (http://existentialtype.wordpress.com/2011/05/01/of-course-ml-has-monads/) Just an observation, in case I am missing something - being fairly new to Haskell. I suppose this is just an adjustment to proper treatment of the impurity of IO, but the effect on program structure was not good. :-) ------------------------------------------- > -----Original Message----- > Subject: Haskell-Cafe Digest, Vol 93, Issue 58 > Now, I have a personal pedagogical comment. >A few months ago I gave to some 30 students an >The results? A true DISASTER! > The OpenGL bindings in Haskell are hardly "functional". > You make us sweat with generic functional patterns, laziness, exquisite typing, non-determinism monad, parsing tools, etc., > and then you throw us into this ugly imperativism !

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(no subject)
by Fernando Henrique Sanches 13 Jun '11

13 Jun '11

http://maipenarai.com/lindex02.html

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Unexpected type error
by Alexander Solla 13 Jun '11

13 Jun '11

Hi everybody, I'm having a bit of a problem with Parsec. The most relevant error message is: Couldn't match expected type `Text.Parsec.Prim.ParsecT String () Control.Monad.Identity.Identity String' against inferred type `parsec-2.1.0.1:Text.ParserCombinators.Parsec.Prim.Parser [Char]' In the expression: countMinMax ... So apparently, the module/package that defines countMinMax and the current module are using different versions of Parsec. My global GHC database has Parsec 3.1.1 installed, but each package has the constraint parsec >=2.1 && <2.2 Both modules use the exact same import line: import Text.ParserCombinators.Parsec -- Lime.Data.IPAddress import Text.ParserCombinators.Parsec -- Lime.Utility.Parsec countMinMax :: (Integral n) => n -> n -> Parser a -> Parser [a] and the expression GHC is complaining about above has type Parser String. The cabal-dev package database contains only one version of Parsec: parsec-2.1.0.1-190ec41e7316b7eb7fddf355acddd826.conf What's going on here?

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Profiling with Data.PQueue.Min
by blackcat＠pro-ns.net 13 Jun '11

13 Jun '11

I'm having trouble with getting a profiling version of Data.PQueue.Min. I've set the profiling options in ~/.cabal/config and used it to build profiling versions of other packages. When I run "cabal install --reinstall pqueue", it runs successfully and properly installs the package...but it isn't building a profiling version. Has anyone else gotten this to work? I'd appreciate some pointers. thanks Lee Short

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question about interruptable state runner
by Alexander V Vershilov 13 Jun '11

13 Jun '11

Hello. I'm writing a small tcp server with that can handle connections and answer by rules writen in a small script that can be interpreted by server. For this purpose I've written an interpreter that has type ErrorT MyError (StateT ScriptState IO) so I can call "native" IO function in that script, and define new one. I can run this script with runState (runErrorT (...)) oldState. But there is one problem: in script i should be able to call functions that will stop script interpretation and wait for some server event. To continue interpretation. Can smb give an advice what is the best way to do it? -- Best regards Alexander V Vershilov

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GHC handles badly with math formulas
by Grigory Sarnitskiy 13 Jun '11

13 Jun '11

I've noted several times that GHC doesn't evaluate formulas for Double values during compilation. That is using 4.2326514735445615 instead of (512 / 0.844)**(1/3) or 0.906179845938664 instead of (1/3) * sqrt(5 + 2 * sqrt(10 / 7)) This can lead to significant slowdown if such values are called often. It's not always convenient to use where or let to reduce the number of calculations of such constants so I wonder how to force GHC to evaluate them during compilation?

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Re: [Haskell-cafe] [Haskell] ANNOUNCE: Parallel and Concurrent programming in Haskell, a tutorial
by Henning Thielemann 13 Jun '11

13 Jun '11

On Thu, 19 May 2011, Simon Marlow wrote: > I've put together a tutorial on Parallel and Concurrent programming in > Haskell, here: > > http://community.haskell.org/~simonmar/par-tutorial.pdf > > The main reason for writing this was that I needed some lecture notes for a > course at the CEFP summer school next month, but I hope the material is > useful to the Haskell community in general. Thank you a lot for this overview! Concerning the parallelisation of K-means: It seems to be difficult to choose a size for the pieces of points. Since the 'combine' function is associative something map-reduce-like seems to be applicable. That is, if there is a library function for a parallelised 'reduce' and I promise, that I call it with an associative function as argument, then the library could choose an appropriate size for the pieces. Is there already library support for parallelism of this style?

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Input and output of mathematical expressions
by Jacek Generowicz 13 Jun '11

13 Jun '11

Greetings Cafe, What would you recommend as a Haskell-based means of interactively reading and writing mathematical formulae? As a toy example, what might I use to write a program which presents the user with Please simplify the expression: \pi x^2 + 3\pi x^2 (Where the TeX-style expression would be presented with a greek pi and superscript twos on the xs.) The user should then have the ability to reply with something that looks like the result of TeXing 5 \pi x^2 Whatever means the user uses to enter this expression, he should be able to preview the "typeset" version of his input before submitting. Any ideas? Thanks.

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location of IEEE Viz code ?
by briand＠aracnet.com 13 Jun '11

13 Jun '11

Hi, I hate to even ask this because the answer is probably staring me in the face. This trac page http://hackage.haskell.org/trac/PolyFunViz/wiki/IEEEVisCode talks about the code being available through darcs but I can't seem to put my hands on the http address I would need to pull the code. This is all relating to the paper, "Huge Data but Small Programs: Visualization Design via Multiple Embedded DSLs". I was wondering if anyone has accessed it and/or knows where it's hiding. Thank you, Brian

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