October 2010 - Haskell-Cafe

ANNOUNCE: darcs 2.5 release candidate 1
by Reinier Lamers 15 Oct '10

15 Oct '10

The darcs team would like to announce the immediate availability of darcs 2.5 release candidate 1 (also known as darcs 2.4.99.1 due to Cabal restrictions). If no blocking issues are found in the coming week, we will finally releease darcs 2.5. Important changes since darcs 2.4.4 are: * trackdown can now do binary search with the --bisect option * darcs always stores patch metadata encoded with UTF-8 * diff now supports the --index option * amend-record now supports the --ask-deps option * apply now supports the --match option * amend-record has a new --keep-date option * inventory-changing commands (like record and pull) now operate in constant time with respect to the number of patches in the repository * the push, pull, send and fetch commands no longer set the default repository by default * the --edit-description option is now on by default for the send command Changes since the last beta release are: * Fix a bug that let users add files outside the working copy directory * Fix excessive network usage when pulling from an old-fashioned repository * Make sure darcs builds on Windows with Haskell Platform 2010.2.0.0 If you have installed the Haskell Platform or cabal-install, you can install this beta release by doing: $ cabal update $ cabal install darcs-beta Alternatively, you can download the tarball from http://darcs.net/releases/darcs-2.4.99.1.tar.gz and build it by hand as explained in the README file. Kind Regards, the darcs release manager, Reinier Lamers

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Client-extensible heterogeneous types
by Jacek Generowicz 15 Oct '10

15 Oct '10

Greetings, I'm looking for dynamic dispatch on extensible sets of types. Here is a more detailed explanation of what I mean: ######################################################################## # Consider the following Python code as representative of something # you might see in Object-Orineted programs in all sorts of languages. # Create an abstract type from abc import ABCMeta, abstractmethod class Abstract: __metaclass__ = ABCMeta @abstractmethod def method(self): pass # Provide some reifications of the abstract type class Variant1(Abstract): def method(self): return "Variant 1 stuff" class Variant2(Abstract): def method(self): return "Variant 2 stuff" # Provide some utilities to process these data def heterogeneousProcessor(data): return [datum.method() for datum in data] # If you wrap all the above up in a library, clients can easily extend # it with their own new types which will still work within the # framework class ClientDefined(Abstract): def method(self): return "Client-defined stuff" heterogeneousContainer = [Variant1(), Variant2(), ClientDefined()] result = heterogeneousProcessor(heterogeneousContainer) ######################################################################## ------------------------------------------------------------------------ -- In Haskell, on the one hand, the heterogeneity is easily provided -- by algebraic data types data AbstractHeterogeneous = VariantHeterogeneous1 | VariantHeterogeneous2 methodHeterogeneous VariantHeterogeneous1 = "Variant 1 stuff" methodHeterogeneous VariantHeterogeneous2 = "Variant 2 stuff" heterogeneousProcessor dataa = [methodHeterogeneous datum | datum <- dataa] heterogeneousContainer = [VariantHeterogeneous1, VariantHeterogeneous2] resultHeterogeneous = heterogeneousProcessor heterogeneousContainer -- But in order to extend the set of variants, the client would have -- to modify the source code, changing the definiton of -- AbstractHeterogeneous and methodHeterogeneous, both of which belong -- to the library. ------------------------------------------------------------------------ -- On the other hand, the extensibility is easily provided by type -- classes -- Library code: class AbstractExtensible a where methodExtensible :: a -> String instance AbstractExtensible () where methodExtensible _ = "Variant 1 stuff" instance AbstractExtensible Char where methodExtensible _ = "Variant 2 stuff" -- Client extension: instance AbstractExtensible Int where methodExtensible _ = "Client-defined stuff" -- but in this case, there is no heterogeneity: you cannot create the -- equivalent of heterogeneousContainer above -- heterogeneousExtensibleContainer = ???? resultExtensible :: [String] resultExtensible = [methodExtensible (), methodExtensible 'a', methodExtensible (1::Int)] ------------------------------------------------------------------------ I can't see a Haskell solution which combines both of these orthogonal features without losing the benefits of the type system. (For example, I could create my own, weak, type system with tags to identify the type and maps to do the dispatch.) So my question is, what would be the Haskell approach to combining heterogeneity (as provided by variant (algebraic) types) with type-extensibility (as provided by type classes), without losing the benefits of Haskell's type system? I haven't looked into Template Haskell, but i guess that it is likely to provide a solution. But is there a solution in plain Haskell? Thanks.

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dph question
by Warren Harris 15 Oct '10

15 Oct '10

I trying to learn a bit about data parallel haskell, and started from the wiki page here: http://www.haskell.org/haskellwiki/GHC/Data_Parallel_Haskell. Two questions: The examples express the dot product as: dotp_double xs ys = sumP [:x * <http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#v:.> y | x <- xs | y <- ys:] Unless I'm missing something, shouldn't this actually be: dotp_double xs ys = sumP [:x * <http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#v:.> y | x <- xs, y <- ys:] Second, when I run Main with the prescribed 10000 element array, everything seems to work quite nicely. The task takes about 2 seconds on my 4 processor x86_64, and threadscope shows all processors nicely utilized. However, when bumping this to 100000 elements, rather than taking 10x longer as I expected, the process never terminates. During one run I even lost control of my machine and needed to do a hard reset. Are there known limits to the array sizes that can be handled with dph, or can someone suggest what might be going wrong here? Thanks, Warren

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All binary strings of a given length
by rgowka1 15 Oct '10

15 Oct '10

Hi - How can I generate all binary string of a given length? The type signature would something like - genbin :: Int -> [String] For example genbin 2 would give ["00","11","01","10"] and genbin 3 would give ["000","001","010","011","100","101","110","111"] etc.. thanks..

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Fwd: [Haskell-cafe] Fuzzy time deltas
by Alberto G. Corona 15 Oct '10

15 Oct '10

Michael, The package Workflow has persistent timeouts (can wait for years and restart on system failure if embedded in the workflow monad, although it can run in the IO monad, with no recovery). They are composable with any action in the STM monad with orElse: flag <- getTimeoutFlag $ 5*24*60 * 60 -- wait exactly 5 days. even -- if the program restart ap <- step . atomically $ readSomewhere >> return False `orElse` waitUntilSTM flag >> return True case ap of False -> print "something received" True -> print "timeout" step" lift it from the IO to the workflow monad, and gives it persistence and recovery. without "step", it runs in the IO monad (No recovery on system failure): flag <- transientTimeout $ 5*24*60 * 60 -- wait 5 days, timeout -- restarts in case of failure ap <- atomically $ readSomewhere >> return False `orElse` waitUntilSTM flag >> return True case ap of False -> print "something received" True -> print "timeout" ------ transientTimeout t= do flag <- atomically $ newTVar False forkIO $ threadDelay(t * 1000000) >> atomically (writeTVar flag True) >> myThreadId >>= killThread return flag 2010/10/14 Michael Snoyman <michael(a)snoyman.com>: > Hey all, > > Is there a library that supports fuzzy time deltas? For example, given > two UTCTimes (or something like that) it could produce: > > 43 seconds > 13 minutes > 17 hours > 4 days > 8 months > > I want to use it for the news feature on Haskellers. It's not that > hard to write, just wondering if it's already been done. > > Michael > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe >

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allocation for "pure" FFI functions
by Johannes Waldmann 15 Oct '10

15 Oct '10

Hi. I wonder how to do the following properly. I have one (large) C type, let's call it T, and I want to sell it as an abstract type in Haskell. I want to use C functions as if they were of type T -> T (pure function, returns a modified copy of the input) and the question is, how to do the memory allocation for that, in particular, how to avoid IO showing up in the (visible) types on the Haskell side: I don't want IO because I don't want to declare some artificial order of execution - instead I want lazy evaluation. E.g., I might have some Haskell record with a T component which may or may not be evaluated (accessed) at all. I came up with two straightforward approaches that don't work: Version 1: the C function does "malloc" on its own, and returns a Ptr. So far, this is pure (no IO). Problem: to make the "free" happen, I need to add a finalizer in Haskell, and this is IO. Version 2: allocation on the Haskell side with mallocForeignPtr. Problem: this is already in IO. Is there a solution? Workaround? Better approach? If it boils down to unsafePerformIO, then where do I put it, and what should I watch out for? J.W.

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deploying applications that use plugins
by Stefan Kersten 15 Oct '10

15 Oct '10

hi all, i am working on an application that evaluates haskell code entered by the user using the plugins package [1]; now i want to deploy the application as a self-contained .app bundle on OSX, i.e. the end user should not have to install anything in addition. what do i need to bundle in order for plugins to work on a vanilla machine? i suppose i need ghc in PATH and have to ship modified versions of the package.conf files that point to the corresponding libraries within the application bundle? is there a parser for package.conf files? any pointers would be greatly appreciated! thanks, <sk> [1] http://hackage.haskell.org/package/plugins

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Thunks
by Mihai Maruseac 15 Oct '10

15 Oct '10

Hi, Is there a way to determine the order in which thunks are created and expanded/evaluated in Haskell (GHC)? I'm looking mainly at some existing interface but if there is only something in the GHC source it will suffice. Thanks, -- Mihai

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Re: MonadCatchIO, finally and the error monad
by oleg＠okmij.org 15 Oct '10

15 Oct '10

Michael Snoyman wrote: > I have a recommendation of how to fix this: the MonadCatchIO typeclass > should be extended to include finally, onException and everything > else. We can provide default definitions which will work for most > monads, and short-circuiting monads like ErrorT (and I imagine ContT > as well) will need to override them. It seems that `finally' can be fixed without all these proposed additions. The method catch is the only necessary method of the class. The subject of catching errors in non-IO monads has a long history, some of which is documented at http://okmij.org/ftp/Haskell/index.html#catch-MonadIO The page points out to an old CaughtMonadIO file. I have just updated it for new Exceptions, and added the final test: > test3c go = runErrorT $ go `gfinally` (liftIO $ putStrLn "sequel called") > test31 = test3c (return "return" :: ErrorT String IO String) *CaughtMonadIO> test31 sequel called Right "return" > test32 = test3c (error "error" :: ErrorT String IO String) *CaughtMonadIO> test32 sequel called *** Exception: error > test33 = test3c (throwError "throwError" :: ErrorT String IO String) *CaughtMonadIO> test33 sequel called *** Exception: ErrorException "\"throwError\"" As we can see, sequel is always called. Here is the updated file: http://okmij.org/ftp/Haskell/CaughtMonadIO.lhs Incidentally, one should be very careful of using `finally' with the continuation monad. The Cont monad lets us ``enter the room once, and exit many times''. So, finally may be called more than once. We need the ugly dynamic-wind -- or try to use less powerful monads if they suffice.

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Cobbled together a version of Haskell Platform 2010.2 that works for GHC 7.1
by Ryan Newton 15 Oct '10

15 Oct '10

Well, it's not 2010.2 anymore because I had to upgrade a few packages ;-), but if you want to try out some of the new GHC 7 features this can get you a working cabal-install. Download here: http://parfunk.blogspot.com/2010/10/hacking-together-working-version-of.html Apologies if this is already available somewhere, but I didn't see it. Cheers, -Ryan

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