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ANN: Future 1.1.0 concurrency library
by ChrisK 10 Mar '09

10 Mar '09

Hello, As a side effect of the discussion of the new C++ future/promise features at http://lambda-the-ultimate.org/node/3221 I have implemented a Haskell package called "future" at http://hackage.haskell.org/cgi-bin/hackage-scripts/package/future This ought to do what C++ standard futures/promises do, plus a bit more. The main operation is > forkPromise :: IO a -> IO (Promise a) This sets the "IO a" operation running in a fresh thread. The eventual result can be accessed in many ways (non-blocking, blocking, blocking with timeout). > let one :: Int; one = 1 > p <- forkPromise (return (one+one)) > x <- get > y <- wait x is an Int with value 2. y is an (Either SomeException Int) with value (Right 2). The useful thing about futures, as opposed to various IVar packages, is handling the case where the forked operation ends with an exception. The exception becomes the return value of the promise. The "get" operation rethrows it, the "wait" operation returns it as (Left ...). There is also an "abort" command to kill a promise. The dead promise may then have an exceptions as its value. The "plus a bit more" than C++ is the nonblocking "addTodo" feature. This takes a continuation function from the "Either SomeException a" to an IO operation. These continuation functions get queued and they are run immediately after the the forked operation completes. Once completed any new "addTodo" continuations run immediately. These continuations allow you to race a list of action and take the first one done, or to collect the answers as they complete into a Chan. Both of those options are demonstrated in Future.hs as racePromises and forkPromises. It should be safe to use "unsafePerformIO . get" or "unsafePeformIO . wait" to get lazy access to the result, which is itself immutable once set. Cheers, Chris

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Re: [Haskell-cafe] where does ghc specify ar options
by Duncan Coutts 10 Mar '09

10 Mar '09

On Mon, 2009-03-09 at 21:08 -0400, Alexy Khrabrov wrote: > Duncan -- > > On Mar 9, 2009, at 8:14 PM, Duncan Coutts wrote: > > BTW, how did you get the package installed in that location? Did it > > involve copying into a temp dir and copying again? I believe that on > > OSX, copying a .a file breaks the ar index. This is because for > > reasons > > best known to themselves Apple decided that the index is only valid if > > its timestamp is the same as the last modification time of the file. > > Oh -- and that's because instead of the supplied bootstrap.sh I used > one off some website, and it used /tmp which somehow put stuff in > there and registered it there. I then unregistered it and removed it > and redid the cabal-install, and it worked with my patched ar. Ahhh. So is there anything in Cabal or the bootstrap.sh that needs to be fixed for this problem? Duncan

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where does ghc specify ar options
by Alexy Khrabrov 10 Mar '09

10 Mar '09

When bootstrapping cabal-install 0.6.2 on Mac OSX Leopard (Intel), I get a problem when linking: Linking dist/build/cabal/cabal ... ld: in /private/var/folders/mw/mwJSf7ErEa4w8nWyKyyqD++++TY/-Tmp-/zlib-0.5.0.0/dist/build/libHSzlib-0.5.0.0.a, archive has no table of contents This had happened with other ar'chives on my Mac before where it hadn't for some others. In all cases the fix was to make sure ranlib is run on the archive in question, or s is passed to ar during the creation. I have the latest XCode 312m gcc build 5490, and ghc 6.10.1 from MacPorts. In order to ensure ar cr becomes ar crs, I need to find out where in the ghc build process is it known how to create .a files? Where can an "ar cr" be changed to "ar crs" on a system-wide ghc build basis? Cheers, Alexy

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Re: ANN: Future 1.1.0 concurrency library
by Neal Alexander 10 Mar '09

10 Mar '09

ChrisK wrote: > Hello, > > As a side effect of the discussion of the new C++ future/promise > features at http://lambda-the-ultimate.org/node/3221 I have implemented > a Haskell package called "future" at > > http://hackage.haskell.org/cgi-bin/hackage-scripts/package/future > > This ought to do what C++ standard futures/promises do, plus a bit > more. The main operation is > > > forkPromise :: IO a -> IO (Promise a) > > This sets the "IO a" operation running in a fresh thread. The eventual > result can be accessed in many ways (non-blocking, blocking, blocking > with timeout). > > > let one :: Int; one = 1 > > p <- forkPromise (return (one+one)) > > x <- get > > y <- wait > > x is an Int with value 2. > y is an (Either SomeException Int) with value (Right 2). > > The useful thing about futures, as opposed to various IVar packages, is > handling the case where the forked operation ends with an exception. > The exception becomes the return value of the promise. The "get" > operation rethrows it, the "wait" operation returns it as (Left ...). > > There is also an "abort" command to kill a promise. The dead promise > may then have an exceptions as its value. > > The "plus a bit more" than C++ is the nonblocking "addTodo" feature. > This takes a continuation function from the "Either SomeException a" to > an IO operation. These continuation functions get queued and they are > run immediately after the the forked operation completes. Once > completed any new "addTodo" continuations run immediately. > > These continuations allow you to race a list of action and take the > first one done, or to collect the answers as they complete into a Chan. > Both of those options are demonstrated in Future.hs as racePromises and > forkPromises. > > It should be safe to use "unsafePerformIO . get" or "unsafePeformIO . > wait" to get lazy access to the result, which is itself immutable once set. > > Cheers, > Chris I used something similar to this for a monadic opengl "RPC" mechanism. The need was for transparently creating OGL objects from any context. Returning thunks that block on the result when finally evaluated works out real nice in practice. Its easy to manage, and performs better than dealing with several OS threads and GL contexts.

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I want to write a compiler
by Loup Vaillant 10 Mar '09

10 Mar '09

This is a homework question. I am mainly looking for guidance or pointers. Source code, even. (Not GHC's, though, it is too complicated for me right now. The AST of STG is fine, but the rest kinda scares me.) Ideally, I would very much like to compile to C. The requirements are easily stated. My language must -> be lazy by default, -> support algebraic data types and case expressions (unless I can get away with encoding them as functions), -> have some kind of FFI with C (I suppose it imply some support for unboxed values). There is also the first class functions thing, but lambda lifting is okay. I don't care about any kind of execution efficiency. I just want my compiler to be As Simple As Possible. Eventually, I intend to bootstrap. Then, I will start dreaming about doing better than the Mighty Simons. (I can dream, right?) I figured I would start by the back-end. I began to write 15 pathetic lines of code to start an STG machine. Needles to say, I am stuck. I don't know where to start. I am already familiar with some papers. In particular, [1] (on template instantiation), [2] and [3]. I once wrote a simple graph reducer using template instantiation (about 20 lines at most). I have chosen the STG machine because I thought i would be easier to get an FFI and case exprs out of it. Maybe I am wrong, and template instantiation is really easier. There is also the BRISK machine, and GRIN. But the paper I read were less readable to me than the three mentioned. So, If any of you can give me some pointer or guidance or advice about where to start, I would be very grateful. Loup [1] http://www.cs.otago.ac.nz/cosc459/books/pjlester.pdf [2] http://research.microsoft.com/~simonpj/Papers/spineless-tagless-gmachine.ps… [3] http://research.microsoft.com/~simonpj/Papers/eval-apply/index.htm

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Minimum Int?
by Colin Paul Adams 10 Mar '09

10 Mar '09

Is there a function that yields the minimum value of Int on an implementation? -- Colin Adams Preston Lancashire

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Partial pattern matching
by Peter Verswyvelen 10 Mar '09

10 Mar '09

In Haskell, a data constructor can be used partially applied: data Pair a b = P a b f = P 1 however, I cannot do "partial pattern matching", e.g firstCoord (P x) = x does not work. I guess a very important reason must exist why this is the case?

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Parsing floating point numbers
by Bjorn Buckwalter 09 Mar '09

09 Mar '09

Hi all, What is your preferred method of parsing floating point numbers (from String to Float/Double)? Parsec it seems only does positive floats out of the box and PolyParse requires the float to be on scientific form (exponential). While I've worked around these shortcomings in the past I feel that I am reinventing the wheel as surely I am not the only to run into these limitations. How do you parse your floats? Can you recommend a parsing library that handles them solidly? (For my current needs the formats accepted by "read" are sufficient, but I want reasonable error handling (Maybe or Either) instead of an exception on bad inputs.) Thanks, Bjorn Buckwalter

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Purely Functional Data Structures
by Gü?nther Schmidt 09 Mar '09

09 Mar '09

Hi, is the above mentioned book still *the* authority on the subject? I bought the book, read about 10 pages and then put it back on the shelf. Um. In my app I have to deal with 4 csv files, each between 5 - 10 mb, and some static data. I had put all that data into an Sqlite3 database and used SQL on it. But, as the requirements keep changing the SQL becomes a bit messy. I guess we've all had that experience. So I'm wondering if I will find clues in this book how to do my querying and handling of moderately large data in a more haskellish way and be able to drop the SQL. Your suggestions appreciated. Günther

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How to make a dock window for xmonad using gtk2hs?
by Magicloud Magiclouds 09 Mar '09

09 Mar '09

Hi, I am using gtk2hs to make some sidebar thing for xmonad. Well, after I tried a few ways, I still cannot make it dock like xmobar. It just covered other windows at the edge of the screen. Could someone give me a sample or something I could learn from? Thanks. -- 竹密岂妨流水过山高哪阻野云飞

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STG's? [was: I want to write a compiler]
by Rick R 09 Mar '09

09 Mar '09

Where can I find more information on STGs? Google search doesn't bring up anything too enlightening. My curiosity was piqued by http://www.cs.chalmers.se/~gedell/ssc/index.html. Of course it doesn't indicate how these should be built or the format. On Sun, Mar 8, 2009 at 6:41 PM, Loup Vaillant <loup.vaillant(a)gmail.com>wrote: > Thanks to the courage you all gave me, I found the strength to go on. > > Template instantiation seems to be the easiest path, so I decided to > follow it. I think I solved the problem about primitives and FFI I > though it had. Namely, how the hell do I compile primitive calls with > an arbitrary number of arguments. > > Starting from the Tutorial of Peyton Jones and Lester, I added a new > node type to help guide the order of evaluation. Basically, this "Seq" > node takes the address of two other nodes. It pushes them both on the > stack, like an application node would. (That is because I don't want a > dump, but I could as well create a new stack and and push the previous > one on the dump.) > > Note the nodes which are evaluated this way must not have a functional > head normal form (primitive or supercombinator). That would leak past > the beginning of the stack. > > When the node is finally evaluated (e.g. points to a HNF, like a boxed > integer), the topmost and second topmost pointers on the stack are > swapped. (or the topmost pointer of the stack and the topmost pointer > of the topmost stack on the dump are swapped). > > With a suitable compilation scheme, that should guarantee that > primitives have their arguments already evaluated. (And avoid infinite > loops). > > Now, I just have to implement it. :-) > > Cheers, > Loup. > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe > -- We can't solve problems by using the same kind of thinking we used when we created them. - A. Einstein

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ANN : happs-tutorial 0.8
by Creighton Hogg 09 Mar '09

09 Mar '09

Hello, I'd like to announce the release of the happstack-0.2 compatible release of happs-tutorial on hackage and available for perusing on tutorial.happstack.com. A number of changes occurred in this release: * General cleanup of code for readability * Migration to the new Happstack.Server.SimpleHTTP api * Addition of a chapter on multimaster, the Spread Toolkit based system for syncing your applications across multiple Happstack instances. * Revised chapters on introducing Happstack.Server and Happstack.State, with an emphasis on simple, well-commented examples showing the orthogonality of these separate libraries. * Minor bug fixes Cheers, Creighton

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Cabal and package changelog
by Manlio Perillo 09 Mar '09

09 Mar '09

Hi. The Cabal package description includes a property `license-file`. This property may be used by Hackage to provide a link to the license file. However I would like to have a `changelog-file`, too. So that from Hackage I can quickly check what's changed with the new version of a package. Is this feasible? Thanks Manlio Perillo

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Looking for literature
by Gü?nther Schmidt 09 Mar '09

09 Mar '09

Hi all, I'm trying to catch up with all the wonderful Haskell Types, classes, Abstract Data Types, Algebraic Data Types, Types that give peoples headaches and all the other, deeper stuff I have been happily putting off. Now to look smart I have a bookshelf full of books for smart people and TaPL is amoung them. Should I try that or will RWH teach me enough to get by? Günther

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cabal-install 0.6.2 does not bootstrap with ghc-6.10.1 debian distribution
by Ahn, Ki Yung 09 Mar '09

09 Mar '09

Dear Haskellers and especially who are working on cabal-install and debian packaging, I sometimes clean up .ghc and .cabal in my home directory to start from scratch because of dependency loopholes (cabal-install does not have remove option yet, so it's hard to fix when such loophole happens). Today, I had some time in the airport and decided to start from scratch again because of the dependency loophole with process 1.0.1.1 and haddock. I downloaded the most recent version of cabal-install the version 0.6.2, and found out that the ./bootstrap.sh does not work. So, I had to bootstrap from version 0.6.2 and do "cabal update" and "cabal upgrade cabal-install" to upgrade to 0.6.2. I am not sure whether this is a cabal-install problem or debian dstribution ghc-6.10.1 packaging probelm, since I have not tried to test this with any other ghc-6.10.1 distribution. If anyone who are not using debian distribution ghc-6.10.1 (e.g., general linux binary ghc-6.10.1 or source compiled one) can try bootstrapping cabal-install 0.6.2 from scratch also finds the same problem, I think someone should make a ticket for cabal-install. Thanks, Ahn, Ki Yung

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Looking for a co-founder for a startup using Haskell
by Ed McCaffrey 09 Mar '09

09 Mar '09

Hello, I'm turning a project involving music into a startup, and I will be using Haskell for most and possibly all of it. I had an angel investor interested until the market collapse forced him to turn his focus away from new investments. Other investors I have spoken with want me to contact them again when it is further developed; basically, I think that I am optimistic at the prospect of funding with the right team. I am looking for one or more co-founders in the Boston, MA, area who would be willing to work on this full-time in the near future, after funding is secured, and in their spare time until then. Since many investors are currently moving towards larger investments in growth-stage companies, I am going to apply to incubators who specialize in early-stage companies. Anyone who wants more info send a reply to ed(a)edmccaffrey.net Thanks, Ed McCaffrey ed(a)edmccaffrey.net

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Re: [Haskell] string type class
by Duncan Coutts 09 Mar '09

09 Mar '09

On Fri, 2009-03-06 at 19:16 +0000, Chris Kuklewicz wrote: > Matthew Pocock wrote: > > It seems every time I look at hackage there is yet another stringy > > datatype. For lots of apps, the particular stringy datatype you use > > matters for performance but not algorithmic reasons. Perhaps this is a > > good time for someone to propose a stringy class? > > Not likely. > > I did define my own (private) class for regular expressions, to abstract over > String, the ByteStrings, and Seq Char. But it is used in one place and is a > wart that should be removed. > > The simple task of looping over the contents of a String (once, forward) is > quite different from a Strict ByteString (using an index and a lookup). > > This means for decent efficiency I need two copies of my code, hand specialized > to each case. > > "tail" or "(x:xs)" : very efficient for String (no allocation) > "tail" or "uncons" : not efficient for ByteString (allocation, might as well > convert to [Char] head/tail/uncons on a strict or lazy ByteString is perfectly ok. I would recommend it over using length+index. If you are using tail in a tail recursion then ghc can almost always optimise it to unbox the various ByteString components and so there is no allocation of fresh BS constructors, just adjusting offsets and lengths held in registers. It's also possible to use unsafeHead and unsafeTail if you've checked for null. Duncan

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3D Rendering Engine in Haskell (Re: [Haskell-cafe] FRP + physics / status of hpysics)
by Peter Verswyvelen 09 Mar '09

09 Mar '09

Awesome! When you say you took Ogre as a base, do you mean your engine will be able to load the meshes and material files, or are you also wrapping the C++ code base? Did you also consider Collada? 2009/3/8 Csaba Hruska <csaba.hruska(a)gmail.com> > Hi! > > This should be a bit offtopic, but now I'd like to announce that I'm > working on a 3D rendering rengine in Haskell. > I take ogre3d (www.ogre3d.org) as a base. My code can import import a > subset of ogre model files (currently xml version) and material scripts > (http://www.ogre3d.org/docs/manual/manual_14.html#SEC23) > You can easily import an existing 3d content or create new one because > there are ogre exporter plugins for every major modeler programs. > Currently I have some working code. I hope I'll be ready to release it in a > month. It should act as graphical backend for FRP libraries. > I use yampa in the example program but it's just an option, only example > program depends on it. > > Cheers, > Csaba Hruska > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe > >

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Arch Linux: New package updates for Mar 8, 2008
by Don Stewart 09 Mar '09

09 Mar '09

New Haskell packages for the week ending Mar 8. http://archhaskell.wordpress.com/2009/03/08/arch-haskell-news-mar-8-2009/ Arch now has 962 Haskell packages in AUR. That’s 17 new packages this week, and lots of updates as well. Notable releases this week * htar-0.3: Command-line tar archive utility * haskell-llvm-0.6.5.0: Bindings to the LLVM compiler toolkit * haskell-harpy-0.4.1: Runtime code generation for x86 machine code * haskell-future-1.0.0: Supposed to mimics and enhance proposed C++ ”future” features * haskell-holumbus-mapreduce-0.0.1: a distributed MapReduce framework * haskell-holumbus-storage-0.0.1: a distributed storage system * haskell-holumbus-distribution-0.0.1: intra- and inter-program communication * hevolisa-dph-0.0.1: Genetic Mona Lisa problem in Haskell - using Data Parallel Haskell * doctest-0.0.0: Test interactive Haskell examples * haskell-happstack-0.2.1: The haskell application server stack + code generation See the full list of new Haskell packages this week on the blog, broken down by category, http://archhaskell.wordpress.com/2009/03/08/arch-haskell-news-mar-8-2009/

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derive & Drift - some help please
by Gü?nther Schmidt 09 Mar '09

09 Mar '09

Hi, I need to derive Data, Typeable for Data.Time.LocalTime. I had hoped to use either derive or DrIFT to generate the instance declarations for it. I tried to understand the respective user manuals but it's still all Greek to me. Someone please help! Günther

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Help with c2hs, structs and typedefs?
by Antoine Latter 09 Mar '09

09 Mar '09

Hey folks, I was trying to create some FFI bindings using c2hs and I ran into trouble: I can't use the 'sizeof' and 'set' hooks for structs that aren't defined inside a typedef. I don't know a whole lot about C - is there any declaration I can make to make something like my second struct work in c2hs? Because the second struct is what's provided in <sys/uio.h> :-) In the following code, if you change all of the directives in the haskell code from my_iovec_2 to my_iovec, the "unknown indentifier" errors go away. Thanks, Antoine >>>>> -- Works! #c typedef struct { void * my_iov_base; int my_iov_len; } my_iovec; #endc -- Broken! I get lots of "unknown identifier" errors for -- the #sizeof and #set hooks - but the #pointer hook seems to work. #c struct my_iovec_2 { void * my_iov_base; int my_iov_len; }; #endc {#pointer *my_iovec_2 as IOVec newtype#} withIOVec :: [(Ptr a, CSize)] -> (Ptr IOVec -> Int -> IO b) -> IO b withIOVec [] f = f nullPtr 0 withIOVec bufers f = let len = length buffers in alocaBytes (length bufers * {#sizeof my_iovec_2#}) $ \ptr -> vecAry <- castPtr ptr foldM (go vecAry) 0 bufers f vecAry len where go vecAry off (ptr, size) = do vec <- vecAry `plusPtr` off * {#sizeof my_iovec_2#} {#set my_iovec_2->my_iov_base#} vec $ castPtr ptr {#set my_iovec_2->my_iov_len#} vec $ cIntConv size return $ off + 1 <<<<<

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ANNOUNCE: Holumbus-MapReduce 0.0.1
by Stefan Schmidt 08 Mar '09

08 Mar '09

The Holumbus framework is known for building specialized search-engines like the Hayoo! Haskell API search. Now, I'd like to announce the newest libraries of the Holumbus framework: Holumbus-MapReduce 0.0.1 Holumbus-Distribution 0.0.1 Holumbus-Storage 0.0.1 As part of my Master's Thesis at the University of Applied Sciences in Wedel, Germany, I have developed a library for building distributed MapReduce systems in Haskell. The existing Holumbus-Searchengine library already offers a MapReduce implementation, but it does not provide mechanisms for distributed computing. The newly developed Holumbus-MapReduce library contains of tools and modules for building individualized distributed MapReduce systems. Holumbus-MapReduce is based on two libraries: Holumbus-Distributed and Holumbus-Storage. Maybe they could be useful for building other distributed systems, not only a MapReduce system. Holumbus-Distribution consists of modules and tools for the implementation of distributed systems in general. Besides common data types and small helper functions, this library provides a mailbox-based data transfer mechanism. It offers an easy-to-use interface for inter- and intra-process communication like in Erlang or Mozart/Oz. Holumbus-Storage is designed to build a distributed storage system which can be linked into other programs. It is used by the MapReduce system for the data management. All three libraries are still under heavy development and should not be considered for creating applications in productive environments. But I think, it's time to give them to the community for suggestions on further development and features. Please email comments, bugs, etc. to sts(a)holumbus.org Stefan Schmidt Holumbus on Hackage: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/Holumbus-MapRedu… http://hackage.haskell.org/cgi-bin/hackage-scripts/package/Holumbus-Distrib… http://hackage.haskell.org/cgi-bin/hackage-scripts/package/Holumbus-Storage link to my thesis: http://www.holumbus.org/redmine/attachments/37/thesis-mapreduce.pdf project homepage: http://www.holumbus.org/redmine/wiki/holumbus darcs repository (latest snapshots): http://holumbus.fh-wedel.de/holumbus/src

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Naturality condition for inits
by R J 08 Mar '09

08 Mar '09

Here's another Bird problem that's stymied me: The function "inits" computes the list of initial segments of a list; its type is inits :: [a] -> [[a]]. What is the appropriate naturality condition for "inits"? The only discussion in the text concerning naturality conditions concerns map, where the naturality conditions are stated in what seem to be quasi-commutativity laws over the composition operator, as follows: f . head = head . map f, where f is strict (i.e., f _|_ = _|_) map f . tail = tail . map f map f (xs ++ ys) = map f xs ++ map f ys map f . reverse = reverse . map f map f . concat = concat . map (map f) I believe that none of the following naturality conditions, extrapolated from those above, hold: a. head . inits = inits [head] b. tail . inits = inits . tail c. reverse . inits = inits . reverse d. concat . inits = inits . concat In case the definition of "inits" is relevant, my definition is: inits :: [a] -> [[a]] inits xs = [take n xs | n <- seglengths] where seglengths = [0..length xs] Thanks. _________________________________________________________________ Windows Live™: Life without walls. http://windowslive.com/explore?ocid=TXT_TAGLM_WL_allup_1a_explore_032009

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Haskell Weekly News: Issue 108 - March 7, 2009
by Brent Yorgey 08 Mar '09

08 Mar '09

--------------------------------------------------------------------------- Haskell Weekly News http://sequence.complete.org/hwn/20090307 Issue 108 - March 07, 2009 --------------------------------------------------------------------------- Welcome to issue 108 of HWN, a newsletter covering developments in the [1]Haskell community. The ICFP programming contest will be held from 26-29th June! It's not too early to start thinking about putting a team together. Announcements tar 0.3.0.0. Duncan Coutts [2]announceda major new release of the [3]tar package for handling ".tar" archive files. This release has a completely new and much improved API. storable 0.1 -- Storable type class for variable-sized data. Tomáš Janoušek [4]announced the first release of the [5]storable library, which fills the gap between Foreign.Storable and Data.Binary by adding support for marshalling (finite) values of variable-sized data types, like lists or trees, while preserving the performance and memory efficiency one expects from the Storable class. It also provides a (monadic) syntactic sugar that takes care of alignment restrictions by itself and makes instance deriving easy. CFP: Submit a talk proposal to CUFP. Kathleen Fisher [6]requested talk proposals for [7]CUFP. The Industrial Haskell Group. Duncan Coutts [8]announced the creation of the [9]Industrial Haskell Group (IHG). The IHG is an organisation to support the needs of commercial users of the Haskell programming language. Currently, the main activity of the IHG is a collaborative development scheme, in which multiple companies fund work on the Haskell development platform to their mutual benefit. The scheme has started with three partners of the IHG, including Galois and Amgen. pandoc 1.2. John MacFarlane [10]announced the release of [11]pandoc version 1.2. The most significant new feature is support for literate Haskell; you can now use pandoc directly on literate Haskell source files to produce syntax-highlighted HTML output. A Haskell binding for the Augeas API. Jude [12]announced a [13]Haskell FFI binding for the Augeas configuration editing API. Haskell Logo Voting will start soon!. Eelco Lempsink [14]announced that [15]voting for the new Haskell logo will begin on March 16! Everyone subscribed to haskell-cafe will receive a ballot; if you are not subscribed but would like to vote, email Eelco with the subject "haskell logo voting ballot request" and include a short, unique message. Happstack 0.2 Released. Matthew Elder [16]announced the [17]release of [18]Happstack 0.2. Extensible and Modular Generics for the Masses: emgm-0.3. Sean Leather [19]announced the third major release of [20]Extensible and Modular Generics for the Masses (EMGM), a library for generic programming in Haskell using type classes and a sum-of-products view. Deriving is now greatly improved, and there are several new functions, including case, everywhere, and everywhere'. major speed improvement: regex-tdfa reaches 1.0.0. ChrisK proudly [21]announced the version 1.0.0 release of [22]regex-tdfa. This is is not just a bug fix release; it is a serious improvement in the asymptotic running time of the library algorithms. Discussion Definitions of purity and Lazy IO. Oleg began a [23]discussion on lazy IO. Left fold enumerator - a real pearl overlooked?. Günther Schmidt began a [24]discussion of left-fold enumerators and their current status within the community. Jobs Looking for a co-founder for a startup using Haskell. Ed McCaffrey [25]is looking for a co-founder to work on a startup music project in Haskell. Email Ed for more information. Fully-funded doctoral studentships in dependently type programming at Oxford and Strathclyde. Jeremy Gibbons [26]announced two fully-funded doctoral student positions in dependently-typed programming at [27]Oxford and [28]Strathclyde. Blog noise [29]Haskell news from the [30]blogosphere. * Holumbus: [31]First release of Holumbus-MapReduce. * FP Lunch: [32]Breadth first labelling. * Sean Leather: [33]Experiments with EMGM: Emacs org files. * Galois, Inc: [34]Call for Proposals: CUFP 2009. * Matthew Elder: [35]Happstack 0.2 Released. * Yi: [36]Lazy and Incremental Parsing: the paper. * Xmonad: [37]xmonad as a multi-head sliding block puzzle. * Don Stewart (dons): [38]Playing with GHC's parallel runtime. * Bjorn Buckwalter: [39]Extended sessions with the Haskell Curl bindings. * Manuel M T Chakravarty: [40]Installing GtK2Hs on a Mac with the native GTK+ OS X Framework.. * Alex Mason: [41]If you need speed, don't talk to main!. * Luke Palmer: [42]New game: SpaceBattle. * Galois, Inc: [43]Galois Tech Talk: Specializing Generators for High-Performance Monte-Carlo Simulation in Haskell. * Bjorn Buckwalter: [44]Programmer Man. * Conrad Parker: [45]Random code: Pretty printing durations in Haskell. * The GHC Team: [46]New paper: Runtime Support for Multicore Haskell. * ganesh: [47]London Haskell Users Group. * Manuel M T Chakravarty: [48]This is the performance of a dot product of two vectors of 10.... * Sean Leather: [49]Template Haskell 2.3 or Cabal 1.2? EMGM can't have both!. * Tom Schrijvers: [50]Complete and Decidable Type Inference for GADTs. * Galois, Inc: [51]Trustworthy Voting Systems. * Well-Typed.Com: [52]The Industrial Haskell Group. * Sean Leather: [53]Incremental fold, a design pattern. * Darcs: [54]darcs weekly news #19. * FP Lunch: [55]Container eating. About the Haskell Weekly News New editions are posted to [56]the Haskell mailing list as well as to [57]the Haskell Sequence and [58]Planet Haskell. [59]RSS is also available, and headlines appear on [60]haskell.org. To help create new editions of this newsletter, please see the information on [61]how to contribute. Send stories to byorgey at cis dot upenn dot edu. The darcs repository is available at darcs get [62]http://code.haskell.org/~byorgey/code/hwn/ . References 1. http://haskell.org/ 2. http://www.haskell.org//pipermail/haskell-cafe/2009-March/056870.html 3. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/tar 4. http://article.gmane.org/gmane.comp.lang.haskell.general/16934 5. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/storable 6. http://article.gmane.org/gmane.comp.lang.haskell.general/16904 7. http://cufp.galois.com/2009/call.html 8. http://article.gmane.org/gmane.comp.lang.haskell.general/16903 9. http://industry.haskell.org/ 10. http://article.gmane.org/gmane.comp.lang.haskell.general/16899 11. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/pandoc 12. http://article.gmane.org/gmane.comp.lang.haskell.general/16897 13. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/augeas 14. http://article.gmane.org/gmane.comp.lang.haskell.cafe/54201 15. http://community.haskell.org/~eelco/poll.html 16. http://article.gmane.org/gmane.comp.lang.haskell.cafe/54144 17. http://blog.happstack.com/2009/03/04/happstack-02-released 18. http://blog.happstack.com/2009/03/04/happstack-02-released 19. http://article.gmane.org/gmane.comp.lang.haskell.cafe/54035 20. http://www.cs.uu.nl/wiki/GenericProgramming/EMGM 21. http://article.gmane.org/gmane.comp.lang.haskell.cafe/53943 22. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/regex%2Dtdfa 23. http://thread.gmane.org/gmane.comp.lang.haskell.general/16914 24. http://thread.gmane.org/gmane.comp.lang.haskell.cafe/53873 25. http://article.gmane.org/gmane.comp.lang.haskell.cafe/54246 26. http://article.gmane.org/gmane.comp.lang.haskell.general/16927 27. http://web.comlab.ox.ac.uk/news/72-full.html 28. http://personal.cis.strath.ac.uk/~conor/phds/ 29. http://planet.haskell.org/ 30. http://haskell.org/haskellwiki/Blog_articles 31. http://holumbus.fh-wedel.de/blog/?p=17 32. http://sneezy.cs.nott.ac.uk/fplunch/weblog/?p=205 33. http://feedproxy.google.com/~r/splonderzoek/~3/6bxI33NIRMc/experiments-with… 34. http://www.galois.com/blog/2009/03/05/call-for-proposals-cufp-2009/ 35. http://blog.happstack.com/2009/03/04/happstack-02-released/ 36. http://yi-editor.blogspot.com/2009/03/lazy-and-incremental-parsing-paper.ht… 37. http://xmonad.wordpress.com/2009/03/04/xmonad-as-a-multi-head-sliding-block… 38. http://donsbot.wordpress.com/2009/03/04/playing-with-ghcs-parallel-runtime/ 39. http://flygdynamikern.blogspot.com/2009/03/extended-sessions-with-haskell-c… 40. http://justtesting.org/post/83333955 41. http://axman6.homeip.net/blog/2009/03/if-you-need-speed-dont-talk-to-main.h… 42. http://lukepalmer.wordpress.com/2009/03/04/new-game-spacebattle/ 43. http://www.galois.com/blog/2009/03/03/galois-tech-talk-specializing-generat… 44. http://flygdynamikern.blogspot.com/2009/02/programmer-man.html 45. http://blog.kfish.org/2009/03/random-code-pretty-printing-durations.html 46. http://ghcmutterings.wordpress.com/2009/03/03/new-paper-runtime-support-for… 47. http://www.londonhug.net/2009/03/03/80/ 48. http://justtesting.org/post/83014052 49. http://feedproxy.google.com/~r/splonderzoek/~3/WqqB1QrNZ_4/template-haskell… 50. http://tomschrijvers.blogspot.com/2009/03/complete-and-decidable-type-infer… 51. http://www.galois.com/blog/2009/03/02/trustworthy-voting-systems/ 52. http://blog.well-typed.com/2009/03/the-industrial-haskell-group/ 53. http://feedproxy.google.com/~r/splonderzoek/~3/tj_MkKqIlkQ/incremental-fold… 54. http://blog.darcs.net/2009/03/darcs-weekly-news-19.html 55. http://sneezy.cs.nott.ac.uk/fplunch/weblog/?p=201 56. http://www.haskell.org/mailman/listinfo/haskell 57. http://sequence.complete.org/ 58. http://planet.haskell.org/ 59. http://sequence.complete.org/node/feed 60. http://haskell.org/ 61. http://haskell.org/haskellwiki/HWN 62. http://code.haskell.org/~byorgey/code/hwn/

1 0

Converting list comprehensions to combinatory style
by R J 08 Mar '09

08 Mar '09

Can anyone help with this problem from Bird: a. Convert the following list comprehensions to combinatory style: i. [(x, y) | x <- [1..n], odd x, y <- [1..n]] ii. [(x, y) | x <- [1..n], y <- [1..n], odd x] b. Are they equal? c. Compare the costs of evaluating the two expressions. I gather that by combinatory style, he means the use of concat, map, and filter. While Bird provides the following conversion rules, he doesn't prove them, justify them, or even provide an example using them: R1. [ f x | x <- xs ] = map f xs R2. [ x | x <- xs, p x ] = filter p xs R3. [ e | Q, P ] = concat [[e | P] | Q] R4. [ e | x <- [d | P] ] = [e [x := d] | Q, P] Thanks. _________________________________________________________________ Windows Live™ Groups: Create an online spot for your favorite groups to meet. http://windowslive.com/online/groups?ocid=TXT_TAGLM_WL_groups_032009

2 1

do you have to use fix with forkio?
by Daryoush Mehrtash 08 Mar '09

08 Mar '09

In this chat server implementation http://www.haskell.org/haskellwiki/Implement_a_chat_server forkIO is used with fix as in: reader <- forkIO $ <http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#v:.> fix $ <http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#v:.> \loop -> do (nr', line) <- readChan chan' when (nr /= <http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#v:/=> nr') $ <http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#v:.> hPutStrLn hdl line loop Do you have to use fix? Or is there a way to write this with a "let"? -- Daryoush

8 11

Ambiguous type with PolymorphicComponents
by Maurício 08 Mar '09

08 Mar '09

Hi, When reading this code in ghci, I get an ambiguous type at last line: {-# LANGUAGE PolymorphicComponents #-} {-# LANGUAGE RankNTypes #-} import Graphics.UI.Gtk data Test = Test (forall w. WidgetClass w => w) toAction (Test w) = toWidget w It's interesting that if I replace 'Integral' for 'WidgetClass' and 'toInteger' for 'toWidget', I get no error messages, and I see no essencial diference between both classes and both functions. Do you know what am I missing? Thanks, Maurício

2 3

serializing large data structures, stack overflow
by friggin friggin 08 Mar '09

08 Mar '09

I'm playing around with Netflix, implementing a simple KNN-algorithm, I will later try SVD which seems to be the most successful approach. Using a database like Postgresqk is to slow so I want to serialize a datastructure containing the ratings. I'm not sure about the representation I will use just yet, if I should use multiple arrays or an Map/IntMap. However I tried Data.Binary and already for small sizes I get stack overflow when deserializing. The serializing works fine but when bringing it back it overflows. How can I solve this? This is just 2MB, I will eventually need soemthing like 2-500MB to store everything depending on what representatin I choose. module Serialize where import qualified Data.Binary as B import qualified Data.Binary.Put as P import qualified Data.Map as M import qualified Data.List as L genTest :: Int -> M.Map (Int,Int) Int genTest n = let movies = take n $ repeat 1 grades = take n $ repeat 4 in M.fromList $ ([1..n] `zip` movies) `zip` grades main = do let a = genTest 50000 B.encodeFile "C:/users/saftarn/desktop/bintest.txt" a print "Success" dec = B.decodeFile "C:/users/saftarn/desktop/bintest.txt" >>= \a -> return $ (a :: M.Map (Int,Int) Int)

3 2

Re: [Haskell-cafe] serializing large data structures, stack overflow
by Don Stewart 08 Mar '09

08 Mar '09

import Data.Binary and then write a variant of something like how Maps are currently serialised: instance (Ord k, Binary k, Binary e) => Binary (Map.Map k e) where put m = put (Map.size m) >> mapM_ put (Map.toAscList m) get = liftM Map.fromDistinctAscList get So you might want something that avoids flattening it to a list first -- Don frigginfriggins: > can you link to a good example of writing your own because I couldn't find one. > > On Sat, Mar 7, 2009 at 8:57 PM, Don Stewart <dons(a)galois.com> wrote: > > Increase the stack size, or use a different serialiser (they're only a > half dozen lines to write), or different data structure? > > -- Don > > frigginfriggins: > > I'm playing around with Netflix, implementing a simple KNN-algorithm, I > will > > later try SVD which seems to be the most successful approach. > > > > Using a database like Postgresqk is to slow so I want to serialize a > > datastructure containing the ratings. I'm not sure about the > > representation I will use just yet, if I should use multiple arrays or an > Map/ > > IntMap. > > > > However I tried Data.Binary and already for small sizes I get stack > overflow > > when deserializing. > > The serializing works fine but when bringing it back it overflows. > > How can I solve this? This is just 2MB, I will eventually need soemthing > like > > 2-500MB to store everything depending on what representatin I choose. > > > > module Serialize where > > import qualified Data.Binary as B > > import qualified Data.Binary.Put as P > > import qualified Data.Map as M > > import qualified Data.List as L > > > > genTest :: Int -> M.Map (Int,Int) Int > > genTest n = let movies = take n $ repeat 1 > > grades = take n $ repeat 4 in > > M.fromList $ ([1..n] `zip` movies) `zip` grades > > > > main = do > > let a = genTest 50000 > > B.encodeFile "C:/users/saftarn/desktop/bintest.txt" a > > print "Success" > > > > dec = B.decodeFile "C:/users/saftarn/desktop/bintest.txt" >>= \a -> > > return $ (a :: M.Map (Int,Int) Int) > > > > > > > > > > > _______________________________________________ > > Haskell-Cafe mailing list > > Haskell-Cafe(a)haskell.org > > http://www.haskell.org/mailman/listinfo/haskell-cafe > > >

1 0

Zippers
by Cristiano Paris 08 Mar '09

08 Mar '09

Hi, I'm trying to catch the connection between delimited continuations and zippers so I wrote a (kinda) zipper interface to a simple tree structure. Here's the code: ------- module Main where import Data.Maybe data Tree a = Leaf a | Fork (Tree a) (Tree a) deriving Show tree = Fork (Fork (Leaf 1) (Leaf 2)) (Fork (Leaf 3) (Fork (Leaf 4) (Leaf 5))) data ZContext a = ZContext { moveUp :: Maybe (ZContext a), moveLeft :: Maybe (ZContext a), moveRight :: Maybe (ZContext a), this :: Maybe a } initZ t = doInitZ Nothing t where doInitZ c (Leaf a) = ZContext c Nothing Nothing $ Just a doInitZ c t@(Fork l r) = ZContext c (Just $ doInitZ s l) (Just $ doInitZ s r) Nothing where s = Just $ doInitZ c t ------- You access the tree in the following way (session from ghci): *Main> this $ fromJust . moveLeft $ fromJust . moveLeft $ initZ tree I read Haskell book's Chapter about Zippers on Wikibooks and I think I understood the underlying concept even if the implementation still seems to me a bit arbitrary (i.e. different implementation can be provided even if the proposed one is neat thinking of differentiation). Hence, I decided to go experimenting myself and came up with the above solution. I know that the interface to a tree having values only on leaves is pointless as the main advantage of using a Zipper is to get O(1) performance on updating but I wanted to keep it as simple as possible. So, can you provide some comments on that implementation? Thank you all, as usual! -- Cristiano

4 8

Running HUnit tests when pushing to darcs on haskell.org
by Colin Paul Adams 07 Mar '09

07 Mar '09

Hello cafe, I've just followed the instructions at http://www.haskell.org/haskellwiki/How_to_write_a_Haskell_program to create my first library project on haskell.org, using darcs. I tried using the darcs setpref test command to execute: runhaskell tests/*.hs (which works locally) but it fails because it can't find Test.HUnit. When I looked back at the webpage, I see it specifically mentions QuickCheck and not HUnit, so I guess I should not have been surprised. But I would like to know if this is possible, and if so, how to arrange that runhaskell can find Test.HUnit. -- Colin Adams Preston Lancashire

1 0

Interesting problem from Bird (4.2.13)
by R J 07 Mar '09

07 Mar '09

Could someone provide an elegant solution to Bird problem 4.2.13? Here are the problem and my inelegant solution: Problem ------- Since concatenation seems such a basic operation on lists, we can try to construct a data type that captures concatenation as a primitive. For example, data (CatList a) = CatNil | Wrap a | Cat (CatList a) (CatList a) The intention is that CatNil represents [], Wrap x represents [x], and Cat x y represents x ++ y. However, since "++" is associative, the expressions "Cat xs (Cat ys zs)" and "Cat (Cat xs ys) zs" should be regarded as equal. Define appropriate instances of "Eq" and "Ord" for "CatList". Inelegant Solution ------------------ The following solution works: instance (Eq a) => Eq (CatList a) where CatNil == CatNil = True CatNil == Wrap z = False CatNil == Cat z w = ( z == CatNil && w == CatNil ) Wrap x == CatNil = False Wrap x == Wrap z = x == z Wrap x == Cat z w = ( Wrap x == z && w == CatNil ) || ( Wrap x == w && z == CatNil ) Cat x y == CatNil = x == CatNil && y == CatNil Cat x y == Wrap z = ( x == Wrap z && y == CatNil ) || ( x == CatNil && y == Wrap z ) Cat x y == Cat z w = unwrap (Cat x y) == unwrap (Cat z w) unwrap :: CatList a -> [a] unwrap CatNil = [] unwrap (Wrap x) = [x] unwrap (Cat x y) = unwrap x ++ unwrap y instance (Eq a, Ord a) => Ord (CatList a) where x < y = unwrap x < unwrap y This solution correctly recognizes the equality of the following, including nested lists(represented, for example, by Wrap (Wrap 1), which corresponds to [[1]]): Wrap 1 == Cat (Wrap 1) CatNil Cat (Wrap 1) (Cat (Wrap 2) (Wrap 3)) == Cat (Wrap 1) (Cat (Wrap 2) (Wrap 3)) Wrap (Wrap 1) == Wrap (Cat (Wrap 1) CatNil) Although this solution works, it's a hack, because unwrap converts CatLists to lists. The question clearly seeks a pure solution that does not rely on Haskell's built-in lists. What's the pure solution that uses cases and recursion on CatList, not Haskell's built-in lists, to capture the equality of nested CatLists? _________________________________________________________________ Windows Live™ Contacts: Organize your contact list. http://windowslive.com/connect/post/marcusatmicrosoft.spaces.live.com-Blog-…

6 7

Type question re: map
by R J 07 Mar '09

07 Mar '09

Given the following (usual) definition of "map": map :: (a -> b) -> [a] -> [b] map f [] = [] map f (x : xs) = f x : map f xs What's the type of "map map"? GHCi's :t command reveals: *Main> :t map map map map :: [a -> b] -> [[a] -> [b]] I'd be grateful if anyone could provide a systematic type calculation so that I can reason through more complicated examples myself. Thanks. _________________________________________________________________ Windows Live™: Life without walls. http://windowslive.com/explore?ocid=TXT_TAGLM_WL_allup_1a_explore_032009

3 2

Test if a file is empty or stat in haskell
by Anish Muttreja 07 Mar '09

07 Mar '09

I am looking for a way to test if a file is empty. Something like isFileEmpty along the lines of System.Directory.doesFileExist? A function that wraps stat would also serve the purpose. I get the feeling that someone must have felt the need for this before me, but Google search not yield anything, hence the question. Thanks! Anish

5 6

Re: [Haskell-cafe] Data.Binary stack overflow with Data.Sequence String
by Gwern Branwen 07 Mar '09

07 Mar '09

On Tue, Mar 3, 2009 at 11:50 PM, Spencer Janssen <spencerjanssen(a)gmail.com> wrote: > On Tue, Mar 3, 2009 at 10:30 PM, Gwern Branwen <gwern0(a)gmail.com> wrote: >> So recently I've been having issues with Data.Binary & Data.Sequence; >> I serialize a 'Seq String' >> >> You can see the file here: http://code.haskell.org/yi/Yi/IReader.hs >> >> The relevant function seems to be: >> >> -- | Read in database from 'dbLocation' and then parse it into an 'ArticleDB'. >> readDB :: YiM ArticleDB >> readDB = io $ (dbLocation >>= r) `catch` (\_ -> return empty) >> where r x = fmap (decode . BL.fromChunks . return) $ B.readFile x >> -- We read in with strict bytestrings to guarantee the >> file is closed, >> -- and then we convert it to the lazy bytestring >> data.binary expects. >> -- This is inefficient, but alas... >> >> My current serialized file is about 9.4M. I originally thought that >> the issue might be the recent upgrade in Yi to binary 0.5, but I >> unpulled patches back to past that, and the problem still manifested. >> >> Whenever yi tries to read the articles.db file, it stack overflows. It >> actually stack-overflowed on even smaller files, but I managed to bump >> the size upwards, it seems, by the strict-Bytestring trick. >> Unfortunately, my personal file has since passed whatever that limit >> was. >> >> I've read carefully the previous threads on Data.Binary and Data.Map >> stack-overflows, but none of them seem to help; hacking some $!s or >> seqs into readDB seems to make no difference, and Seq is supposed to >> be a strict datastructure already! Doing things in GHCi has been >> tedious, and hasn't enlightened me much: sometimes things overflow and >> sometimes they don't. It's all very frustrating and I'm seriously >> considering going back to using the original read/show code unless >> anyone knows how to fix this - that approach may be many times slower, >> but I know it will work. >> >> -- >> gwern > > Have you tried the darcs version of binary? It has a new instance > which looks more efficient than the old. > > > Cheers, > Spencer Janssen I have. It still stack-overflows on my 9.8 meg file. (The magic number seems to be somewhere between 9 and 10 megabytes.) -- gwern

3 5

MPI
by FFT 07 Mar '09

07 Mar '09

Are MPI bindings still the best way of using Haskell on Beowulf clusters? It's my feeling that the bindings stagnated, or are they just very mature?

4 6

Re: [Haskell-cafe] Hint and Ambiguous Type issue
by Daniel Gorín 07 Mar '09

07 Mar '09

I think you can achieve what you want but you need to use the correct types for it. Remember that when you write: getFilterMainStuff :: Deliverable a => FilePath -> Interpreter (Path, Filter a) the proper way to read the signature is "the caller of getFilterMainStuff is entitled to pick the type of a, as long as it picks an instance of Deliverable". Contrast this with a method declaration in Java where: public Set getKeys() is to be read: "The invoked object may pick the type of the result, as long as it is a subclass of (or implements) Set". When you say that you want "to apply fMain to a (Config, Email) and get back a Deliverable a", I think you mean that fMain picks the type for a (and has to be an instance of Deliverable). There two ways to do this in Haskell: 1) You don't. If you know that your possible Deliverables are just FlatEmail and MaildirEmail, then the idiomatic way of doing this would be to turn Deliverable into an ADT: data Deliverable = FlatEmail .... | MaildirEmail .... deriving (Typeable) getFilterMainStuff :: FilePath -> Interpreter (Path, Filter Deliverable) 2) Existential types. If, for some reason, you need your "dynamic code" to be able to define new "deliverables", then you need to use the extension called "existential types". -- using GADT syntax data SomeDeliverable where Wrap :: Deliverable a => a -> SomeDeliverable getFilterMainStuff :: FilePath -> Interpreter (Path, Filter SomeDeliverable) This basically resembles the contract of the Java world: if you run fMain you will get a value of type SomeDeliverable; you can pattern- match it and will get something whose actual type you don't know, but that it is an instance of class Deliverable. See http://www.haskell.org/haskellwiki/Existential_type Good luck! Daniel On Mar 6, 2009, at 2:33 AM, Joseph Fredette wrote: > Okay, I think I understand... I got so hung up thinking the error > had to be in the Interpreter code, I didn't bother to look in the > caller... > > But every answer breeds another question... The practical reason for > inferring fMain as being of type "Deliverable a => Filter a", is to > apply it (via runReader) to a (Config, Email) and get back a > Deliverable a, then to use the deliverIO method on the result -- my > question is, it appears I have to "know" the specific type of a in > order to get the thing to typecheck, but in order to use it, I need > to not know it... > > Perhaps, in fact, I'm doing this wrong. Thanks for the help Daniel, > everyone... > > /Joe > > Daniel Gorín wrote: >> Ok, so I've pulled the latest version and the error I get now is: >> >> Hackmain.hs:70:43: >> Ambiguous type variable `a' in the constraint: >> `Deliverable a' >> arising from a use of `getFilterMainStuff' at Hackmain.hs: >> 70:43-60 >> Probable fix: add a type signature that fixes these type >> variable(s) >> >> Function getFilterMainStuff compiles just fine . The offending line >> is in buildConf and reads: >> >> > (inboxL, fMain) <- runUnsafeInterpreter . getFilterMainStuff $ >> filterMainL >> >> The problem is that GHC can't figure out the type of fMain. It >> infers (Filter a), but doesn't know what is a and therefore how to >> build a proper dictionary to pass to getFilterMainStuff. >> >> Observe that you would get a similar error message if you just >> defined: >> >> > f = show . read >> >> I can get it to compile by providing a type annotation for fMain: >> >> > (inboxL, fMain) <- runUnsafeInterpreter . getFilterMainStuff $ >> filterMainL >> > let _ = fMain :: Filter MaildirEmail >> >> So once you use fMain somewhere and GHC can infer it's type, >> everything should work fine. >> >> Daniel >> >> On Mar 5, 2009, at 11:26 PM, Joseph Fredette wrote: >> >>> Oh, crap- I must have never pushed the latest patches, I did put >>> the typeable instances in all the appropriate places. And provided >>> a (maybe incorrect? Though I'm fairly sure that shouldn't affect >>> the bug I'm having now) Typeable implementation for Reader, but I >>> still get this ambiguous type. I'll push the current version asap. >>> >>> Thanks. >>> >>> /Joe >>> >>> Daniel Gorín wrote: >>>> Hi >>>> >>>> I've downloaded Hackmain from patch-tag, but I'm getting a >>>> different error. The error I get is: >>>> >>>> Hackmain.hs:63:10: >>>> No instance for (Data.Typeable.Typeable2 >>>> Control.Monad.Reader.Reader) >>>> arising from a use of `interpret' at Hackmain.hs:63:10-67 >>>> >>>> Hint requires the interpreted values to be an instance of >>>> Typeable in order to check, in runtime, that the interpreted >>>> value matches the type declared at compile. Therefore, you need >>>> to make sure that (Filter a) is indeed an instance of Typeable. >>>> >>>> Since you have Filter a = Reader (Config, Email) a, you probably >>>> need to >>>> >>>> - Derive Config and Email instances for Filter, >>>> >>>> - Manually provide Typeable instances for Reader a b, something >>>> along the lines of: >>>> >>>> instance (Typeable a, Typeable b) => Typeable (Reader a b) where... >>>> >>>> (I don't know why this isn't done in the mtl....) >>>> >>>> - Change the signature to: >>>> >>>> getFilterMain :: (Typeable a, Deliverable a) => FilePath -> >>>> Interpreter (Filter a) >>>> >>>> Also, you can try using "infer" instead of "as :: ...." >>>> >>>> Hope that helps >>>> >>>> Daniel >>>> >>>> On Mar 5, 2009, at 8:47 PM, Joseph Fredette wrote: >>>> >>>>> So, I tried both of those things, both each alone and together. >>>>> No dice. Same error, so I reverted back to the >>>>> original. :( >>>>> However, I was, after some random type signature insertions, >>>>> able to convert the problem into a different one, via: >>>>> >>>>> getFilterMain :: Deliverable a => FilePath -> Interpreter >>>>> (Filter a) getFilterMain MainLoc = do >>>>> loadModules [fMainLoc]; setTopLevelModules [(takeWhile (/ >>>>> ='.') fMainLoc)] fMain <- >>>>> (interpret "(filterMain)" infer) >>>>> return (fMain :: Deliverable a => Filter a) >>>>> >>>>> Inferred type is less polymorphic than expected >>>>> Quantified type variable `a' is mentioned in the environment: >>>>> fMain :: Filter a (bound at Hackmain.hs:77:1) >>>>> In the first argument of `return', namely >>>>> `(fMain :: (Deliverable a) => Filter a)' >>>>> In the expression: return (fMain :: (Deliverable a) => Filter a) >>>>> In the expression: >>>>> do loadModules [fMainLoc] >>>>> setTopLevelModules [(takeWhile (/= '.') fMainLoc)] >>>>> fMain <- (interpret "(filterMain)" infer) >>>>> return (fMain :: (Deliverable a) => Filter a) >>>>> I'm >>>>> thinking that this might be more easily solved -- I do think I >>>>> understand the issue. somehow, I need to tell the compiler >>>>> that the 'a' used in the return statement (return >>>>> (fMain :: ...)) is the same as the 'a' in the type sig for the >>>>> whole function. >>>>> >>>>> While I ponder this, and hopefully receive some more help -- >>>>> thanks again Dan, Ryan -- Are there any other options besides >>>>> Hint that might -- at least in the short term -- make this >>>>> easier? I'd really like to finish this up. I'm _so_ close to >>>>> getting it done. >>>>> >>>>> Thanks, >>>>> >>>>> /Joe >>>>> >>>>> Ryan Ingram wrote: >>>>>> So, by using the Haskell interpreter, you're using the >>>>>> not-very-well-supported dynamically-typed subset of Haskell. >>>>>> You can >>>>>> tell this from the type signature of "interpret": >>>>>> >>>>>> >>>>>>> interpret :: Typeable a => String -> a -> Interpreter a >>>>>>> >>>>>> >>>>>> >>>>>>> as :: Typeable a => a >>>>>>> as = undefined >>>>>>> >>>>>> >>>>>> (from http://hackage.haskell.org/packages/archive/hint/0.2.1/doc/html/src/Languag…) >>>>>> >>>>>> In particular, the "as" argument to interpret is specifying >>>>>> what type >>>>>> you want the interpreted result to be typechecked against; the >>>>>> interpretation fails if it doesn't match that type. But you >>>>>> need the >>>>>> result type to be an instance of Typeable; (forall a. >>>>>> Deliverable a => >>>>>> Filter a) most certainly is not. >>>>>> >>>>>> >>>>>> Off the top of my head, you have a couple of directions you can >>>>>> take this. >>>>>> >>>>>> (1) Make Typeable a superclass of Deliverable, saying that all >>>>>> deliverable things must be dynamically typeable. Then derive >>>>>> Typeable >>>>>> on Filter, and have the result be of type "Filter a" using >>>>>> ScopedTypeVariables as suggested before. (You can also pass >>>>>> "infer" to >>>>>> the interpreter and let the compiler try to figure out the >>>>>> result type >>>>>> instead of passing (as :: SomeType).) >>>>>> >>>>>> (2) Make a newtype wrapper around Filter and give it an >>>>>> instance of >>>>>> Typeable, and add a constraint to filterMain that the result >>>>>> type in >>>>>> the filter is also typeable. Then unwrap the newtype after the >>>>>> interpreter completes. >>>>>> >>>>>> Good luck; I've never tried to use the Haskell interpreter >>>>>> before, so >>>>>> I'm curious how well it works and what problems you have with it! >>>>>> >>>>>> >>>>>> -- ryan >>>>>> >>>>>> 2009/3/5 Joseph Fredette <jfredett(a)gmail.com>: >>>>>> >>>>>>> I've been working on a little project, and one of the things I >>>>>>> need to do is >>>>>>> dynamically compile and import a Haskell Source file >>>>>>> containing filtering >>>>>>> definitions. I've written a small monad called Filter which is >>>>>>> simply: >>>>>>> >>>>>>> type Filter a = Reader (Config, Email) a >>>>>>> >>>>>>> To encompass all the email filtering. The method I need to >>>>>>> import, >>>>>>> filterMain, has type: >>>>>>> >>>>>>> filterMain :: Deliverable a => Filter a >>>>>>> >>>>>>> where Deliverable is a type class which abstracts over >>>>>>> delivery to a path in >>>>>>> the file system. The notion is that I can write a type like: >>>>>>> >>>>>>> data DEmail = {email :: Email, path :: FilePath} >>>>>>> newtype Maildir = MD DEmail >>>>>>> >>>>>>> instance Deliverable Maildir where >>>>>>> {- ... omitted -} >>>>>>> >>>>>>> However, Filter a should not be restricted to Deliverable >>>>>>> types- it also >>>>>>> encompasses the results of regular expression matching, etc, >>>>>>> which are not >>>>>>> -- in general -- Deliverable instances. >>>>>>> >>>>>>> My question is this, when importing the file containing the >>>>>>> definitions of >>>>>>> filterMain, I have the following code to grab filterMain and >>>>>>> return it as a >>>>>>> function. >>>>>>> >>>>>>> getFilterMain :: Deliverable a => FilePath -> Interpreter >>>>>>> (Filter a) >>>>>>> getFilterMain fMainLoc = do >>>>>>> loadModules >>>>>>> [fMainLoc]; setTopLevelModules [(takeWhile (/='.') fMainLoc)] >>>>>>> fMain <- (interpret "(filterMain)" (as :: >>>>>>> Deliverable a >>>>>>> => Filter a)) return (fMain) >>>>>>> >>>>>>> However, when I try to >>>>>>> compile >>>>>>> this, I get the type error: >>>>>>> >>>>>>> Hackmain.hs:70:43: >>>>>>> Ambiguous type variable `a' in the constraint: >>>>>>> `Deliverable a' >>>>>>> arising from a use of `getFilterMainStuff' at >>>>>>> Hackmain.hs:70:43-60 >>>>>>> Probable fix: add a type signature that fixes these type >>>>>>> variable(s) >>>>>>> >>>>>>> My understanding is that a type like "Foo a => Bar a" (where >>>>>>> Foo is a class >>>>>>> and Bar is a datatype) would simply restrict >>>>>>> the values of a to only those implementing Foo. But evidently >>>>>>> I'm wrong. Is >>>>>>> there a good (read, easy... :) ) fix to this? >>>>>>> >>>>>>> Any help would be greatly appreciated. >>>>>>> >>>>>>> /Joe >>>>>>> >>>>>>> PS. All the actual code is on patch-tag, here >>>>>>> http://patch-tag.com/repo/Hackmail/home -- if anyone prefers >>>>>>> to look at that >>>>>>> directly, the relevant files are in Src, namely, Hackmain.hs, >>>>>>> Filter.hs, and >>>>>>> Deliverable.hs >>>>>>> >>>>>>> _______________________________________________ >>>>>>> Haskell-Cafe mailing list >>>>>>> Haskell-Cafe(a)haskell.org >>>>>>> http://www.haskell.org/mailman/listinfo/haskell-cafe >>>>>>> >>>>>>> >>>>>>> >>>>>> >>>>>> >>>>> <jfredett.vcf>_______________________________________________ >>>>> Haskell-Cafe mailing list >>>>> Haskell-Cafe(a)haskell.org >>>>> http://www.haskell.org/mailman/listinfo/haskell-cafe >>>> >>>> >>> <jfredett.vcf> >> >> > <jfredett.vcf>

2 1

Re: Migration to QuickCheck 2.0
by John Goerzen 06 Mar '09

06 Mar '09

John Goerzen wrote: > Hi, > > My google skills must be faulty, because I can't find much stuff on > migrating from QuickCheck 1.0 to 2.0. > > I've got a number of questions: > > What's the deal with Result and reason being in two different places > in QuickCheck with two different definitions? > > All the QC.Config -- configMaxTest, defaultConfig, arguments, etc. are > missing. Are there direct replacements? OK, ignore the second one. That was an HUnit question; sorry. > > Thanks, > > -- John

1 0

Re: [Haskell] Lazy IO breaks purity
by Simon Marlow 06 Mar '09

06 Mar '09

Lennart Augustsson wrote: > I don't see any breaking of referential transparence in your code. > Every time you do an IO operation the result is basically > non-deterministic since you are talking to the outside world. > You're assuming the IO has some kind of semantics that Haskell makes > no promises about. > > I'm not saying that this isn't a problem, because it is. > But it doesn't break referential transparency, it just makes the > semantics of IO even more complicated. > > (I don't have a formal proof that unsafeInterleaveIO cannot break RT, > but I've not seen an example where it does yet.) So the argument is something like: we can think of the result of a call to unsafeInterleaveIO as having been chosen at the time we called unsafeInterleaveIO, rather than when its result is actually evaluated. This is on dodgy ground, IMO: either you admit that the IO monad contains an Oracle, or you admit it can time-travel. I don't believe in either of those things :-) Cheers, Simon > -- Lennart > > On Thu, Mar 5, 2009 at 2:12 AM, <oleg(a)okmij.org> wrote: >> >> We demonstrate how lazy IO breaks referential transparency. A pure >> function of the type Int->Int->Int gives different integers depending >> on the order of evaluation of its arguments. Our Haskell98 code uses >> nothing but the standard input. We conclude that extolling the purity >> of Haskell and advertising lazy IO is inconsistent. >> >> Henning Thielemann wrote on Haskell-Cafe: >>> Say I have a chain of functions: read a file, ... write that to a file, >>> all of these functions are written in a lazy way, which is currently >>> considered good style >> Lazy IO should not be considered good style. One of the common >> definitions of purity is that pure expressions should evaluate to the >> same results regardless of evaluation order, or that equals can be >> substituted for equals. If an expression of the type Int evaluates to >> 1, we should be able to replace every occurrence of the expression with >> 1 without changing the results and other observables. >> >> The expression (read s) where s is the result of the (hGetContents h1) >> action has the pure type, e.g., Int. Yet its evaluation does more than >> just producing an integer: it may also close a file associated with >> the handle h1. Closing the file has an observable effect: the file >> descriptor, which is a scarce resource, is freed. Some OS lock the >> open file, or prevent operations such as renaming and deletion on the >> open file. A file system whose file is open cannot be >> unmounted. Suppose I use an Haskell application such as an editor to >> process data from a removable drive. I mount the drive, tell the >> application the file name. The (still running) application finished >> with the file and displayed the result. And yet I can't unplug the >> removable drive, because it turns out that the final result was >> produced without needing to read all the data from the file, and the >> file remains unclosed. >> >> Some people say: the programmer should have used seq. That is the >> admission of guilt! An expression (read s)::Int that evaluates to 1 is >> equal to 1. So, one should be able substitute (read s) with 1. If the >> result of evaluating 1 turns out not needed for the final outcome, >> then not evaluating (read s) should not affect the outcome. And yet it >> does. One uses seq to force evaluation of an expression even if the >> result may be unused. Thus the expression of a pure type >> has *side-effect*. >> >> The advice about using seq reminds me of advice given to C programmers >> that they should not free a malloc'ed pointer twice, dereference a >> zero pointer and write past the boundary of an array. If lazy IO is >> considered good style given the proper use of seq, then C should be >> considered safe given the proper use of pointers and arrays. >> >> Side affects like closing a file are observable in the external >> world. A program may observe these effects, in an IO monad. One can >> argue there are no guarantees at all about what happens in the IO >> monad. Can side-effects of lazy IO be observable in _pure_ Haskell >> code, in functions with pure type? Yes, they can. The examples are >> depressingly easy to write, once one realizes that reading does have >> side effects, POSIX provides for file descriptor aliasing, and sharing >> becomes observable with side effects. Here is a simple Haskell98 code. >> >> >>> {- Haskell98! -} >>> >>> module Main where >>> >>> import System.IO >>> import System.Posix.IO (fdToHandle, stdInput) >>> >>> -- f1 and f2 are both pure functions, with the pure type. >>> -- Both compute the result of the subtraction e1 - e2. >>> -- The only difference between them is the sequence of >>> -- evaluating their arguments, e1 `seq` e2 vs. e2 `seq` e1 >>> -- For really pure functions, that difference should not be observable >>> >>> f1, f2:: Int -> Int -> Int >>> >>> f1 e1 e2 = e1 `seq` e2 `seq` e1 - e2 >>> f2 e1 e2 = e2 `seq` e1 `seq` e1 - e2 >>> >>> read_int s = read . head . words $ s >>> >>> main = do >>> let h1 = stdin >>> h2 <- fdToHandle stdInput >>> s1 <- hGetContents h1 >>> s2 <- hGetContents h2 >>> print $ f1 (read_int s1) (read_int s2) >>> -- print $ f2 (read_int s1) (read_int s2) >> >> One can compile it with GHC (I used 6.8.2, with and without -O2) and >> run like this >> ~> /tmp/a.out >> 1 >> 2 >> -1 >> That is, we run the program and type 1 and 2 as the inputs. The >> program prints out the result, -1. If we comment out the line >> "print $ f1 (read_int s1) (read_int s2)" and uncomment out the last >> line the transcript looks like this >> ~> /tmp/a.out >> 1 >> 2 >> 1 >> >> Running the code with Hugs exhibits the same behavior. Thus a pure >> function (-) of the pure type gives different results depending on the >> order of evaluating its arguments. >> >> Is 1 = -1? >> >> >> >> >> >> >> _______________________________________________ >> Haskell mailing list >> Haskell(a)haskell.org >> http://www.haskell.org/mailman/listinfo/haskell >> > _______________________________________________ > Haskell mailing list > Haskell(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell

6 5

Difficulties in accessing inner elements of data types
by David Miani 06 Mar '09

06 Mar '09

Hi, I'm working on a Haskell library for interacting with emacs org files. For those that do not know, an org file is a structured outline style file that has nested headings, text, tables and other elements. For example: * Heading 1 Some text, more text. This is a subelement of Heading 1 1. You can also have list 1. and nested lists 2. more... ** Nested Heading (subelement of Heading 1) text... (subelement of Nested Heading) ** Another level 2 heading (subelement of Heading 1) | Desc | Value | |-----------+--------------------------------------| | Table | You can also have tables in the file | | another | row | | seperator | you can have seps as well, eg | |-----------+--------------------------------------| * Another top level heading There are many more features, see orgmode.org My library enables read and write access to a subset of this format (eg lists aren't parsed atm). The data structures used for writing are: data OrgFile = OrgFile [OrgFileElement] data OrgFileElement = Table OrgTable | Paragraph String | Heading OrgHeading -- heading level, title, subelements data OrgHeading = OrgHeading Int String [OrgFileElement] data OrgTable = OrgTable [OrgTableRow] data OrgTableRow = OrgTableRow [String] | OrgTableRowSep To write a file you contruct a OrgFile out of those elements, and pass it to a writeOrgFile func. Eg: writeOrg $ OrgFile [Heading (OrgHeading 1 "h1" [Paragraph "str"])] would produce: * h1 str I was going to use the same data structures for reading an org file, but it quickly became apparent that this would not be suitable, as you needed the position of the file of an element to be able to report errors. Eg if you needed to report an error that a number was expected, the message "'cat' is not a number" is not very useful, but "Line 2031: 'cat' is not a number" is. So the data structures I used were: data FilePosition = FilePosition Line Column data WithPos a = WithPos { filePos :: FilePosition, innerValue :: a } data OrgTableP = OrgTableP [WithPos OrgTableRow] data OrgFileElementP = TableP OrgTableP | ParagraphP String | HeadingP OrgHeadingP data OrgHeadingP = OrgHeadingP Int String [WithPos OrgFileElementP] data OrgFileP = OrgFileP [WithPos OrgFileElementP] Finally there is a function readOrg, which takes a string, and returns an OrgTableP. Now, this all works as expected (files are correctly being parsed and written), however I am having a lot of trouble trying to come up with a decent API to work with this. While writing an OrgFile is fairly easy, reading (and accessing inner parts) of an org file is very tedious, and modifying them is horrendous. For example, to read the description line for the project named "Project14" in the file: * 2007 Projects ** Project 1 Description: 1 Tags: None ** Project 2 Tags: asdf,fdsa Description: hello * 2008 Projects * 2009 Projects ** Project14 Tags: RightProject Description: we want this requires the code: type ErrorS = String listToEither str [] = Left str listToEither _ (x:_) = Right x get14 :: OrgFileP -> Either ErrorS String get14 (OrgFileP elements) = getDesc =<< (getRightProject . concatProjects) elements where concatProjects :: [WithPos OrgFileElementP] -> [OrgHeadingP] concatProjects [] = [] concatProjects ((WithPos _ (HeadingP h)) : rest) = h : concatProjects rest concatProjects (_ : rest) = concatProjects rest getRightProject :: [OrgHeadingP] -> Either ErrorS OrgHeadingP getRightProject = listToEither "Couldn't find project14" . filter (\(OrgHeadingP _ name _) -> name == "Project14") getDesc :: OrgHeadingP -> Either ErrorS String getDesc (OrgHeadingP _ _ children) = case filter paragraphWithDesc (map innerValue children) of [] -> Left $ show (filePos $ head children) ++ ": Couldn't find desc in project" ((ParagraphP str):_) -> Right str _ -> error "should not be possible" paragraphWithDesc :: OrgFileElementP -> Bool paragraphWithDesc (ParagraphP str) = str =~ "Description" paragraphWithDesc _ = False If you think that is bad, try writing a function that adds the Tag "Hard" to Project2 :( What I really need is a DSL that would allow sql like queries on an OrgFileP. For example: select (anyHeading `next` headingWithName "Project14" `withFailMsg` "couldn't find p14" `next` paragraphMatchingRegex "Description" `withFailMsg` "no desc") org `output` paragraphText would return a String OR select (anyHeading `next` headingWithName "Project2" `next` paragraphMatchingRegex "Tag:") org `modify` paragraphText (++ ",Hard") would return an OrgFile, with the new Hard tag added. However, I don't know if this is even possible, how to do it, or if there is a better alternative to this. I would really apreciate any hints with regards to this. It would be useful to know if there are other libraries that also face this problem, and how they solved it. Finally, I would be grateful for any other advice regarding my code. One thing that has bugged me is my solution for having file position info - my solution never seemed very elegant. Thanks, David

5 5

Hint and Ambiguous Type issue
by Joseph Fredette 06 Mar '09

06 Mar '09

I've been working on a little project, and one of the things I need to do is dynamically compile and import a Haskell Source file containing filtering definitions. I've written a small monad called Filter which is simply: type Filter a = Reader (Config, Email) a To encompass all the email filtering. The method I need to import, filterMain, has type: filterMain :: Deliverable a => Filter a where Deliverable is a type class which abstracts over delivery to a path in the file system. The notion is that I can write a type like: data DEmail = {email :: Email, path :: FilePath} newtype Maildir = MD DEmail instance Deliverable Maildir where {- ... omitted -} However, Filter a should not be restricted to Deliverable types- it also encompasses the results of regular expression matching, etc, which are not -- in general -- Deliverable instances. My question is this, when importing the file containing the definitions of filterMain, I have the following code to grab filterMain and return it as a function. getFilterMain :: Deliverable a => FilePath -> Interpreter (Filter a) getFilterMain fMainLoc = do loadModules [fMainLoc]; setTopLevelModules [(takeWhile (/='.') fMainLoc)] fMain <- (interpret "(filterMain)" (as :: Deliverable a => Filter a)) return (fMain) However, when I try to compile this, I get the type error: Hackmain.hs:70:43: Ambiguous type variable `a' in the constraint: `Deliverable a' arising from a use of `getFilterMainStuff' at Hackmain.hs:70:43-60 Probable fix: add a type signature that fixes these type variable(s) My understanding is that a type like "Foo a => Bar a" (where Foo is a class and Bar is a datatype) would simply restrict the values of a to only those implementing Foo. But evidently I'm wrong. Is there a good (read, easy... :) ) fix to this? Any help would be greatly appreciated. /Joe PS. All the actual code is on patch-tag, here http://patch-tag.com/repo/Hackmail/home -- if anyone prefers to look at that directly, the relevant files are in Src, namely, Hackmain.hs, Filter.hs, and Deliverable.hs

4 7

Calculating with list comprehension
by R J 06 Mar '09

06 Mar '09

I can calculate non-nested list comprehensions without a problem, but am unable to calculate nested comprehensions involving, for example, the generation of a list of pairs where the first and separate elements are drawn from two separate lists, as in: [(a, b) | a <- [1..3], b <- [1..2]] How does one calculate the expansion of this list? The two rules for expanding list comprehensions are: 1. Generator rule: [e | x <- xs, Q] = concat (map f xs) where f x = [e | Q] 2. Guard rule: [e | p, Q] = if p then [e | Q] else [] There is a third rule that I've seen on the Internet, not in an authoritative text: [e | Q1 , Q2] = concat [ [e | Q 2] | Q1 ] I don't understand what this third rule means, or whether it's relevant. Concat and map are defined as: concat :: [[a]] -> [a] concat [] = [] concat (xs:xss) = xs ++ concat xss map :: (a -> b) -> [a] -> [b] map f [] = [] map f (x:xs) = f x : (map f xs) Any help is appreciated. _________________________________________________________________ Windows Live™: Life without walls. http://windowslive.com/explore?ocid=TXT_TAGLM_WL_allup_1a_explore_032009

2 1

Re: Left fold enumerator - a real pearl overlooked?
by John Lato 06 Mar '09

06 Mar '09

> John A. De Goes schrieb: > >> Elsewhere, laziness can be a real boon, so I don't understand your >> question, "Why have laziness in Haskell at all?" > > As I have written, many libaries process their data lazily (or could be > changed to do so without altering their interface) but their interface > can forbid application to data that is fetched from the outside world. > Say you are used to 'map', 'filter', 'foldr' - you cannot use them on > data fetched by the iteratee/enumerator approach. > Thank you for replying to this; it's good to see what features people would like to make iteratees more useful. Where did you get the idea that you can't use 'map'? IterateeGM is a Monad and Functor, so fmap is available for the resultant value. If you want to map over the elements of the stream, use the 'mapStream' function ('map_stream' in Oleg's writings). I never thought about writing a general iteratee fold because that's all iteratees are anyway; it's simple enough to make a foldl. It should be possible to write foldr for seekable resources by enumerating the stream backwards. That certainly wouldn't work for network I/O, unfortunately. Note that this is essentially the approach taken by lazy bytestrings, for example. There are other elements I'd want to address before adding this, however. There are also complications dealing with nested streams. I have some ideas, but the way forward is far from clear. filter should be fairly simple to implement with peek and drop. I'll look into it. Cheers, John

3 2

Theory about uncurried functions
by Peter Verswyvelen 06 Mar '09

06 Mar '09

Lambda calculus is a nice theory in which every function always has one input and one output. Functions with multiple arguments can be simulated because functions are first class and hence a function can "return" a function. Multiple outputs cannot be done, one must embed these outputs in some data type, e.g. using a tuple, or one must use continuation passing style. Now, does a similar theory exist of functions that always have one input and one output, but these inputs and outputs are *always* tuples? Or maybe this does not make any sense?

10 22

missing support to Data.Binary for uvector package
by Manlio Perillo 06 Mar '09

06 Mar '09

Hi. I'm still having problems with the uvector package. I have an IntMap (UArr xxx) data type, and I want to serialize it to disk, in binary format. I'm using the uvector package from http://patch-tag.com/repo/pumpkin-uvector/home The problem is with missing instance declarations, for Hyperstrict data type and Data.Array.Vector.UArr.UPrim. For the former, the instance is simple, but for the latter I have no idea about what to do. The full error message (GHC 6.8.2): No instance for (uvector-0.2:Data.Array.Vector.UArr.UPrim (Word32 uvector-0.2:Data.Array.Vector.Prim.Hyperstrict.:*: Word8)) arising from a use of `encode' at bin/process-data-1.hs:72:18-36 Possible fix: add an instance declaration for (uvector-0.2:Data.Array.Vector.UArr.UPrim (Word32 uvector-0.2:Data.Array.Vector.Prim.Hyperstrict.:*: Word8)) Thanks Manlio Perillo

3 2

monadic MapReduce
by Manlio Perillo 06 Mar '09

06 Mar '09

Hi. I have a function that do some IO (take a file path, read the file, parse, and return some data), and I would like to parallelize it, so that multiple files can be parsed in parallel. I would like to use the simple mapReduce function, from Real Word Haskell: mapReduce :: Strategy b -- evaluation strategy for mapping -> (a -> b) -- map function -> Strategy c -- evaluation strategy for reduction -> ([b] -> c) -- reduce function -> [a] -- list to map over -> c mapReduce mapStrat mapFunc reduceStrat reduceFunc input = mapResult `pseq` reduceResult where mapResult = parMap mapStrat mapFunc input reduceResult = reduceFunc mapResult `using` reduceStrat Is this possible? Thanks Manlio Perillo I

4 7

Re: [Haskell-cafe] Re: [Haskell] Lazy IO breaks purity
by Svein Ove Aas 06 Mar '09

06 Mar '09

On Thu, Mar 5, 2009 at 2:08 PM, Simon Marlow <marlowsd(a)gmail.com> wrote: > > So the argument is something like: we can think of the result of a call to > unsafeInterleaveIO as having been chosen at the time we called > unsafeInterleaveIO, rather than when its result is actually evaluated. This > is on dodgy ground, IMO: either you admit that the IO monad contains an > Oracle, or you admit it can time-travel. I don't believe in either of > those things :-) > As it turns out, time-travel as a vehicle for computation has a long tradition in the more advanced systems; see http://www.frc.ri.cmu.edu/~hpm/project.archive/general.articles/1991/TempCo… for an example.

1 0

Finding the Convex Hull (Problem 12 of Real World Haskell)
by Rob Crowther 05 Mar '09

05 Mar '09

I wrote a "solution" to this problem, but it appears to return incorrect results. There's a pastebin of the code at http://hpaste.org/fastcgi/hpaste.fcgi/view?id=2121 and a picture of the inputs, outputs, and expected results graphed at http://img510.imageshack.us/img510/9971/resultsg.jpg I'm wondering if this is a flaw in my code, my understanding of the problem, or both. Any ideas on how to track this one down would be very much appreciated. Thank you! -- ヽ(^o^)ノ -rob

3 2

possible memory leak in uvector 0.1.0.3
by Manlio Perillo 05 Mar '09

05 Mar '09

Hi. In the "help optimizing memory usage for a program" I discovered some interesting things: 1) Using lazy ByteStrings to read files it not a good choice, since the garbage collector is not able to proper garbage cleanup. Running with -F1 RTS flag, however, keeps memory usage down. 2) UArr from uvector leaks memory. I'm rather sure about this. In fact the memory usage of my program is 815 MB, when: - I use lazy ByteString with -F1 flag or - if I use strict ByteString Using UArray and strict ByteString, memory usage is 660 MB. In total I have 17770 arrays, so there is a leak of about 9146 bytes per array. In another program, where I do a lot of array concatenations, memory leak is much more evident. Even using -F1 flag, memory usage grows too much. Is this a know bug? How can I verify if it is really a memory leak bug? pumpkin_ (on #haskell.it) suggested me to use a more recent version of uvector package, from http://patch-tag.com/publicrepos/pumpkin-uvector Using this version memory usage is, finally, 643 MB! (and execution if a bit faster, too). The other program, with a lot of array concatenations, still eats a lot of memory... Thanks Manlio Perillo

8 20

help optimizing memory usage for a program
by Manlio Perillo 05 Mar '09

05 Mar '09

Hi. After some work I have managed to implement two simple programs that parse the Netflix Prize data set. For details about the Netflix Prize, there was a post by Kenneth Hoste some time ago. I have cabalized the program, and made available here: http://haskell.mperillo.ath.cx/netflix-0.0.1.tar.gz The process-data-1 program parse the training data set, grouping ratings by movies. The process-data-2 program, instead, groups ratings by users. The structure of the two programs is similar: I create singletons IntMap and then use foldl + union to merge them together. The data structure used is: type Rating = Word32 :*: Word8 -- id, rating type MovieRatings = IntMap (UArr Rating) UArr is from uvector package. The training data set contains about 100 million ratings, so, in theory, the amount of memory required to store pairs of (id, rating) is about 480 MB. The process-data-1 program parse the entire dataset using about 1.4 GB of memory (3x increment). This is strange. The memory required is proportional to the number of ratings. It may be IntMap the culprit, or the garbage collector than does not release the memory to the operating system (or, worse, does not deallocate all used temporary memory). The process-data-2 has much more problems. When I try to parse *only* 500 movie ratings, the memory usage is 471 MB; 780 MB after all data has been fully evaluated (array concatenations). I'm using ghc 6.8.2 on Debian Etch. I would like to do some tests with recent GHC versions, but it may be a problem. Thanks Manlio Perillo

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