- Haskell-Cafe - Haskell.org

syntactic sugar for heterogeneous lists
by Alberto G. Corona 12 Apr '11

12 Apr '11

Hi Cafe: http://hackage.haskell.org/trac/ghc/ticket/1245 I also want some kind of syntactic sugar for H. Lists. The absence of syntactic sugar makes heterogeneous list to look scary. People in haskell is accostumed to syntactic sugaring, so people think of not sugared expressions as second class. I´ don't know the details, but it seems that tuples are in the language to cover the heterogeneous flexibility thing that homogeneous lists may not provide, but they introduce its own inflexibilities; These repeating instances for two, three, four and so on tuples makes them artificial and repetitive. In its comparison, a complex but unique H. List instance look elegant. specially if it is sugarized. An alternative to sugarize H.Lists preserving tuples could be to use {} to sugarize H. lists as {x,y,z} and desugarize it into: x :*: y :*: z :*: {} Just like [] means empty list, {} would mean HNil, the empty heterogeneous list. But this alternative , if implemented, would soon render tuples obsolete. These (,,) (,,,) constructors may create marginally faster and compact structures, but they are much less manageable.

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redefining a type class for one instance?
by Edward Amsden 12 Apr '11

12 Apr '11

I am trying as an experiment to write something as follows: data Wrapper a b where Pure :: (a -> b) -> Wrapper a b Nullable :: (Maybe a -> b) -> Wrapper a b class Liftable a b where liftTo :: (b -> c) -> a b c instance Liftable Wrapper a where liftTo = Pure instance Liftable Wrapper (Maybe a) where liftTo = Nullable Obviously this code (with suitable extensions enabled) complains of overlapping type instances, but is there a way to accomplish redefining a class instance for one particular group of types while having it defined for all others? (Maybe a vs. a in this instance) -- Edward Amsden Student Computer Science Rochester Institute of Technology www.edwardamsden.com

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Current heap size and other runtime statistics -- API for accessing in GHC?
by Ryan Newton 12 Apr '11

12 Apr '11

Hi cafe, The rtsopts (-s etc) can provide some nice debugging information regarding memory management. And System.Mem.performGC can initiate garbage collection. But are there APIs for querying the current state of the heap? I've googled and come up dry. In this case I'm running benchmarks and for the sake of fair comparison I want to make sure that everything from a previous run is cleaned up before the next run. Thanks, -Ryan

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ghc6: Re: Update: ... compiling ghc6
by Svante Signell 12 Apr '11

12 Apr '11

Addressing both Haskell and Hurd people here. Any hints by anyone? On Wed, 2011-04-06 at 09:30 +0200, Svante Signell wrote: ... > #5 0x011d3ce0 in __libc_read (fd=DWARF-2 expression error: DW_OP_reg > operations must be used either alone or in conjuction with DW_OP_piece > or DW_OP_bit_piece. > ) at ../sysdeps/mach/hurd/read.c:27 > #6 0x084919c8 in s9qJ_ret () > #7 0x0861f842 in StgRun () > #8 0x087c44e0 in ?? () Looking into this further, this looks like an error. What does it mean? from eglibc-2.11.2/sysdeps/mach/hurd/read.c /* Read NBYTES into BUF from FD. Return the number read or -1. */ ssize_t __libc_read (int fd, void *buf, size_t nbytes) { error_t err = HURD_FD_USE (fd, _hurd_fd_read (descriptor, buf, &nbytes, -1)); return err ? __hurd_dfail (fd, err) : nbytes; } The call is from the haskell binary cabal-bin but I don't have any debug info from that executable. Anybody fluent in Haskell? (How to get debug info in Haskell with gdb?) Below is the (stripped-down) call and the source code cabal-bin.hs: cd libraries/random: ../cabal-bin /usr/bin/ghc6 ../bootstrapping.conf configure --verbose=3 --with-compiler=../../ghc/stage1-inplace/ghc --with-hc-pkg=../../utils/ghc-pkg/install-inplace/bin/ghc-pkg module Main (main) where import Control.Monad import Data.Maybe import Distribution.PackageDescription import Distribution.PackageDescription.Parse import Distribution.Simple import Distribution.Simple.Utils import Distribution.Verbosity import System.Directory import System.Environment import System.FilePath import qualified Distribution.Make as Make import qualified Distribution.Simple as Simple setupProg :: FilePath setupProg = "./Setup" main :: IO () main = do unprocessedArgs <- getArgs let verbosity = verbose case unprocessedArgs of ghc : packageConf : args -> doit verbosity ghc packageConf args _ -> die "Bad args" doit :: Verbosity -> FilePath -> FilePath -> [String] -> IO () doit verbosity ghc packageConf args = do exists <- doesFileExist setupProg if exists then rawSystemExit verbosity setupProg args else do gpdFile <- defaultPackageDesc verbosity gpd <- readPackageDescription verbosity gpdFile let pd = packageDescription gpd case buildType pd of Just Simple -> Simple.defaultMainArgs args Just Make -> Make.defaultMainArgs args Just Configure -> defaultMainWithHooksArgs autoconfUserHooks args _ | packageName pd == PackageName "Cabal" -> -- Cabal is special...*sigh* Simple.defaultMainArgs args | otherwise -> runSetup verbosity ghc packageConf args runSetup :: Verbosity -> FilePath -> FilePath -> [String] -> IO () runSetup verbosity ghc packageConf args = do -- Don't bother building Setup if we are cleaning. If we need to -- build Setup in order to build, and Setup isn't built already, -- then there shouldn't be anything to clean anyway. unless cleaning $ rawSystemExit verbosity ghc ["-package-conf", packageConf, "--make", "Setup", "-o", "Setup"] rawSystemExit verbosity "./Setup" args where cleaning = case args of "clean" : _ -> True _ -> False

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Re: [Haskell-cafe] Assimp FFI Library
by John Lato 12 Apr '11

12 Apr '11

> > From: Joel Burget <joelburget(a)gmail.com> > Subject: [Haskell-cafe] Assimp FFI Library > To: haskell-cafe(a)haskell.org > Message-ID: <BANLkTimW-71NcBRqnr68kRAvKZytE1uVwA(a)mail.gmail.com> > Content-Type: text/plain; charset="iso-8859-1" > 5. I've reduced a lot of boilerplate in Vec.hs by using the CPP > preprocessor > extension. I could reduce the boilerplate by another factor of 3 if I could > recursively call templates but that's not allowed. I would like to have one > template to generate both of these lines: > > > data Vec N2 Double t = Vec2D !Double !Double deriving (Show, Eq) > > data Vec N3 Double t = Vec3D !Double !Double !Double deriving (Show, Eq) > > Notice there is an extra !Double in the second. Is there an easy way to do > this? I don't know much about Template Haskell, would that work? Would it > be > easy? > Yes, it works, and it's pretty easy. My adaptive-tuple package, http://hackage.haskell.org/package/adaptive-tuple, provides this (as well as strict vectors up to 20 elements). Tuple is a bit of a misnomer. You'll need to download the source to get the Template Haskell splices because they aren't exported (tarball from hackage, or via http://www.tiresiaspress.us/haskell/adaptive-tuple/) John L.

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Call for Contributions - Haskell Communities and Activities Report, May 2011 edition
by Janis Voigtländer 12 Apr '11

12 Apr '11

Dear all, I would like to collect contributions for the 20th edition of the ================================================================ Haskell Communities & Activities Report http://www.haskell.org/haskellwiki/Haskell_Communities_and_Activities_Report Submission deadline: 1 May 2011 (please send your contributions to hcar at haskell.org, in plain text or LaTeX format) ================================================================ This is the short story: * If you are working on any project that is in some way related to Haskell, please write a short entry and submit it. Even if the project is very small or unfinished or you think it is not important enough -- please reconsider and submit an entry anyway! * If you are interested in an existing project related to Haskell that has not previously been mentioned in the HCAR, please tell me, so that I can contact the project leaders and ask them to submit an entry. * Feel free to pass on this call for contributions to others that might be interested. More detailed information: The Haskell Communities & Activities Report is a bi-annual overview of the state of Haskell as well as Haskell-related projects over the last, and possibly the upcoming six months. If you have only recently been exposed to Haskell, it might be a good idea to browse the November 2010 edition -- you will find interesting topics described as well as several starting points and links that may provide answers to many questions. Contributions will be collected until the submission deadline. They will then be compiled into a coherent report that is published online as soon as it is ready. As always, this is a great opportunity to update your webpages, make new releases, announce or even start new projects, or to talk about developments you want every Haskeller to know about! Looking forward to your contributions, Janis (current editor) FAQ: Q: What format should I write in? A: The required format is a LaTeX source file, adhering to the template that is available at: http://haskell.org/communities/05-2011/template.tex There is also a LaTeX style file at http://haskell.org/communities/05-2011/hcar.sty that you can use to preview your entry. If you do not know LaTeX, then use plain text. If you modify an old entry that you have written for an earlier edition of the report, you should already have received your old entry as a template (provided I have your valid email address). Please modify that template, rather than using your own version of the old entry as a template. Q: Can I include Haskell code? A: Yes. Please consider using lhs2tex syntax (http://people.cs.uu.nl/andres/lhs2tex/) The report is compiled in mode polycode.fmt. Q: Can I include images? A: Yes, you are even encouraged to do so. Please use .jpg format, then. Q: Should I send files in .zip archives or similar? A: No, plain file attachements are the way. If you have several entries to submit, it helps if you send them in separate emails. Q: How much should I write? A: Authors are asked to limit entries to about one column of text. This corresponds to approximately one page, or 40 lines of text, with the above style and template. A general introduction is helpful. Apart from that, you should focus on recent or upcoming developments. Pointers to online content can be given for more comprehensive or "historic" overviews of a project. Images do not count towards the length limit, so you may want to use this opportunity to pep up entries. There is no minimum length of an entry! The report aims for being as complete as possible, so please consider writing an entry, even if it is only a few lines long. Q: Which topics are relevant? A: All topics which are related to Haskell in some way are relevant. We usually had reports from users of Haskell (private, academic, or commercial), from authors or contributors to projects related to Haskell, from people working on the Haskell language, libraries, on language extensions or variants. We also like reports about distributions of Haskell software, Haskell infrastructure, books and tutorials on Haskell. Reports on past and upcoming events related to Haskell are also relevant. Finally, there might be new topics we do not even think about. As a rule of thumb: if in doubt, then it probably is relevant and has a place in the HCAR. You can also ask the editor. Q: Is unfinished work relevant? Are ideas for projects relevant? A: Yes! You can use the HCAR to talk about projects you are currently working on. You can use it to look for other developers that might help you. You can use it to write "wishlist" items for libraries and language features you would like to see implemented. Q: If I do not update my entry, but want to keep it in the report, what should I do? A: Tell the editor that there are no changes. The old entry will typically be reused in this case, but it might be dropped if it is older than a year, to give more room and more attention to projects that change a lot. Do not resend complete entries if you have not changed them.

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Assimp FFI Library
by Joel Burget 12 Apr '11

12 Apr '11

Hello, I've been working on an ffi library for the Assimp asset import library( http://assimp.sourceforge.net) It should be useful for people doing graphics in Haskell. I've been working on it so I can import models into a ray-tracer I've been working on. My current progress is here: https://github.com/joelburget/assimp. A couple weeks ago I managed to import models and do some simple (and slightly buggy) renders, but since then I've been redoing a lot of the guts of the library. At this moment it doesn't build because of the changes I'm in the middle of. Hopefully I'll be able to release the first version in the next week or so. I've run into a few roadblocks that I would be thankful for help with. 1. In Graphics.Formats.Assimp.Vec I'm trying to get the fastest and smallest possible vector types since a program could easily have millions of vectors in memory and operations on them take a huge fraction of the time in an average program. One problem I ran into is how to make the vectors type-safe. The assimp library has separate data types for colors and vectors, but represented the same in memory. I have a few ideas about how to represent these: a. The way I've been doing it: >data N2 >data N3 >data N4 >data Color >class Num a => Vector n a where > data Vec n a :: * -> * > ... rest of class >-- A 3-dimensional vector of doubles. t is the (phantom) tag type, like Color, Direction, etc >-- This is meant to improve type safety >instance Vector N3 Double where > data Vec N3 Double t = Vec3D !Double !Double !Double deriving (Show, Eq) > ... rest of instance This seems very close to what I want, but it presents a few problems. First, I create matrices from vectors, so the matrix must have an implicit type, the type of the tags from the vectors it is made of. I made this () by convention but then to multiply a vector by this matrix I have to use the dot product and other vector operations, but then the tag type from the matrix is different from that of the matrix I am multiplying. This leads to the type `dot :: Vec n a t1 -> Vec n a t2 -> a`. Notice how there are two different tag types, reducing type safety. Another problem is importing vectors from the library. I would like to have different tags for distance and direction, again to increase type safety, but they are both represented the same way in the library, so the Storable instance can't always know what type to create. I would probably always have to return a vector with tag type (). b. Just create separate Vec and Color datatypes. I would be mirroring the library directly. Both would be instances of Vector. c. Use newtype wrappers around Vec to represent the different types. I don't really like this solution because the newtypes are too painful to use. d. I've thought about some other solutions that I've forgotten now. Maybe you have something better! Remember, I would like to have a high degree of type safety if possible, but the top consideration is performance. Another thing I've been wondering about: could I use overlapping instances to allow the vectors to be of any numeric type, but specialize and unbox them for the common ones? I'm assuming unboxing can't be done unless we have a specific type because the compiler won't know how large the type is. 2. How do people feel about the vector and matrix operators (eg |*|, |*||, ||*)? I like them a lot but I would like to see how other people feel about them. 3. Do I have to worry about marshalling float, double, and int straight to Haskell? The report says Float and Double should conform to the IEEE standard. I'm more interested in Int since the report only requires a 30 bit Int. On my computer they appear to be 64 bits. Does this vary on 32 bit computers? 4. There are several places the original library uses unsigned ints. Obviously they can't be negative so I used CUInt in the Haskell code. I never see CUInt instead of Int in real code. Should I change these? 5. I've reduced a lot of boilerplate in Vec.hs by using the CPP preprocessor extension. I could reduce the boilerplate by another factor of 3 if I could recursively call templates but that's not allowed. I would like to have one template to generate both of these lines: > data Vec N2 Double t = Vec2D !Double !Double deriving (Show, Eq) > data Vec N3 Double t = Vec3D !Double !Double !Double deriving (Show, Eq) Notice there is an extra !Double in the second. Is there an easy way to do this? I don't know much about Template Haskell, would that work? Would it be easy? I should mention that I'm going to convert all the Storable instances from something like this: > peek p = do > w <- (#peek aiQuaternion, w) p > x <- (#peek aiQuaternion, x) p > y <- (#peek aiQuaternion, y) p > z <- (#peek aiQuaternion, z) p > return $ Quaternion w x y z to something like this: > peek p = Quaternion <$> (#peek aiQuaternion, w) p > <*> (#peek aiQuaternion, w) p > <*> (#peek aiQuaternion, w) p > <*> (#peek aiQuaternion, w) p So there's no need to tell me to change those. But please let me know of other places that are in poor style. Oh yes, patches and collaborators are welcome. Thanks! Joel

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Stateful iteratees
by Ertugrul Soeylemez 12 Apr '11

12 Apr '11

Hello fellow Haskellers, I'm trying to solve a very practical problem: I need a stateful iteratee monad transformer. Explicit state passing is very inconvenient and would destroy the elegance of my library. There are two approaches to this: 1. type MyT a m = Iteratee a (StateT MyConfig m) 2. type MyT a m = StateT MyConfig (Iteratee a m) Both work well except in two very specific corner cases: - I need to convert the transformer to 'Iteratee a m', i.e. remove the state layer. This is obviously trivial with the second variant, but seems very difficult with the first one, if it's possible at all. - I need to use control structures of Iteratee like catchError. This is obviously trivial with the first variant, but very inconvenient with the second, because I would need to reinvent many wheels. Does someone know a cleaner, more elegant solution? Encapsulating the state in the iteratee's input type is not an option. Many thanks in advance. Greets, Ertugrul -- nightmare = unsafePerformIO (getWrongWife >>= sex) http://ertes.de/

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Re: [Haskell-cafe] Foreign function performance: monadic vs pure
by John Lato 12 Apr '11

12 Apr '11

> > From: Gregory Collins <greg(a)gregorycollins.net> > > On Mon, Apr 11, 2011 at 3:55 PM, Serguei Son <serguei.son(a)gmail.com> > wrote: > > So if I must use a safe function returning IO a, > > there is no way to improve its performance? To give you > > a benchmark, calling gsl_ran_ugaussian a million times > > in pure C takes only a second or two on my system. > > In the C version, are you also producing a linked list containing all > of the values? Because that's what mapM does. Your test is mostly > measuring the cost of allocating and filling ~3 million machine words > on the heap. Try mapM_ instead. > I've done some rough tests of a few versions, and I believe Gregory's analysis is correct. These are for calculating a sum of sines of 1..10^7 with ghc-7.0.3. version execution time IO, safe - 4.08s Pure, safe - 3.07s IO, unsafe - 2.77s Pure, unsafe - 1.95s For the IO versions, I used the following: main = foldM (\ !acc n -> fmap (acc +) $ c_sin_io n) [1..10^7] >>= print (this can possibly be improved; it was the first thing I thought of) The most important factor (as often the case with Haskell) is to pay attention to the space properties of your algorithm. Instead of using mapM, which forces the spine of the list, it's much better to interleave the fold step with the other IO. Note that the "acc" parameter is strict, this is necessary to avoid building a thunk. John L.

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Tuple
by Anwar Bari 12 Apr '11

12 Apr '11

HI Cafe I have to make a function to check that I have one occurrence of the last element (z) of the same list [a,b] in the tuple [([a,b],z)] For example [([1,2],3),([1,1],5),([1,3],6).......] this is true because there is one single z for each single list. while this one is false [([1,2],3),([1,2],5),([1,3],6).......] because 3&5 were found for the same list [1,2] any Idea how to code this Fn. Thanks

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