- Haskell-Cafe - Haskell.org

compilation succeeds -- execution fails
by Jason Dusek 31 Mar '08

31 Mar '08

I have a program here: https://svn.j-s-n.org/public/haskell/cedict currently at revision 302, which compiles okay but I can't get it to work. I'm using the FFI to take a (currently small) array and translate it into a Map. It compiles fine and loads fine -- but it doesn't run fine: GHCi, version 6.8.2: http://www.haskell.org/ghc/ :? for help Loading package base ... linking ... done. Prelude> import Data.Char.CEDICT Prelude Data.Char.CEDICT> traditional 'a' Loading package array-0.1.0.0 ... linking ... done. Loading package containers-0.1.0.1 ... linking ... done. Loading package parsec-2.1.0.0 ... linking ... done. Loading package utf8-string-0.2 ... linking ... done. Loading package cedict-0.1.1 ... linking ... <interactive>: unknown symbol `___stginit_cedictzm0zi1zi1_DataziCharziCEDICTziMatter_' ghc-6.8.2: unable to load package `cedict-0.1.1' Prelude Data.Char.CEDICT> I'm on a Mac -- Leopard. A whole bunch of things *could* be wrong -- I'd appreciate some help in narrowing the list. -- _jsn

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Re: [Haskell-cafe] Usage of Read Class
by Ariel J. Birnbaum 31 Mar '08

31 Mar '08

> > <interactive>:1:1: > > Ambiguous type variable `a' in the constraint: > > `Read a' arising from a use of `read' at <interactive>:1:1-4 > > Probable fix: add a type signature that fixes these type variable(s) > > What the error message says is, it doesn't know which type to read to. > read has the type Read a => String -> a, and unless you tell it what > 'a' is going to be, how would it know? If you say '(read.show) x :: > Color' it should work just fine. Just to sharpen the point: read cannot know from the string it processes which type to deliver. For example, try the following at the ghci prompt: read "3" -> Yields the same error as above read "3" + 3 -> 6 read "3" + 3.3 -> 6.3 read "3.3" + 3.3 -> 6.6 read "3.3" + 3 -> Error: no parse What's happening is that the type of the expression is determined by the literal on the _right_side_ of the + sign, and not by the contents of the string on the left side. The last example is a bit tricky. ghci sees 3 on the rhs, which can be of any type in the Num class (as opposed to 3.3, which has to be of a Fractional type). If unresolved, it will default to Integer (or Int?), and try to parse "3.3" as such -- which will of course fail. -- Ariel J. Birnbaum

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Usage of Read Class
by Simeon Mattes 31 Mar '08

31 Mar '08

Hello, I would like to ask something that results in when I have the following commands data Color = Red | Green | Blue | Indigo | Violet deriving (Enum,Show,Read) (read.show) x <interactive>:1:1: Ambiguous type variable `a' in the constraint: `Read a' arising from a use of `read' at <interactive>:1:1-4 Probable fix: add a type signature that fixes these type variable(s) I also receive the same message with the following: *Test > let x = Branch (Branch (Branch (Leaf 'a') (Leaf 'b')) (Leaf 'c') ) (Leaf 'd') *Test > x <<<'a'|'b'>|'c'>|'d'> *Test > (read.show) x <interactive>:1:1: Ambiguous type variable `a' in the constraint: `Read a' arising from a use of `read' at <interactive>:1:1-4 Probable fix: add a type signature that fixes these type variable(s) data Tree a = Leaf a | Branch (Tree a) (Tree a) --type ShowS = String -> String showsTree :: (Show a) => Tree a -> ShowS showsTree (Leaf x) = shows x showsTree (Branch l r) = ('<':).showsTree l . ('|':) . showsTree r . ('>':) --type ReadS a = String -> [(a,String)] readsTree :: (Read a) => ReadS (Tree a) readsTree s = [(Branch l r, x) | ("<",t) <- lex s, (l,u) <- readsTree t, ("|",v) <- lex u, (r,w) <- readsTree v, (">",x) <-lex w] ++ [(Leaf x, t) | (x,t) <- read s] instance Show a => Show (Tree a) where showsPrec _ x = showsTree x instance Read a => Read (Tree a) where readsPrec _ s = readsTree s Why is this happen? Maybe an example with a simple application of the Class Read would be helpful. Thanks -- View this message in context: http://www.nabble.com/Usage-of-Read-Class-tp16381441p16381441.html Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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Announce: Decimal arithmetic package
by Paul Johnson 31 Mar '08

31 Mar '08

I've just uploaded version 0.1.0 of a decimal arithmetic package to Hackage. Decimal numbers are stored as an Integral mantissa and a Word8 exponent, where the number stored is mantissa * 10^(-exponent). In other words the exponent is the number of decimal places stored. There are also routines for partitioning a value such that the sum of the parts is equal to the original value. This is useful in financial arithmetic. This came out of my ongoing work on an AMQP binding for Haskell. Version 0-10 of the AMQP spec includes decimal fractions defined in this way for financial applications, so I thought I'd make a proper job of it. A darcs repository will be set up shortly. Paul.

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announce: Glome.hs raytracer
by Jim Snow 31 Mar '08

31 Mar '08

I have recently posted a haskell port of my ocaml raytracer, Glome: http://syn.cs.pdx.edu/~jsnow/glome/ It supports spheres and triangles as base primitives, and is able to parse files in the NFF format used by the standard procedural database (http://tog.acm.org/resources/SPD/) It uses a bounding interval heirarchy acceleration structure, so it can render fairly complicated scenes in a reasonable amount of time. Shadows and reflections are supported, but not specular highlights or refraction. It's still slower than the ocaml version, but at least they're in the same ballpark (and a good part of that difference may be inefficiencies in my BIH traversal). I would welcome any advice on making it go faster or use less memory. I compile the program with "ghc Glome.hs --make -fasm -O2 -threaded -fglasgow-exts -funbox-strict-fields -fbang-patterns -fexcess-precision -optc-ffast-math -optc-O2 -optc-mfpmath=sse -optc-msse2". (I assume the -optc options don't do anything unless you compile via C.) Here are some of my current concerns: -Multi-core parallelism is working, but not as well as I'd expect: I get about a 25% reduction in runtime on two cores rather than 50%. I split the default screen size of 512x512 into 16 blocks, and run "parMap" on those blocks with a function that turns the screen coordinates of that block into a list of (x,y,r,g,b) tuples that get drawn as pixels to the screen through OpenGL by the original thread. -Memory consumption is atrocious: 146 megs to render a scene that's a 33k ascii file. Where does it all go? A heap profile reports the max heap size at a rather more reasonable 500k or so. (My architecture is 64 bit ubuntu on a dual-core amd.) -Collecting rendering stats is not easy without global variables. It occurs to me that it would be neat if there were some sort of write-only global variables that can be incremented by pure code but can only be read from within monadic code; that would be sufficient to ensure that the pure code wasn't affected by the values. The sorts of things I'm looking for are the number of calls to "trace" per image, the number of BIH branches traversed and ray/triangle and ray/sphere intersections per pixel. (Disclaimer: I don't really fully understand monads, so I may be oblivious to an obvious solution.) -Is there a fast way to cast between Float and Double? I'm using Float currently, and the only reason is because that's what the OpenGL api expects. I'd like to be able to use either representation, but the only way to cast that I've found so far is "float_conv x = fromRational(toRational x)", which is too slow. thanks, -jim

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announce: hgdbmi (GDB/MI interface)
by Evan Martin 30 Mar '08

30 Mar '08

Just in case someone else needed this, here you go: GDB/MI lets programs drive GDB. It can be used, for example, by GDB frontends. This module wraps attaching GDB to a process and parsing the (surprisingly complicated) GDB/MI output. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/hgdbmi

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ANNOUNCE: xmonad 0.7 released
by Don Stewart 30 Mar '08

30 Mar '08

http://xmonad.org The xmonad dev team is pleased to announce xmonad 0.7! The headlines: The 0.7 release of xmonad provides several improvements over 0.6, including improved integration with GNOME, more flexible "rules", various stability fixes, and of course, many new and interesting features in the extension library (general support for window decorations, utf8 support, scratch pad terminals, pointer control) and more! New GNOME support: Active, automated support for GNOME utilities. We know our users often like to use GNOME menus, tools and status bars, and we'd like to announce that xmonad actively supports GNOME! Extensions for communicating with and utilising gnome utilities come in the library suite, as well as extensive documentation and support. For more information see the GNOME/xmonad integration page on the wiki. A period of active development: In the past year, the xmonad development team received contributions from over 60 developers, making xmonad one of the largest window manager development teams around, and dwarfing other tiling window manager projects. Yet, at the same time, the core code base remains at around 1000 lines of code, with another 7000 lines in the extension library -- a significant achievment! Change logs: http://haskell.org/haskellwiki/Xmonad/Notable_changes_since_0.6 http://xmonad.org/changelog-0.7.txt http://xmonad.org/changelog-xmc-0.7.txt About: xmonad is a tiling window manager for X. Windows are arranged automatically to tile the screen without gaps or overlap, maximising screen use. Window manager features are accessible from the keyboard: a mouse is optional. xmonad is extensible in Haskell, allowing for powerful customisation. Custom layout algorithms, key bindings and other extensions may be written by the user in config files. Layouts are applied dynamically, and different layouts may be used on each workspace. Xinerama is fully supported, allowing windows to be tiled on several physical screens. Features: * Very stable, fast, small and simple. * Automatic window tiling and management * First class keyboard support: a mouse is unnecessary * Full support for tiling windows on multi-head displays * Full support for floating, tabbing and decorated windows * Full support for Gnome and KDE utilities * XRandR support to rotate, add or remove monitors * Per-workspace layout algorithms * Per-screens custom status bars * Compositing support * Powerful, stable customisation and reconfiguration * Large extension library * Excellent, extensive documentation * Large, active development team, support and community Get it! Information, screenshots, documentation, tutorials and community resources are available from the xmonad home page: http://xmonad.org The 0.7 release, and its dependencies, are available from hackage.haskell.org: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/xmonad xmonad packages are available in the package systems of at least: Debian, Gentoo, Arch, Ubuntu, OpenBSD, NetBSD, FreeBSD, Gobo, NixOS, Source Mage, Slackware and 0.7 packages will appear in coming days (some are already available). On the fly updating to xmonad 0.7 is supported. You should be able to upgrade to xmonad 0.7 from 0.6 and earlier, transparently, without losing your session. Load the new code with mod-q and enjoy. Extensions: xmonad comes with a huge library of extensions (now more than 7 times the size of xmonad itself), contributed by viewers like you. Extensions enable pretty much arbitrary window manager behaviour to be implemented by users, in Haskell, in the config files. For more information on using and writing extensions see the webpage. The library of extensions is available from hackage: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/xmonad-contrib Full documentation for using and writing your own extensions: http://xmonad.org/contrib.html This release brought to you by the xmonad dev team: Spencer Janssen Don Stewart Jason Creighton David Roundy Brent Yorgey Devin Mullins Braden Shepherdson Roman Cheplyaka Lucas Mai Featuring code contributions from over 60 developers: Aaron Denney Adam Vogt Alec Berryman Alex Tarkovsky Alexandre Buisse Andrea Rossato Austin Seipp Bas van Dijk Ben Voui Brandon Allbery Chris Mears Christian Thiemann Clemens Fruhwirth Daniel Neri Daniel Wagner Dave Harrison David Glasser David Lazar Dmitry Kurochkin Dominik Bruhn Dougal Stanton Eric Mertens Ferenc Wagner Gwern Branwen Hans Philipp Annen Ivan Tarasov Jamie Webb Jeremy Apthorp Joachim Breitner Joachim Fasting Joe Thornber Joel Suovaniemi Juraj Hercek Justin Bogner Kai Grossjohann Karsten Schoelzel Klaus Weidner Mathias Stearn Mats Jansborg Matsuyama Tomohiro Michael Fellinger Michael Sloan Miikka Koskinen Neil Mitchell Nelson Elhage Nick Burlett Nicolas Pouillard Nils Anders Danielsson Peter De Wachter Robert Marlow Sam Hughes Shachaf Ben-Kiki Shae Erisson Simon Peyton Jones Stefan O'Rear Tom Rauchenwald Valery V. Vorotyntsev Will Farrington Yaakov Nemoy timthelion As well as the support of many others on the #xmonad and #haskell IRC channels, and the wider Haskell and window manager communities. Thanks to everyone for their support!

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Haddock Help Required
by Dominic Steinitz 29 Mar '08

29 Mar '08

What should I be using for the file name for the read-interface option in haddock? Trying to use the file on www.haskell.org gives this: dom@lagrange:~/asn15/asn1> haddock -html -o hdoc Pretty.hs -B /usr/lib/ghc-6.8.2 --optghc="-fglasgow-exts" --read-interface=http://www.haskell.org/ghc/docs/latest/html/libraries/base,http://www.haskell.org/ghc/docs/latest/html/libraries/base/base.haddock haddock: internal Haddock or GHC error: http://www.haskell.org/ghc/docs/latest/html/libraries/base/base.haddock: openBinaryFile: does not exist (No such file or directory) But if I download the file and try and use it locally, I get this dom@lagrange:~/asn15/asn1> haddock -v -html -o hdoc Pretty.hs -B /usr/lib/ghc-6.8.2 --optghc="-fglasgow-exts" --read-interface=http://www.haskell.org/ghc/docs/latest/html/libraries/base,base.haddock Warning: Cannot read base.haddock: "Magic number mismatch: couldn't load interface file: base.haddock" Skipping this interface. Warning: main:Language.ASN1: could not find link destinations for: GHC.Enum.Enum GHC.Base.Eq GHC.Base.Ord GHC.Show.Show GHC.Num.Integer Language.ASN1.ComponentIndex Language.ASN1.Tagged GHC.Base.String Data.Maybe.Maybe Language.ASN1.Octet Data.Map.Map Warning: main:Pretty: could not find link destinations for: Text.PrettyPrint.HughesPJ.Doc Pretty.Pretty Pretty.PrettyVal Warning: main:ConstrainedType: could not find link destinations for: Data.Maybe.Maybe GHC.Base.Bool GHC.Num.Integer Pretty.Pretty Pretty.PrettyVal GHC.Base.String GHC.Base.Eq GHC.Show.Show GHC.Base.Int GHC.Word.Word8 GHC.Base.Ord Data.Monoid.Monoid Data.Binary.Strict.BitPut.BitPut GHC.Real.Integral Control.Monad.State.Class.MonadState Data.ByteString.Internal.ByteString Control.Monad.Error.Class.MonadError GHC.Base.Char Data.Binary.Strict.BitGet.BitGet Any help would be appreciated. Dominic.

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Monad instance for Data.Set, again
by Henning Thielemann 29 Mar '08

29 Mar '08

The blog article http://www.randomhacks.net/articles/2007/03/15/data-set-monad-haskell-macros describes a variant of the Monad class which allows to restrict the type of the monadic result, in order to be able to make Data.Set an instance of Monad (requiring Ord constraint for the monadic result). The same problem arises for container data structures with restricted element types, where the element type restriction depends on the implementation of the container structure (such as UArray). It would be cumbersome to have several class parts, even more, type constraints in type signatures may reveal implementation details. E.g. constraint (Container c x y z) might be needed for a 'zipWith' function, whereas (Container c y x z) is needed if you use 'zipWith' with swapped arguments. Here is another approach that looks tempting, but unfortunately does not work, and I wonder whether this can be made working. module RestrictedMonad where import Data.Set(Set) import qualified Data.Set as Set class AssociatedMonad m a where class RestrictedMonad m where return :: AssociatedMonad m a => a -> m a (>>=) :: (AssociatedMonad m a, AssociatedMonad m b) => m a -> (a -> m b) -> m b instance (Ord a) => AssociatedMonad Set a where instance RestrictedMonad Set where return = Set.singleton x >>= f = Set.unions (map f (Set.toList x)) GHC says: RestrictedMonad.hs:21:13: Could not deduce (Ord b) from the context (RestrictedMonad Set, AssociatedMonad Set a, AssociatedMonad Set b) arising from use of `Data.Set.unions' at RestrictedMonad.hs:21:13-22 Probable fix: add (Ord b) to the class or instance method `RestrictedMonad.>>=' In the definition of `>>=': >>= x f = Data.Set.unions (map f (Data.Set.toList x)) In the definition for method `RestrictedMonad.>>=' In the instance declaration for `RestrictedMonad Set'

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Glome raytracer bug: bad output with -O2 -fasm
by Jim Snow 29 Mar '08

29 Mar '08

I was trying to get Blinn highlights working with my raytracer, and kept getting ugly artifacts. After trying a bunch of things, I finally compiled without -O2, and the artifacts went away. Here's what I mean: http://syn.cs.pdx.edu/~jsnow/glome/Glome.hs-noartifact.png http://syn.cs.pdx.edu/~jsnow/glome/Glome.hs-artifact.png Here's the offending code, run "./make" and "./run" and you should see the artifacts if your setup is the same as mine. (Requires OpenGL.) http://syn.cs.pdx.edu/~jsnow/glome/glome.hs-0.2-bug.tar.gz The artifacts also go away if I use -fvia-C. It doesn't seem to matter whether I use Floats or Doubles in the rendering code. The artifacts also show up with -O1. Have I stumbled across a known compiler bug? Or perhaps an OpenGL bug? (The bug could, of course, be in my code, but then one might expect to get the same erroneous output every time regardless of compiler flags.) To reiterate, I'm using ghc 8.6.2. thanks, -jim

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Sound typeable via type families?
by Dan Doel 29 Mar '08

29 Mar '08

Hello, Recently, someone (I forgot who, exactly) mentioned in #haskell this article: http://okmij.org/ftp/Haskell/types.html#unsound-typeable where Oleg demonstrates that Typeable makes the type system unsound, and one can get back unsafeCoerce (which is used in the implementation, I suspect). However, this evening, Ryan Ingram showed how to construct a safe Typeable using GADTs. The problem with that, of course, is that GADTs aren't open, so one can only have a predefined number of Typeables. However, it occurred to me that data families don't have that problem, so I took a shot and came up with the following: {-# LANGUAGE TypeFamilies, ScopedTypeVariables, GADTs , MultiParamTypeClasses, FlexibleInstances, OverlappingInstances #-} class Typeable t where data TypeRep t :: * typeRep :: TypeRep t instance Typeable Int where data TypeRep Int = TInt typeRep = TInt instance Typeable Integer where data TypeRep Integer = TInteger typeRep = TInteger instance Typeable () where data TypeRep () = TUnit typeRep = TUnit data TEq a b where Refl :: TEq a a class (Typeable a, Typeable b) => TEquality a b where teq :: TypeRep a -> TypeRep b -> Maybe (TEq a b) instance (Typeable a) => TEquality a a where teq _ _ = Just Refl instance (Typeable a, Typeable b) => TEquality a b where teq _ _ = Nothing cast :: forall a b. (TEquality a b) => a -> Maybe b cast a = do Refl <- teq ta tb return a where ta :: TypeRep a ta = typeRep tb :: TypeRep b tb = typeRep Which, somewhat to my surprise, actually works. And, of course, if one tries to pull off Oleg's trick: newtype Oleg a = Oleg { unOleg :: a } instance Typeable (Oleg a) where TypeRep (Oleg a) = ... typeRep = ?? Well, it won't work, because TypeRep () ~/~ TypeRep (Oleg a). However, obviously, this depends on overlapping instances (if there's some other way, I'd be happy to know; if type inequality contexts are available, I wasn't able to find them), and I've heard that type families don't play well with overlap. Does that not apply to data families? Will this construction still work in 6.10 and beyond? Also, this doesn't seem to be a suitable basis for Dynamic. Trying to extend the GADT solution presented resulted in errors unless incoherent instances were turned on (clearly not a good sign), and even then, it didn't actually work. Is it possible to do better, and come away with something that will actually work for Dynamic, and be sound? Is there some other trick waiting to pull unsafeCoerce out of this setup (seems doubtful, as it isn't used in the code, and the type families folks have no doubt done plenty of work to ensure soundness)? Comments appreciated. -- Dan

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What Does This Message Mean? (Re: [Haskell-cafe] compilation succeeds -- execution fails)
by Jason Dusek 29 Mar '08

29 Mar '08

The message unknown symbol `___stginit_cedictzm0zi1zi1_DataziCharziCEDICTziMatter_' says that it can't find the initializer for` Data.Char.CEDICT.Matter` in `cedict-0.1.1` (this is 'z-encoding', if I remember correctly). So, the odd thing is, that is not the part with the C FFI stuff in it -- that's in `Data.Char.CEDICT.Lists`. Is there a usual thing that is wrong when the initializer can not be found? -- _jsn

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[gsoc] mingw64 ghc port
by Bulat Ziganshin 29 Mar '08

29 Mar '08

Hello haskell-cafe, it's probably a bit too late, but i recalled that there is one project that will be very useful - it's porting ghc to mingw64 platform, allowing it to generate 64-bit windows platforms. may be someone will find it interesting -- Best regards, Bulat mailto:Bulat.Ziganshin@gmail.com

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[GSoC] Parallel Benchmarking and Profiling
by Etienne Laurin 29 Mar '08

29 Mar '08

Hello, I am putting together a student proposal to participate in Google's Summer of Code with one of the following project ideas. Parallel programming benchmarking and benchmark suite - http://hackage.haskell.org/trac/summer-of-code/ticket/1544 Are there open source projects and real world applications that rely on GHC's parrallel programming primitives and libraries? I have found many references to LOLITA, but it seems to be old and not available online. The idea page suggests porting existing benchmark suites such as PARSEC, but PARSEC is 5G of C code. Most of it seems to come from existing applications already written in C. It might also be interesting to automate the discovery of optimal strategies through empirical data, and to modify the thresholds dynamically. Parallel profiling tools for GHC - http://hackage.haskell.org/trac/summer-of-code/ticket/1559 Simon Marlow wrote on the idea page that Gransim was ported to a more up-to-date GHC. The documentation available on the web seems to be for GHC 0.29 but it describes many options for logging and visualising the activity of threads and processors over time. Getting GHC to display that information on the frontpanel would make a nice project. Do you have any comments or suggestions? Thank you, Etienne Laurin

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Re: [Haskell-cafe] Web server libraries
by Sterling Clover 28 Mar '08

28 Mar '08

Yipe. It's just been pointed out to me that the hvac repo was missing a key file. I just committed it and tried a fresh pull and build, and it seems to work properly now. Apologies to all who couldn't get it working. Regards, Sterl. On Fri, 28 Mar 2008, Sterling Clover wrote: > While hvac, which I announced here recently, is not yet ready for > primetime, so to speak, you may want to take a look at it -- with a > few tweaks it should match your specs. (darcs get http:// > community.haskell.org/~sclv/hvac/<http://community.haskell.org/%7Esclv/hvac/> ) > [...]

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MonadMemory class
by Ariel J. Birnbaum 28 Mar '08

28 Mar '08

Two questions came to mind while thinking of memory references in Haskell: 1. Is there a "standard" equivalent of the following: class (Monad m) => MonadMemory m r | m -> r where new :: a -> m (r a) read :: r a -> m a write :: r a -> a -> m () What kind of axioms should an instance of this class satisfy? 2. How would a "pure" instance of this class look like (obvious unsafePerformIO-based solutions aside)? Is it even possible in pure Haskell? Thanks much! -- Ariel J. Birnbaum

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Recursion problem in infinite list model
by Hans Aberg 28 Mar '08

28 Mar '08

When experimenting with list index sets (i.e. lists more general than provided by Haskell), I arrived at the problem that in the example code below for example h(list 6) does not (in Hugs) write out the beginning of the list lazily. It does work for list 6 first (list 6) rest (list 6) 10 -+ (list 3) and so on. So it seems to be a problem with h, also suggested by heap profiling, perhaps similar to that of "foldl". So is it possible to fix it? The function h x is derived by unwinding the monadic construct corresponding to (for lists) do {i <- [x..]; return i} So there are not many possibilities of change. Below, (-+) corresponds, for lists, to ":", "first" to "head", and "rest" to "tail". Hans Aberg -------- data List a = List(Integer->a) instance Show a => Show (List a) where show (List f) = "[" ++ show (f(0)) ++ concat ["," ++ show (f(toInteger i))| i<-[1..]] ++ "]" list :: Integer -> List Integer list x = List(\z -> x+z) (-+) :: a -> List a -> List a x -+ (List y) = List(f) where f 0 = x f k = y(k-1) first :: List a -> a first (List f) = f 0 rest :: List a -> List a rest (List y) = List(f) where f k = y(k+1) h :: List a -> List a h x = (-+) (first x) (h (rest x)) --------

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Running classical logic
by Jim Apple 28 Mar '08

28 Mar '08

We can't safely extract a t out of IO t, but we can run an IO t, interact with the environment, and then see the t. Griffin ( http://citeseer.ist.psu.edu/griffin90formulaeastypes.html ) explained that we can do the same thing with (t -> r) -> r by interacting with the context. That is, he defined a function c :: ((t -> r) -> r) -> t buy interaction with the reduction context of calls to c. I don't expect such a function to be definable in Haskell without some magic, but even with magic, I'm not sure how to write such a function. On #haskell a couple of days ago, we came up with data Empty deriving Typeable toEmpty :: a -> Empty toEmpty = unsafeCoerce fromEmpty :: Empty -> a fromEmpty = unsafeCoerce type Bot = forall x . x griffinC :: Cont Bot a -> IO a griffinC (Cont f) = do key <- newUnique catch (evaluate $ f $ \v -> throwDyn (hashUnique key,toEmpty v)) (\x -> case x of DynException d -> case fromDynamic d of Just (u,y) -> if u == hashUnique key then return $ fromEmpty y else throwIO x Nothing -> throwIO x _ -> throwIO x) The Unique is used to keep the later griffinC calls from being cast to the wrong types. The more I look at this, the less I understand it. Any clues? Jim P.S. Thanks to #haskell, and especially roconnor for the fine help

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Web server libraries
by John Goerzen 28 Mar '08

28 Mar '08

Hi again, I'm currently working on a project that has a Web interface to data stored in a SQL database. I wrote this thing in WASH a few years back. Overall, this has been acceptable, but the non-Haskell-adepts around here run away screaming from the code. Not only that, but we don't get control over the names of the form elements, which can make interacting with other software that sometimes plonks users down halfway through the process -- well, interesting. Also, WASH supports nothing better than CGI, which is a big minus when talking to SQL databases. What I really want is some sort of simple tool that supports FastCGI or some such, has basic support for form data input validation and marshalling to/from Haskell types, and basic control flow. I don't need, or really particularly want, something that uses hs-plugins to compile pages on demand (I'm using Haskell for it's static safety, after all). Nor do I need fancy templating systems or session support (we use HTTP auth and are fine with it for now.) So I've looked around a bit at the landscape. Any recommendations? I've found these: HSP: big on "dynamic pages". I don't want to make my webserver able to compile Haskell code. Develop code, compile, test, make sure it's right, then push to production every 6 months around here. WASH: Pretty much the right niche, but the event model is hard to use. HAppS: Frankly it is a big collection of confusing packages to me. What documentation exists doesn't seem to be relevant anymore, and it seems to be designed to not use a SQL database. FastCGI: Relies upon CGI for a good part of processing. Does not seem to have any form data validation support built in. Thanks again, -- John

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reactive programming, what's hot?
by Greg Fitzgerald 28 Mar '08

28 Mar '08

A new version of yampa<http://hackage.haskell.org/cgi-bin/hackage-scripts/package/Yampa-0.9.2>was uploaded to hackage today, but it's website <http://www.haskell.org/yampa/> hasn't been updated since 2004. SpaceInvaders<http://hackage.haskell.org/cgi-bin/hackage-scripts/package/SpaceInvaders-0.…>depends on HGL and Yampa. There's been more noise around Phooey<http://haskell.org/haskellwiki/Phooey>and TV <http://haskell.org/haskellwiki/TV> in recent months, but afaik, hasn't been used for any applications yet. There was also ParseP <http://twan.home.fmf.nl/parsep/> which seems to tackle a similar problem, but from the perspective of a more general parsing library. Are there other purely functional reactive libraries in active development? Why might one choose one over another? Thanks, Greg

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GSoC Project Proposal: Parallel Profiling Tools for GHC.
by Donnie Jones 28 Mar '08

28 Mar '08

Hello Everyone,GSoC Project: Parallel Profiling Tools for GHC. Student: Donnie Jones Mentor: Simon Marlow Project based upon suggestion by Simon Marlow: http://hackage.haskell.org/trac/summer-of-code/ticket/1559 Overview: For Haskell there are currently no tools to investigate the performance of parallel programs. This project would extend the Haskell compiler GHC to support profiling of parallel programs. Parallel programs are inherently difficult to analyze and the prevalence of multi-core processors has increased the need for tools to facilitate gaining performance from parallelism. The ability to profile parallel programs in GHC could lead to improvements in GHC's RTS, and insights into writing parallel Haskell code. Goals: * Investigate the use of the GranSim framework for parallel profiling in GHC i. Decide what is needed to commit the GranSim port to the GHC tree i. Build additional profiling tools upon the GranSim framework * Testing of performance hit accrued from parallel profiling i. Investigate improving performance, possible modifications to GranSim port * Construct tests to measure the performance improvements of parallel programs i. Building benchmarks for parallel programs, related to ticket: http://hackage.haskell.org/trac/summer-of-code/ticket/1544 * Improve upon GranSim visualization of profiles to clarify parallel profiling i. Possibly add new visualization profiles, if time allows I am sure that further details will need to be developed with the help of Simon Marlow; however, I am unsure if some of these details can be defined without first investigating the potential use of GranSim. I look forward to your feedback. Thank you. __ Donnie Jones

1 1

Terminating GLUT/GLFW programs
by hjgtuyl＠chello.nl 28 Mar '08

28 Mar '08

L.S., I am trying GLUT and GLFW (on Windows XP, with GHC 6.8.2); the sample programs do not terminate when I close the window by clicking on the cross in the upper right corner of the window. The sample program for GLUT is at http://blog.mikael.johanssons.org/archive/2006/09/opengl-programming-in-has… the GLFW program: http://haskell.org/haskellwiki/GLFW I tried in the GLUT program: close = exitWith ExitSuccess closeCallback $= Just close -- => User error (unknown GLUT call getCloseFunc, check for freeglut) this needs freeglut (not documented); I downloaded freeglut.dll and placed it in the windows\system32 directory. The error message remained. What is needed to let these programs terminate properly? -- Met vriendelijke groet, Henk-Jan van Tuyl -- http://functor.bamikanarie.com http://Van.Tuyl.eu/ --

4 7

Libraries need a new owner
by Adrian Hey 27 Mar '08

27 Mar '08

Hello Folks, As some of you will be aware, I have been working on various Map implementations that currently live here.. http://code.haskell.org/collections/collections-ghc6.8 The libs in question being Data.Tree.AVL, Data.Trie.General and a few other bits like Data.COrdering and the AVL based Data.Map/Set clones. Well, I have decided to stop work on these. So they need a new owner if they're going to go anywhere. If anyone is interested in the job then I suggest they contact myself or Jean-Philippe Bernardy. Of course I will be happy to provide any help or advise anyone who takes over these libs may feel they need from me. I might even contribute a few patches from time to time myself :-) Thanks -- Adrian Hey

3 7

No newlines in the whitespace for Parsec lexeme parsers
by Paul Keir 27 Mar '08

27 Mar '08

Hi, I'm looking to parse a Fortran dialect using Parsec, and was hoping to use the ParsecToken module. Fortran though is unlike the Parsec examples, and prohibits default line continuation (requiring instead an explicit ampersand token &). The lexical parsers of the ParsecToken module skip whitespace after each symbol parsed (lexeme parsers); including the newline. I was thinking of making a minor change to Parsec: In Token.hs, lexeme, whiteSpace, and simpleSpace are defined. lexeme uses whiteSpace which uses simpleSpace. simpleSpace is defined: simpleSpace = skipMany1 (satisfy isSpace) I could replace this locally with: simpleSpace = skipMany1 (satisfy isSpacePlusAmpersandMinusNewline) This approach may have other problems though. Has anyone experience of using Parsec to parse such a language? For example assembly or a preprocessor? Thanks in advance, Paul

4 4

FW: containers-0.1.0.1
by john lask 27 Mar '08

27 Mar '08

who is maintainer of containers package ... ---------------------------------------- > From: jvlask(a)hotmail.com > To: libraries(a)haskell.org > Subject: containers-0.1.0.1 > Date: Wed, 12 Mar 2008 23:48:33 +0000 > > > > containers-0.1.0.1 will not build with ghc-6.6, the lines > #include "Typeable.h" INSTANCE_TYPEABLE1(Set,setTc,"Set") > > cause a parsing error, due no doubt to Typeable.h being different to 6.8 version > _________________________________________________________________ > What are you waiting for? Join Lavalife FREE > _________________________________________________________________ Are you paid what you're worth? Find out: SEEK Salary Centre http://a.ninemsn.com.au/b.aspx?URL=http%3A%2F%2Fninemsn%2Eseek%2Ecom%2Eau%2…

1 0

Unboxed arrays
by Andrew Coppin 27 Mar '08

27 Mar '08

Somebody asked me, so now I'm asking you... In Haskell, you can make "unboxed" arrays of certain value types. These are typically more efficient in space, and probably time too, and also make the array strict in its values. However, you can only do this magic trick for certain types - not for *all* types. Why is that? Is it because nobody has done anything about it yet? Is it because it's thought to be "not necessary" in some way? Is there some theoretical problem? Has somebody got a better idea? I did think it was along the lines of "oh, well, if you want to unbox a type of your own, you just need to write your own instance". The thing that makes me suspicious of this logic is the absense of an instance for tuples. Surely this would be trivial to write, and yet it's not present. If we had instances for a couple of sizes of tuples, it would surely be quite easy to write your own custom instances that just sit on top of this and tuple/untuple your custom values... Any insights here?

7 13

Type synonyms
by Hugo Pacheco 26 Mar '08

26 Mar '08

Hi guys, There is something I think not to fully understand: what are the differences between these two type synonyms? type FInt x = Either One x type FInt = Either One Their kind is the same, so do they differ or are exactly the same type? Thanks, hugo

2 1

[GSoC] student applications deadline approaching
by Malcolm Wallace 26 Mar '08

26 Mar '08

Reply-To: haskell-cafe(a)haskell.org Just to remind students who are interested in Google Summer of Code. Student applications are now open, and the final deadline for submitting your proposals is 2400 UTC, 31st March. http://code.google.com/soc/2008/ Regards, Malcolm

1 0

Haskell in Web Browser: draft tutorial
by Dimitry Golubovsky 26 Mar '08

26 Mar '08

Hi, I have written a draft of the tutorial on Haskell programming for client side of Web applications (related to Yhc/Javascript backend): http://www.haskell.org/haskellwiki/Haskell_in_web_browser Any feedback, suggestions, and additions (like your own code examples) are welcome. Thanks. PS This part of the Tutorial does not cover XMLHTTP programming; this is yet to be written, but there is Haddock documentation under http://www.golubovsky.org:5984/_utils/yhcws/index.html Yhc web service online (where you may experiment with your code): http://www.golubovsky.org:5984/static/yhcws/MainGUI.html Users guide: http://www.haskell.org/haskellwiki/Yhc_web_service -- Dimitry Golubovsky Anywhere on the Web

1 0

Gtk2hs on GHC6.8.2?
by Ryan Ingram 26 Mar '08

26 Mar '08

The WinXP binary release of Gtk2hs won't install for me; it seems to expect GHC6.8.1 and refuses to install on 6.8.2. Is there any significant difference in the compilers that would cause it not to work on 6.8.2? Is there a way I can trick it into installing? I really don't want to have to build from source. I've had nothing but trouble trying to get large projects to build within cygwin/mingw/msys. -- ryan

2 2

Parsec (Zero or One of)
by Paul Keir 26 Mar '08

26 Mar '08

Hi, I'm having some difficulty using the Parsec library, perhaps you could help. I've reduced my problem as shown below. I would like the 'only_prod' parser to require the reserved string "only", _optionally_ followed by an identifier. As part of 'mytest', this should then be followed by the reserved string "end". mytest = do { only_prod; end } only_prod = do { reserved "only"; try identifier } end = reserved "end" i.e. I'd like both > parseTest mytest "only end" and >parseTest mytest "only green end" to parse successfully. As it stands, only the second is successful. The first fails with: parse error at (line 1, column 9): unexpected end of input expecting letter or digit or "end" Does anyone have an idea how I could repair the situation? Thanks in advance, Paul

4 7

True parallelism missing :-(
by Dusan Kolar 26 Mar '08

26 Mar '08

Dear all, I've thought the following three (dummy) programs would run some of their parts in parallel (on dual core) if compiled with option threaded (smp). The truth is that only the first one exploits multicore CPU. Why? Moreover, using RTS option -sstderr makes runtime not to evaluate in parallel even for the first program. Why? Thanks for tips Dusan My arch: Linux pcx 2.6.24-ARCH #1 SMP PREEMPT Sun Feb 10 15:44:59 CET 2008 x86_64 Intel(R) Core(TM)2 CPU 6600 @ 2.40GHz GenuineIntel GNU/Linux My ghc: The Glorious Glasgow Haskell Compilation System, version 6.8.2 /64bit, binary distro for FC/ ---------------------------------------------------- Prog 1: module Main() where import Control.Parallel import Control.Parallel.Strategies fibs :: Integer -> Integer fibs n | n > 1 = fibs (n-1) + fibs (n-2) | n == 1 = 1 | True = 0 fib n = if n<0 then error "Negative input to fib!" else f1+f2 where [f1,f2] = parMap rnf fibs [(n-1),(n-2)] main = do putStrLn "Starting..." putStrLn $ "Fib 43: " ++ show (fib 43) putStrLn "Done!" ---------------------------------------------------- Prog 2: module Main() where import Control.Concurrent import Control.Concurrent.MVar fibs :: Integer -> Integer fibs n | n > 1 = fibs (n-1) + fibs (n-2) | n == 1 = 1 | True = 0 fib n = if n<0 then error "Negative input to fib!" else do v1 <- newEmptyMVar v2 <- newEmptyMVar h1 <- forkIO $ putMVar v1 $ fibs (n-1) h2 <- forkIO $ putMVar v2 $ fibs (n-2) f1 <- takeMVar v1 f2 <- takeMVar v2 killThread h1 killThread h2 return (f1+f2) main = do putStrLn "Starting..." f <- fib 43 putStrLn $ "Fib 43: " ++ show f putStrLn "Done!" ---------------------------------------------------- Prog 3: module Main() where import Control.Concurrent import Control.Concurrent.MVar fibs :: Integer -> Integer fibs n | n > 1 = fibs (n-1) + fibs (n-2) | n == 1 = 1 | True = 0 fib n = if n<0 then error "Negative input to fib!" else do v1 <- newEmptyMVar v2 <- newEmptyMVar h1 <- forkOS $ putMVar v1 $ fibs (n-1) h2 <- forkOS $ putMVar v2 $ fibs (n-2) f1 <- takeMVar v1 f2 <- takeMVar v2 killThread h1 killThread h2 return (f1+f2) main = do putStrLn "Starting..." f <- fib 43 putStrLn $ "Fib 43: " ++ show f putStrLn "Done!"

4 5

ANNOUNCE: Haskell Server Pages v 0.4
by Niklas Broberg 26 Mar '08

26 Mar '08

Greetings fellow Haskelleers, I am very pleased to announce a new chapter in the Haskell Server Pages saga. After a two-year long hiatus, while we in the team have been busy with Other Stuff, we have resumed work on Project HSP, and this release marks the first milestone. ================= === Project HSP === ================= Haskell Server Pages (HSP) is a programming model for writing dynamic web pages in Haskell, both server-side and client-side. One of its core features is the use of literal XML syntax mixed with ordinary Haskell code. * Project HSP home (with bug/feature tracker): http://code.google.com/p/hsp * Project HSP mailing list: haskell-server-pages(a)googlegroups.com * Project HSP darcs repos: http://code.haskell.org/HSP For flavor, here is a simple Hello World application written in HSP: ----- module HelloWorld where import HSP import HSP.HJScript import HJScript import HJScript.DOM -- A simple HTML page holding a button with onClick action. page :: HSP XML page = <html> <head> <title>Hello World</title> </head> <body> <% <button>click</button> `onClick` (window # alert (string "Hello World !")) %> </body> </html> ----- Note that literal XML elements are first class expressions in HSP, unlike languages like PHP. Also note that using literal XML syntax is by no means required, if you don't feel like writing the XML manually (I sure don't!). There are combinators for building XHTML pages (although they are currently missing from version 0.4, will be added shortly, but release early and all that...). The most interesting new feature in version 0.4 is that literal XML syntax (or the equivalent combinators) can now be used to create XML elements both server-side and client-side, with the same code. Here is another short example script showing this feature: ------------------ module AddParagraph where import HJScript hiding (test) import HJScript.DOM import HSP import HSP.HJScript makeP x = <p><b><% x %></b></p> page :: HSP XML page = <html> <head><title>Add paragraph</title></head> <% do (r,b) <- ref <body><% makeP "Paragraph created Server-side!" %></body> let fun = do let x = document # getElementById (string r) x <: makeP "Paragraph created Client-side!" return () b <: (<button>Click Me!</button> `onClick` fun) %> </html> -------------------- Project HSP v 0.4 consists of a set of packages with related functionality: == package hsx == Haskell Source with XML (HSX, hsx) is a package that contains everything pertaining to literal XML syntax. In particular it contains a) the trhsx preprocessor that translates hsx source files into vanilla Haskell, and b) modules defining the functions that are injected by trhsx. It also nominally contains generic combinators for creating values of the same types as the literal XML syntax, though these modules are not present in 0.4. darcs get --partial http://code.haskell.org/HSP/hsx http://hackage.haskell.org/cgi-bin/hackage-scripts/package/hsx-0.4 == package hsp == The core HSP package defines the datatypes and functions for writing server-side dynamic web pages. Also defines how to use the HJScript functionality in HSP pages, to allow for client-side dynamics as well. darcs get --partial http://code.haskell.org/HSP/hsp http://hackage.haskell.org/cgi-bin/hackage-scripts/package/hsp-0.4 == package HJavaScript == This package defines an abstract syntax and (not-so-)pretty printer for a large subset of JavaScript, as Language.HJavaScript. However, a significant difference from JavaScript is that HJavaScript is typed, even on the abstract syntax level using GADTs. The subset of JavaScript that is supported is those parts that lend themself to typing (i.e. no prototyping of classes). darcs get --partial http://code.haskell.org/HSP/hjavascript http://hackage.haskell.org/cgi-bin/hackage-scripts/package/HJavaScript-0.4 == package HJScript == HJScript is a DSL built on top of HJavaScript, for writing client-side dynamic web pages. The programming model is fairly low-level, resembling the actual JavaScript code quite a lot, but should be easy to extend with higher-level functionality. Notable is that HJScript supports the use of literal XML syntax for creating DOM ElementNodes. Also notable is that HJScript supports Ajax functionality. HJScript and HJavaScript can be used independently of HSP, for creating JavaScript code. darcs get --partial http://code.haskell.org/HSP/hjscript http://hackage.haskell.org/cgi-bin/hackage-scripts/package/HJScript-0.4 == package hsp-cgi == Run HSP pages as CGI scripts. darcs get --partial http://code.haskell.org/HSP/hsp-cgi http://hackage.haskell.org/cgi-bin/hackage-scripts/package/hsp-cgi-0.4 (Note: Earlier versions of HSP have come with a server application, hspr, providing the runtime environment for HSP. As of 0.4, this server has been discontinued and should be considered deprecated.) ===================== Notable items on the TODO list for Project HSP: - More functionality, allowing for higher-level programming models. - Writing larger applications, in order to stress test, and serve as inspiration for higher-level functionality. - Integrate with HAppS, to allow stateful computation (among other things). - A home on the web for HSP, with a web page written in HSP. We welcome both users (of course) and contributors to the project. There is much work to be done, and we welcome all help we can get. If you think this is interesting stuff, drop by our mailing list (haskell-server-pages(a)googlegroups.com) Like to do web programming in Haskell? Want to do a GSoC project? We in the project would be happy to see applications for HSP projects, and the list above is a good starting place. See for instance (http://hackage.haskell.org/trac/summer-of-code/ticket/1538) and (http://hackage.haskell.org/trac/summer-of-code/ticket/1539) Cheers, /Niklas on behalf of the Project HSP team

1 0

separate input calculation output
by Thomas Engel 25 Mar '08

25 Mar '08

Hello, I'm new to haskell and want to write a small program for some winder calcucations. I want to input 10-15 values and based on this input doing some calculations. After this calculations I want to print out the results. My problem is: How can I get the input values into the calculations and back the result to the output. In an other language I would use global variables for this. So what should I use in haskell? An other point is that I want to separate the input from the calculations and the output. How can I achieve this with haskell? Modules? Data types? See some pseudo code below. inputvalues :: IO() inputvalues = do hSetBuffering stdout NoBuffering putStr "Berechnung Bremsgenerator \n\n" ve <- readNum "Bitte Maschinengeschwindigkeit (m/min) angeben: " de <- readNum "Bitte Durchmesser der vollen Rolle eingeben (mm): " dewalze <- readNum "Bitte Durchmesser der Walze eingeben (mm): " dewand <- readNum "Bitte Wandstaerke der Walze eingeben (mm): " and so on .............. readNum :: String -> IO Float readNum text = do putStrLn text readLn -- calculations nvr :: Float-> Float-> Float nvr ve de = ve / de /pi *1000 pzugmin :: Float-> Float-> Float->Float pzugmin ve femin be = 1.666 * ve * femin * be * 0.00000001 pzugmax :: Float-> Float-> Float->Float pzugmax ve femax be = 1.666 * ve * femax * be * 0.00000001 and so on .............. outputvalues = do putStr "\n\nErgebnisse Bremsgenerator \n\n" putStr "Drehzahl leere Trommel: " putStr(show(round(nvr))) putStr " U/min \n" putStr "Drehzahl volle Trommel: " putStr(show(round(drehbgmax))) putStr " U/min \n" and so on ...................... Thanks Thomas

4 5

getChar
by Cetin Sert 25 Mar '08

25 Mar '08

Hi, is there a version of getChar that doesn't buffer keyboard input until enter is pressed? specialReadln :: IO String specialReadln = do c ← getChar if c == '#' then do return [] else do cs ← specialReadln return (c:cs) I want the input process to terminate when '#' or any other specific key has been pressed. Best Regards, Cetin Sert

2 1

Problem with OpenAL
by George Giorgidze 25 Mar '08

25 Mar '08

I tried OpenAL binding (the one which is on the Hackage), but with no luck. I can not hear anything from speakers and also according to generated output on console it seems that "AL.play" never completes playback of a buffer as buffer remains registered as "unprocessed" in OpenAL context. Here is the piece of code. I am not getting any error messages from OpenAL library functions. playOpenAL :: Int -> IO () playOpenAL sr = do mDevice <- AL.openDevice Nothing when (isNothing mDevice) $ error "opening OpenAL device" let device = fromJust mDevice mContext <- AL.createContext device [ AL.Frequency (fromIntegral sr) , AL.Refresh (fromIntegral sr) ] when (isNothing mContext) $ error "creating OpenAL context" let context = fromJust mContext AL.currentContext AL.$= (Just context) let sampleNumber = 256 bufSize = sampleNumber * (sizeOf (undefined :: CShort)) buf2 <- mallocBytes bufSize -- here I am filling buf2 with noise .... [source] <- AL.genObjectNames 1 [buffer] <- AL.genObjectNames 1 let region = AL.MemoryRegion buf2 (fromIntegral bufSize) AL.bufferData buffer AL.$= (AL.BufferData region AL.Mono16 (fromIntegral sr)) AL.queueBuffers source [buffer] AL.loopingMode source AL.$= AL.OneShot let waitForSource = do putStrLn "waiting" s <- AL.get (AL.buffersProcessed source) putStrLn $ show s s <- AL.get (AL.buffersQueued source) putStrLn $ show s state <- AL.get (AL.sourceState source) case state of AL.Playing -> do threadDelay 1024 waitForSource _ -> return () putStrLn "Start Playing ... " AL.play [source] waitForSource AL.currentContext AL.$= Nothing AL.destroyContext context b <- AL.closeDevice device when (not b) $ error "closing device" Is this library still maintained? Best, George

2 2

How to program with sqlite?
by Deng Chao 25 Mar '08

25 Mar '08

Hi all, I'm learning sqlite, and as I know haskell has some libraries like HDBC or HSQL can access sqlite DB. Can anybody give me a small example to show how to use it? It will be very appreciate? Thanks! Best Regards, Deng Chao

4 4

Random Monad
by Matthew Pocock 25 Mar '08

25 Mar '08

Hi, Who currently maintains the Random monad code? I have some patches to contribute. Matthew

4 4

deconstruction of the list/backtracking applicative functor?
by Conal Elliott 25 Mar '08

25 Mar '08

Is there a known deconstruction of the list/backtracking applicative functor (AF)? If I decompose the list type into pieces (Maybe, product, composition), I think I can see where the ZipList AF comes from, but not the list/backtracking AF. Is there some construction simpler than lists (non-recursive) that introduces cross products? Thanks, - Conal

2 3

ghc warn-unused-imports
by Evan Laforge 25 Mar '08

25 Mar '08

So it appears that GHC will warn about "unused" imports when you import qualified if you could have gotten the symbol from somewhere else. For instance, if you write: import qualified Control.Monad.Trans as Trans import qualified Control.Monad.Writer as Writer and use "Trans.lift", it will complain because you could have used "Writer.lift". Then if you go import State, it will want you to use State.liftIO instead of Trans.liftIO (Writer doesn't have liftIO). Now, one question is why these modules are exporting random stuff from Trans, but in general the practice of warning when you use qualified imports seems not only misleading (since if you comment out the import your code breaks, and it's not obvious where it wanted you to get the symbol), but brittle since a messy export list on some module you import can suddenly trigger a flood of warnings. So I'd suggest that if you say "import qualified M" and then use "M.something", then "M is unused" msgs shouldn't appear. warn-unused-imports is genuinely useful most of the time, so I don't like to put on -fno-warn-unused-imports. Is there a better way to suppress the "false positives" here?

2 1

BUG: genObjectNames dies on Win32
by Jefferson Heard 25 Mar '08

25 Mar '08

Could this get forwarded on to another more appropriate maling list? Confirmed on GHC and GHCi 6.6 and 6.8, Graphics.Rendering.OpenGL.GL.genObjectNames n is dying if I ask it to return an IO :: [DisplayList] For an example, just open GHCI and change context to Graphics.Rendering.OpenGL.GL and do genObjectNames 4 :: IO [DisplayList] GHCi will merely die. Compiled with GHC, I'm simply running out of memory. It eats RAM indefinitely. -- Jeff

1 0

HXT and types in Control.Arrow.ArrowTree
by Robert Vollmert 24 Mar '08

24 Mar '08

Hello, I'm having some trouble with HXT, the types of some functions in particular. This may well be caused by a lack of understanding for arrow programming -- I'd appreciate any hints. In short, I'm constantly running into what appear to be artificial type restrictions in Control.Arrow.ArrowTree. For example, the signature of "deep" is deep :: (Tree t, ArrowTree a) => a (t b) (t b) -> a (t b) (t b) instead of the more general deep :: (Tree t, ArrowTree a) => a (t b) c -> a (t b) c Say I have an arrow getLink :: (ArrowXml a) => a XmlTree Link that converts an HTML link (<a href="url">text</a>) into some appropriate data type (code below). To collect all links in a document, I'd like to use deep getLink instead of deep (hasName "a") >>> getLink Is this the wrong approach to converting XML into Haskell data types? Cheers Robert module Links where import Text.XML.HXT.Arrow data Link = Link { url, text :: String } deriving Show getLink :: (ArrowXml a) => a XmlTree Link getLink = isElem >>> hasName "a" >>> (getAttrValue0 "href" &&& getAllText) >>> arr (uncurry Link) getAllText :: (ArrowXml a) => a XmlTree String getAllText = listA (deep isText >>> getText) >>> arr concat

2 2

more on FFI build error
by Galchin Vasili 24 Mar '08

24 Mar '08

line #102 ... allocaBytes (#const sizeof(struct mq_attr)) $ \ p_attrs -> do definition of struct mq_attr on Linux ... struct mq_attr { long int mq_flags; /* Message queue flags. */ long int mq_maxmsg; /* Maximum number of messages. */ long int mq_msgsize; /* Maximum message size. */ long int mq_curmsgs; /* Number of messages currently queued. */ long int __pad[4]; }; build errors received .... vigalchin@ubuntu:~/FTP/Haskell/unix-2.2.0.0$ runhaskell Setup.hs build Preprocessing library unix-2.2.0.0... MQueue.hsc: In function 'main': MQueue.hsc:102:0: error: invalid application of 'sizeof' to incomplete type 'struct mq_attr' MQueue.hsc:102:0: error: invalid application of 'sizeof' to incomplete type 'struct mq_attr' MQueue.hsc:102:0: error: invalid application of 'sizeof' to incomplete type 'struct mq_attr' compiling dist/build/System/Posix/MQueue_hsc_make.c failed comma ??? Kind regards, vasili

4 3

RE: How to implement Read instance for user defined type
by Verma Anurag-VNF673 24 Mar '08

24 Mar '08

That works, but is probably not what you want. You can use the lex function to parse identifiers not enclosed in quotes: > instance Read Mark where > readsPrec _ str = [(Mark x, t') | ("mark",t) <- lex str, > (x,t') <- reads t I played a bit around with lex function and it seems that under certain circumstances it doesn't read the string of token properly For e.g. -- IP address *Mark> lex "192.168.0.1" [("192.168",".0.1")] -- digit + char Also lex "8021p" output is *Mark> lex "8021p" [("8021","p")] Is there any other lex variant available? Or should I switch to using Parsec library, in that case, will I still be able to use it with Read instances? If so, how? Anurag

1 0

HFuse: ls fails in HelloFS
by Georg Neis 24 Mar '08

24 Mar '08

Hello, I've installed the HFuse package from hackage and am playing with the HelloFS example in the System/Posix/HFuse directory. The problem that I encounter is that listing the directory doesn't work: % ghc --make HelloFS.hs [1 of 1] Compiling Main ( HelloFS.hs, HelloFS.o ) Linking HelloFS ... % mkdir bla % ./HelloFS bla % cd bla % cat hello Hello World, HFuse! % ls ls: cannot open directory .: Function not implemented Here's the relevant output from strace: open(".", O_RDONLY|O_NONBLOCK|O_LARGEFILE|O_DIRECTORY|0x80000) = -1 ENOSYS (Function not implemented) write(2, "ls: ", 4ls: ) = 4 write(2, "cannot open directory .", 23cannot open directory .) = 23 write(2, ": Function not implemented", 26: Function not implemented) = 26 Some system information: - Debian unstable - Linux 2.6.24 - Fuse 2.7.3 - GHC 6.8.2 - HFuse 0.1 Any ideas? Thanks, Georg

7 10

Any ideas for "->" (function type) in a simply-typed lambda calculus version of Brett Victor's Alligator Eggs game?
by Benjamin L. Russell 24 Mar '08

24 Mar '08

Brett Victor has invented an Alligator Eggs game (very loosely resembling Conway's Game of Life), using formal rules to determine feeding and breeding patterns for alligators and their eggs, but the game represents the *untyped* lambda calculus: Alligator Eggs! http://worrydream.com/AlligatorEggs/ I am interested in adapting this to the *simply typed* lambda calculus with Curry-style typing, to aid students of Haskell. Brett Victor's game currently aids students of LISP, and I think that it would be helpful to have a Haskell counterpart (see http://wadler.blogspot.com/2007/05/oh-no-alligators.html for a comment on how the Alligator Eggs game has been used by EE students at Caltech in learning LISP): Simply typed lambda calculus - Wikipedia, the free encyclopedia http://en.wikipedia.org/wiki/Simply_typed_lambda_calculus Thereafter, I would like to create a new entry for this Haskell version of Alligator Eggs on HaskellWiki, and create a link to it under a new subsection, "Board Games," under the current "Learning Haskell" category: http://www.haskell.org/haskellwiki/Learning_Haskell In Brett Victor's version, the following representations hold: * a hungry alligator: lambda abstraction * an old alligator: a pair of parentheses * an egg: a variable * the Eating Rule: beta-reduction * the Color Rule: (over-cautious) alpha-conversion * the Old Age Rule: the rule that a pair of parentheses containing a single term may be removed However, in thinking about how to adapt the game, I am not quite sure how to incorporate the representation of "->" (function type): * ???: "->" (function type) What ideas, if any, would anybody have on how "->" (function type) could be represented in a simply-typed lambda calculus version of Brett Victor's Alligator Eggs? Benjamin L. Russell

2 2

hoisting as well as lifting and returning
by Matthew Pocock 24 Mar '08

24 Mar '08

Hi, Happy Easter! I've been using monad transformers for the first time in anger this week. They certainly do the job. However, there's an operation that I keep defining over and over again. It is sort of like lift and return in that it's glue to get operations and values into the monad from outside it. Here are two examples. hoistList :: (Monad m) => [a] -> ListT m a hoistList = foldr (mplus . return) mzero hoistMaybe :: (Monad m) => Maybe a -> MaybeT m a hoistMaybe = maybe mzero return The general idea is that you have some legacy, non-transform operation in some monad MonadFoo, and you are writing an operation in MonadFooT. You want to get the monadFoo value into MonadFooT. So, you say something like: do vInMFT <- hoist vInMF Is this a common enough operation on a monad transformer and it's 'raw' monad to warrant a class of its own? I have 'hoist' methods for several monad transformers, including RamdomMonadT, although I've probably defined them in ways that are overly eager or eat stack/heap, so I'd prefer knowledgable people to write them :) There must be other operations that link the base monad with its transformed version, other than 'hoist' - the runFooT method for example. Perhaps 'hoist' already exists and I'm re-inventing the wheel? Matthew

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Type checking of partial programs
by ac 24 Mar '08

24 Mar '08

Is anybody interested in working on this? This is a project I've been interested in for some time, but recognize I probably need some guidance before I go off and start hacking on it. As dcoutts pointed out on #haskell-soc, this may be of particular interest to people working on yi and HaRe. Other interesting and related projects include parsing partial programs to insert "placeholders" in appropriate places. An example of a partial program could be: foo :: [Foo] -> <placeholder 1> foo xs = map <placeholder 2> xs What are the possible type signatures for placeholder 1 and the possible expressions for placeholder 2? I would like to stir up a discussion about this, and eventually write some useful code. -Abram

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"doctest" for haskell -- a good project?
by Shaun Cutts 24 Mar '08

24 Mar '08

Hello, I am an experienced programmer, currently learning Haskell. Currently I write many things in python. I use both the "doctest" and "unittest" modules extensively. As I write code, I simultaneously write doctest code in the doc strings to explain/set out the "typical narrative" of how the code is used. Then finishing off a module I write unittests for boundary conditions, more complex test cases, and generally code that would be annoying to write & read in doctests. I note that there is a unit testing framework for Haskell, but I don't see any doctest module. Might this be a good project? If so, suggestions as to resources would be greatly appreciated. I believe I can't just "introspect" Haskell modules to get at documentation/comments, like I can in python? (Why not? :)) I notice that there are a few "documentation generators". Should I try to write an extension of one of these? Haddock, for instance? Are there any Haddock developers hanging out on this list, to encourage or dissuade me? :) (And where is the Haddock doc for Haddock?) In any case, thanks in advance for any comments & advice. - Shaun Cutts

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Re: Haskell-Cafe Digest, Vol 55, Issue 30
by Deng Chao 23 Mar '08

23 Mar '08

Hi Sebastian Sylvan, Thank you very much! I thick the last column return a string because of the Haskell "Show" function, for when I made test, I got such an error message: Show ([SqlValue] -> IO [[SqlValue]]). Any way, thank you very much! Deng Chao 在 2008-03-22六的 13:55 -0400，haskell-cafe-request@haskell.org写道： > Send Haskell-Cafe mailing list submissions to > haskell-cafe(a)haskell.org > > To subscribe or unsubscribe via the World Wide Web, visit > http://www.haskell.org/mailman/listinfo/haskell-cafe > or, via email, send a message with subject or body 'help' to > haskell-cafe-request(a)haskell.org > > You can reach the person managing the list at > haskell-cafe-owner(a)haskell.org > > When replying, please edit your Subject line so it is more specific > than "Re: Contents of Haskell-Cafe digest..." > > > Today's Topics: > > 1. compile error with FFI code (Galchin Vasili) > 2. "doctest" for haskell -- a good project? (Shaun Cutts) > 3. Re: [GSoC] Plugins for GHC (Bulat Ziganshin) > 4. Re: "doctest" for haskell -- a good project? (Roman Cheplyaka) > 5. Re: "doctest" for haskell -- a good project? (Don Stewart) > 6. Re: [GSoC] Plugins for GHC (Neil Mitchell) > 7. Re: "doctest" for haskell -- a good project? (Neil Mitchell) > 8. Re: An ugly zip3 problem.. (Michael Feathers) > 9. Re: An ugly zip3 problem.. (Michael Feathers) > 10. How to program with sqlite? (Deng Chao) > 11. Re: "doctest" for haskell -- a good project? (Thomas Schilling) > 12. Re: How to program with sqlite? (Sebastian Sylvan) > 13. Re: "doctest" for haskell -- a good project? (Sebastian Sylvan) > 14. HXT and types in Control.Arrow.ArrowTree (Robert Vollmert) > 15. Opening Windows .lnk Files (Dominic Steinitz) > 16. RE: "doctest" for haskell -- a good project? (Shaun Cutts) > 17. Re: An ugly zip3 problem.. (Bryan O'Sullivan) > 18. Re: "doctest" for haskell -- a good project? (Paul Johnson) > 19. Re: AMQP framing layer: design help requested. (Paul Johnson) > 20. Re: Problem with OpenAL (Antoine Latter) > 21. Re: AMQP framing layer: design help requested. (Derek Elkins) > 22. Re: Opening Windows .lnk Files (Neil Mitchell) > 23. Re: "doctest" for haskell -- a good project? (Neil Mitchell) > 24. Re: Opening Windows .lnk Files (Dominic Steinitz) > 25. RE: "doctest" for haskell -- a good project? (Shaun Cutts) > > > ---------------------------------------------------------------------- > > Message: 1 > Date: Fri, 21 Mar 2008 23:21:26 -0500 > From: "Galchin Vasili" <vigalchin(a)gmail.com> > Subject: [Haskell-cafe] compile error with FFI code > To: Haskell-cafe <haskell-cafe(a)haskell.org> > Cc: Galchin Vasili <vigalchin(a)gmail.com> > Message-ID: > <5ae4f2ba0803212121w17fe482ek105c13cf6999e603(a)mail.gmail.com> > Content-Type: text/plain; charset="iso-8859-1" > > Hello, > > In my blah.hsc, I have "allocbytes (#const (struct bozo)) .. where > > struct bozo is a bunch of "long int" ,,, In the "runhaskell Setup.hs build" > step I get a nasty error message about an "incomplete type". I have look at > many times but this error doesn't make sense to me. ?? > > Kind regards, Vasili >

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