- Haskell-Cafe - Haskell.org

Something like optimistic evaluation
by Daniil Elovkov 30 Apr '08

30 Apr '08

Hello Somewhat on the topic of optimistic evaluation, I've just thought of another way to evaluate thunks. When the program is about to block on some IO, what if we start a thread to evaluate (any) unevaluated thunks. We'll have additional system thread, but the blocked one will not actually consume any processor time. This would take place only when the program is compiled as threaded and run with -N k, k>1. The RTS knows at least about some operations that will block, those which IO operations are implemented with. for example. It could merely start a process of evaluating any (or something more clever) outstanding thunks right before going into one of those operations and stop it when it's back. Of course, it's not like optimistic evaluation because we don't avoid creating thunks. But in a sense it's similar. It could also be compared with incremental garbage collection :) Has something like that been done, discussed?

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Mutual recursive data types
by rodrigo.bonifacio 29 Apr '08

29 Apr '08

Hi all, I have the following data types: type Id = String type Action = String type State = String type Response = String data Scenario = Scenario Description [Step] data Step = Step Id Scenario Action State Response So, there is a mutual recursion between Scenario and Step. Now, consider the following function: xmlScenario2Scenario :: XmlScenario -> Scenario xmlScenario2Scenario (XmlScenario description steps) = Scenario description [xmlStep2Step x | x <-steps] How can I send "scenario" as an argument for xmlStep2Step? I've tried let and where but I get in a loop. Thanks a lot, Rodrigo. ----------------------------------- Rodrigo Bonifácio de Almeida Universidade Católica de Brasília - Grupo de Engenharia de Software - JavaComBr (www.ucb.br/java)

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pls help about subtree
by cetin tozkoparan 29 Apr '08

29 Apr '08

Assume a tree is a subtree of the other if all elements of the first tree is included in the second with the exact structure; all parent-child relations are preserved with their order. data Tree = Empty | Leaf Int | Node (Int,Tree,Tree) subtree:: Tree -> Tree -> Bool How can i start to write this code with this data structure? Example; tree1 tree2 3 / \ 5 6 / \ 7 4 13 / \ 3 12 / \ / \ 5 6 9 6 / \ / \ 7 4 11 7 / / \ 2 3 4 ____________________________________________________________________________________ Be a better friend, newshound, and know-it-all with Yahoo! Mobile. Try it now. http://mobile.yahoo.com/;_ylt=Ahu06i62sR8HDtDypao8Wcj9tAcJ

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force inlining in GHC
by Henning Thielemann 29 Apr '08

29 Apr '08

Whenever I try to inline a lot of nested function calls, GHC decides to specialise one of the functions and the specialised function is no longer inlined. I hoped to get the function inlined anyway by specialising it manually. Say, I want to inline genericFunc {-# INLINE genericFunc #-} genericFunc :: RealFrac a => a -> Int but GHC inserts a call to genericFunc1 specialised to a=Double where I apply genericFunc to a Double argument. Now I define {-# INLINE doubleFunc #-} doubleFunc :: Double -> Int doubleFunc = genericFunc However, in the Core output 'doubleFunc' does not get the __inline_me tag and thus will not be inlined, too. :-(

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FFI Query: How to free a CString allocated in Haskell
by Verma Anurag-VNF673 29 Apr '08

29 Apr '08

Hi, I am calling a Haskell function from my C code, wherein I export a Haskell function xyz that takes as an argument string and returns a string: Foreign export ccall xyz::CString -> IO CString xyz = do -- extract the input string and act on it-- -- at the end I return a string like this newCString str Now once I call this function from C code, I am freeing the allocated memory using free function. I want to confirm that this is the right thing to do. - Anurag

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Confused by instances
by Fraser Wilson 29 Apr '08

29 Apr '08

Hello, Haskellers, I feel like I'm missing something obvious, but here's some example code: > module Instance where > > data Value = Value Integer > > class ValueClass a where > fromValue :: Value -> a > > instance ValueClass Bool where > fromValue (Value n) = n /= 0 > > instance ValueClass String where > fromValue (Value n) = show n > > instance (Num a) => ValueClass a where > fromValue (Value n) = fromInteger n > The Bool instance compiles fine. The String instance fails, and that would be because String is a synonym for [Char], and we can't create instances for those. There's a slight hack in the Prelude which allows you to define your own list show function, and that's exactly what gets done by Show Char, right? I'm sorry about calling it a hack but, well, it is. :) What I'm really confused by is the response to instance (Num a) => ValueClass a -- what I am trying to say is "if a is an instance of Num, then can be an instance of ValueClass too, and here's how". But ghc says: Instance.hs:14:0: Illegal instance declaration for `ValueClass a' (All instance types must be of the form (T a1 ... an) where a1 ... an are distinct type *variables* Use -XFlexibleInstances if you want to disable this.) In the instance declaration for `ValueClass a' I don't understand why what I'm doing above is different from, for example, instance Show a => Show [a], or Monad m => MonadState s (StateT s m) ... I imagine that it's related to the fact that the difference between those two and what I have is that they have either the same class on each side of the =>, or new type variables on the right. But I'm having trouble abstracting from that to the general rule. Any help would be greatly appreciated. In context, I'm attempting to wrap up different types inside a data type, and rather than extracting the value with getBoolean, getInteger, getString etc, I thought I would let type classes work for me, and just say fromValue instead. Is this silly? You can tell me, I won't be hurt. cheers, Fraser.

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announce: Glome.hs-0.3 (Haskell raytracer)
by Jim Snow 29 Apr '08

29 Apr '08

A new version of my raytracer is out. It now supports cones, cylinders, disks, boxes, and planes as base primitives (previously it only supported triangles and spheres), as well as transformations of arbitrary objects (rotate, scale, translate) and the CSG operations difference and intersection. Perlin noise and Blinn highlights have been added, as well. http://syn.cs.pdx.edu/~jsnow/glome/ Glome can parse NFF-format scene files (see http://tog.acm.org/resources/SPD/) but many features are only accessible via raw Haskell, since NFF doesn't support very many kinds of primitives. I included a TestScene.hs file that demonstrates how to create a scene with various kinds of geometry (including a crude attempt at a recursively-defined oak tree) in haskell. There isn't any documentation yet, but many of the primitives have constructors that resemble their equivalents in povray, so anyone familiar with povray's syntax should be able to figure out what's going on. -jim

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Re: [Haskell-cafe] Trouble compiling collections-0.3 (from Hackage)
by Adrian Hey 29 Apr '08

29 Apr '08

ccing Haskell Cafe in case anyone else is interested in my answer.. Bryan O'Sullivan wrote: > Adrian Hey wrote: > >> I don't think anyone is interested in working on this or maintaining >> it, so it's probably best not to use it for new stuff. > > If nobody has stepped up yet, I'll take it over. It would be a shame to > see it bitrot. I think the main problem is that the collections package contained a number of different sub-projects that should really be separated out (and indeed have been to some extent), as they were progressing at rather different rates and it was never clear when would be a good time to do a new release of collections as a whole, so it was never done. There's the abstract collections class API's which were really what Jean-Philippe Bernardy was working on. I'm not sure if he's still interested in that so you'd better ask him. I think he's more interested in Yi these days :-) Then there's the AVL tree lib which has been put in hackage as a separate package already. I kinda washed my hands of this too, but I guess I'd better change my mind about that as if I don't maintain it I don't think any one else will. Then there's the relatively trivial AVL based Data.Set/Map clones which aren't in hackage yet but I guess could be put there without too much work. Then there's all the generalised trie stuff which has stalled for the time being, at least until type families are available. You'll probably see some more action on this in the summer as it's the subject of Jamie Brandons GSoC project. Regards -- Adrian Hey

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Simplest possible Fasta shootout entry. How do I zap the ugly line? Suggest any other improvements.
by Richard Kelsall 29 Apr '08

29 Apr '08

(Extracting these questions from my previous thread for clarity.) Below is my simplest possible program to solve the Fasta shootout benchmark. http://shootout.alioth.debian.org/gp4/benchmark.php?test=fasta&lang=all http://haskell.org/haskellwiki/Shootout/Fasta I can see one remaining flaw - the line marked 'Ugly'. What's the best way to get rid of this line? Any other suggestions for simplifying or improving the program would also be interesting. This code is about three or four times slower that the current fastest GHC entry for the Fasta benchmark. I'll elaborate it for speed when I've produced the best version regardless of speed. Richard. {-# OPTIONS -O -fexcess-precision #-} -- The Computer Language Shootout : Fasta -- http://shootout.alioth.debian.org/ -- Simple solution by Richard Kelsall. -- http://www.millstream.com/ import System main = do n <- getArgs >>= readIO . head title "ONE" "Homo sapiens alu" writeLined (cycle alu) (n * 2) title "TWO" "IUB ambiguity codes" let (r1, r2) = splitAt (fromIntegral (n * 3)) (rand 42) -- Ugly !! writeLined (map (look iubs) r1) (n * 3) title "THREE" "Homo sapiens frequency" writeLined (map (look homs) r2) (n * 5) title :: String -> String -> IO () title a b = putStrLn $ ">" ++ a ++ " " ++ b look :: [(Char, Float)] -> Float -> Char look [(c, _)] _ = c look ((c, f) : cfs) r = if r < f then c else look cfs (r - f) lineWidth = 60 writeLined :: [Char] -> Integer -> IO () writeLined cs 0 = return () writeLined cs n = do let w = min n lineWidth (cs1, cs2) = splitAt (fromInteger w) cs putStrLn cs1 writeLined cs2 (n - w) rand :: Int -> [Float] rand seed = newran : (rand newseed) where im = 139968 ia = 3877 ic = 29573 newseed = (seed * ia + ic) `rem` im newran = fromIntegral newseed / fromIntegral im alu = "GGCCGGGCGCGGTGGCTCACGCCTGTAATCCCAGCACTTTGGGAGGCCGAGGCGGGCGGA\ \TCACCTGAGGTCAGGAGTTCGAGACCAGCCTGGCCAACATGGTGAAACCCCGTCTCTACT\ \AAAAATACAAAAATTAGCCGGGCGTGGTGGCGCGCGCCTGTAATCCCAGCTACTCGGGAG\ \GCTGAGGCAGGAGAATCGCTTGAACCCGGGAGGCGGAGGTTGCAGTGAGCCGAGATCGCG\ \CCACTGCACTCCAGCCTGGGCGACAGAGCGAGACTCCGTCTCAAAAA" iubs = [('a', 0.27), ('c', 0.12), ('g', 0.12), ('t', 0.27), ('B', 0.02), ('D', 0.02), ('H', 0.02), ('K', 0.02), ('M', 0.02), ('N', 0.02), ('R', 0.02), ('S', 0.02), ('V', 0.02), ('W', 0.02), ('Y', 0.02)] homs = [('a', 0.3029549426680), ('c', 0.1979883004921), ('g', 0.1975473066391), ('t', 0.3015094502008)]

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CPU simulation library?
by Justin Bailey 29 Apr '08

29 Apr '08

I want to simulate various process/thread scheduling algorithms for a small project. I'm trying to compare the performance versus power consumption of several algorithms. If anyone knows of some Haskell code along those lines I'd love to hear about it. Thanks in advance! Justin p.s. I can find non-Haskell code on my own - I'm hoping there is something in my favorite language, but any particularly good suggestions are also welcome.

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maintainer of StorableVector
by Henning Thielemann 28 Apr '08

28 Apr '08

I sent some patches to the maintainer of StorableVector, Spencer Janssen http://code.haskell.org/~sjanssen/storablevector but did not get some feedback and the patches are still not applied. Any idea what might went wrong?

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Haskell as a general purpose programming language...
by Martin Grabmueller 28 Apr '08

28 Apr '08

Hi all, just a thought: is it just me, or is Haskell really really really the coolest general-purpose programming language? If I had the time, I would love to do a re-implementation of Harpy [1] using category-extras [2]. Martin [1] http://hackage.haskell.org/cgi-bin/hackage-scripts/package/harpy-0.4 [2] http://hackage.haskell.org/cgi-bin/hackage-scripts/package/category-extras-…

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ANNOUNCE: Haskell XML Toolbox Version 8.0.0
by Uwe Schmidt 28 Apr '08

28 Apr '08

Haskell XML Toolbox 8.0.0 I would like to announce a new version of the Haskell XML Toolbox. The main change is the separation of the old filter interface into a separate package "hxt-filter". This makes the API simpler and more readable. The filter API will not be developed further. But all the functionality ist still available. The filter package hxt-filter must be installed on top of hxt. The darcs repos for HXT are at http://darcs.fh-wedel.de/hxt http://darcs.fh-wedel.de/hxt-filter Furthermore there is a new wiki page http://www.haskell.org/haskellwiki/HXT/Conversion_of_Haskell_data_from/to_X… describing the serialization and deserialization of Haskell values to/from XML data. More information and download: http://www.fh-wedel.de/~si/HXmlToolbox/index.html There's also a copy on Hackage. Please email comments, bugs, etc. to hxmltoolbox(a)fh-wedel.de or si(a)fh-wedel.de Uwe -- University of Applied Sciences, Wedel, Germany http://www.fh-wedel.de/~si/index.html

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ANNOUNCE: category-extras 0.2
by Dan Doel 28 Apr '08

28 Apr '08

Hello, all. After quite a bit of collaboration with Edward Kmett over the past few days, I've rolled up a new release of category-extras. Perhaps the most significant addition is the generalized hylomorphism he first blogged about here: http://comonad.com/reader/2008/generalized-hylomorphisms/ added to Control.Recursion under the name 'refoldWith', as all other combinators in the library turn out to be special cases of it. Other changes include: * Data.BranchingStream moves to Control.Comonad.Cofree, and various functions are renamed accordingly, as it is the cofree comonad of a functor: Cofree F A = nu X. A * FX * Similarly, a new module has been added: Control.Monad.Free, the free monad of a functor. Free F A = mu X. A + FX * Several new recursion combinators have been added to Control.Recursion. The generalized hylomorphism 'refoldWith' was mentioned above, but we now also have: - Futumorphisms (futu, g_futu) - Chronomorphisms (chrono, g_chrono; name coined by edwardk; they're a combination of a histomorphism and a futumorphism). - Dynamorphisms (dyna, g_dyna; formalize dynamic programming) - Codynamorphisms (codyna, g_codyna; these apparently don't appear in any literature (like chronomorphisms), and we didn't attempt to give them a name, but once one sees dynamorphisms in terms of chronomorphisms/ g_hylomorphisms, this becomes fairly obvious). As can be seen, there's quite a glut of these combinators, and they all correspond to choices of distributive laws for refoldWith, so in the future, it may be better to talk about those directly. For now, though, we have lots of functions with fancy-sounding names. I believe those are the significant updates for this release. Enjoy. hackage: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/category-extras-… darcs: darcs get http://code.haskell.org/~dolio/category-extras/ -- Dan

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Trouble compiling collections-0.3 (from Hackage)
by David F. Place 28 Apr '08

28 Apr '08

Hi, I am not able to build collections-0.3 because on an error. I hope someone can give me a hint. Below, please find a short transcript with various version information. Cheers, David [david@congo ~]$ cd Desktop/collections-0.3 [david@congo collections-0.3]$ runghc Setup.hs configure Configuring collections-0.3... [david@congo collections-0.3]$ runghc Setup.hs build Preprocessing library collections-0.3... Building collections-0.3... Data/Collections.hs:154:17: Could not find module `Data.ByteString.Lazy': it is a member of package bytestring-0.9.0.4, which is hidden [david@congo collections-0.3]$ uname -a Linux congo 2.6.16-1.2111_FC5 #1 SMP Thu May 4 21:16:04 EDT 2006 x86_64 x86_64 x86_64 GNU/Linux [david@congo collections-0.3]$ ghc --version The Glorious Glasgow Haskell Compilation System, version 6.8.2 [david@congo collections-0.3]$ ghc-pkg list /usr/local/lib/ghc-6.8.2/package.conf: ALUT-2.1.0.0, Cabal-1.2.3.0, GLUT-2.1.1.1, HUnit-1.2.0.0, OpenAL-1.3.1.1, OpenGL-2.2.1.1, QuickCheck-1.1.0.0, array-0.1.0.0, base-3.0.1.0, bytestring-0.9.0.1, bytestring-0.9.0.4, cgi-3001.1.5.1, containers-0.1.0.1, directory-1.0.0.0, fgl-5.4.1.1, filepath-1.1.0.0, (ghc-6.8.2), haskell-src-1.0.1.1, haskell98-1.0.1.0, hpc-0.5.0.0, html-1.0.1.1, mtl-1.1.0.0, network-2.1.0.0, old-locale-1.0.0.0, old-time-1.0.0.0, packedstring-0.1.0.0, parallel-1.0.0.0, parsec-2.1.0.0, pretty-1.0.0.0, process-1.0.0.0, random-1.0.0.0, readline-1.0.1.0, regex-base-0.72.0.1, regex-compat-0.71.0.1, regex-posix-0.72.0.2, rts-1.0, stm-2.1.1.0, template-haskell-2.2.0.0, time-1.1.2.0, unix-2.3.0.0, xhtml-3000.0.2.1 [david@congo collections-0.3]$

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Help with complicated type inference
by Paul Johnson 28 Apr '08

28 Apr '08

I'm trying to write an AMQP framing layer. AMQP has two very similar union types: there is a "variant" that contains a single item, and an "array" which consists of a list of elements of the same type. So I thought I could define a "Unit" type container thus: > newtype Unit a = Unit {unUnit :: a} So now I can say: > type AmqpVariant = AmqpVariantBase Unit > type AmqpArray = AmqpVariantBase [] Then the AmqpVariantBase type looks something like this (except that it doesn't work, see below): > data forall a . (AmqpWire a, AmqpWire (c a)) => > AmqpVariantBase c = AmqpVarBin8 (c Bin8) > | AmqpVarInt8 (c Int8) > | AmqpVarUint8 (c Word8) > | AmqpVarChar (c Word8) > | AmqpVarBoolean (c Bool) > | AmqpVarBin16 (c Bin16) > | AmqpVarInt16 (c Int16) > | AmqpVarUint16 (c Word16) > | AmqpVarBin32 (c Bin32) > | AmqpVarInt32 (c Int32) > -- And on for about 20 more types, including compound types. All AMQP types have to be seralised, so I've defined a class "AmqpWire" for serialisation in AMQP format. All the individual types (Bin8, Int8 etc) are instances of this class. I've also defined instances for Unit and [] such as: > instance (AmqpWire a) => AmqpWire (Unit a) where > amqpPut = amqpPut . unUnit > amqpGet = map Unit amqpGet The problem is with the type constraint for AmqpVariantBase. I need to say "AmqpWire (c a)" without explicitly listing all the values of "a" (i.e. Bin8, Int8, etc) because any time I use AmqpVariantBase I have to repeat the same constraint. How do I do this? Paul.

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some questions about type improvement, FDs vs TFs, and Hugs vs GHCi
by Claus Reinke 28 Apr '08

28 Apr '08

consider class FD a b | a -> b instance CFD a b => FD a b class CFD a b instance CFD Bool Char and the constraint 'CFD Bool c'. logically, we know that if we have 'CFD Bool c', we also have 'FD Bool c', which implies that 'c' is uniquely determined, which implies that 'c~Char'. practically, the need to find an instance for 'CFD Bool c' does not generate a demand for implied instances that could lead to this improvement. q1: should there be such a forward demand? ie, instead of failing to find the instance for 'CFD Bool c', should the transitive closure of forward implications be taken into account for FDs, similar to the transitive closure of backward implications? without such forward inferences, consistency of FDs seems hard to guarantee, unless one conservatively rules out everything that might lead to an inconsistency. without such forward inference, implementations differ in their interpretation of this kind of code (as so often): Hugs rejects the program ("Instance is more general than a dependency allows"), which is conservative, but seems to make perfect sense: there's no FD for class 'CFD', so there's no guarantee that all those implied instances of class 'FD' will conform to its own FD. that is easily fixed, it seems: class FD a b | a -> b instance CFD a b => FD a b class FD a b => CFD a b instance CFD Bool Char now, there cannot be an instance of 'CFD' unless there also is an instance of 'FD', in such a way that the parameters of 'CFD' obey the FD of 'FD'. and, in fact, Hugs does now accept this code, and is able to improve 'CFD Bool c' to 'CFD Bool Char', reducing the latter as given: :t undefined :: CFD Bool c => c undefined :: Char but wait a second: we have given an instance of 'CFD', which according to the superclass constraint for that class, is valid only if there is a corresponding instance of 'FD', and there is such an instance of 'FD' if there is an instance of 'CFD'. in other words: if the 'CFD' instance is accepted, it is valid, if it isn't accepted, it is not valid. q2: is this treatment of recursion for class instances intended, or is it an unexpected artifact of the special treatment of superclass constraints? GHCi also accepts this code, but it doesn't seem to do the reduction or improvement: :t undefined :: CFD Bool c => c undefned :: CFD Bool c => c :: (CFD Bool c) => c actually, GHCi accepts the first program, too! perhaps it is delaying the FD consistency check until there is an actual rather than a possible FD conflict? unfortunately not, as we can see if we add a conflicting 'CFD' instance: class FD a b | a -> b instance CFD a b => FD a b class {- FD a b => -} CFD a b instance CFD Bool Char instance CFD Bool Bool note that the conflict is not in 'CFD' but in the implied instances of 'FD', which GHCi doesn't seem to take into account for any FD consistency check: *Main> :t undefined :: CFD Bool c => c undefined :: CFD Bool c => c :: (CFD Bool c) => c *Main> :t undefined :: CFD Bool Char => c undefined :: CFD Bool Char => c :: c *Main> :t undefined :: FD Bool c => c undefined :: FD Bool c => c :: (CFD Bool c) => c *Main> :t undefined :: FD Bool Char => c undefined :: FD Bool Char => c :: c only if we explicitly mention the inconsistent constraints together, and directly in terms of 'FD', not just in terms of 'CFD', do we get the expected error, and even then, the FD isn't used for improvement: *Main> :t undefined :: (CFD Bool Char,CFD Bool Bool) => c undefined :: (CFD Bool Char,CFD Bool Bool) => c :: c *Main> :t undefined :: (FD Bool Char,FD Bool Bool) => c <interactive>:1:0: Couldn't match expected type `Char' against inferred type `Bool' When using functional dependencies to combine FD Bool Bool, arising from the polymorphic type `forall c. (FD Bool Char, FD Bool Bool) => c' at <interactive>:1:0-44 FD Bool Char, arising from the polymorphic type `forall c. (FD Bool Char, FD Bool Bool) => c' at <interactive>:1:0-44 *Main> :t undefined :: (FD Bool Char,FD Bool c) => c undefined :: (FD Bool Char,FD Bool c) => c :: (CFD Bool c) => c this looks like a bug to me, or am i missing something? perhaps we fare better with TFs, as FD development in GHC was put on hold while the new foundations for TFs were put in place? unfortunately, we can't even define TFs in terms of classes, as has been discussed here before (in the context of using FD-based libaries in TF-based client code). what we'd like to write is something like: type family TF a type instance TF a = CTF a b => b -- won't work.. class {- FD a b => -} CTF a b instance CTF Bool Char instance CTF Bool Bool as with the FD-based variant, the definition of TF should not work unless there is a functional relationship between the parameters of 'CTF', but the 'TF' instance isn't even accepted syntactically ("Illegal polymorphic type in type instance"). q3: (a) are qualified types in type instances problematic, or just not yet implemented, or not yet investigated in theory? (b) what about the intended interaction of TFs and FDs in the example. it would seem to permit the use of FD-based code in TFs, as requested here earlier, but is this ever going to be supported? questions, questions,.. does anybody have answers?-) claus

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Ann: Lekash.org
by Jürgen Nicklisch-Franken 28 Apr '08

28 Apr '08

The Haskell IDE written in Haskell (currently in early infancy) called Leksah has now a website (with reference to a Google Code issue tracker): http://leksah.org/ Contributions of all kind welcome. (The released version is still 0.11 but the development version 0.3 has made some progress.) Jürgen

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Finite Difference Implementation
by Fernan Bolando 27 Apr '08

27 Apr '08

Hi all I was wondering if anyone has or knows of a Finite Difference or Finite Element Implementation in pure haskell. I would like to write this myself, but my haskell skills is not that good. I did a bit of google search I've seen it bieng discussed, but did not point to an actual code. thanks fernan -- http://www.fernski.com

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lookup tables & style guidelines
by Cetin Sert 27 Apr '08

27 Apr '08

Dear Haskell Devs ^_^, 1) what is the most performant lookup table/hashtable/dictionary solution for Haskell? 1.1) should I use size-balanced binary trees for that or is there a more common way? 2) are there any established style guidelines for haskell code? Best Regards, Cetin Sert

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asserting the type of a binding in a "do" expression
by Ken Takusagawa 27 Apr '08

27 Apr '08

This works (with -fglasgow-exts): foo::IO Int; foo = do{ (x::Int) <- bar; return x;}; bar = undefined But this does not: foo::IO a; foo = do{ (x::a) <- bar; return x;}; Error message: A pattern type signature cannot bind scoped type variables `a' unless the pattern has a rigid type context. How can I accomplish my goal of asserting the type of x, as a programmer sanity check, when there are type variables? Thanks, --ken

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Replacing RDMS - global lock and STM preventing retrying?
by Marc Weber 27 Apr '08

27 Apr '08

What would be the right way to go to replace RDBMS (postgres/ mysql) etc using haskell only for small to medium sized (web)applications? I guess one way to go is using STM. But what happens if you have some tables each row represented as TVar and you'd like to do a full backup? Some occasionally occuring updates on single rows will make the atomic action get all rows and write them to disk retry again and again? Is there a way to make the update action retry in this case? And then you can implement something like: Try it 20 times, if the aciton doesn't succeed aquire global lock ! Wow. Has anyone already implemented such a RDBMS replacement? Anyone interested in working on this? Marc Weber One solution I came up within minutes :) I love haskell. You write it down fix error and it works :) Would you prefer another way to solve this? --packages: containers, binary, stm, mtl, random module Main where import System.IO.Unsafe import Random import Control.Concurrent import Control.Monad import Control.Concurrent.STM import Control.Monad.Trans -- running count of actions. if set to -1 a transaction has aquired global lock globalLock = unsafePerformIO $ newTVarIO (0::Int) modifyTVar tvar f = do v <- readTVar tvar writeTVar tvar $ f v -- of course this should be in it's own monad to force using this function -- myAtomically: aquires global lock -- of course I don't need 5 atomically calls, but this way an action will not be retried if only the global count changes myAtomically aquireGlobalLock stmAction = if aquireGlobalLock then do atomically $ do runningCount <- readTVar globalLock when (runningCount /= 0) retry writeTVar globalLock (negate 1) -- other actions should be retrying atomically $ do stmAction writeTVar globalLock 0 else do atomically $ do runningCount <- readTVar globalLock when (runningCount == (negate 1)) retry modifyTVar globalLock (+1) atomically stmAction atomically $ modifyTVar globalLock (\x -> x -1) -- log utility printing start / stop of an action stsp :: (MonadIO m) => String -> m r -> m r stsp msg act = do liftIO $ putStrLn $ "start: " ++ msg r <- act liftIO $ putStrLn $ "stop: " ++ msg return r data Table rec = Table { table :: [TVar rec] } newTable rowcount = liftM Table $ mapM newTVarIO [(1::Int)..rowcount] dumpTable fn t = do dat <- myAtomically True $ mapM readTVar $ table t writeFile fn $ show dat disturb t@(Table (row:_)) = do stsp "disturbing" $ do v <- randomRIO (0,1000000) myAtomically False $ writeTVar row v threadDelay 1000000 disturb t -- loop main = do stsp "application" $ do table <- newTable 100000 forkIO $ disturb table stsp "dumping" $ dumpTable "dump" table

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functional update
by Evan Laforge 26 Apr '08

26 Apr '08

Here's some haskell to update a particular field in a data structure (the below run in a StateT / ErrorT context): > set_track_width :: (UiStateMonad m) => > Block.ViewId -> Int -> Block.Width -> m () > set_track_width view_id tracknum width = do > view <- get_view view_id > track_views <- modify_at (Block.view_tracks view) tracknum $ \tview -> > tview { Block.track_view_width = width } > update_view view_id (view { Block.view_tracks = track_views }) Plus some utilities: > modify_at xs i f = case post of > [] -> throw $ "can't replace index " ++ show i > ++ " of list with length " ++ show (length xs) > (elt:rest) -> return (pre ++ f elt : rest) > where (pre, post) = splitAt i xs > update_view view_id view = modify $ \st -> st > { state_views = Map.adjust (const view) view_id (state_views st) } A similar imperative update would look something like this: > state.get_view(view_id).tracks[tracknum].width = width Has there been any work on improving update syntax in haskell? Possibly some improvement could be made with a typeclass or two and a few custom operators, to unify some of the disparate syntax. Maybe more improvement could be made with some TH hackery. A better record update syntax I'm sure could improve things even more. Or maybe there's a way to structure existing code to improve the above? Does anyone know of work that's been done on analysing functional update syntax issues, and coming up with something concise and clear? Other functional languages that do this better? It seems like an obvious candidate for improvement. Probably not an easy one though. There are various overlapping issues: monadic binds not mixing easily with non-monadic functions, different data structures (maps, lists, ...) having different update functions, record update syntax being super wordy, and probably some other things I'm not thinking of. As an aside, on the record system side, I would love to see one of the proposed record systems implemented in GHC, whether it be trex or the "lighter weight" version proposed by one of the Simons. Does anyone know what the main hangup is? Unresolved issues in the proposal? No consensus? Backward incompatibility? Not enough interested GHC devs? All I can say is that as a heavy user of (.) I wouldn't mind replacing them all with (#) or (@) or whatever if I got a nicer record syntax. It wouldn't be hard to write a search/replace for (.).

6 6

Why doesn't GHC use the Hugs definition of splitAt to avoid traversing the first part of the list twice?
by Richard Kelsall 26 Apr '08

26 Apr '08

I've just been investigating a performance oddity in using splitAt on a long stream of random numbers. I don't understand why GHC appears to want to traverse the first part of the list twice. GHC seems to implement the splitAt function something like splitAt n xs = (take n xs, drop n xs) whereas Hugs is something like splitAt n (x : xs) = (x : xs', xs'') where (xs', xs'') = splitAt (n-1) xs which seems much more sensible to me. Wouldn't it be better to change GHC to the Hugs method? Have I misunderstood something? Richard.

3 4

Function Type Calculation (revisited)
by PR Stanley 26 Apr '08

26 Apr '08

Hi I know we've already looked at the topic of function type calculation though last time I didn't have the chance to go through it thoroughly. So here it is again. Apologies for the repetition. I've had a try at calculating function types for two examples below. So to start with I'd be grateful for an assessment of my efforts. All comments are welcome. Thanks, Paul [1] funk f x = f (funk f) x f :: a x :: b funk f x :: c therefore funk :: a -> b -> c RHS f (funk f) x :: c f (funk f) :: d -> c x :: d f :: e -> d -> c funk :: h -> e f :: h unification f :: a = h = (e -> d -> c) x b = d therefore funk :: ((h -> e) -> b -> c) -> b -> c [2] w f = f f Assigning Types f :: a w f :: b therefore w :: a -> b RHS f f :: b f :: c -> b f :: c f :: a = b = c = (c -> b) therefore w :: (a -> a) -> a

1 0

n00b circular dep question
by Jennifer Miller 26 Apr '08

26 Apr '08

Hi all, This is my first post, and I'm a n00b, so go easy if I breach protocol. :-) I have a graph where the nodes contain a number of fields of various types. The values for one of those fields -- call it Mass -- need to reside in a separate file from the main graph definition (this is for workflow reasons and is given to me as a constraint). In order to set the Mass for a particular node, I need access to the other fields of that node. The other constraint is that the graph needs to be available in an interpreter (eg GHCi). So, I have a circular dependency in my modules that I don't know how to resolve in Haskell. I went looking for the equivalent of #include which is how I would have solved this in C++. I'm sure there is a simple answer to this and I'm hoping this group can point me in the right direction. Many thanks, ogina

6 7

a faster, accumulating mapM (was Re: [Haskell-cafe] mapM vs mapM_ performance)
by Ben 26 Apr '08

26 Apr '08

Hello Luke and other Haskellers, Thanks for the reply, but as I noted before, the amount of memory allocated (and resident) is roughly the same. Anyhow it's definitely not a GC issue because I wrote an accumulating version of mapM and got close to mapM_ 's performance. In the code below, main1 is mapM_, main2 is the current mapM (basicallly sequence . map), map3 is a hand-coded accumulating parameter version, mapM2 is the accumulating parameter mapM and main4 uses mapM2. The timings I get are about 15, 175, 20 and 20 seconds for main1, main2, main3 and main4 respectively. main2 uses about 2% less memory than main3 or main4 on this particular run, though I don't know if that is true generally. Unless someone can see a reason why mapM2 is not as good as mapM, can I suggest replacing the implementation of mapM by the implementation of mapM2. A 10x speedup seems to be a bigger deal than GCing 2% more memory. best regards, Ben module Main where import System.IO (openFile, IOMode(..), hPutStr) testlst = let ls = [(i, [(j, (fromIntegral j)::Float) | j <- [1..5]::[Int]]) | i <- [1..500000]::[Int]] in ls main = do h <- openFile "bardump" WriteMode mapM_ ((hPutStr h) . show) testlst main2 = do h <- openFile "bardump2" WriteMode result <- mapM ((hPutStr h) . show) testlst print $ length result main3 = do h <- openFile "bardump3" WriteMode result <- dump h testlst [] print $ length result where dump h (x:xs) accum = do hPutStr h $ show x dump h xs $ ():accum dump _ [] accum = return accum mapM2 :: Monad m => (a -> m b) -> [a] -> m [b] {-# INLINE mapM2 #-} mapM2 fn lst = mapM2accum fn lst [] where mapM2accum _ [] accum = return accum mapM2accum fn (x:xs) accum = do r <- fn x mapM2accum fn xs (r:accum) main4 = do h <- openFile "bardump2" WriteMode result <- mapM2 ((hPutStr h) . show) testlst print $ length result On Thu, Apr 24, 2008 at 1:37 AM, Luke Palmer <lrpalmer(a)gmail.com> wrote: > On Tue, Apr 22, 2008 at 11:32 AM, Ben <midfield(a)gmail.com> wrote: > > Hello Haskellers, > > > > I'm running ghc 6.8.2 on vista 64. Consider the following program, > > which is compiled with -02 -prof -auto-all: > > > > module Main where > > > > import System.IO (openFile, IOMode(..), hPutStr) > > > > testlst = let ls = [(i, [(j, (fromIntegral j)::Float) | j <- > > [1..5]::[Int]]) | i <- [1..500000]::[Int]] > > in ls > > > > main2 = do > > h <- openFile "bardump" WriteMode > > mapM_ ((hPutStr h) . show) testlst > > > > > > main = do > > h <- openFile "bardump2" WriteMode > > mapM ((hPutStr h) . show) testlst > > return () > > > > main and main2 are different in only that mapM_ versus mapM_ are used. > > But the mapM version runs about 20x slower! I'm running with +RTS -p > > -hc -RTS and I see that the amount of memory allocated is about the > > same, and I think the resident memory is about the same too. But the > > mapM_ version runs in about 8.7 seconds, and the mapM version takes > > 167 seconds. > > My first guess is that the garbage collector is not running at all in > the mapM_ version, but is working it's ass off in the mapM version > cleaning up the list that will never be used. > > > > You may ask, why use mapM if you're discarding the values? > > Unfortunately in my real app I need the values, which are more > > interesting than IO (). > > If you need the values, then you've got to pay that price I suppose. > If you need the values, I'm going to take a stab that in your real app > you use a lot of memory because of this (because presumably you're > keeping the values around), whereas you're just seeing a speed hit on > this small test program. > > Luke >

8 13

Trying to build a stand-alone executable GLUT app with ghc, Windows XP
by Peter Schmitz 26 Apr '08

26 Apr '08

Problem summary Trying to build a stand-alone executable GLUT app with ghc, Windows XP Problem description I compile and link (without errors) a simple GLUT application under Windows XP. When I run it, XP pops an error window saying the app cannot start due to a missing "glut32.dll". I want to do a static build to create a stand-alone executable GLUT app under Windows XP, without using DLL files, or placing any files in the Windows system dir. This is my first GUI code in Haskell, and I chose GLUT because it is a standard library. Following are some details. Thanks much for any advice. Source code -- Simple GLUT app to create a window module Main(main) where import Graphics.Rendering.OpenGL import Graphics.UI.GLUT main = do (progname, _) <- getArgsAndInitialize createWindow "Hello World" mainLoop Compile/run Environment ghc-6.8.2 on a USB flashdrive under a non-admin Windows XP account ghc dir is not on C: E:\apps\ghc\ghc-6.8.2 XP shell used: cmd.exe shell path E:\ghcTest>path PATH=C:\WINDOWS\system32;C:\WINDOWS;E:\apps\ghc\ghc-6.8.2\bin;.\ ghc library path E:\ghcTest>ghc --print-libdir E:/apps/ghc/ghc-6.8.2 compile/link output E:\ghcTest>ghc --make x -package GLUT [1 of 1] Compiling Main ( x.hs, x.o ) Linking x.exe ... E:\ghcTest> files (sizes in bytes) 186 x.hs 387 x.hi 3,184 x.o 498 x.exe.manifest 609,222 x.exe When application is run Error dialog window pops up window title a.exe - Unable To Locate Component window text The application has failed to start because glut32.dll was not found. Re-installing the application may fix this problem. No output in shell; no glut window is created. Other builds tried; same runtime error ghc --make x -package GLUT -static ghc -package GLUT x.hs -o x ghc --make x -package GLUT -LE:\apps\ghc\ghc-6.8.2\lib\GLUT-2.1.1.1 ghc --make x -LE:\apps\ghc\ghc-6.8.2\lib\GLUT-2.1.1.1 --

3 2

My try for a CoroutineT monad tranformer
by Joachim Breitner 26 Apr '08

26 Apr '08

Hi, (for those who follow planet.haskell.org this is old news, but I thought I’d tell the others) In http://www.joachim-breitner.de/blog/archives/291-Pausable-IO-actions-for-be… I describe how I wrote a monad transformer that allows me to pause a computation from within by returning another computation that I can use to re-start the computation (or to throw it away if I want). I needed this for a long running drawing computation in a gtk2hs program that I want to pause at convenient points (to allow user interaction), and that I need to abort when what I’m drawing is not up-to-date anymore. The API basically consists of the function > runCoroutineT :: Monad m => CoroutineT m () -> m (Maybe (CoroutineT m ())) which runs the pausable computation, any Maybe returns Just the resume action, and the function > pause :: Monad m => CoroutineT m () to be used inside the computation, which pauses it. I have put the complete module in the darcs repository that might later also contain the GUI program at http://darcs.nomeata.de/FrakView/ What do you think of CoroutineT? Could it have been easily implemented using the existing monad transformers? Is it useful enough so that it should be included somewhere, and where? Are there any problems with strictness or other tripping points that I have overlooked? Greetings, Joachim -- Joachim Breitner e-Mail: mail(a)joachim-breitner.de Homepage: http://www.joachim-breitner.de ICQ#: 74513189 Jabber-ID: nomeata(a)joachim-breitner.de

3 4

I hate Haskell's typeclasses
by Ryan Ingram 26 Apr '08

26 Apr '08

WARNING: RANT AHEAD. Hopefully this fires off some productive discussion on how to fix these problems! Don't get me wrong: I think the idea of typeclasses is great. Their implementation in Haskell comes so close to being awesome and then falls short, and that's almost worse than not being awesome in the first place! Some examples of things I think you should be able to do, that just Do Not Work. Examples like these are trivial in many other languages, and they shouldn't be that hard here, either! 1) You can't make sensible default implementations. For example, it'd be nice to make all my Monads be Applicatives and Functors without resorting to Template Haskell or infinite boilerplate. Why can't I just write this? instance Monad m => Applicative m where pure = return (<*>) = ap Sure, I get that there might be ambiguity of which instance to choose. But why not warn me about that ambiguity, or let me choose somehow on a case-by-case basis when it happens? 2) You can't add sensible superclasses. I was playing with QuickCheck and wanted to write "equal with regards to testing". So I wrote up a class for it: class TestableEq a where (~=) :: a -> a -> Property instance Eq a => TestableEq a where -- should be a superclass of Eq instead! a ~= b = a == b instance (Arbitrary a, TestableEq b) => TestableEq (a -> b) where f ~= g = forAll arbitrary (\a -> f a ~= g a) But this doesn't work without overlapping & undecidable instances! Sure, there is an alternative: I could manually declare instances of TestableEq for EVERY SINGLE TYPE that is an instance of Eq. I am sure nobody here would actually suggest that I do so. And sure, these extensions are both safe here, because the intent is that you won't declare instances of TestableEq for things that are already instances of Eq, and you won't do something stupid like "instance TestableEq a => Eq a". But why do I need to jump through these hoops for a perfectly safe & commonly desired operation? 3) There's no reflection or ability to choose an implementation based on other constraints. In QuickCheck, (a -> b) is an instance of Arbitrary for appropriate a, b. But you can't use this instance in forAll or for testing functions without being an instance of Show. Now, this is probably a design mistake, but it's the right choice with the current typeclass system (see (2)). But it'd be a million times better to have something like the following: class Arbitrary a => MkArbitrary a where mkArbitrary :: Gen (a, String) case instance MkArbitrary a where Show a => mkArbitrary = do x <- arbitrary return (x, show x) otherwise => mkArbitrary = do st <- getGenState x <- arbitrary return (x, "evalGen arbitrary " ++ show st) With this, QuickCheck could print reproducible test cases painlessly without adding the requirement that everything is an instance of Show! Now, you could say that mkArbitrary should be a member function of Arbitrary, but then you clutter up your instance definitions with tons of "mkArbitrary = defaultMkArbitrary" for types that have a Show instance. 4) Every concrete type should be an instance of Typeable without having to do anything, and Typeable should give you typecase & reflection: genericShow :: Typeable a => a -> String genericShow x = typecase x of String -> x (Show t => t) -> show x -- any instance of Show _ -> "<unknown>" -- ryan P.S. I'd actually love to work on any or all of these problems, but I can't get GHC to compile! See http://hpaste.org/5878

8 16

looking for examples of non-full Functional Dependencies
by Tom Schrijvers 25 Apr '08

25 Apr '08

Hello, I'm looking for practical examples of non-full functional dependencies and would be grateful if anyone could show me some or point to applications using them. A non-full functional dependency is one involves only part of the parameters of a type class. E.g. class C a b c | a -> b has a non-full functional dependency a -> b which does not involve c. Thanks, Tom -- Tom Schrijvers Department of Computer Science K.U. Leuven Celestijnenlaan 200A B-3001 Heverlee Belgium tel: +32 16 327544 e-mail: tom.schrijvers(a)cs.kuleuven.be url: http://www.cs.kuleuven.be/~toms/

13 35

Higher order randomness and the fat tails
by Steve Lihn 25 Apr '08

25 Apr '08

Hi, Haskellers, This email is only remotely connected to this list. Recently I am doing some research on the fat tails (in fact, I simply bumped into it) on the stock market. I came up with an equation which I think solves the fat tails -- which is really the Levy stable distribution. During the complex arguments that led to the solution, I coined a term called "Higher order randomness". This is inspired by the concept of higher order function in Haskell. * Higher order function operates on another function, instead on an ordinary value. So higher order randomness operates on another randomness, instead of directly producing the time series. The request is the following. I know there are many scientists on this list. If you or your colleagues happen to be involved or know any scenario that has fat tail distribution, can you let me know? I only "solved" one scenario -- numerically, not even analytically. Probability-wise, it is a dangerous thing to claim I solve "the problem". So I'd like to collect as many scenarios as I can. It can be financial related, or in other fields of science. PS. I'd like to have some operational details and numbers. Not just a general statement like "fat tail is observed in astronomy". Thanks and sorry for the spam, Steve

1 0

how can this code be less?
by cetin tozkoparan 25 Apr '08

25 Apr '08

I wrote this code and Can it be less? [2,4,5]list is sub list of [3,7,2,4,5,9] list and return True but not of[3,7,4,2,5,9] list ; return False sublist :: Eq a => [a] -> [a] -> Bool sublist [] _ = True sublist (_:_) [] = False sublist (x:xs) (y:ys) | x == y = if isEqual (x:xs) (y:ys) == False then sublist (x:xs) ys else True | otherwise = sublist (x:xs) ys isEqual :: Eq a => [a] -> [a] -> Bool isEqual [] _ = True isEqual (_:_) [] = False isEqual (x:xs) (y:ys) | x==y = isEqual xs ys | otherwise = False ____________________________________________________________________________________ Be a better friend, newshound, and know-it-all with Yahoo! Mobile. Try it now. http://mobile.yahoo.com/;_ylt=Ahu06i62sR8HDtDypao8Wcj9tAcJ

5 5

Help me refactor this type!
by Ryan Ingram 25 Apr '08

25 Apr '08

More FRP stuff: a new type for Future that came to me in an inspiration this morning. But it's ugly and I need someone with better theoretical underpinnings than me to tell me what I've really built :) data Future t a = Known t a | Unknown (t -> IO (STM (Maybe (Future t a)))) Given Eq t, Ord t, Bounded t, this type is at least member of Monad, Applicative, Functor, and MonadPlus/Monoid. But the derivation gives me that "needs refactoring" feeling; here's an example: force :: (Ord t, Bounded t) => Future t a -> IO (t, a) force (Known t a) = return (t, a) force (Unknown f) = do stmF <- f maxBound mF <- atomically stmF case mF of Nothing -> return (maxBound, error "never") Just fut' -> force fut' delayF :: Ord t => t -> Future t a -> Future t a delayF t0 (Known t a) = Known (max t0 t) a delayF t0 (Unknown f) = Unknown $ \t -> fmap (fmap (fmap (delayF t0))) (f t) instance (Ord t, Bounded t) => Monad (Future t) where return = Known minBound Known t a >>= g = delayF t (g a) Unknown f >>= g = Unknown $ \t -> do -- IO stmF <- f t return $ do -- STM mF <- stmF return $ do -- Maybe fut' <- mF return (fut' >>= g) This code makes me sad; so many nested blocks. There's got to be a refactoring of this that I am missing! It's clearly got something to do with Fix, Either, ReaderT, and MaybeT, and type composition, but none of those seem to answer the whole question. Any thoughts? -- ryan

3 2

Stronger STM primitives needed? Or am I just doing it wrong?
by Ryan Ingram 25 Apr '08

25 Apr '08

How can I implement the following operation efficiently in STM? Given a TVar "now", waitFor t0 = do t <- readTVar now if (t < t0) then retry else return () This naive implementation has the problem that the transaction gets restarted every time "now" gets updated, even if the new value is still less than t0. One primitive that would be strong enough is this: retryUntil :: TVar a -> (a -> Bool) -> STM () although it would still do some computation every time "now" changed. The thought I originally had was to register the "waitFor" time with some helper which kept track of the current time and fired off a notice when it was ready. But the problem with that is that "retry" undoes all the work of setting that up; the goal is still to block, but I want a stronger blocking primitive. Does anyone have any ideas? -- ryan Here's the background: I'm following along with Conal's excellent FRP paper at http://conal.net/papers/simply-reactive/ In his implementation of futures, Conal uses a race primitive which spawns two threads each computing a value that (if successful) is guaranteed to agree. I was (and still am) sure that an implementation using STM could avoid needing to race, with careful use of orElse & retry. Here's the datatype I'm using for Future: > -- semantic type for future > -- t = type of Time, an instance of Bounded. a = value type > type F t a = (t, a) > force :: Future t a -> F t a > data Future t a = Fut > { waitFor :: t -> STM () > , value :: STM (t, a) > } > firstFuture :: Future t a -> Future t a -> Future t a > -- with semantics: > -- force (firstFuture f1 f2) = (min t1 t2, if t1 <= t2 then v1 else v2) > -- where > -- (t1, v1) = force f1 > -- (t2, v2) = force f2 Each future lives in some universe; imagine you have the following: ] type Universe t -- abstract ] P :: Bounded t => Universe t -- universe of pure values ] R :: Universe Time -- universe of the real world ] univ :: Future t a -> Universe t ] now :: Universe t -> STM t ] -- now P = return maxBound ] -- now R = current time The main thrust of this is that each universe has its own idea of what time it is; but when combining futures, we get a new universe which tracks the later of the times in the two universes. The problem is combining two futures: Lets say we have: anyKey :: Future Time () -- fires when the user first presses a key thousand :: Num t => Future t () -- fires at tick 1000 thousand = exactly 1000 () and we are evaluating force (firstFuture anyKey thousand) Now, "clock" is going to fire at tick 1000; we know this because it lives in the pure universe and its value is always available. So firstFuture can get the following: x1 <- maybeSTM (value anyKey) -- x1 = Nothing x2 <- maybeSTM (value clock) -- x2 = Just (1000, ()) ... -- synchronize P with R at tick 1000 waitFor anyKey 1000 -- if nothing has changed before tick 1000 return (1000, ()) -- converts a possibly-retry-calling STM into one that never fails maybeSTM m = fmap Just m `orElse` return Nothing So if we use the naive implementation for "waitFor" in terms of "now", the whole transaction will get re-evaluated every tick. I only want it to get re-evaluated if the user presses a key (changing some TVar evaluated in "value anyKey"), or tick 1000 passes, whichever comes first. Is there a way to do this? Is my choice of "waitFor" as the basic "universe synchronization" operation too weak?

9 25

Kieran Beltran/Watson/IBM is out of the office.
by Kieran Beltran 25 Apr '08

25 Apr '08

I will be out of the office starting 04/23/2008 and will not return until 04/28/2008. I will be on vacation for a few days. If this is urgent please reach my Admin Kristine Smester ((917) 472-3387), otherwise I will respond to your message when I return.

1 0

Call for Contributions - Haskell Communities and Activities Report, May 2008 edition
by Janis Voigtlaender 25 Apr '08

25 Apr '08

Dear Haskellers, so much has happened in the Haskell world in the past months. Therefore, we would very much like to collect contributions for the 14th edition of the ================================================================ Haskell Communities & Activities Report http://www.haskell.org/communities/ Submission deadline: 10 May 2008 (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 to the us. 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 any project related to Haskell that has not previously been mentioned in the HC&A Report, please tell us, so that we 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 6 months. If you have only recently been exposed to Haskell, it might be a good idea to browse the December 2007 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! As the purpose of the report is to collect recent or current activities, we encourage you to update all existing summaries and reports. We will probably drop any topics that have not had any activity for the past year, i.e., since May 2007, but we would very much prefer you to present an updated description of the topic. Of course, new entries are more than welcome. Reports should generally be kept brief and informative, ranging from a few sentences to a few hundred words, to keep the whole report reasonably sized. Looking forward to your contributions, Andres and Janis (current editors) FAQ: 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 over 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 don't 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 editors. 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: How much should I write? A: There's no formal limit. But generally, entries should be short and to the point. A general introduction is helpful. Apart from that, you should focus on recent or upcoming developments. There also is no minimum length of an entry! The report aims at being as complete as possible, so please consider writing an entry, even if it's only a few lines long. Q: If I don't update my entry, but want to keep it in the report, what should I do? A: Tell us that there are no changes. We will reuse the old entry in this case, but we might drop it if it's older than a year, to give more room and more attention to projects that change a lot. Don't resent complete entries if you haven't changed them. Q: What format should I write in? A: The best format is a LaTeX source file, adhering to the template that's available at: http://haskell.org/communities/05-2008/template.tex There's also a LaTeX style file at http://haskell.org/communities/05-2008/hcar.sty that you can use to preview your entry. If you don't know LaTeX, then use plain text. If you modify an old entry, we will send you your old entry as a template within the next hours (provided we have your valid email address). Please modify that template, rather than using your own version of the old entry as a template. Don't worry about writing correct LaTeX, we will be able to handle your file. Please don't use HTML or even DOC. -- Dr. Janis Voigtlaender http://wwwtcs.inf.tu-dresden.de/~voigt/ mailto:voigt@tcs.inf.tu-dresden.de

2 1

Terminal manipulation in windows?
by Jonathan Coveney 25 Apr '08

25 Apr '08

I have a program that requires the user to be in raw mode. This doesn't seem to be an issue on linux, but in Windows, it's a bit tricky (System.Posix.Terminal isn't an option). Is there a possible way to set this in Haskell? Or even outside of Haskell? It doesn't appear cygwin has a raw mode.

1 0

curl binding examples?
by brad clawsie 25 Apr '08

25 Apr '08

-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 i was once given this snippet with reference to an earlier incarnation of the curl bindings: http://hpaste.org/3529 my guess is that this is no longer accurate documentation (?) anything that could be provided in the way of examples would be nice, i have placed a stub here: http://haskell.org/haskellwiki/Network.Curl it would be nice of people inlined small examples directly into the haddock docs for libraries. putting a minimal "hello world" example illustrating the basics of a library would preclude 90% of the followup questions on lists and irc. -----BEGIN PGP SIGNATURE----- Version: GnuPG v2.0.8 (FreeBSD) iEYEARECAAYFAkgQrvMACgkQxRg3RkRK91NuEQCfUdInLYcjioLRsHfYGkwMW2Dp jI8An3uEuuF/aaXimZwhweJJ4zGz3CsY =qs80 -----END PGP SIGNATURE-----

2 1

searching via zipper over a tree
by Graham Fawcett 25 Apr '08

25 Apr '08

Hi folks, I'm studying zippers (and by extension, the State monad), and have written a function for finding the first node-location in a 'zippered' tree that satisfies a predicate: http://paste.lisp.org/display/59636 (The code uses Krasimir Angelov's Data.Tree.Zipper.) My code works, but I suspect that I'm working too hard. I would appreciate any comments! Regards, Graham

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Fw: I have a problem
by cetin tozkoparan 24 Apr '08

24 Apr '08

I have a problem which i can'tsolve. Is there any one who has an idea? Two lists is sent as parameter to a function. If second list contains first list, return true, else return false. This comparision must be in order of first list. You can look at examples. function type as follows: sublist:: [a] -> [a] -> Bool examples: For instance [2,4,5]list is sub list of [3,7,2,4,5,9] list but not of[3,7,4,2,5,9] list. sublist [2,8][1,5,6,2,4,7,8,2] False sublist [1,2,3][0,1,2,3,4,5,6] True sublist [5,4] [1,4,5,7,8,3,5,4] True sublist [1,2,4,3,4,5,7,8,9,5] [8,2,3,1,2,4,3,4,5,7,8,9,5,1,6,2] True ____________________________________________________________________________________ Be a better friend, newshound, and know-it-all with Yahoo! Mobile. Try it now. http://mobile.yahoo.com/;_ylt=Ahu06i62sR8HDtDypao8Wcj9tAcJ

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mapM vs mapM_ performance
by Ben 24 Apr '08

24 Apr '08

Hello Haskellers, I'm running ghc 6.8.2 on vista 64. Consider the following program, which is compiled with -02 -prof -auto-all: module Main where import System.IO (openFile, IOMode(..), hPutStr) testlst = let ls = [(i, [(j, (fromIntegral j)::Float) | j <- [1..5]::[Int]]) | i <- [1..500000]::[Int]] in ls main2 = do h <- openFile "bardump" WriteMode mapM_ ((hPutStr h) . show) testlst main = do h <- openFile "bardump2" WriteMode mapM ((hPutStr h) . show) testlst return () main and main2 are different in only that mapM_ versus mapM_ are used. But the mapM version runs about 20x slower! I'm running with +RTS -p -hc -RTS and I see that the amount of memory allocated is about the same, and I think the resident memory is about the same too. But the mapM_ version runs in about 8.7 seconds, and the mapM version takes 167 seconds. You may ask, why use mapM if you're discarding the values? Unfortunately in my real app I need the values, which are more interesting than IO (). ? Take care, Ben

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Re: [Haskell] How to define tail function for Even/Odd GADT lists?
by David Roundy 24 Apr '08

24 Apr '08

2008/4/23 Martijn Schrage <martijn(a)cs.uu.nl>: > It depends a bit on what you want to use these lists for, but the following > encoding works for your examples and doesn't need the type class. > > data E > data O > > type Even = (E,O) > type Odd = (O,E) That's a nice little trick! I like how you achieve type signatures relating two distinct types just by sticking them in a tuple. :) David

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Hackage being too strict?
by John Goerzen 24 Apr '08

24 Apr '08

When I went to make my upload of MissingH 1.0.1, Hackage rejected it, saying: Instead of 'ghc-options: -XPatternSignatures' use 'extensions: PatternSignatures' It hadn't rejected MissingH 1.0.0, even though it had the same thing. Now, my .cabal file has this: -- Hack because ghc-6.6 and the Cabal the comes with ghc-6.8.1 -- does not understand the PatternSignatures extension. -- The Cabal that comes with ghc-6.8.2 does understand it, so -- this hack can be dropped if we require Cabal-Version: >=1.2.3 If impl(ghc >= 6.8) GHC-Options: -XPatternSignatures which was contributed by Duncan Coutts. It seems arbitrary that Hackage would suddenly reject this valid usage. Thoughts?

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compressed pointers?
by Ketil Malde 24 Apr '08

24 Apr '08

One of the downsides of a 64-bit environment is the increased size of pointers. This means that the cost of a String increases from something like 12 bytes per char to something like 24. I notice BEA uses something called "compressed pointers" to get the 64-bit (more registers, etc) benefits without paying the (cache-thrashing) cost. http://e-docs.bea.com/jrockit/releases/5026x/relnotes/relnotes.html#wp10797… Something for GHC? -k -- If I haven't seen further, it is by standing in the footprints of giants

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zipper for rose trees? (Data.Tree)
by Graham Fawcett 24 Apr '08

24 Apr '08

Hi folks, Is there a common zipper implementation for the "Tree a" datatype, defined in Data.Tree? The wiki gives examples for binary trees and B-trees, but not for these. Thanks, Graham

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Re: [Haskell] Matrix multiplication
by Sebastian Sylvan 24 Apr '08

24 Apr '08

On Wed, Apr 23, 2008 at 5:01 PM, Tillmann Vogt <Tillmann.Vogt(a)rwth-aachen.de> wrote: > Hi, > > I am currently experimenting with parallelizing C-programs. I have > therefore written a matrix vector multiplication example that needs 13 > seconds to run (5 seconds with OpenMP). Because I like Haskell I did the > same in this language, but it takes about 134 seconds. Why is it so slow? > Does someone have an idea? > > Yes, in the C version you use unboxed arrays, in the Haskell version you use a linked list of linked lists. Naturally the latter will take up more space, require more work to index, and will thrash the cache quite a bit.

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PLMMS - Last call for papers
by Jacques Carette 24 Apr '08

24 Apr '08

LAST CALL FOR PAPERS Second Workshop on Programming Languages for Mechanized Mathematics (PLMMS 2008) http://events.cs.bham.ac.uk/cicm08/workshops/plmms/ As part of CICM / Calculemus 2008 Birmingham, UK, 28-29 July 2008 This workshop is focused on the intersection of programming languages (PL) and mechanized mathematics systems (MMS). The latter category subsumes present-day computer algebra systems (CAS), interactive proof assistants (PA), and automated theorem provers (ATP), all heading towards fully integrated mechanized mathematical assistants that are expected to emerge eventually (cf. the objective of Calculemus). The two subjects of PL and MMS meet in the following topics, which are of particular interest to this workshop: * Dedicated input languages for MMS: covers all aspects of languages intended for the user to deploy or extend the system, both algorithmic and declarative ones. Typical examples are tactic definition languages such as Ltac in Coq, mathematical proof languages as in Mizar or Isar, or specialized programming languages built into CA systems. Of particular interest are the semantics of those languages, especially when current ones are untyped. * Mathematical modeling languages used for programming: covers the relation of logical descriptions vs. algorithmic content. For instance the logic of ACL2 extends a version of Lisp, that of Coq is close to Haskell, and some portions of HOL are similar to ML and Haskell, while Maple tries to do both simultaneously. Such mathematical languages offer rich specification capabilities, which are rarely available in regular programming languages. How can programming benefit from mathematical concepts, without limiting mathematics to the computational worldview? * Programming languages with mathematical specifications: covers advanced "mathematical" concepts in programming languages that improve the expressive power of functional specifications, type systems, module systems etc. Programming languages with dependent types are of particular interest here, as is intentionality vs extensionality. * Language elements for program verification: covers specific means built into a language to facilitate correctness proofs using MMS. For example, logical annotations within programs may be turned into verification conditions to be solved in a proof assistant eventually. How need MMS and PL to be improved to make this work conveniently and in a mathematically appealing way? These issues have a very colorful history. Many PL innovations first appeared in either CA or proof systems first, before migrating into more mainstream programming languages. Some examples include type inference, dependent types, generics, term-rewriting, first-class types, first-class expressions, first-class modules, code extraction etc. However, such innovations were never aggressively pursued by builders of MMS, but often reconstructed by programming language researchers. This workshop is an opportunity to present the latest innovations in MMS design that may be relevant to future programming languages, or conversely novel PL principles that improve upon implementation and deployment of MMS. We also want to critically examine what has worked, and what has not. Why are all the languages of mainstream CA systems untyped? Why are the (strongly typed) proof assistants so much harder to use than a typical CAS? What forms of polymorphism exist in mathematics? What forms of dependent types may be used in mathematical modeling? How can MMS regain the upper hand on issues of "genericity" and "modularity"? What are the biggest barriers to using a more mainstream language as a host language for a CAS or PA/ATP? Invited Talk ------------ Conor McBride (Alta Systems, Northern Ireland) will give an invited talk "Theorem Proving for the Lazy Programmer" Submission ---------- Submission works through EasyChair http://www.easychair.org/conferences/?conf=plmms2008 Two kinds of papers will be considered: * Full research papers may be up to 12 pages long. Authors of accepted papers are expected to present their work on the workshop in a regular talk. * Position papers may be up to 4 pages long. The workshop presentation of accepted position papers consists of two parts: a stimulating statement of certain issues or challenges by the author, followed by a discussion in the plenum. Papers should use the usual ENTCS style http://www.entcs.org/prelim.html (11 point version), and will be reviewed by the program committee. Informal workshop proceedings will be circulated as a technical report. Moreover there will be post-workshop proceedings of improved research papers, or position papers that have been completed into full papers, to appear in a special issue of the Journal of Automated Reasoning. There will be a separate submission and review phase for this, where papers from both PLMMS 2007 and 2008 will be considered. Programme Committee ------------------- Jacques Carette (Co-Chair) (McMaster University, Canada) John Harrison (Intel Corporation, USA) Hugo Herbelin (INRIA, Ecole polytechnique, France) James McKinna (Radboud University Nijmegen, Netherlands) Ulf Norell (Chalmers University, Sweden) Bill Page Christophe Raffalli (Universite de Savoie, France) Josef Urban (Charles University, Czech Republic) Stephen Watt (ORCCA, University of Western Ontario, Canada) Makarius Wenzel (Co-Chair) (Technische Universitaet Muenchen, Germany) Freek Wiedijk (Radboud University Nijmegen, Netherlands) Important Dates --------------- * Submission deadline - 5 May 2008 * Notification of acceptance - 6 June 2008 * Final version - 7 July 2008 (approximately) * Workshop - 28-29 July 2008

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announcing franchise 0.0
by David Roundy 24 Apr '08

24 Apr '08

I'm pleased to announce the existence (not release, properly) of franchise, a new configuration/build system for Haskell programs and packages. Franchise ========= Franchise is a configuration and build system for Haskell projects. The configure system employed in franchise is designed to be easily forward and backward compatible, meaning that you shouldn't need to change your Setup.hs file in order to compile with a new version of ghc, and if you *do* need to make a change in your Setup.hs file, it shouldn't force users who have an older version of franchise and/or ghc to upgrade either their compiler or their copy of franchise. The latter goal is really only going to be realized if and when a stable version of franchise is released... as it is currently is something of a pre-alpha state (but useable, for instance, for compiling darcs). One goal of franchise is to not require developers to provide redundant information. For instance, you've already listed all the modules you use, and ghc already knows which modules are present in which packages, so there's in general no need for you to list the packages that you require, much less their versions. This enhances both forwards and backwards compatibility, and just plain makes your life easier. If a particular module is provided by more than one package, you may need to disambiguate, but that's not the common case. Perhaps also worth mentioning is that franchise supports parallel builds similar to make -j. Currently the number of simultaneous builds is fixed at four. Franchise does not, however, compute an optimized parallel build order, with the result that on the darcs repository a franchise build is a few percent slower than make -j4. Franchise is currently ghc-specific and won't run on Windows, but patches to extend either of these limitations would be welcome. It also currently won't work when any flags contain space characters (e.g. with --prefix="/home/user/My stuff"), but the fix for lack of space support is the same as the fix for Windows support, so far as I can tell. The package name "franchise" stands for "Fun, relaxing and calming Haskell into Saturday evening." It is also something of an antonym of "cabal," since "franchise" means the right to vote. Which also fits in with the concept of allowing the code to decide on its own dependencies. Franchise is made up of some pretty ugly code, with a small amount of pretty beautiful code. But it was all code that was fun and relaxing to write. It you want to have argumentative, stressful conversations, please don't do so on the subject of fun, relaxing and calming code. Note: franchise is almost entirely undocumented. It does only export a couple of dozen functions, but still might be hard to learn to use. This is because writing documentation is not as fun, relaxing or calming as writing Haskell. Also, franchise is not yet at the stage where it's likely to be useful to you without any features added, unless you've got a very simple project, in which case you should be able to pretty easily copy and modify an existing franchise Setup.hs file. To get franchise, run darcs get http://darcs.net/repos/franchise (note that this will require darcs 2.0.0) To build and install franchise, simply execute runghc Setup.hs install --user --prefix=$HOME if you don't want to actually install it, just run runghc Setup.hs build I think that's all (and obviously, in no particular order). I hope you enjoy franchise, or if you don't enjoy franchise, I hope you don't tell me about it. David Roundy P.S. A franchise build file for darcs is included below. This is a work-in-progress. It doesn't build the documentation, doesn't allow the user to configure on the command-line which packages they want to use (e.g. bytestring/curl/libwww) and doesn't build the Workaround.hs module which codes replacements for missing or broken library functions. But it *is* able to build darcs, and if these features are added, it will be able to replace darcs' configure and build system without loss of features or compatibility. #!/usr/bin/runhaskell import Distribution.Franchise configure = do findPackagesFor "src/darcs.lhs" addEnv "GHC_FLAGS" "-DPACKAGE_VERSION=\"2.0.0\"" addEnv "GHC_FLAGS" "-O2 -Wall -Werror" addEnv "CFLAGS" "-DPACKAGE_VERSION=\"2.0.0\"" -- look for libz checkLib "z" "zlib.h" "gzopen(\"temp\",\"w\")" -- look for libwww do systemOut "libwww-config" ["--cflags"] >>= addEnv "CFLAGS" systemOut "libwww-config" ["--libs"] >>= addEnv "LDFLAGS" addEnv "GHC_FLAGS" "-DHAVE_LIBWWW" putStrLn "Found libwww" `catch` \_ -> putStrLn "Libwww isn't present!" -- look for libcurl do systemOut "curl-config" ["--cflags"] >>= addEnv "GHC_FLAGS" systemOut "curl-config" ["--cflags"] >>= addEnv "CFLAGS" systemOut "curl-config" ["--libs"] >>= addEnv "LDFLAGS" addEnv "GHC_FLAGS" "-DHAVE_CURL" putStrLn "Found libcurl" `catch` \_ -> putStrLn "Libcurl isn't present!" -- look for libcurses do checkLib "curses" "term.h" "tgetnum(\"hello world\")" checkLib "curses" "curses.h" "" addEnv "GHC_FLAGS" "-DHAVE_CURSES" `catch` \_ -> putStrLn "Couldn't find libcurses." -- look for http packages do requireModule "Network.HTTP ()" addEnv "GHC_FLAGS" "-DHAVE_HTTP" `catch` \_ -> putStrLn "Couldn't find http package." findPackagesFor "src/darcs.lhs" main = build configure $ executable "darcs" "src/darcs.lhs" ["src/c_compat.c", "src/maybe_relink.c", "src/atomic_create.c", "src/fpstring.c", "src/umask.c", "src/hslibwww.c", "src/Crypt/sha2.c", "src/hscurl.c"] -- David Roundy Department of Physics Oregon State University

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ANNOUNCE: Galois web libraries for Haskell released
by Don Stewart 24 Apr '08

24 Apr '08

Galois, Inc. is pleased to announce the open source release of a suite of web programming libraries for Haskell! The following libraries are available, providing support for a wide range of Haskell web programming scenarios: * json JSON (JavaScript Object Notation) is a lightweight data-interchange format. It is easy for humans to read and write. It is easy for machines to parse and generate. It is based on a subset of the JavaScript Programming Language, Standard ECMA-262 3rd Edition - December 1999. This library provides a validating parser and pretty printer for converting between Haskell values and JSON. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/json * xml A simple, lightweight XML parser/generator. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/xml * utf8-string A UTF8 layer for IO and Strings. The utf8-string package provides operations for encoding UTF8 strings to Word8 lists and back, and for reading and writing UTF8 without truncation. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/utf8-string * selenium Haskell bindings to communicate with a Selenium Remote Control server. This package makes it possible to use Haskell to write test scripts that exercise web applications through a web browser. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/selenium * curl libcurl is a client-side URL transfer library, supporting FTP, FTPS, HTTP, HTTPS, SCP, SFTP, TFTP, TELNET, DICT, LDAP, LDAPS and FILE. libcurl supports SSL certificates, HTTP POST, HTTP PUT, FTP uploading, HTTP form based upload, proxies, cookies, user+password authentication (Basic, Digest, NTLM, Negotiate, Kerberos4), file transfer resume, http proxy tunneling and more! This package provides a Haskell binding to libcurl. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/curl * sqlite Haskell binding to sqlite3 <http://sqlite.org/>, a light, fast database. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/sqlite * feed Interfacing with RSS (v 0.9x, 2.x, 1.0) and Atom feeds http://hackage.haskell.org/cgi-bin/hackage-scripts/package/feed * mime Haskell support for working with MIME types. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/mime Together these fill in a big chunk of the web programming stack for Haskell. Get the code! You can find all the cabalised packages on hackage.haskell.org. About: Galois <http://galois.com> researches, designs and develops high assurance technologies for security-critical systems, networks and applications. We use Haskell as a primary development tool for producing robust components for a diverse range of clients. Web-based technologies are increasingly important in this area, and we believe Haskell has a key role to play in the production of reliable, secure web software. The culture of correctness Haskell encourages is ideally suited to web programming, where issues of security, authentication, privacy and protection of resources abound. In particular, Haskell's type system makes possible strong static guarantees about access to resources, critical to building reliable web applications. We hope that the release of this suite of libraries to the community will push further the adoption of Haskell in the domain of web programming. This release brought to you by: Iavor Diatchki Trevor Elliott Sigbjorn Finne Andy Gill Eric Mertens Isaac Potoczny-Jones Don Stewart Aaron Tomb

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