- Haskell-Cafe - Haskell.org

ANNOUNCE: wxHaskell 0.10.3 rc1
by Jeremy O'Donoghue 23 Mar '08

23 Mar '08

We are pleased to announce the first release candidate of wxHaskell 0.10.3. This is the first update with binary packages available since June 2005, and is the result of a great deal of work by a new team of contributors. We are hoping to make a full release shortly, and issues and bug reports either through the wxHaskell user mailing list (wxhaskell-users(a)sourceforge.net) or via the Sourceforge bug tracker (http://sourceforge.net/tracker/?group_id=73133&atid=536845) Highlights of 0.10.3 rc1 include: - Support for Unicode builds of wxWidgets - Support for additional widgets including calendar, toolbar divider, styled text control (wxScintilla), media control - Support for clipboard, drag and drop - Support for 64bit (Linux) targets - Support for wxWidgets 2.6.x (support for wxWidgets 2.4.2 retained if you compile from source) - Support for building with GHC 6.6.x and 6.8.x - Parts of wxHaskell are now built with Cabal - New test cases - Removed support GHC version < 6.4 - Profiling support - Smaller generated binary sizes (using --split-objs) Binary packages are available from the wxHaskell download site at http://sourceforge.net/project/showfiles.php?group_id=73133, for the following platforms: - Debian - Windows - OS X (Intel and PPC platforms) - Source code .tar.gz and .zip - Documentation (cross-platform) About wxHaskell wxHaskell is a Haskell binding to the wxWidgets GUI library. It provides a native look and feel on Windows, OS X and Linux, and a medium level programming interface. The main project page for wxHaskell is at http://wxhaskell.sourceforge.net. The latest source code for wxHaskell can always be obtained from http://darcs.haskell.org/wxhaskell. There are developer (wxhaskell-devel(a)lists.sourceforge.net and user (wxhaskell-users(a)lists.sourceforge.net) mailing lists, and a wiki page at http://haskell.org/haskellwiki/WxHaskell which can provide more information to those interested. wxHaskell was originally created by Daan Leijen. The contributors to this new release include: - Eric Kow - shelarcy - Arie Middelkoop - Mads Lindstroem - Jeremy O'Donoghue - Lennart Augustson -- Jeremy O'Donoghue jeremy.odonoghue(a)gmail.com

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compile error with FFI code
by Galchin Vasili 23 Mar '08

23 Mar '08

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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Using HaskellDb to connect to PostgresSql
by Marc Mertens 23 Mar '08

23 Mar '08

Hello, I'm trying to learn to use HaskellDb. I have managed to finally compile and install it on my linux box (I have ghc 6.8.2). But when I try to create a database description (as described in http://haskelldb.sourceforge.net/getting-started.html) (using DBDirect-dynamic instead of DBDirect) I'm stuck. I have tried different names for the driver but I always get the message 'DBDirect-dynamic: user error (Missing field driver)'. Has someone get this working for PostgresSQL and if so, can he/she help me out with some instructions. Also if someone has some pointers to more recent documentation of HaskellDB, I would really be pleased if I could get these. Marc Mertens

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Opening Windows .lnk Files
by Dominic Steinitz 23 Mar '08

23 Mar '08

Does anyone know how to do this? If I open a file on Windows e.g. "asn_application.h.lnk" then I get the link data rather than the data in the linked file. Thanks, Dominic.

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Re: [Haskell-cafe] AMQP framing layer: design help requested.
by Paul Johnson 23 Mar '08

23 Mar '08

Dean Herington wrote: > At 6:41 PM -0700 3/21/08, Adam Langley wrote: >> >> Also >>> getter <- fmap (amqpGetTable !) getWord8 >>> getter >> >> is just >> >> > fmap (amqpGetTable !) getWord8 > > I don't think so. Aren't there two "gettings": the first to get the > "type" byte and the second to get the item? > Yes. I didn't use it because it seemed obfuscated, but in fact the point-free version is > fmap (amqpGetTable !) getWord8 >>= id And I've also got an AmqpWire class similar to Dean's AmqpValue class. But both of these miss the point (which is why I didn't clutter up my simplified explanation with them). I'm looking for an alternative to the honking big AmqpVariant and AmqpArray types. I think I want something along the lines of: > class (Binary v) => VariantClass v where > typeCode :: v -> Word8 > fromVariant :: AmqpVariant -> Maybe v > > newtype AmqpVariant = forall a . (VariantClass a) => Variant a > > newtype AmqpArray = forall a . (VariantClass a) => AmqpArray [a] But I can't see how to write "fromVariant". I'm also unsure what the "amqpGet" and "amqpPut" methods might look like. Or maybe these two types are actually the best design. They do let you pattern-match on the contents. Extracting the contents of a variant from the design above would be something like: > processVariant (Variant v) = > case typeCode v of > 0x01 -> processWord8 $ fromJust $ fromVariant v > 0x02 -> ... and so on. Compare this with: > processVariant (VariantWord8 v) = processWord8 v > processVariant (VariantWord16 v) = processWord16 v > ... and so on. What do you think? Paul.

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[GSoC] Plugins for GHC
by Max Bolingbroke 22 Mar '08

22 Mar '08

Hi, I'm interested in working on a plugins system for the Glasgow Haskell Compiler. The idea here is to allow third parties to add more capabilities to the compiler in a way that doesn't involve messing around with modifying GHC itself. Potential use cases are: * Allow Haskell code to be selectively run on GPUs * Idiosyncratic application-specific optimization passes * And whatever clever ideas the community comes up with! This project has previously been proposed as an Microsoft Research internship project (http://hackage.haskell.org/trac/ghc/wiki/Internships).I have a preliminary wiki page with details at http://hackage.haskell.org/trac/ghc/wiki/Plugins I'd particularly like to get feedback from the list on: * Other design goals people would like to see * Improvements to the proposed (very half-baked!) user interface * Other hooks people think they might like to have into the compiler * Interested mentors. Simon PJ has indicated he would be interested in mentoring this, but there might be other projects that need him more? This project would satisfy a dependency that Manuel mentioned in http://groups.google.com/group/fa.haskell/msg/3ca260fb075a901e to implement an "array DSL" for GPUs in Haskell. It would mean GHC taking a dependency on hs-plugins (http://www.cse.unsw.edu.au/~dons/hs-plugins/) as that's our current strategy to allow GHC to dynamically link with the plugins. A little about my experience with GHC: this year I worked on implementing comprehensive comprehensions (http://research.microsoft.com/~simonpj/papers/list-comp/index.htm) as my final year undergraduate project, supervised by Simon PJ. I have subsequently implemented a few other patches, adding some core-level optimizations / bug fixes (some of which are pending review). Thanks in advance for your comments! Max

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AMQP framing layer: design help requested.
by Paul Johnson 22 Mar '08

22 Mar '08

I'm trying to implement the AMQP framing layer. This is the bottom layer of the protocol that packs and unpacks data in lazy ByteStrings. I'm using the Binary package with its "Get" and "Put" monads to do the unpacking, and that works well. But I've run into a problem and I can't find an elegant solution. I'm wondering if someone can help. The following contains a simplified version of my current design. I've mapped the AMQP basic types to Haskell types. Sometimes this is a basic mapping, sometimes to something more complex. Strings have several AMQP representations, so I've created corresponding newtype declarations and instances. Some AMQP types can contain tagged types. For instance a "map" contains (key, tag, value) triples, where the tag is a single byte indicating the type of the value. I've defined the following type: data AmqpVariant = AmqpVarWord8 Word8 | AmqpVarWord16 Word16 | AmqpVarStr8 Str8 -- And so on for about 20 more types instance Binary AmqpVariant where put (AmqpVarWord8 v)= putWord8 0x01 >> putWord8 v put (AmqpVarWord16 v) = putWord8 0x02 >> putWord16be v put (AmqpVarStr8 v) = putWord8 0x03 >> put v -- And so on for about 20 more types get = do getter <- fmap (amqpGetTable !) getWord8 getter amqpGetTable :: Array Word8 (Get AmqpVariant) amqpGetTable = array (0,255) [ (0x01, fmap AmqpVarWord8 getWord8), (0x02, fmap AmqpVarWord16 getWord16be), (0x03, fmap AmqpVarStr8 get), -- And so on for about 20 more types ] This is a Brute Force and Ignorance solution, but it will probably work. But now I've come up against another type, which AMQP calls an "array". This has a single type tag at the start, followed by N items of that type. I'm wondering how to do this. One option would be to declare data AmqpArray = AmqpArrWord8 [Word8] | AmqpArrWord16 [Word16] -- and so on This effectively duplicates the AmqpVariant solution. It would work, but it feels horrible. I get the feeling that there ought to be a better way, possibly using classes or existential types. Any suggestions? Thanks, Paul.

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ANN: Takusen 0.8.1
by Alistair Bayley 22 Mar '08

22 Mar '08

http://hackage.haskell.org/cgi-bin/hackage-scripts/package/Takusen-0.8.1 This is a fairly minor bugfix release. although there are some API changes. Mainly, we've re-exported a lot of the types from Database.InternalEnumerator in Database.Enumerator. This will hopefully address a common complaint that it's impossible to always write type sigs for functions doing database stuff. We've also tidied up some of the exports; there was stuff exposed that shouldn't have been. There's an outstanding bug involving datetime parameter marshaling in ODBC, which I have not had time to resolve (parameters after the datetime parameter get mangled somehow). I will be on holiday for a few weeks, so this will not be addressed by me for some time. Caveat user. The test suite highlights it, should someone else feel sufficiently motivated; I've been testing against PostgreSQL. Alistair

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YOrd and ysort
by Krzysztof Skrzętnicki 21 Mar '08

21 Mar '08

The ysort function I've written for YOrd class was incredibly inefficient. Here is better one: ysort :: (YOrd a) => [a] -> [a] ysort = head . mergeAll . wrap wrap :: [a] -> [[a]] wrap xs = map (:[]) xs mergeAll :: (YOrd a) => [[a]] -> [[a]] mergeAll [] = [] mergeAll [x] = [x] mergeAll (a:b:rest) = mergeAll ((merge a b) : (mergeAll rest)) merge :: (YOrd a) => [a] -> [a] -> [a] merge [] [] = [] merge xs [] = xs merge [] xs = xs merge xs ys = let ((sm:smS),gts) = xs `ycmp` ys in smS `seq` gts `seq` sm : (merge smS gts)

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ANN: The Disciplined Disciple Compiler - alpha 1
by Ben Lippmeier 21 Mar '08

21 Mar '08

Hi All, I'm pleased to announce the initial alpha release of the Disciplined Disciple Compiler (DDC). Disciple is an explicitly lazy dialect of Haskell which includes: - first class destructive update of arbitrary data. - computational effects without the need for state monads. - type directed field projections. All this and more through the magic of effect typing. More information (and download!) available from: http://www.haskell.org/haskellwiki/DDC or http://code.google.com/p/disciple DDC: more than lambdas. Onward! Ben.

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Help me understand this pattern
by Justin Bailey 21 Mar '08

21 Mar '08

All, Yesterday I wrote some code I thought was very clever (even though I stole most of it) and I wonder what the pattern is. I have seen it in at least one other place, and I suspect it's well known to long-time Haskellers. The problem I wanted to solved was creating a combinator that could be defined to take any number of differently typed arguments and convert them to a more fundamental representation. To be concrete, I have a data type Expr: data Expr a = ... where a is a phantom type. I wanted users of the library (haskelldb) to be able to define functions[1] such as: -- Trim whitespace from the expression given. trim :: Expr a -> Expr String -- Pad the expression given to the correct length with the given string, if one is given. rpad :: Expr a -> Expr Int -> Maybe String -> Expr String I wanted them to be able to do this using one combinator from the library and I did not want to expose the guts of the library so they could define the functions directly in the untyped, primitive representation. That is, I did NOT want to define my combinator as: func :: String -> [PrimExpr] -> Expr a Using ideas already in haskelldb, I came up with this: data ExprNil = ExprNil data ExprCons h tl = ExprCons h tl func :: String -> (ExprNil -> ExprCons (Expr e) tl) -> Expr o arg :: (Expr a) -> (tl -> ExprCons (Expr a) tl) Using these two functions, I could define trim and rpad via composition and I did not have to expose a primitive representation or use an existentially quantified data type[2]: trim str = func "trim" $ arg str rpad str len (Just pad_str) = func "rpad" $ arg str . arg len . arg (toExpr pad_str) where toExpr :: String -> Expr String rpad str len Nothing = func "rpad" $ arg str . arg len Notice how in the second case of 'rpad', the function defined only has two arguments since no padding character was provided. With that background, I'm hoping others can tell me what this generalizes too. It's pretty clear I'm working with type-level lists, but I'm sure there is more. Is this encoding existentials? Is the (ExprCons hd tl) type really a continuation? I'm positive this is a well-known pattern and I'd really like to learn more about it (history, other uses, libraries that depend on it, etc.). Thanks in advance! Justin [1] These functions are used to generate SQL code and do not actually do the operations given. For example, trim causes the "trim" function to be applied to the column given when the SQL is actually generated. [2] Just a note that these functions are NOT in haskelldb right now - this is my own experimentation.

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ANN: Network.MiniHTTP 0.2
by Adam Langley 21 Mar '08

21 Mar '08

Since many people are interested in a ByteString HTTP library, here's network-minihttp 0.2 [1]. * Has code for both clients and servers * HTTP and HTTPS support (the latter is a little shaky, but mostly works) * DNS lookups (in the client code) don't block the whole process The Haddock documentation is at: http://darcs.imperialviolet.org/minihttp-docs/index.html Half the interfaces are ill-thought out, and the rest are just as bad. However, if you send me your ideas for how it should look I'll certainly consider them. The major omission at the moment are defensive timeouts for talking to the network. However, these shouldn't be too hard. [1] http://hackage.haskell.org/cgi-bin/hackage-scripts/package/network-minihttp… -- Adam Langley agl(a)imperialviolet.org http://www.imperialviolet.org

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Termination of substitution
by Neil Mitchell 21 Mar '08

21 Mar '08

Hi I'm trying to show that a system of rules for manipulating Haskell expressions is terminating. The rules can be applied in any order, to any subexpression - and there is a problem if there is any possible infinite sequence. The rule that is giving me particular problems is: (\v -> x) y => x[v/y] (I realise this rule may duplicate the computation of y, but that is not relevant for this purpose.) In particular, given the expression (\x -> x x) (\x -> x x) we can keep applying this rule infinitely. However, I don't believe this expression is type safe in Haskell. Are any such expressions that would cause this to non-terminate not type safe? What are the necessary conditions for this to be safe? Does GHC perform lambda reduction, probably by introducing a let binding? Does the combination of reducing lambdas and creating let bindings give similar problems, particularly if bindings are inlined? I'm wondering if this rule is genuinely unsafe in Haskell. If it is, why isn't it an issue in real simplifiers. If it isn't, what makes it safe. Thanks for any help, Neil

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Three Cabal/Hackage GSoC project proposals
by Duncan Coutts 21 Mar '08

21 Mar '08

All, I've proposed three GSoC proposals on different aspects of Cabal and Hackage: * Cabal 'make-like' dependency framework http://hackage.haskell.org/trac/summer-of-code/ticket/1552 * cabal-install package dependency resolution http://hackage.haskell.org/trac/summer-of-code/ticket/1553 * Hackage build reporting http://hackage.haskell.org/trac/summer-of-code/ticket/1554 This is in addition to the project that Neil proposed: * Hoogle 4 (using it as the main interface to the hackage website) http://hackage.haskell.org/trac/summer-of-code/ticket/1429 I'd like to invite interested people to help improve the proposals and to invite students to look at these and consider them. I'd also like other people interested in mentoring any of these projects to sign up. In particular, students who are interested should email the cabal-devel and/or libraries list (though they'd have to subscribe I think) to express their interest and work with us on improving the detail of their proposal. Duncan

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How to implement Read instance for user defined type
by Verma Anurag-VNF673 20 Mar '08

20 Mar '08

Hi, I am trying to understand how to define instances for Read class for a user defined type. This is the sample code I wrote, but I am not able to get it correctly. So, let me know what's wrong with this program: module Mark where data Mark = Mark Int deriving (Show) instance Read Mark where readsPrec _ str = [(Mark x, t') | ("mark",t) <- reads str, (x,t') <- reads t When I run it in ghci: *Mark> reads "mark 5" :: [ (Mark,String)] ( I am intentionally using mark in lowercase) [] Why is the output coming out as [] as against expected output [(Mark 5, "")] ? -Anurag

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Scoping in template haskell
by Lars Oppermann 20 Mar '08

20 Mar '08

Hi all, I am trying to get familiar with template haskell and I'm wondering whether the following is possible and if so, how it would be done properly... I have two templates, and I'd like to use one from the other. For instance the first one might be gen1 :: Int -> ExpQ gen1 i = [| "<<" ++ (show i) ++ ">>" |] Thus, I gould do > $(gen1 42) and I'd get "<<42>>" now I'd like to use gen2 in another generated expression so that I could so > $(gen2 [42, 66]) and get "<<42>><<66>>" My naive intention was to write gen2 :: [Int] -> ExpQ gen2 is = [| map (\x -> $(gen1 x)) is |] which gives me "Stage error: `x' is bound at stage 2 but used at stage 1 So I guess my actual question would be: how do I pass a variable bound outside a quotation into another splice used in that quotation? Bests, Lars

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Type parameters in type families
by Hugo Pacheco 20 Mar '08

20 Mar '08

Hi, I am trying to understand some differences of parameterizing or not some arguments of type families. I have some code such as *type family G a :: * -> * instance Functor (G Int) where fmap f (Left ()) = Left () fmap f (Right x) = Right (f x) ggg :: Functor (G a) => G a x -> G a x ggg = fmap id* and it works fine. However, I need to parameterize one extra argument (due to type equality): *type family F a x :: * class FunctorF d where fmapF :: (x -> y) -> F d x -> F d y fff :: (FunctorF a) => F a b -> F a b fff = fmapF id* This second scenario fails to compile because the compiler cannot unify types a and b with types d and x from the fmapF declaration. Is there any other option than passing dummy variables to fmapF? * class FunctorF d where fmapF :: d -> x -> (x -> y) -> F d x -> F d y fff :: (FunctorF a) => a -> b -> F a b -> F a b fff a b = fmapF a b id* Thanks, hugo

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Error handling in calculations
by hjgtuyl＠chello.nl 20 Mar '08

20 Mar '08

L.S., When playing with exceptions, I noticed the following strangeness: *Main> 1 / 0 Infinity *Main> 1 `div` 0 *** Exception: divide by zero This is in GHCi 6.8.2; WinHugs Sep 2006 gives: Main> 1 / 0 1.#INF Main> 1 `div` 0 Program error: divide by zero Is this difference between fractional and integral calculation intentional? It makes error handling more difficult. -- Met vriendelijke groet, Henk-Jan van Tuyl -- http://functor.bamikanarie.com http://Van.Tuyl.eu/ --

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libmad and os/x coreaudio wrappers
by Chris Waterson 20 Mar '08

20 Mar '08

Hi there! I've taken my first stab at writing some (admittedly minimal) libraries for Haskell, and would love to get feedback on them: * hmad: a wrapper for the libmad MP3 decoder. http://maubi.net/~waterson/REPO/hmad * CoreAudio: a wrapper for OS/X CoreAudio. http://maubi.net/~waterson/REPO/CoreAudio (You should be able to "darcs get" the above links, if you want.) I wrote the libmad wrapper to generate a "stream" (i.e., a lazy list) of audio samples. CoreAudio allows the input stream to be lazy, as well. So, here's a simple MP3 player: > module Main where > > import Sound.CoreAudio > import Codec.Audio.MP3.Mad > import qualified Data.ByteString.Lazy as B > import System > import System.IO > > main :: IO () > main = do files <- getArgs > mapM_ playFile files > > playFile :: String -> IO () > playFile file = > withBinaryFile file ReadMode $ \ inHandle -> > do xs <- B.hGetContents inHandle > samples <- decode xs > play samples I do have a couple questions... * The CoreAudio library requires its users to be compiled with "-threaded". Is there a way to specify that in the Cabal file? * I wanted to be able to generate a variety of streams from libmad. Besides stereo linear PCM data, it also seemed like it might be worth-while to produce a stream of MP3 frame headers, the unsynthesized frequency domain data, and so on. I tried to accomplish this with a the DecoderSink class, but I'm not sure I succeeded. Any thoughts here would be appreciated! I hope someone else finds these useful. The FFI was a joy to use once I figured it out... :) chris

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ghc 6.8.2 and timer_create error
by Anatoly Yakovenko 20 Mar '08

20 Mar '08

I have a gentoo box with ghc 6.8.2, and the binaries that ghc builds on that box do not work on redhat EL 4 or ubuntu 7.10. When I try to run the binary, i get an error: Main: timer_create: Invalid argument so is there any way to get ghc to build a binary that doesn't use timer_create? or is that something that is configured when ghc itself is built, if so, how do I turn that feature off? Thanks

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Satnam Singh of Microsoft Research
by Galchin Vasili 19 Mar '08

19 Mar '08

Hello, Recently Satnam Singh of Microsoft Research gave a talk at Google about concurrency, Haskell STM, etc. Was there a transcript of this talk? Thanks, Vasili

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Dynamic typing makes you more productive?
by Justin Bailey 19 Mar '08

19 Mar '08

>From a recent interview[1] with the guy leading Ruby development on .NET at Microsoft: "You spend less time writing software than you spend maintaining software. Optimizing for writing software versus maintaining software is probably the wrong thing to do. Static typing makes it harder to maintain software because it's harder to change it." Two years ago I would have agreed with that statement. Now - no way. Make the compiler work for you. I've done a lot of Ruby development and I would never use it for a project of more than 3 or 4 people. It's an awesome language but I don't think it would scale to programming "in the large." Any object can be modified at any time. Determining where a particular method comes from can be an exercise in Sherlockian deduction. Give an organization of 100 developers that much freedom and I can only imagine chaos would result. Justin [1] http://www.regdeveloper.co.uk/2008/03/17/ironruby_work_schedule/

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Re: [Haskell-cafe] build recent(ish) ghc on macos 10.3.9 powerpc?
by Uwe Hollerbach 19 Mar '08

19 Mar '08

Hello, haskellers, a few days ago I had asked about building recent ghc on macos 10.3.9. I have made a bit of progress along those lines, here's a small update on what worked and what didn't. Instead of trying to proceed with the porting procedure, I went back to an install: I ended up going to 6.2.2. After a bit of fiddling, I found that another bit of messing-about I had done to the machine was messing me up: I had upgraded gcc from 3.3.x to 3.4.6 by just downloading the sources and building. That produced a working gcc 3.4.6, but apparently the apple-aware ghc 6.2.2 and the apple-unaware gcc 3.4.6 didn't play nicely together. Once I disabled gcc 3.4.6, stuff worked again. Then I tried building ghc 6.4.2 using the installed 6.2.2. This took a lot of time, the compiler was able (as far as I could tell) to go through all of its own bootstrapping, but after I happily installed what I thought was a working ghc 6.4.2, the installed version was unable to do anything: it segfaulted while trying to do "ghc --version". Oops... Out of somewhat morbid curiosity, I tried running that inside the debugger, but gave up after several hours of elapsed time. Back to the drawing board. So I tried jumping directly from 6.2.2 to 6.6.1; that is, using 6.2.2 to build 6.6.1. That took another lot of time, but I'm happy to report that it has produced a working executable: it did proceed all the way through its bootstrapping, I verified that the executable in compiler/stage2 was able to run "ghc --version" in-place, and after I installed it, it was still able to do that; and (extra bonus! :-) ) it's also able to compile and link at least one small Haskell program written by me! Cool! I'm going to keep going forward and try to build 6.8.2, and I'll report back if that succeeds, but this is already pretty good progress... regards, Uwe

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Re: [Haskell-cafe] intersection of sets
by Tim Chevalier 19 Mar '08

19 Mar '08

[ccing to list] On 3/18/08, Walt Potter <potter(a)southwestern.edu> wrote: > Tim, > > We're using -O2 to optimize, but not changing the heap size. We're > trying to get the profiling running on our file. > The sets have about 25 million elements each. Each element is a sequence > of length 16. > > We'll want to compute larger examples! > > I do not know how intersection works in Data.Set. I'm worried that we're > having problems with stack overflow due to recursion. We'll try to > remove the lazy stuff. > Run profiling before you "try to remove the lazy stuff". There's no point in trying to optimize code until you know what's actually taking up space. If you have problems with profiling, then ask on the glasgow-haskell-users mailing list. Cheers, Tim -- Tim Chevalier * http://cs.pdx.edu/~tjc * Often in error, never in doubt "Aw, honey, you can keep what's in my pockets, but send me back my pants." --Greg Brown

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intersection of sets
by Walt Potter 19 Mar '08

19 Mar '08

All, We're running out of space when we intersect two large sets. We've imported Data.Set. Any suggestions? Thanks, Walt

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[Haskell-cafe] type families, fun deps, lattices imposed on/by types
by Isto Aho 19 Mar '08

19 Mar '08

Hi all, I was trying to solve a problem and by chance found texts on functional dependencies and then on type families (papers, ghc docs). Please, consider the example 03 of "Understanding functional dependencies via Constraint Handling rules" by Sulzmann, Duck, Peyton-Jones and Stuckey. There we are defining a multiplication over different numeric types: class Mul a b c | a b -> c where (*) :: a -> b -> c instance Mul Int Int Int where ... instance Mul Int Float Float where ... Ok, we could add instance Mul Float Int Float where ... but if we want to make everything work nicely together, Integer, Complex, Rational etc, there will be a lot of instances, especially if we have to give both "Float Int" and "Int Float" instances. And now the question on "lattices of types" (didn't know any better name to refer to the following): Is the following possible? Or can something similar achieved with type families (easily)? type lattice T a b -- Each of the following gives a "<"-relation between types, -- "upper" gives a method, how we get the larger (2nd) from -- the smaller (1st). lattice instance T Int Integer where upper = fromIntegral lattice instance T Int Float where upper = fromIntegral lattice instance T Integer (Ratio Integer) where upper = ... lattice instance T (Ratio Integer) Double where ... lattice instance T Float Double ... lattice instance T Double (Complex Double) ... -- e.g. Now the compiler can check that the top type is -- unique and that there is a path from every type to the top type. -- If the compiler founds this not to be the case, an error is output. -- But otherwise there can be types that are not comparable but -- the common top is quaranteed to exist. class Mul a b c where lattice T (*) :: a -> b -> c (*) x y = (upper x y x) * (upper x y y) -- Now there is no need for the instances like. instance Mul Int Float Float where ... instance Mul Float (Ratio Integer) Double where ... The compiler can generate those instances, because we have given the "upper"-methods. There is always the top available. Function (*) x y = (upper x y x) * (upper x y y) might could be replaced with (*) x y = x * y because of the presence of lattice T and thus, the "upper"-function. If this were possible, the benefits would be: - No need to state "Int Float" and "Float Int" instances on cases where the operands do commute. - No need to write a large number of instances if you have several "types on lattice" (e.g. some more or less well defined relation). - If the relation between types is not clear, we could define another lattice T2 to our Mul2 class for the other uses. Continuing the idea: we could override the default instances generated by the compiler with our own instances when needed. Ok, so, is it possible to write something like that already? (I just wouldn't like write a lot of instances...) If not, would it be possible to extend, say ghc, to work that way or are there too much inconsistencies in the above presentation? (Yes, where is the bottom. Why top and not bottom? Should it be possible to tell, which one to use, if both present? e.g.) Thanks for reading this far! -- br, Isto

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Two Lectureships in Nottingham
by Graham Hutton 19 Mar '08

19 Mar '08

Dear all, We are advertising two new Lectureships (Assistant Professorships) in Computer Science at the University of Nottingham in England. Applications in the area of functional programming are particularly welcome. Further details are available from: http://jobs.nottingham.ac.uk/vacancies.aspx?cat=160#j2797 The closing date for applications is 28th March 2008. Best wishes, Graham Hutton +-----------------------------------------------------------------+ | Dr Graham Hutton Email : gmh(a)cs.nott.ac.uk | | School of Computer Science | | University of Nottingham Web : www.cs.nott.ac.uk/~gmh | | Jubilee Campus, Wollaton Road | | Nottingham NG8 1BB, UK Phone : +44 (0)115 951 4220 | +-----------------------------------------------------------------+ This message has been checked for viruses but the contents of an attachment may still contain software viruses, which could damage your computer system: you are advised to perform your own checks. Email communications with the University of Nottingham may be monitored as permitted by UK legislation.

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Re: ANN: Parsec 3.0.0
by John MacFarlane 19 Mar '08

19 Mar '08

> * A "compatibility" Text.ParserCombinators.Parsec tree for the old > Parsec. It's not perfect, but it should work with most Parsec 2 > code. A data point: I recompiled pandoc with the new Text.ParserCombinators.Parsec compatibility module, and performance is much worse than with parsec 2.1 (approximately twice as slow on my standard benchmark). That aside, this is a very welcome release! John

2 1

Converting image2PS function
by iliali16 19 Mar '08

19 Mar '08

Hi guys again just want to ask you I have this function ready -- Creates a Postscript file from an Image -- page shows -200 <= x <= 200, 300 <= y <= 300 -- boundaries, plus origin plotted on output image2PS:: Image -> String -> IO() image2PS (Image contents) filename = writeFile filename (header++contents++footer) where header = unlines ["%!PS-Adobe-3.1" ,"%%BoundingBox: 100 125 500 725" ,"%%PageOrder: Ascend" ,"%%Pages: 1" ,"%%BeginDocument" ,"%%Page: 1 1" ,"100 125 moveto 500 125 lineto 500 725 lineto 100 725 lineto closepath gsave stroke grestore clip" ,"275 425 moveto 50 0 rlineto stroke" ,"300 400 moveto 0 50 rlineto stroke" ] footer = unlines ["showpage" ,"%%EndDocument" ] but when I use it for example image2PS spiral iliali it tells me that the iliali is undefined. What do you thing how should I use it. Thanks in advance! PS: I need to know how this works so that I make a similar function that converts logo2PS since I have a Logo spiral and I want to see it in PostScript. Thanks again! -- View this message in context: http://www.nabble.com/Converting-image2PS-function-tp16122095p16122095.html Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

3 3

build recent(ish) ghc on macos 10.3.9 powerpc?
by Uwe Hollerbach 18 Mar '08

18 Mar '08

[Augh! gmail won't send! Apologies if this shows up more than once...] Hi, all, I have an old iMac G3, running OSX 10.3.9, to which I have a sentimental attachment. I'd like to get ghc running on it, but the pre-built binaries I can find are all for more-recent iMacs, so I thought I would try to build it myself. I believe I read somewhere that gcc 3.3.X didn't work quite right for recent ghc -- I'm trying for now to build ghc 6.6.1 -- so I started by upgrading gcc to 3.4.6. That's working. So, with that in place, I went to the "porting ghc to a new arch" page and started going through the steps. I'm using a laptop running linux as the "host" computer, so that's i386-unknown-linux, some Fedora core derivative. It's using gcc 3.4.4. I think I did all the initial stuff right, but in the section in the porting guide where it says to start diving into all the subdirectories and do a bunch of "make boot && make", I start getting errors, and by the time it says to do that in .../libraries, it just croaks. Did I miss something earlier? Is this a completely hopeless endeavor from the get-go? I'm not even sure what intermediate files or error messages I should post. Any hints appreciated!. If you want to tell me, "Uwe, you're a bozo", that's fine, too, and you could well be right, just tell me why... Thanks! Uwe

3 3

Small question about something easy
by iliali16 18 Mar '08

18 Mar '08

Hi guys I am a bit new to haskell but I am doing good till now. I have to write a function that takes 2 inputs and then reutns one composite output. Now my problem is that I have to make composition of that function meaning that I have to access in some way the output of the function before it is really computed. I will show you part of my code which is working prefectly: play :: Logo -> TurtleState -> (Image, TurtleState) play DoNothing (pen, (x,y), angle) = (emptyImage, (pen, (x,y), angle)) play PenDown (pen, (x,y), angle) = (emptyImage,(pen, (x,y), angle)) play PenUp (pen, (x,y), angle) = (emptyImage,(pen, (x,y), angle)) play (Forward n) (pen, (x,y), angle) |pen == True = ((line (x,y) (x+n,y+n)), (True, (x+n,y+n), angle)) |otherwise = (emptyImage,(pen, (x+n,y+n), angle)) play (Turn n) (pen, (x,y), angle) = (emptyImage, (pen, (x,y), (angle+n))) play (DoNothing :>: p2) (pen, (x,y), angle) = play p2 (pen, (x,y), angle) play (p1 :> DoNothing) (pen, (x,y), angle) = play p1 (pen, (x,y), angle) play (PenDown :>: PenUp) (pen, (x,y), angle) = (emptyImage,(pen, (x,y), angle)) play (PenDown :>: (Forward n)) (pen, (x,y), angle) = play (Forward n) (True, (x,y), angle) play (PenDown :>: PenDown) (pen, (x,y), angle) = (emptyImage,(pen, (x,y), angle)) play (PenDown :>: (Turn n)) (pen, (x,y), angle) = play (Turn n) (pen, (x,y), angle) play (PenUp :>: PenUp) (pen, (x,y), angle) = (emptyImage,(pen, (x,y), angle)) play (PenUp :>: (Forward n)) (pen, (x,y), angle) = play (Forward n) (False, (x,y), angle) play (PenUp :>: (Turn n)) (pen, (x,y), angle) = play (Turn n) (pen, (x,y), angle) play (PenUp :>: PenDown) (pen, (x,y), angle) = (emptyImage,(pen, (x,y), angle)) Now the problem comes here: play (p1 :>: p2) state |play p1 state == (i1,state1) && play p2 state1 == (i2,state2) = (i1+++i2,state2) I know that if I manage to do that function the one above with this sign :>: do not need to be impelmented since this one will cater for all the cases. Can you please help me? Thanks in advance! -- View this message in context: http://www.nabble.com/Small-question-about-something-easy-tp16119618p161196… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

3 3

Need Cabal/library building help for windows
by Ryan Ingram 18 Mar '08

18 Mar '08

For reference, I'm using GHC6.8.1 on WinXP. I'm trying to package up some code into a library. But cabal fails to configure my project: >runhaskell setup.hs configure Configuring Prompt-1.0... setup.hs: ld is required but it could not be found. Prompt.cabal contains: name: Prompt version: 1.0 cabal-version: >= 1.2 build-type: Simple library exposed-modules: Control.Monad.Prompt Does GHC require additional tools in order to build libraries on Win32? If so, I hope this can be remedied; it builds executables without additional tools after all. What do I need to do next? -- ryan

4 5

haskell.org is accepted to GSoC
by Malcolm Wallace 18 Mar '08

18 Mar '08

Google announced last night that Haskell.org has been accepted as a mentoring organisation for this year's Summer of Code (amongst 175 Open Source organisations). http://code.google.com/soc/2008/ The game is on! Student applications open at 1900 UTC on Mon 24th March, and close at 2400 UTC on Mon 31st March. In the meantime, my _strong_ suggestion is for students to search for ideas amongst the many lists available on the web: http://hackage.haskell.org/trac/summer-of-code/ http://haskell.org/haskellwiki/Hac_2007_II/Projects http://haskell.org/haskellwiki/Hac4/Projects and when crafting your proposal: * post it to an appropriate mailing list for comments e.g. haskell-cafe, libraries, wxHaskell, cabal-devel (prefix your subject line with [GSoC] to make it easy to find) * use feedback from the list to improve your proposal. Potential mentors will rank your final submitted proposal based on your ability and commitment to engage with the wider Haskell community, in addition to technical merit. Proposals that will benefit the entire Haskell world will generally be ranked more highly than niche projects with only a few potential users. So things like Cabal-hacking, darcs-hacking, compiler-hacking, are good, as are new libraries to do tasks that are in high demand. (This is not an exhaustive list.) Regards, Malcolm

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ANN: haskell-src-exts 0.3.2
by Niklas Broberg 18 Mar '08

18 Mar '08

Hi all, I'm pleased to announce a new release for the haskell-src-exts package. haskell-src-exts 0.3.2 =========================== haskell-src-exts is a package for handling and manipulating Haskell source code. It is based on the haskell-src package that is part of the standard libraries, but extends this to support a number of syntactic extensions, e.g. MPTCs, fundeps, GADTs, TH etc. It is intended as a drop-in replacement for the standard haskell-src package, and exports the same functions and data types, plus some more. Apart from the more standard extensions supported by e.g. GHC, haskell-src-exts also provides support for HaRP (Haskell Regular Patterns) and HSX (Haskell Source with XML) syntax. Note that as of 0.3, haskell-src-exts /= HSX. * cabal sdist: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/haskell-src-exts… * darcs repo: darcs get http://code.haskell.org/HSP/haskell-src-exts === Changes from 0.2: === * Added support for - Indexed type families (including associated types/datatypes) - Explicit kind signatures - Standalone deriving * haskell-src-exts is now decoupled from hsx/trhsx and harp. - Modules renamed to Language.Haskell.Exts.* - Module Transform is removed from the package (now in package hsx) * New repository (i.e. darcs pull in an old repo won't work, use darcs get), containing only the haskell-src-exts package (no hsx or harp). * Builds with 6.8.2 (thanks Duncan Coutts) === Complete list of supported extensions === * Multi-parameter type classes (MPTCs) * Functional dependencies * Associated types, indexed type families * Liberal class and instance heads * Implicit parameters * Explicit kind signatures * Pattern guards * Generalized algebraic data types (GADTs) * Template Haskell (TH) * Universal and existential quantification (forall) * Empty data type declarations * Unboxed tuples (# #) * Standalone deriving * Regular patterns * Haskell XML, HSX style === Build Requirements === * happy >= 1.17 - It might work with 1.16 though I haven't tested. In that case change the cabal file. - It would work with older versions as well, though they insert a dependency on Array (haskell98) instead of Data.Array. If that's all you have to work with, update the cabal file with a dependency on haskell98. * Cabal >= 1.2 Patches are more than welcome. :-) Cheers, /Niklas

5 9

Haskell board game, anyone?
by Benjamin L. Russell 18 Mar '08

18 Mar '08

Maybe off topic, but when I saw this, I thought, "Why isn't there a Haskell version of this game?" (Or is there?) C-jump is a neat idea, mapping something fun (downhill skiing) with programming. I look at this game and wonder what it would look like in the wonderful world of higher-order functions. Begin forwarded message from "Geoff Knauth" (with subject "[plt-scheme] Scheme board game, anyone?" on "PLT-list Mailing List" <plt-scheme(a)list.cs.brown.edu>, dated "Mon, 17 Mar 2008 12:38:32 -0400"): > Maybe off topic, but when I saw this, I thought, "Why > isn't there a > Scheme version of this game?" (Or is there?) > > C-jump is a neat idea, mapping something fun (downhill > skiing) with programming. I look at this game and > wonder what it would look like > in the wonderful world of continuations. > > Begin forwarded message from Chase Turner: > > > Subject: Is there a Scheme version of this game? > > > > http://www.c-jump.com Benjamin L. Russell

2 1

problem with type equality constraints
by Ganesh Sittampalam 18 Mar '08

18 Mar '08

Hi, When I try to compile this code with ghc-6.9.20080310: module Test2 where type family Id a type instance Id Int = Int type instance Id (a, b) = (Id a, Id b) class Id a ~ ida => Foo a ida instance Foo Int Int instance (Foo a ida, Foo b idb) => Foo (a, b) (ida, idb) I get these errors: Test2.hs:12:0: Couldn't match expected type `ida' against inferred type `Id a' `ida' is a rigid type variable bound by the instance declaration at Test2.hs:12:16 When checking the super-classes of an instance declaration In the instance declaration for `Foo (a, b) (ida, idb)' Test2.hs:12:0: Couldn't match expected type `idb' against inferred type `Id b' `idb' is a rigid type variable bound by the instance declaration at Test2.hs:12:27 When checking the super-classes of an instance declaration In the instance declaration for `Foo (a, b) (ida, idb)' It seems to me that since Foo a ida and Foo b idb are superclassess, Id a ~ ida and Id b ~ idb should be known and so this should have worked - am I missing something? Cheers, Ganesh

2 4

Network.Browser and getCookies
by Adam Smyczek 18 Mar '08

18 Mar '08

Somehow I cannot get cookies from the Response using Network.Browser module (HTTP 3001.0.4). The cookie header part seams to be empty and getCookies returns empty list as well. Any idea how to debug this or does someone have a working example? Thanks, Adam

2 2

2-level type analysis of introducing naming into data types
by Greg Meredith 18 Mar '08

18 Mar '08

All, The following Haskell code gives a 2-level type analysis of a functorial approach to introducing naming and name management into a given (recursive) data type. The analysis is performed by means of an example (the type of Conway games). The type is naturally motivated because through the application of one functor we arrive at a lambda calculus with an embedded type for Conway games (giving access to field operations for arithmetic on virtually every type of number ever considered) -- while another functor provides a process calculi with a similarly embedded data type. Moreover, the technique extends to a wide variety of algebra(ic data type)s. The recipe is simple. 1. Provide a recursive specification of the algebra. Example. data Game = Game [Game] [Game] 2. Abstract the specification using parametric polymorphism. Example. data PolyGame g = PolyGame [g] [g] 2.a. Identify the 2-Level knot-tying version of the recursive type Example. data RGame = RGame (PolyGame RGame) 3. Insert name management type into the knot-tying step of the 2-Level version Example. data DefRefGame var = DefRefGame (PolyDefRef var (PolyGame (DefRefGame var))) given the basic form data PolyDefRef var val = Def var (PolyDefRef var val) (PolyDefRef var val) | Ref var | Val val is the name management technology We illustrate the idea with two other forms of name management technology: the lambda calculus and the pi-calculus. Then we show that names themselves can be provided as the codes of the terms of the new type. At the root of this work is the proposal that communication begins from the concrete -- a "closed" recursive type -- representing some observation or proposal of how something works in the universe (of discourse). As said proposal develops or is explored, calculations need to rely more and more on naming (i.e. "let x stand for ... in ..."). Thus, this capability is "injected" into the data type -- moving from the concrete to the general. The analysis above provides a concrete, disciplined procedure for achieving this in both a sequential and concurrent computational setting. Best wishes, --greg -- This code summarizes the previous post. It shows that for any -- "closed" recursive type we can devise a closed extension of this -- type embedding the type as a value in the lambda calculus module Grho( Game, PolyGame, PolyDefRef, DefRefGame, QDefRefGame ,Term, GTerm, QGTerm ,Sum, Agent, Process, GProcess, QGProcess ) where -- Conway's original data type data Game = Game [Game] [Game] deriving (Eq,Show) -- Abstracting Conway's data type data PolyGame g = PolyGame [g] [g] deriving (Eq,Show) -- Recovering Conway's data type in terms of the abstraction newtype RGame = RGame (PolyGame RGame) deriving (Eq,Show) -- RGame ~ Game -- Expressions with references and values data PolyDefRef var val = Def var (PolyDefRef var val) (PolyDefRef var val) | Ref var | Val val deriving (Eq,Show) -- Games with references and values data DefRefGame var = DefRefGame (PolyDefRef var (PolyGame (DefRefGame var))) deriving (Eq,Show) -- Games with references and values in which variables are quoted games newtype QDefRefGame = QDefRefGame (DefRefGame QDefRefGame) deriving (Eq,Show) -- Lambda terms with values data Term var val = Var var | Abs [var] (Term var val) | App (Term var val) [Term var val] | Value val deriving (Eq,Show) -- Lambda terms with games as values data GTerm var = Term var (PolyGame (GTerm var)) deriving (Eq,Show) -- Lambda terms with games as values and variables that are quoted lambda terms data QGTerm = GTerm QGTerm -- And the following defn's/eqns take it one step further by providing a notion of process with -- Conway games as embedded values -- Process terms with values -- Sums data Sum var val agent = PValue val | Locate var agent | Sum [Sum var val agent] deriving (Eq,Show) -- One of the recent observations i've had about the process calculi -- is that '0' is Ground, and as such is another place to introduce -- new Ground. Thus, we can remove the production for '0' and replace -- it with the types of values we would like processes to 'produce'; -- hence the PValue arm of the type, Sum, above. Note that this -- design choice means that we can have expressions of the form -- v1 + v2 + ... + vk + x1A1 + ... xnAn -- i am inclined to treat this as structurally equivalent to -- x1A1 + ... xnAn -- but there is another alternative: to allow -- sums of values to represent superpositions -- Agents data Agent var proc = Input [var] proc | Output [proc] deriving (Eq,Show) -- Processes data Process var sum = Case sum | Deref var | Compose [Process var sum] deriving (Eq,Show) -- Processes over games data GProcess var = Process var (Sum var (PolyGame (GProcess var)) (Agent var (GProcess var))) deriving (Eq,Show) -- Processes over games with quoted process for variables newtype QGProcess = QGProcess (GProcess QGProcess) deriving (Eq,Show) -- L.G. Meredith Managing Partner Biosimilarity LLC 806 55th St NE Seattle, WA 98105 +1 206.650.3740 http://biosimilarity.blogspot.com

2 2

floating point operations and representation
by Jacob Schwartz 18 Mar '08

18 Mar '08

I have two questions about using the Double data type and the operations in the Floating typeclass on a computer that uses IEEE floating point numbers. I notice that the Floating class only provides "log" (presumably log base 'e') and "logBase" (which, in the latest source that I see for GHC is defined as "log y / log x"). However, in C, the "math.h" library provides specific "log2" and "log10" functions, for extra precision. A test on IEEE computers (x86 and x86-64), shows that for a range of 64-bit "double" values, the answers in C do differ (in the last bit) if you use "log2(x)" and "log10(x)" versus "log (x) / log(2)" and "log(x) / log(10)". I am under the restriction that I need to write Haskell programs using Double which mimic existing C/C++ programs or generated data sets, and get the same answers. (It's silly, but take it as a given requirement.) If the C programs are using "log2", then I need "log2" in the Haskell, or else I run the risk of not producing the same answers. My first thought is to import "log2" and "log10" through the FFI. I was wondering if anyone on Haskell-Cafe has already done this and/or has a better suggestion about how to get specialized "log2" and "log10" (among the many specialized functions that the "math.h" library provides, for better precision -- for now, I'm just concerned with "log2" and "log10"). My second question is how to get at the IEEE bit representation for a Double. I am already checking "isIEEE n" in my source code (and "floatRadix n == 2"). So I know that I am operating on hardware that implements floating point numbers by the IEEE standard. I would like to get at the 64 bits of a Double. Again, I can convert to a CDouble and use the FFI to wrap a C function which casts the "double" to a 64-bit number and returns it. But I'm wondering if there's not a better way to do this natively in Haskell/GHC (perhaps some crazy use of the Storable typeclass?). Or if someone has already tackled this problem with FFI, that would be interesting to know. Thanks. Jacob

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STM example code
by Galchin Vasili 18 Mar '08

18 Mar '08

Hello, I am playing around with the STM API. I would like to see examples of STM other than the Santa.hs as I am having problems with STM vs IO. Thanks, Vasili

3 2

Re: [Haskell-cafe] QuickCheck
by rodrigo.bonifacio 18 Mar '08

18 Mar '08

Hi all, Is it possible to define a limit for the size of children list bellow? I've tried: children <- resize (10 (listGen featureGenNormal)) But it didn't work. Thanks a lot, Rodrigo. > Sebastian Sylvan: > >> featureGenNormal = do > >> id <- stringGen > >> name <- stringGen > >> featuretype <- arbitrary > >> grouptype <- arbitrary > >> children <- arbitrary > >> properties <- listGen stringGen > >> return (Feature id name featuretype grouptype children properties) > > Ryan Ingram wrote: > > Also, you can shorten this significantly with liftM or ap (from Control.Monad): > > True, but in this case I like being able to see meaningful > names for each parameter of the constructor. > ----------------------------------- Rodrigo Bonifácio de Almeida Universidade Católica de Brasília - Grupo de Engenharia de Software - JavaComBr (www.ucb.br/java)

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Re: Haskell-Cafe Digest, Vol 55, Issue 20
by Maverick 17 Mar '08

17 Mar '08

Thanks Sterl, you're right, this was because I have don´t compiled with the -threaded flag, this works fine now. > This answer may be way off base, but if differences appear between > ghci and compiled versions, I've often found its as simple as > remembering to compile with the -threaded flag. The ghci runtime is > threaded by default, as I understand it, while compiled binaries are > not, and IO operations will block in very different fashions (i.e. in > their own thread, or stalling the entire app) depending on the runtime. > Regards, > sterl. Alvaro ______________________________________________ ¿Con Mascota por primera vez? Sé un mejor Amigo. Entra en Yahoo! Respuestas http://es.answers.yahoo.com/info/welcome

1 0

QuickCheck
by rodrigo.bonifacio 17 Mar '08

17 Mar '08

Hi all, I'm trying to use the quick-check library for checking some properties of a user defined data type. Bellow the target data type: data Feature = Feature Id Name FeatureType GroupType Children Properties | FeatureError where: Id = String Name = String FeatureType = int GroupType = int Children = [Feature] Propertyes = [String] I've written the following quick-check property: prop_AlternativeFeature :: Feature -> Feature -> QuickCheck.Property prop_AlternativeFeature fm fc = length (children fc) == 0 ==> length (checkAlternativeFeature fm fc) > 0 When I try to check such property, the result is: ERROR "./EshopModelChecking.hs":11 - Type error in instance member binding *** Term : arbitrary *** Type : Feature *** Does not match : Gen Feature I think that I need to write some arbitrary or generator functions, but I didn't realize how to do that with the availalble quick-checking documentation. Any help will be welcome. Thanks in advance. Rodrigo.

4 3

Building HaXML 1.13.3 on Windows fails
by Dusan Kolar 17 Mar '08

17 Mar '08

Hello all, I'm trying to build HaXML 1.13.3 on Windows using build.bat - I have modified it even on places referred as "should work" (SRCS and OBJS variables); now it works till the last command: ghc-pkg register pkg.conf The error is: Reading package info from "pkg.conf" ... ghc-pkg.exe: Line 68: The field main-is was already defined on line 62 Taking a look into the pkg.conf says there are several lines main-is. I'm using ghc 6.6 to build the stuff. What shall I do? Thanks and regards, Dusan

2 1

GHC threaded RTS will call the finaliser of a ForeignPtr while references still exist
by Adam Langley 17 Mar '08

17 Mar '08

I believe that I've managed to distill a crash which has been driving me crazy for a few days into a short enough test case (22 lines) that it might be useful. In short: the threaded RTS will call the finialiser of a ForeignPtr while an exception handler still holds a reference to it: % ghc -make fptest.hs cbits.c -threaded [1 of 1] Compiling Main ( fptest.hs, fptest.o ) Linking fptest ... % ./fptest New object getting created: 66f010 Finaliser getting called for 66f010 Use called for 66f010 cbits.c: http://hpaste.org/6420 fptest.hs: http://hpaste.org/6421 I'm hoping that this is useful to someone who knows the RTS. Cheers, -- Adam Langley agl(a)imperialviolet.org http://www.imperialviolet.org

2 3

"GADT" rhymes with "cat"
by Ashley Yakeley 17 Mar '08

17 Mar '08

"GADT" rhymes with "cat". The "d" is silent, like the Danish "godt", or the German "Stadt", or the American trademark "Bundt". -- Ashley Yakeley

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Re: [Haskell-cafe] QuickCheck
by rodrigo.bonifacio 17 Mar '08

17 Mar '08

Dear Sebastian Sylvan, Thanks for your datailed answer. It saved me a lot of time. Best regards, Rodrigo. > On Sun, Mar 16, 2008 at 5:42 PM, rodrigo.bonifacio < > rodrigo.bonifacio(a)uol.com.br> wrote: > > > Hi all, > > > > I'm trying to use the quick-check library for checking some properties of > > a user defined data type. Bellow the target data type: > > > > data Feature = > > Feature Id Name FeatureType GroupType Children Properties | > > FeatureError > > > > where: > > > > Id = String > > Name = String > > FeatureType = int > > GroupType = int > > Children = [Feature] > > Propertyes = [String] > > > > > > I've written the following quick-check property: > > > > prop_AlternativeFeature :: Feature -> Feature -> QuickCheck.Property > > prop_AlternativeFeature fm fc = length (children fc) == 0 ==> length > > (checkAlternativeFeature fm fc) > 0 > > > > When I try to check such property, the result is: > > > > ERROR "./EshopModelChecking.hs":11 - Type error in instance member binding > > *** Term : arbitrary > > *** Type : Feature > > *** Does not match : Gen Feature > > > > I think that I need to write some arbitrary or generator functions, but I > > didn't realize how to do that with the availalble quick-checking > > documentation. > > > > Any help will be welcome. > > > > > You use the available functions to build up a generator for your data type. > > First, let's give the instanc itself. For this I'm just going to use the > frequency function to use "featureGenNormal" five times more often than > "return FeatureError". This means that will get a FeatureError every now and > then, but mostly you'll get featureGenNormal (see below). You can change > these frequences, of course. > > instance Arbitrary Feature where > arbitrary = do > frequency [ (5, featureGenNormal), (1, return FeatureError) ] > > In order to write featureGenNormal, we need to be able to generate random > values of each of the parts of the data type. Often these types will already > have Arbitrary instances, so generating an isntance for your type is quite > often just a matter of calling "arbitrary" for each component, and then > returning a datatype. However, there is no Arbitrary instance for String, > which is a bit annoying, so let's write our own generator for strings. > > First a generator for a single letter: > > letterGen = oneof $ map return $ ['a'..'z'] ++ ['A'..'Z'] > > Then a combinator for generating a list of values given a generator for a > single value: > > listGen :: Gen a -> Gen [a] > listGen g = do > x <- g > xs <- frequency [ (1, return []), (10, listGen g) ] > return (x:xs) > > And then we use this to build our "stringGen" generator. > > stringGen :: Gen String > stringGen = listGen letterGen > > Now, we have all we need to write the featureGenNormal generator: > > featureGenNormal = do > id <- stringGen > name <- stringGen > featuretype <- arbitrary > grouptype <- arbitrary > children <- arbitrary > properties <- listGen stringGen > return (Feature id name featuretype grouptype children properties) > > > Note that we use "arbitrary" to generate the list of children recursively. > > -- > Sebastian Sylvan > +44(0)7857-300802 > UIN: 44640862 > ----------------------------------- Rodrigo Bonifácio de Almeida Universidade Católica de Brasília - Grupo de Engenharia de Software - JavaComBr (www.ucb.br/java)

1 0

ANNOUNCE: omnicodec
by Magnus Therning 16 Mar '08

16 Mar '08

I've just uploaded to hackage a package containing two command line utilities for encoding and decoding data, called omnicodec. It's built on top of dataenc. /M -- Magnus Therning (OpenPGP: 0xAB4DFBA4) magnus＠therning．org Jabber: magnus．therning＠gmail．com http://therning.org/magnus What if I don't want to obey the laws? Do they throw me in jail with the other bad monads? -- Daveman

1 0

Re: [Haskell-cafe] A question about "monad laws"
by askyle 16 Mar '08

16 Mar '08

> My favorite presentation of the monad laws is associativity of Kliesli > composition: > (a1 >=> a2) x = a1 x >>= a2 -- predefined in 6.8 control.monad > -- The laws > return >=> a = a > a >=> return = a > a >=> (b >=> c) = (a >=> b) >=> c If you use this presentation you also need the following law: (a . b) >=> c = (a >=> c) . b that is, compatibility with ordinary function composition. I like to call this "naturality", since it's instrumental in proving return and bind to be natural transformations. The law looks less alien if we use a flipped version of (>=>): (<=<) = flip (>=>) {- Monad Laws in terms of (<=<) -} return <=< f = f <=< return = f -- Identity f <=< (g <=< h) = (f <=< g) <=< h -- Associativity f <=< (g . h) = (f <=< g) . h -- Naturality (which happens to be my favorite presentation of the laws, followed by the derivations that lead to the 'do' notation, which lead to various 'ah' moments from unsuspecting FP-challenged friends) ----- Ariel J. Birnbaum -- View this message in context: http://www.nabble.com/A-question-about-%22monad-laws%22-tp15411587p15975734… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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Implementing fixed-sized vectors (using datatype algebra?)
by Alfonso Acosta 15 Mar '08

15 Mar '08

Hi, The EDSL implementation (system design) I'm working on would really benefit from an implementation of fixed-sized vectors. I thought this would be a generally desired ADT but it turned out I wasn't able to find an implementation. I was thinking about using datatype algebra plus GADTs to implement a type-level parameter indicating the size of the vector. I'm a total noob with regard to GADTs and type-level algebra trickery. So my questions are: 1) Do you think it is feasible? Can you think about a better approach? 2) An implementation of type-level Naturals would really help. What has already been done? Thanks in advance, Fons

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