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Haskell Weekly News: Issue 100 - January 10, 2009
by Brent Yorgey 11 Jan '09

11 Jan '09

--------------------------------------------------------------------------- Haskell Weekly News http://sequence.complete.org/hwn/20090110 Issue 100 - January 10, 2009 --------------------------------------------------------------------------- Welcome to issue 100 of HWN, a newsletter covering developments in the [1]Haskell community. Welcome to the 100th (!) issue of the Haskell Weekly News, suitably published on your friendly neighborhood HWN editor's 1000th birthday (base 3). If you ever have content to be included in the HWN (announcements, blog posts, major life news) or a suggestion on how the HWN could be more useful to you as a window into the goings-on of the Haskell community, please don't hesitate to send it along, using the contact information at the end of each issue. Announcements Haskell BLAS bindings version 0.7. Patrick Perry [2]announced the [3]release of version 0.7 of the [4]blas package, Haskell bindings to the BLAS (Basic Linear Algebra Subprograms) library. According to Patrick, this release is "a major milestone---it is finally the library with all of the features that I want." X Haskell Bindings. Antoine Latter [5]announced a preview release of the [6]X Haskell Bindings. The goal of the library is to provide low-level access to the X11 protocol, in the spirit of the [7]X C Bindings. Data.TCache 0.5.5. Alberto G. Corona [8]announced the 0.5.5 release of the [9]TCache package, a transactional data cache with configurable persistence. This version adds the the capability to safely handle transactions, and incrementally serialize many data types simultaneously in the same piece of code. haskell-src-exts 0.4.8. Niklas Broberg [10]announced a new release (0.4.8) of the [11]haskell-src-exts package. This is a bug-fix release in the wake of the flurry of bug reports due to [12]hlint. bytestring-trie 0.1.2 (bugfix). wren ng thornton [13]announced a bugfix release for [14]bytestring-trie, efficient finite maps from (byte)strings to values. This release fixes a bug in alterBy, and adds an Eq instance. wxHaskell 0.11.1. Jeremy O'Donoghue [15]announced the release of [16]wxHaskell 0.11.1, a Haskell binding for the wxWidgets GUI library. The main highlights include support for XRC resource files, support for wxWidgets 2.8.x and GHC 6.10, and preliminary support for Cabal and Hackage. cabal2doap 0.1. Greg Heartsfield [17]announced the [18]release of [19]Cabal2doap, which generates Description of a Project (DOAP) XML/RDF data representing a Haskell project. This should make it possible for semantic web project aggregation sites to find and index Haskell projects. Jobs Jane Street is hiring functional programmers. Yaron Minsky [20]reminded everyone that [21]Jane Street is still hiring! Jane Street now has over 30 OCaml developers, and is actively looking to hire more in Tokyo, London and New York. PhD, postdoc, and engineering positions at HATS. CFP [22]announced the availability of 10 PhD, postdoc, and engineering positions within the HATS project (Highly Adaptable and Trustworthy Software using Formal Models), a new Integrated Project funded by the European Union, within the programme "Future and Emerging Technologies" (FET). The goal of HATS is a tool-supported framework and formal methodology for the development of long-lived and trustworthy software systems. Hypothetical Haskell job in New York. Tony Hannan [23]asked how many would be interested in applying to a hypothetical Haskell job in New York, assuming his boss can be convinced to use Haskell. Blog noise [24]Haskell news from the [25]blogosphere. * Lennart Augustsson (augustss): [26]LLVM arithmetic. * GHC / OpenSPARC Project: [27]Bootstrapping 6. * Patrick Perry: [28]New Haskell BLAS bindings!. * Clemens Fruhwirth: [29]Liskell standalone. * The GHC Team: [30]Benchmarking recent improvements in parallelism. * Galois, Inc: [31]Tech Talk: OpenTheory: Package Management for Higher Order Logic Theories. * Eric Kow (kowey): [32]fold diagram revisited?. * GHC / OpenSPARC Project: [33]Bootstrapping 5. * Lennart Augustsson (augustss): [34]LLVM. * Sebastian Fischer: [35]Monadic and Queue-Based Tree Search. * Philip Wadler: [36]Well-typed programs can't be blamed. * GHC / OpenSPARC Project: [37]Bootstrapping 4. * Manuel M T Chakravarty: [38]GPU Kernels as Data-Parallel Array Computations in Haskell.. * GHC / OpenSPARC Project: [39]Bootstrapping 3. * Tom Schrijvers: [40]Monadic Constraint Programming. * Conal Elliott: [41]Another angle on functional future values. * >>> Greg Heartsfield: [42]Cabal2doap. * Christopher Lane Hinson: [43]MaybeArrow?. * GHC / OpenSPARC Project: [44]Bootstrapping 2. * Darcs: [45]darcs 2.2.0pre2 ready for testing!. * Alson Kemp: [46]Sad about Import Cycles. * GHC / OpenSPARC Project: [47]Bootstrapping. * gamr7: [48]Accessing Recursive Haskell Data Structures from C/Python. Quotes of the Week * jml: A wise man once said, "the program isn't debugged until the last user is dead". * pumpkin: OMG I <3 RECORD SYNTAX * Olathe: <Olathe> > floor (1.0/0.0) <lambdabot> 179769... <Olathe> But you can see that Haskell can calculate the maximum Integer. * lilac: <drdozer> gha! I'm drowning in the haskell number hierarchy again <lilac> drdozer: magic 8-ball says 'add calls to fromIntegral' * monopoly: do not exit IO. go directly to the REPL, do not pass any parameters or continuations. * EvilTerran: [on category theory] the same place of nightmares that spawned zygohistomorphic prepromorphisms :P About the Haskell Weekly News New editions are posted to [49]the Haskell mailing list as well as to [50]the Haskell Sequence and [51]Planet Haskell. [52]RSS is also available, and headlines appear on [53]haskell.org. To help create new editions of this newsletter, please see the information on [54]how to contribute. Send stories to byorgey at cis dot upenn dot edu. The darcs repository is available at darcs get [55]http://code.haskell.org/~byorgey/code/hwn/ . References 1. http://haskell.org/ 2. http://article.gmane.org/gmane.comp.lang.haskell.cafe/50195 3. http://quantile95.com/2009/01/09/new-haskell-blas-bindings/ 4. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/blas 5. http://article.gmane.org/gmane.comp.lang.haskell.cafe/50170 6. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/xhb 7. http://xcb.freedesktop.org/ 8. http://article.gmane.org/gmane.comp.lang.haskell.cafe/50169 9. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/TCache 10. http://article.gmane.org/gmane.comp.lang.haskell.cafe/50119 11. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/haskell-src-exts 12. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/hlint 13. http://article.gmane.org/gmane.comp.lang.haskell.cafe/49956 14. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/bytestring-trie 15. http://article.gmane.org/gmane.comp.lang.haskell.cafe/49898 16. http://wxhaskell.sourceforge.net/ 17. http://article.gmane.org/gmane.comp.lang.haskell.general/16718 18. http://blog.gregheartsfield.com/2009/01/04/cabal2doap/ 19. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/cabal2doap 20. http://article.gmane.org/gmane.comp.lang.haskell.general/16741 21. http://janestreet.com/ 22. http://article.gmane.org/gmane.comp.lang.haskell.general/16736 23. http://article.gmane.org/gmane.comp.lang.haskell.cafe/50015 24. http://planet.haskell.org/ 25. http://haskell.org/haskellwiki/Blog_articles 26. http://augustss.blogspot.com/2009/01/llvm-arithmetic-so-we-want-to-compute-… 27. http://ghcsparc.blogspot.com/2009/01/bootstrapping-6.html 28. http://quantile95.com/2009/01/09/new-haskell-blas-bindings/ 29. http://blog.clemens.endorphin.org/2009/01/liskell-standalone.html 30. http://ghcmutterings.wordpress.com/2009/01/09/46/ 31. http://www.galois.com/blog/2009/01/08/tech-talk-opentheory-package-manageme… 32. http://koweycode.blogspot.com/2009/01/fold-diagram-revisited.html 33. http://ghcsparc.blogspot.com/2009/01/bootstrapping-5.html 34. http://augustss.blogspot.com/2009/01/llvm-llvm-low-level-virtual-machine-is… 35. http://www-ps.informatik.uni-kiel.de/~sebf/projects/monad-vs-queue.html 36. http://wadler.blogspot.com/2009/01/well-typed-programs-cant-be-blamed.html 37. http://ghcsparc.blogspot.com/2009/01/bootstrapping-4.html 38. http://justtesting.org/post/68696876 39. http://ghcsparc.blogspot.com/2009/01/bootstrapping-3.html 40. http://tomschrijvers.blogspot.com/2009/01/monadic-constraint-programming.ht… 41. http://conal.net/blog/posts/another-angle-on-functional-future-values/ 42. http://blog.gregheartsfield.com/2009/01/04/cabal2doap/ 43. http://blog.downstairspeople.org/2009/01/04/maybearrow/ 44. http://ghcsparc.blogspot.com/2009/01/test-run-of-patched-ghc-6.html 45. http://blog.darcs.net/2009/01/darcs-220pre2-ready-for-testing.html 46. http://www.alsonkemp.com/haskell/sad-about-import-cycles/ 47. http://ghcsparc.blogspot.com/2009/01/building.html 48. http://gamr7.com/blog/?p=66 49. http://www.haskell.org/mailman/listinfo/haskell 50. http://sequence.complete.org/ 51. http://planet.haskell.org/ 52. http://sequence.complete.org/node/feed 53. http://haskell.org/ 54. http://haskell.org/haskellwiki/HWN 55. http://code.haskell.org/~byorgey/code/hwn/

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Looking for Haskellers on Windows
by Günther Schmidt 11 Jan '09

11 Jan '09

Hi all, I'm looking for Haskell programmer that use Windows as their OS-Plattform. I developing an application in Haskell on Windows and run into problems that seem to have a lower priority with the greater Haskell community as most of them are using Linux where the problems do not occur or are already solved. So it'd be nice to get in touch with other Haskellers that face the same problems. Günther

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Re: [Haskell-cafe] Haskell not ready for Foo [was: Re: Hypothetical Haskell job in New York]
by John A. De Goes 11 Jan '09

11 Jan '09

If you're looking for a project to take on, I would suggest starting with the following: A high-level, type-safe AMQP client written in 100% Haskell, which provides a clean way of handling hundreds of unique message types. Then it would be possible to hook it up to any AMQP broker (most likely RabbitMQ). There are logical steps beyond the above (an embedded Haskell broker), but the above project is far from trivial. And the main undertaking, I think, consists not in coding to the spec (for which there is some help; a JSON representation of the AMQP specification can be processed and used to emit language-specific code), but in finding a design that works well in Haskell. Regards, John On Jan 8, 2009, at 1:16 PM, Creighton Hogg wrote: > On Thu, Jan 8, 2009 at 2:02 PM, John A. De Goes <john(a)n-brain.net> > wrote: >> >> On Jan 8, 2009, at 12:56 PM, Tim Newsham wrote: >>> >>> You replied to someone discussing using Haskell at a CDN to >>> implement >>> things like web servers by saying that Haskell wasn't suitable for >>> the task. >> >> >> That is incorrect. I replied to Achim's message asking for >> elaboration on >> Haskell's unsuitability. It was a convenient point to discuss >> Haskell's >> networking deficiencies. > > Part of the problem I'm having with this discussion, though, is that > it's still not clear to me what critical features are lacking. I know > you've said Haskell doesn't have anything quite like the Erlang OTP > library, but that doesn't really help me much. I've only looked a > little at OTP & I don't think I understand what you get from its > behaviors that you don't get from Control.Concurrent + mtl, i.e. > various forms of state & error handling independent of the concurrency > underneath. > > Can you elucidate a bit more? I don't want the conversation to > degenerate.

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Re: [Haskell-cafe] teaching functional programming at work
by Warren Harris 11 Jan '09

11 Jan '09

Thank you all for your responses on this. These ideas/materials are very helpful. In particular, the John Harrison book looks excellent (http://www.cl.cam.ac.uk/teaching/Lectures/funprog-jrh-1996/index.html ) -- a comprehensive and understandable introduction to all the concepts I had in mind. (I'm not sure what my audience's attention span will be though.) The WhyFP paper is also very good, although doesn't go into monads or types, it does a good job of illustrating modularity/composability. I'll send an update if this study group gets off the ground, to let you all know how it worked out. Warren

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Shared library creating on Mac OS X
by Phil 11 Jan '09

11 Jan '09

Hi, I¹m hitting a problem trying create shared haskell libs to be linked into a C program on Mac OS X. I¹m using the latest download for Leopard from the GHC page: http://www.haskell.org/ghc/dist/6.10.1/witten/ghc-6.10.1-powerpc-apple-darwi n.tar.bz2 I can get basic executables working fine (with a C main() #including ghc¹s stub header), using something like: ghc -optc-O invnorm.c InverseNormal.o InverseNormal_stub.o -o cTest I started off using the following line to try to create a shared lib: ghc --make -no-hs-main -optl '-shared' -o Inv.so InverseNormal.hs This doesn¹t work on mac os x because Apple¹s gcc annoyingly takes different switches, so I changed it to: ghc --make -no-hs-main -optl '-dynamiclib' -o Inv.dylib InverseNormal.hs Which still fails at the final link giving: Linking Inv.dylib ... Undefined symbols: "_environ", referenced from: _environ$non_lazy_ptr in libHSbase-4.0.0.0.a(PrelIOUtils.o) ld: symbol(s) not found I¹ve seen similar things before, and I believe if you have full control over the source you just slip in a: #define environ (*_NSGetEnviron()) Sure enough, I can find references to environ in, for example HsBase.h Problem (as I see it) is that references to environ are already wrapped up in the static lib libHSbase-4.0.0.0.a, so without recompiling Haskell we can¹t alter the C definition now. However, given that packager must have made this behave when he compiled the distribution there must be a way to make Mac gcc accept _envrion symbols?? Has anyone seen this before / can confirm my analysis / and by any chance have a solution? Many thanks, Phil.

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Re: [Haskell-cafe] Haskell not ready for Foo [was: Re: Hypothetical Haskell job in New York]
by John A. De Goes 11 Jan '09

11 Jan '09

Hi John, Take two examples I gave up getting to work: a Haskell wrapper for a popular GUI library; and a Haskell wrapper for a database. I understand the former has a new team of developers, so perhaps it's time to revisit the library. Then again, writing 5000 line GUIs in an imperative style in a Haskell program just doesn't seem very appealing to me. And that's one of the problem with FFI-based libraries: it exposes the functionality (when it works), but in a strictly imperative way. Selection bias applies to us both. To quote myself, "Which means your statements come from your experience, and my statements come from my experience, and the truth is probably somewhere in between." Regards, John On Jan 9, 2009, at 1:42 PM, John Goerzen wrote: > John A. De Goes wrote: >> >> Hi Austin, >> >> How do you know it's not your experience with FFI code that isn't >> biased? As far as I know, there has been no systematic attempt to >> document whether pure Haskell or FFI-based libraries are better >> designed >> and better maintained. Which means your statements come from your >> experience, and my statements come from my experience, and the >> truth is >> probably somewhere in between. >> >> In my experience, FFI code is often based on bulk translation of C >> header files into the IO monad. It requires an exact version of a >> library to work (usually much older than the current), it does not >> compile out of the box, there's scant documentation, and very little >> high-level design has been imposed on the low-level C interface >> (may as >> well use C!). Exceptions to this rule, there are, but as I said >> before, >> my experience leads me to believe they *are* exceptions to a >> *general* rule. > > Which libraries are you talking about? I haven't ever used *any* like > that, as far as I know. > > Current libraries almost always do build right out of the box with > standard cabal commands. > > Maybe part of what you state was accurate a few years ago, but right > now? > > As far as selection bias is concerned, that would apply equally well > to > both of us, would it not? > > -- John

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Re: [Haskell-cafe] Looking for Haskellers on Windows
by Günther Schmidt 10 Jan '09

10 Jan '09

Hi Kyra, thanks for your reply. The problem I was facing is using an MS-Access backend with HDBC-ODBC. I try to export data to MS-Access and I can't get it to work. Otherwise HDBC works fine in other parts of my app but with MS-Access it blows up. I did of course post this on the haskell-cafe list first, but no response and the HDBC package maintainer himself was unable to help too since he's not using Windows. Günther Am 10.01.2009, 15:48 Uhr, schrieb kyra <kyrab(a)mail.ru>: > GЭnther Schmidt wrote: >> Hi all, >> I'm looking for Haskell programmer that use Windows as their >> OS-Plattform. I developing an application in Haskell on Windows and run >> into problems that seem to have a lower priority with the greater >> Haskell community as most of them are using Linux where the problems do >> not occur or are already solved. >> So it'd be nice to get in touch with other Haskellers that face the >> same problems. >> GЭnther > > I don't know what problems You are talking about, but, since Windows is > my development platform and I'm a haskeller, I hope to be able to help > You. > > What problems are your facing? > > Kyra -- Erstellt mit Operas revolutionärem E-Mail-Modul: http://www.opera.com/mail/

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data declarations should provide fold functions
by Tim Newsham 10 Jan '09

10 Jan '09

I've had to use some fold functions recently and I've come to the conclusion that Haskell should automatically generate fold functions for data definitions. To clarify what I mean, for the data definition: data MyData = This Int Char | That String Int Int there should be a matching function definition: foldMyData f g (This a b) = f a b foldMyData f g (That a b c) = g a b c This definition is as natural as the constructors "This" and "That". Consider the tuple definition and its fold: data (,) a b = (a, b) foldTuple f (x, y) = f x y and the definition of Either and its fold: data Either a b = Left a | Right b foldEither f g (Left x) = f a foldEither f g (Right x) = g x In logic these constructors define the introduction rules ((,), Left and Right) and the folds define the elimination rules (exactly for Either, indirectly for tuples) for conjunction and disjunction. Further, while pattern-matching is very convenient for accessing the constituents of the data type, patterns are not first-class objects in Haskell. Fold functions, on the othe hand, are. They can be passed around and used in higher-order functions to extract constituents in a points-free manner. I know the short-term answer is "use TH" to derive folds if I want them, but I think such an important concept should probably be part of the language. Tim Newsham http://www.thenewsh.com/~newsham/

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ANN: Haskell BLAS bindings version 0.7
by Patrick Perry 10 Jan '09

10 Jan '09

New version! The blas package is a set of bindings to the BLAS (Basic Linear Algebra Subprograms) library. On Hackage: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/blas-0.7 What's new: * Get rid of most functional dependencies in favor of type families. There is one remaining functional dependency that cannot be gotten rid of until GHC implements equality constraints in superclass contexts. * Put the right superclass constraints in ReadMatrix/ReadBanded. * Fix freeze/thaw functions. They used to be awkward to use. * Fix a bug in getting a row view of a banded matrix. * Make sure no NaNs/Infinities/Denormals are used when testing. * Documentation. * Export functions for writing QuickCheck tests. * Remove Perm and Diag. These will be in the LAPACK bindings. * Get rid of "UnsafeIOToM" type class. * Lots of INLINE everywhere. This will improve performance (?) * Switch to autoconf for build system. Requisite blog post: http://quantile95.com/2009/01/09/new-haskell-blas-bindings/ Not much example code. Here's some old stuff: http://github.com/patperry/blas/tree/master/examples/LU.hs Thanks for listening. Any feedback is always welcome. Patrick

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Hypothetical Haskell job in New York
by Tony Hannan 10 Jan '09

10 Jan '09

Hello Haskellers, I'm trying to convince my boss to use Haskell. His main concern is finding people who know Haskell when hiring. He is comfortable with Java because he knows he can always find a Java developer. So if I advertised we were looking for Haskell programmers in New York City, how many would respond? Please reply to this email if you would respond. Email me privately if you like and I will post the results to haskell-cafe later. Thanks, Tony

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RE: [Haskell-cafe] HookedBuildInfo and Cabal
by Duncan Coutts 10 Jan '09

10 Jan '09

On Sat, 2009-01-10 at 01:40 +0100, Brian B wrote: > Hi Duncan, > > This works for me too, many thanks. Great. > The only change I needed was that the real pg_config outputs a newline > where your fake one didn't: Ah yes, good point. Duncan

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Possible GC bug
by Anish Muttreja 10 Jan '09

10 Jan '09

Hi, I have a program that seems to run into occasional garbage collection-related core dumps. The problem typically only occurs after the program has been running for a while and is consuming a large amount of memory (5 - 16GB). The large memory consumption is expected because the program analyzes very large traces from verilog simulation and needs to maintain IntMaps with hundreds of thousands of entries. Is this a bug that I should report?I am afraid that my employer will not allow me to share my source code. I do have a stack trace, below. This was obtained using ghc 6.10.1, RTS -N2 on an RHEL 4 machine. Is there something I can do trace the problem or avoid it? Thanks. PS: This is my first Haskell program and one of the most complicated I ever wrote, in any language. Using Haskell has been (mostly :-)) a joy. (gdb) where #0 0x0000000000612f40 in slowIsHeapAlloced () #1 0x000000000060f868 in evacuate () #2 0x0000000000618d12 in scavenge_block () #3 0x0000000000617c8d in scavenge_loop () #4 0x0000000000610b25 in scavenge_until_all_done () #5 0x0000000000610d02 in gc_thread_entry () #6 0x000000000064859d in start_thread (arg=<value optimized out>) at pthread_create.c:297 #7 0x000000000069e739 in clone () #8 0x0000000000000000 in ?? () -- As an adolescent I aspired to lasting fame, I craved factual certainty, and I thirsted for a meaningful vision of human life - so I became a scientist. This is like becoming an archbishop so you can meet girls. -Matt Cartmill, anthropology professor and author (1943- )

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Re: [Haskell-cafe] Haskell not ready for Foo [was: Re: Hypothetical Haskell job in New York]
by John A. De Goes 10 Jan '09

10 Jan '09

Hi Austin, How do you know it's not your experience with FFI code that isn't biased? As far as I know, there has been no systematic attempt to document whether pure Haskell or FFI-based libraries are better designed and better maintained. Which means your statements come from your experience, and my statements come from my experience, and the truth is probably somewhere in between. In my experience, FFI code is often based on bulk translation of C header files into the IO monad. It requires an exact version of a library to work (usually much older than the current), it does not compile out of the box, there's scant documentation, and very little high-level design has been imposed on the low-level C interface (may as well use C!). Exceptions to this rule, there are, but as I said before, my experience leads me to believe they *are* exceptions to a *general* rule. Regards, John On Jan 8, 2009, at 3:47 PM, Austin Seipp wrote: > Excerpts from John A. De Goes's message of Thu Jan 08 12:14:18 -0600 > 2009: >> But really, what's the point? FFI code is fragile, often uncompilable >> and unsupported, and doesn't observe the idioms of Haskell nor take >> advantage of its powerful language features. > > This is a completely unfair generalization. The FFI is an excellent > way to interoperate with an extraordinary amount of external > libraries, and if you ask me, it's *worth* taking those pieces of C > code and wrapping them up in a nice, safe haskell interface. I will > also > mention that Haskell has *the* simplest FFI I have ever used, which to > me only means it's easier to get it right (the fact that there are > customized *languages* like e.g. cython to make writing python > extensions easier makes me wonder.) > > I suggest you take a look at the haskell llvm bindings - they are > extraordinarily well documented, and the high level interface uses > *many* haskell idioms that make the library safe and easy to use: > > http://hackage.haskell.org/cgi-bin/hackage-scripts/package/ > llvm-0.4.2.0 > > This code most certainly takes advantage of powerful features and > idioms that only Haskell can provide. Please do not take your bad > experiences with a few crappy binding (or not even crappy bindings, > perhaps bindings that just aren't very abstract) and extend them to > the bindings that are excellent with a sweeping statement like that. > > Austin > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe

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ANNOUNCE: haskell-src-exts 0.4.8
by Niklas Broberg 10 Jan '09

10 Jan '09

Fellow Haskelleers, it is my pleasure to announce the new release of the haskell-src-exts package, version 0.4.8: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/haskell-src-exts… darcs get http://code.haskell.org/HSP/haskell-src-exts This is a bug-fix release in the wake of the flurry of bug reports I received due to Neil Mitchell's release of hlint. Not all bugs have been squashed, but all the ones that I could handle with fairly small changes to the library should be. Those include deriving for MPTCs, importing constructor symbols, inline pragmas in instance declarations, scoped type variables, and a few more. There are some minor non-backwards compatible changes to the abstract syntax, but nothing too serious. The most pervasive is that the Match and PatBind constructors have an extra argument of type Maybe Type, representing an optional type signature. Derivings also are no longer just a list of class names, since those classes can now have extra parameters. Four things remain on the bug list: - Support for explicitly kinded arguments to type families. Shouldn't be too hard, but will require changes to the AST that I will leave for the next release. - Support for (un-parenthesised) higher-ranked types as arguments. haskell-src-exts supports e.g. foo :: b -> (forall a . [a]) -> b but not foo :: b -> forall a . [a] -> b. Supporting the latter is simply a parser issue, but a rather tricky one. - Correct handling of hyphened vars (an artifact of HSX/HSP) vs minus operators. This one is nasty. - Support for Unicode symbols for e.g. ->. Fixing that would require me to have a Unicode-compliant editor, which it appears I don't. And I couldn't have someone else submit a patch either, since then I couldn't open the file anymore in my editor. So unless someone can point out a good Unicode-aware editor for Windows, I'm afraid this is a feature that won't be implemented. If you find anything else that haskell-src-exts fails on, please report it. Cheers and Happy Haskelling, /Niklas

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ANNOUNCE: X Haskell Bindings
by Antoine Latter 10 Jan '09

10 Jan '09

Folks, I'd like to announce a preview-release of the X Haskell Bindings. The goal of the library is to provide low-level access to the X11 protocol, in the spirit of the X C Bindings This is a "preview" because I expect that the interface will still need to change - but I do plan on bump the minor-version number when that happens. On Hackage: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/xhb What works: All X protocol requests have corresponding functions. I haven't tested all of them. Requests are sent asynchronously - the caller is not blocked until they need to examine the result of the request Errors and Events are unpacked into Haskell data types It should be safe to use the same connection in multiple threads What needs improvement: * There's no documentation - you pretty much need to be sitting down with the X Protocol reference while using this library. Once we can get documentation into the XML that this library is generated from, we should be able to generate Haddock documentation at the same time * A lot of parameters are specified using integer types, even though a proper Haskell enum-like data type exists. * Many functions take a ValueParam argument, which is not fun to work with in a language like Haskell. I have plans for how to make these more type-safe, but this is something else that requires more information in the XML protocol descriptions. * There's no way to share a connection between XHB and libraries built against the Xlib bindings. * I haven't done any benchmarking, so it's possible that all of this is pretty slow. Keep in mind that this is a library for the X Protocol - it alone is not a replacement for XLib - XLib does a lot more than pushing and pulling on a file-handle. Related releases: Spencer Janssen has released FFI bindings to Xauth: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/Xauth-0.1 I've released a small library for determining the byte-ordering of the system: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/byteorder-1.0.0 Related projects: X C Bindings: http://xcb.freedesktop.org/ -Antoine

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ANNOUNCE: Data.TCache 0.5.5
by Alberto G. Corona 10 Jan '09

10 Jan '09

The main addition of this versión is the capablity to safely handle transact, and serialize to permanent storage many datatypes simultaneously in the same piece of code and incrementally. Just register each new datatype (with registerType :: <Type>). So it is not necessary to glue all types in advance in a single algebraic datatype. I suppose taht "enhanced composablility" applies to this feature. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/TCache In this release: Added a Data.TCache.Dynamic. (SEE dynamicsample.hs) - Can handle, transact, and serialize to disk many datatypes simultaneously and incrementally - Dynamic uses the same interface than TCache and add *DResource(s) calls for handling many datatypes simultaneously - Safe dynamic data handling trough a lighter, indexable and serializable version of Data.Dynamic - Added KEY object for retrieving any object of any type. Data.Tcache is a transactional cache with configurable persistence. It tries to simulate Hibernate for Java or Rails for Ruby. The main difference is that transactions are done in memory trough STM. There are transactional cache implementations for some J2EE servers like JBOSS. TCache uses STM. It can atomically apply a function to a list of cached objects. The resulting objects go back to the cache (withResources). It also can retrieve these objects (getResources). Persistence can be syncronous (syncCache) or asyncronous, wtih configurable time between cache writes and configurable cache clearance strategy. the size of the cache can be configured too . All of this can be done trough clearSyncCacheProc. Even the TVar variables can be accessed directly (getTVar) to acceess all the semantic of atomic blocks while maintaining the persistence of the TVar updates. Persistence can be defined for each object: Each object must have a defined key, a default filename path (if applicable). Persistence is pre-defined in files, but the readResource writeResource and delResource methods can be redefined to persist in databases or whatever. There are Samples in the package that explain the main features.

1 0

advanced class constraints in Haskell 98?
by Henning Thielemann 09 Jan '09

09 Jan '09

GHC accepts a class declaration like class Monad (m Maybe) => C m where ... without having any language extension switched on. But it isn't Haskell 98, is it? Hugs 2005 also accepts this.

4 10

Re: [Haskell-cafe] Haskell not ready for Foo [was: Re: Hypothetical Haskell job in New York]
by John A. De Goes 09 Jan '09

09 Jan '09

> On Thu, Jan 08, 2009 at 10:36:32AM -0700, John A. De Goes wrote: >> >> The number of applications requiring the implementation of a custom >> web >> server is an insignificant fraction of the number of applications >> requiring a messaging system. I don't think anyone would dispute >> Haskell's ability to do low-level, raw networking, of the type that >> few >> people actually need to do. It's the higher level stuff where >> there's a >> huge amount of room for improvement. > > I disagree on both points. > > Haskell has had somewhat of a deficit in the low-level networking > stuff, not even supporting IPv6 in the standard stack until just > recently. (That is, things like AF_INET6 were not present.) > > I think it has pretty much caught up by now though. If you think Haskell has caught up by now, then you DO NOT disagree with my statement that "[no one] would dispute Haskell's ability to do low-level, raw networking." Which makes me wonder what you mean by, "I disagree on both points." > On the other hand, I see nothing in Haskell that would prevent its use > for any of your purposes. There are numerous high-level web > infrastructures already. Perhaps they are more or less suited to your > needs, but that's a library issue, not a language issue. In other words, to quote myself, "it's the higher level stuff where there's a huge amount of room for [library] improvement." Nothing in the Haskell language prevents its use for high-level networking. But the sad fact of the matter is that there is no library support for it. So applications that need such networking today are better off written in Erlang. Regards, John

1 0

issues with posix-realtime package
by Manlio Perillo 09 Jan '09

09 Jan '09

Hi. I'm testing the posix-realtime package, but I have found a problem. Configuration, build and install works well, but if I execute some code from ghci I get: GHCi runtime linker: fatal error: I found a duplicate definition for symbol __hsunix_wifexited whilst processing object file /usr/local/lib/posix-realtime-0.0.0.1/ghc-6.8.2/HSposix-realtime-0.0.0.1.o This could be caused by: * Loading two different object files which export the same symbol * Specifying the same object file twice on the GHCi command line * An incorrect `package.conf' entry, causing some object to be loaded twice. GHCi cannot safely continue in this situation. Exiting now. Sorry. If I comment the definition for __hsunix_wifexited, I get a duplicate definition for symbol pPrPr_disableITimers, and so on, for all the definitions in HsUnix.h and execvpe.h Thanks Manlio Perillo

1 0

Staged evaluation, names?
by wren ng thornton 09 Jan '09

09 Jan '09

Dear Cafe, Every now and then I find myself in the position where I'd like to define some hairy value as a CAF instead of a literal, but I'd like for it to be fully evaluated at compile-time rather than postponed until runtime. It'd be possible to bludgeon the CPP into doing this, but it seems easier to use an autocannon like Template Haskell to swat this fly. This seems like a common thing for folks to want but the TH documentation targets much higher-hanging fruit, which actually made it harder to figure out how to do something this basic (which would have been transparent with MetaML/MetaOCaml-like syntax). I've figured it out and circumvented the gotchas, and I was thinking of releasing the code as a counterpoint to all the very-high level TH tutorials. The question for y'all is what should I call it? I've been calling the template-function qaf (for Compiled Applicative Form ;) and the type class with that function would be the only thing in the package, but I'm not sure where QAF.hs should be in the module hierarchy. Thoughts? -- Live well, ~wren

3 5

Marshalling recursive data structures with the FFI
by Ron de Bruijn 09 Jan '09

09 Jan '09

Hi, A few days ago we published an article (http://gamr7.com/blog/?p=66) on using the FFI to marshal recursive data structures between Haskell and C (or Python if you use ctypes). Best regards, Ron de Bruijn

1 0

GHC libraries documentation and links to source files
by Manlio Perillo 09 Jan '09

09 Jan '09

Hi. I have noted that recent versions of the GHC libraries documentation, no longer have links to the source code. What is the reason? I find it very useful. Thanks Manlio Perillo

3 2

How to give unique name/id to nodes outside any monad ?
by minh thu 09 Jan '09

09 Jan '09

Hi, I'd like to process some kind of graph data structure, say something like data DS = A [DS] | B DS DS | C. but I want to be able to discover any sharing. Thus, in b = B a a where a = A [C], if I want to malloc a similar data structure, I have to handle to the node representing B two times the same pointer (the one returned after allocating A [C]). To discover sharing, I thought it would be necessary to give unique name to node and then compare them while traversing the graph. I could give the name by hand but it would be cumbersome. But at least it would not require any monad for the bookkeeping of ungiven names. Is it possible to give those names automatically but outside any monad ? Thanks, Thu

6 15

Blitting one IOUArray into another
by Bueno, Denis 09 Jan '09

09 Jan '09

Hi all, I'm seeing a lot of unexpected memory allocation with some simple code that copies the contents of one vector (IOUArray Int Int64) into another of the same type and dimensions. In particular, profiling reveals that `copyInto' is allocating oodles and oodles of memory. My small test case creates two 50000-element arrays and performs 10000 copies from one into the other. Since the elements are Int64s and the arrays are unboxed, each array should be 50000 elements * 8 bytes per element = 400,000 bytes and so the arrays should only take 800,000 bytes total. I understand there's some overhead for thunks and whatnot, but the profiler reported allocation is around 40 billion bytes. And almost all of that allocation is in my copying function, not in main (main allocates the arrays). I've attached two versions of the code, just for comparison. The only difference is the way the copying is done. One calls `getBounds' to figure out the bounds, and one is given the bounds for copying. They're both about the same speed and allocation (originally I was using IOUArray Int64 Int64, and there was a much greater allocation difference between the two versions; but that went away. Oh well). So, does anyone know why copying takes so much allocation? I expect there is _some_ way to just move memory from one array to another, like a memcpy -- how can I do that? Denis

6 8

Re: State Monad - using the updated state
by John Lato 09 Jan '09

09 Jan '09

It sounds like you've got the monadic solution figured out, so that's good. Even though I would recommend using State (or StateT if necessary), I wanted to be sure this question was answered. > ranq1List :: (Word64 -> a ) -> Word64 -> [a] > ranq1List converter state = converter newState : ranq1List converter > newState > where > newState = ranq1Increment state > > > One more question on this - the other concern I had with the recursive list > approach was that although lazy evaluation prevents me generating numbers > before I 'ask' for them, I figured that if I was going to be asking for say > 10 million over the course of one simulation, that although I request them > one by one, over hours or even days, at the end of the simulation I will > still have a list of 10 million word64s - each of which I could throw away > within minutes of asking for it. This seemed like huge memory bloat, and > thus probably I was taking the wrong approach. This "memory bloat" is a space leak. In general they are to be avoided. GHC's garbage collection is quite efficient, so if you structure your program properly the space will be reclaimed after the values are used. Values will only be collected if they cannot be referenced from live code (just like Java, Python, C#, or other languages with GC). If you are able to structure your code like this: doSomething :: [Word64] -> IO () doSomething (rand:rands) = do someIO rand otherIO rand ... if endFlag then return () else doSomething rands main = do let rands = ranq1List id 1 doSomething rands you won't have a space leak. The head of the list is used within doSomething, while only the tail is passed in the recursion. At the time of the recursive call to doSomething, the head 'rand' value is no longer accessible by any running code, so it can be GC'd. Contrast that with this version (somewhat contrived): doSomething2 :: [Word64] -> Int -> IO () -- the Int is an index into the list. doSomething2 rands i = do let rand = rands !! i someIO rand otherIO rand ... if endFlag then return () else doSomething2 rands (i + 1) This version is Really Bad. Firstly the entire list is passed on each recursion. Every element remains within scope, so nothing is GC'd. That's the space leak. There's another big problem: the current random value is computed by traversing from the head of the list. As the index value grows, it takes progressively longer to traverse the list to retrieve the current value. (N.B. I haven't tested these specific examples, but I think I'm right.) Cheers, John Lato

1 0

Taking Exception to Exceptions
by Immanuel Litzroth 09 Jan '09

09 Jan '09

I'm trying to use the new (for me at least) extensible exceptions and I am little amazed that I cannot get catch, try or mapException to work without telling them which exceptions I want to catch. What is the rationale behind this? How does bracket manage to catch all exceptions? What should onException do? Is there some example code that uses these exceptions, or better documentation? Thanks in advance, Immanuel

5 5

HookedBuildInfo and Cabal
by John Goerzen 09 Jan '09

09 Jan '09

Hi Brian & everyone, Got this bug report, which appears to be related to your autoconf patch. Did you test it on Windows? As a more general question, how can one use Cabal to detect PostgreSQL paths in a way that works with both GHC 6.8 and 6.10? The commit here was: commit 57c723873b020643b9062941a2ece130cd0f36c6 Author: Brian Bloniarz <phunge0(a)hotmail.com> Date: Fri Dec 26 14:33:43 2008 -0500 Update hdbc-postgresql for GHC 6.10 & Cabal 1.6. This switches over to use autoconf rathe r than Cabal hooks to detect postgres paths; there's no backwards-compatible way to use write HookedBuildInfo with Cabal 1.2 and Cabal 1.6 (see http://www.haskell.org/pipermail/glasgow-ha skell-users/2008-October/015707.html) -- John ----- Forwarded message from software-noreply(a)complete.org ----- From: software-noreply(a)complete.org Date: Thu, 08 Jan 2009 12:37:12 -0600 Subject: [HDBC PostgreSQL driver - Bug #131] Throw exception when use data type "timpstamp without time zone" Issue #131 has been updated by Benjamin Cao. by the way, I cannot configure 1.1.6.0.0 successfully on my windows OS because of the Expr command(which is a linux command, even if I got a windows version of it, it keep crash on windows.) The function isFixedPointUnderConversionToLocalTime in 1.1.6.0.0 is the same. So I think it should be a same issue on 1.1.6.0.0 ---------------------------------------- Bug #131: Throw exception when use data type "timpstamp without time zone" http://software.complete.org/software/issues/show/131 Author: Benjamin Cao Status: New Priority: Normal Assigned to: Category: Target version: 1.1.4.1.0 Resolution: The database is like CREATE TABLE test ( "name" character varying(200) NOT NULL, "time" timestamp with time zone, timenotz timestamp without time zone, CONSTRAINT test_pkey PRIMARY KEY (name) ) WITH (OIDS=FALSE); ALTER TABLE test OWNER TO postgres; when call st <- prepare c "select name,time, timenotz from test" execute st [] Will get this error: Time.toClockTime: timezone offset out of range So I modified the function in statement.hsc to: isFixedPointUnderConversionToLocalTime calTime = do let hasError = False calTime' <- Control.Exception.catch (toLocalCalTime calTime) (\_->do let hasError = True return calTime) if hasError then return False else return $ eqParts calTime calTime' ---------------------------------------- You have received this notification because you have either subscribed to it, or are involved in it. To change your notification preferences, please click here: http://software.complete.org/software/my/account ----- End forwarded message -----

2 1

State Monad - using the updated state
by Phil 09 Jan '09

09 Jan '09

Hi, I¹m a newbie looking to get my head around using the State Monad for random number generation. I¹ve written non-monad code that achieves this no problem. When attempting to use the state monad I can get what I know to be the correct initial value and state, but can¹t figure out for the life of me how to then increment it without binding more calls there and then. Doing several contiguous calls is not what I want to do here and the examples I¹ve read all show this (using something like liftM2 (,) myRandom myRandom). I want to be able to do: Get_a_random_number < a whole load of other stuff > Get the next number as defined by the updated state in the first call <some more stuff> Get another number, and so on. I get the first number fine, but am lost at how to get the second, third, forth etc without binding there and then. I just want each number one at a time where and when I want it, rather than saying give 1,2,10 or even n¹ numbers now. I¹m sure it¹s blindly obvious! Note: I¹m not using Haskell¹s built in Random functionality (nor is that an option), I¹ll spare the details of the method I¹m using (NRC¹s ranq1) as I know it works for the non-Monad case, and it¹s irrelevent to the question. So the code is: ranq1 :: Word64 -> ( Double, Word64 ) ranq1 state = ( output, newState ) where newState = ranq1Increment state output = convert_to_double newState ranq1Init :: Word64 -> Word64 ranq1Init = convert_to_word64 . ranq1Increment . xor_v_init -- I¹ll leave the detail of how ranq1Increment works out for brevity. I know this bit works fine. Same goes for the init function it¹s just providing an initial state. -- The Monad State Attempt getRanq1 :: State Word64 Double getRanq1 = do state <- get let ( randDouble, newState ) = ranq1 state put newState return randDouble _________ And then in my main _________ -- 124353542542 is just an arbitrary seed main :: IO() main = do let x = evalState getRanq1 (ranq1Init 124353542542) print (x) As I said this works fine; x gives me the correct first value for this sequence, but how do I then get the second and third without writing the giveMeTenRandoms style function? I guess what I want is a next() type function, imperatively speaking. Many thanks for any help, Phil.

4 4

"Inconsistency" in support for phantom types?
by DavidA 09 Jan '09

09 Jan '09

Hi, I have run into what appears to be an inconsistency in the support for using phantom types to parameterize other types. Here's an example (don't pay too much attention to the maths, it's just there to motivate the example). I want to define types for the finite fields with 2, 3 and 5 elements (clock arithmetic modulo 2, 3 or 5). {-# OPTIONS_GHC -fglasgow-exts #-} class IntegerAsType a where value :: a -> Integer -- three phantom types: data T2 instance IntegerAsType T2 where value _ = 2 data T3 instance IntegerAsType T3 where value _ = 3 data T5 instance IntegerAsType T5 where value _ = 5 newtype Fp n = Fp Integer deriving (Eq,Ord) -- our three finite field types: type F2 = Fp T2 type F3 = Fp T3 type F5 = Fp T5 -- Show and Num instances instance Show (Fp n) where show (Fp x) = show x instance IntegerAsType n => Num (Fp n) where Fp x + Fp y = Fp $ (x+y) `mod` value (undefined :: n) negate (Fp 0) = 0 negate (Fp x) = Fp $ value (undefined :: n) - x Fp x * Fp y = Fp $ (x*y) `mod` value (undefined :: n) fromInteger m = Fp $ m `mod` value (undefined :: n) Now, we can also define a Fractional instance, using the extended Euclid algorithm (given at the end) -- n must be prime instance IntegerAsType n => Fractional (Fp n) where recip 0 = error "Fp.recip 0" recip (Fp x) = let p = value (undefined :: n) (u,v,1) = extendedEuclid x p -- so ux+vp = 1. (We know the gcd is 1 as p prime) in Fp $ u `mod` p Now, the problem I've run into is, what do I do if I want to define a function parameterised over the phantom types, but without doing it as part of a type class instance? For example, suppose that I had just wanted to define "inv" as a synonym for "recip": inv :: IntegerAsType n => Fp n -> Fp n inv 0 = error "Fp,inv 0" inv (Fp x) = let p = value (undefined :: n) (u,v,1) = extendedEuclid x p in Fp $ u `mod` p "inv" has exactly the same code as "recip", but now the IntegerAsType constraint is part of the type signature, rather than an instance constraint. It seems that this means that the constraint is not available to the code during compilation, because when I try to compile this I get Ambiguous type variable `n' in the constraint: `IntegerAsType n' arising from a use of `value' at Test.hs:52:21-42 Probable fix: add a type signature that fixes these type variable(s) It seems to me highly desirable that this code should compile as expected, just as the recip code compiles. Is it a bug in GHC, or a missing language feature, or is there a "better" way to do what I'm trying to do? Thanks, David -- extendedEuclid a b returns (u,v,d) such that u*a + v*b = d extendedEuclid a b | a >= 0 && b >= 0 = extendedEuclid' a b [] where extendedEuclid' d 0 qs = let (u,v) = unwind 1 0 qs in (u,v,d) extendedEuclid' a b qs = let (q,r) = quotRem a b in extendedEuclid' b r (q:qs) unwind u v [] = (u,v) unwind u v (q:qs) = unwind v (u-v*q) qs

4 3

Re: [Haskell-cafe] Defining methods generically for a class
by Jeff Heard 09 Jan '09

09 Jan '09

That's probably the best thing to do, yes. The purpose of doing so was for Data.List.delete, but I see now there's a Data.List.deleteBy, so I can use the regionEquals function as my equality predicate. On Thu, Jan 8, 2009 at 12:26 PM, Martijn van Steenbergen <martijn(a)van.steenbergen.nl> wrote: > Hi Jeff, > > Jeff Heard wrote: >> >> instance Region a => Eq a where >> regiona == regionb = all $ zipWith (==) (bounds regiona) (bounds >> regionb) > > If you want to be Haskell98 compliant, why not define regionEquals :: Region > a => a -> a -> Bool as above and use that everywhere instead of (==)? > > If you insist on using the overloaded (==), then in Haskell98 you will need > to define individual Eq instances for all your custom region types. However, > you can simply define (==) = regionEquals for all those types. > > Hope this helps, > > Martijn. >

1 0

Fwd: [Haskell-cafe] Defining methods generically for a class
by Jeff Heard 09 Jan '09

09 Jan '09

---------- Forwarded message ---------- From: Jeff Heard <jefferson.r.heard(a)gmail.com> Date: Thu, Jan 8, 2009 at 12:26 PM Subject: Re: [Haskell-cafe] Defining methods generically for a class To: Cristiano Paris <frodo(a)theshire.org> Not really... I'm not testing if each of the items a are equal, but rather that in the context of them being a Region, they are equal.As long "dim" (which is in the class) can be defined, then equality is defined over all types Region. On Thu, Jan 8, 2009 at 12:16 PM, Cristiano Paris <frodo(a)theshire.org> wrote: > On Thu, Jan 8, 2009 at 6:04 PM, Jeff Heard <jefferson.r.heard(a)gmail.com> wrote: >> ... >> How do I declare all Regions to be Eqs without putting it in the class >> body (since I define a function over all Regions that is independent >> of datatype that is an instance of Region)? > > Would this be a solution? > > class Eq a => Region a where > ... > > Cristiano >

1 0

Defining methods generically for a class
by Jeff Heard 09 Jan '09

09 Jan '09

I have the following class: class Region a where numDimensions :: a -> Int dim :: Int -> a -> (Double,Double) merge :: a -> a -> a and several ancillary methods defined, the most importance of which is: bounds :: Region a => a -> [(Double,Double)] bounds r = take (numDimensions r) . map dim . iterate (+1) $ 0 Let's say that I want all Regions to also be of class Eq, where regiona == regionb = all $ zipWith (==) (bounds regiona) (bounds regionb) How do I declare all Regions to be Eqs without putting it in the class body (since I define a function over all Regions that is independent of datatype that is an instance of Region)? Is it merely: instance Region a => Eq a where regiona == regionb = all $ zipWith (==) (bounds regiona) (bounds regionb) -- Jeff

3 2

ideas for a phd in the area of paralleism?
by Michael Lesniak 08 Jan '09

08 Jan '09

Hello, currently I'm searching for a topic for my phd-thesis or at least for a workshop-paper (as a starting point, to get my feet wet...). My academic group has specialized itself on (practical, i.e. not too theoretical stuff like verification, etc...) parallel programming and related topics, so my future thesis has to be in this area, but as long as its related I'm free to choose whatever I want. Since I've been interested in FP in general for a few years as a "private hobby" and since Haskell has as a pure language a lot of potential in the area of parallelism, and it's community (esp. #haskell and this list) is really nice and supportive I'd like to do something in this area; I've been hacking in Haskell more intensive for about six month now. I've looked at the different parallelism/concurrency models in Haskell (dph, gph, nested data parallelism) and (after a preliminary overview-reading) it seems that a lot of research has already been done on the basic topics, except for distributed Haskell, where I could not found current papers (i.e. newer than ~2003). If anyone could give me further advice, tips, or hints in general, what could be interesting or where I should take a more deeper look to find a niche I'd really be thankful (and you'll get a special thanks in my foreword in 3 years! ;-)). Best regards, Michael -- Dipl.-Inf. Michael C. Lesniak University of Kassel Programming Languages / Methodologies Research Group Department of Computer Science and Electrical Engineering Wilhelmshöher Allee 73 34121 Kassel Phone: +49-(0)561-804-6269

4 3

Debugging STM
by John Ky 08 Jan '09

08 Jan '09

Hi, Does anyone have any advice on how to inspect complex TVar data structures that may include cycles? They're opaque as far as Show goes. And sometimes I'd like a more comprehensive view of my data structures at the ghci prompt rather than the dribs and drabs I get by typing "x <- atomically $ readTVar var" and the like all the time. Also, do TVars have a printable identity for debugging purposes? Thanks -John

1 0

Template Haskell question
by Jeff Heard 08 Jan '09

08 Jan '09

Alright... I *think* I'm nearly there, but I can't figure out how to derive a class instance using record accessors and updaters... Can anyone help? There are [| XXXf |] instances at the end of the module and they all need replaced, but I can't figure out what to replace them with. The basic idea of the module is that you define your record type, Q, and that record type contains all the state you're interested in. The Hieroglyph system has other basic state, and the idea is that you use $(additions "QWithState" ''Q) $(deriveUIState ''QWithState) to create your final UIState instance. -- - {-# LANGUAGE TemplateHaskell #-} module Graphics.Rendering.Hieroglyph.TH where import Language.Haskell.TH import Language.Haskell.TH.Syntax import Graphics.Rendering.Hieroglyph.UIState import Graphics.Rendering.Hieroglyph.Primitives import Graphics.UI.Gtk.Types (Widget) import Control.Monad {- output of $( fmap (LitE . StringL . show) [| reify ''BasicUIState |] ) TyConI (DataD [] Graphics.Rendering.Hieroglyph.BasicUIState.BasicUIState [] [RecC Graphics.Rendering.Hieroglyph.BasicUIState.BasicUIState [(Graphics.Rendering.Hieroglyph.BasicUIState.mousePosition,NotStrict,ConT Graphics.Rendering.Hieroglyph.Primitives.Point) ,(Graphics.Rendering.Hieroglyph.BasicUIState.mouseLeftButtonDown,NotStrict,ConT GHC.Base.Bool) ,(Graphics.Rendering.Hieroglyph.BasicUIState.mouseRightButtonDown,NotStrict,ConT GHC.Base.Bool) ,(Graphics.Rendering.Hieroglyph.BasicUIState.mouseMiddleButtonDown,NotStrict,ConT GHC.Base.Bool) ,(Graphics.Rendering.Hieroglyph.BasicUIState.mouseWheel,NotStrict,ConT GHC.Base.Int) ,(Graphics.Rendering.Hieroglyph.BasicUIState.keyCtrl,NotStrict,ConT GHC.Base.Bool) ,(Graphics.Rendering.Hieroglyph.BasicUIState.keyShift,NotStrict,ConT GHC.Base.Bool) ,(Graphics.Rendering.Hieroglyph.BasicUIState.keyAlt,NotStrict,ConT GHC.Base.Bool) ,(Graphics.Rendering.Hieroglyph.BasicUIState.key,NotStrict,ConT Graphics.Rendering.Hieroglyph.UIState.Key) ,(Graphics.Rendering.Hieroglyph.BasicUIState.drawing,NotStrict,AppT (ConT Data.Maybe.Maybe) (ConT Graphics.UI.Gtk.Types.Widget)) ,(Graphics.Rendering.Hieroglyph.BasicUIState.sizeX,NotStrict,ConT GHC.Float.Double) ,(Graphics.Rendering.Hieroglyph.BasicUIState.sizeY,NotStrict,ConT GHC.Float.Double) ,(Graphics.Rendering.Hieroglyph.BasicUIState.imageCache,NotStrict,AppT (ConT Data.Maybe.Maybe) (ConT Graphics.Rendering.Hieroglyph.UIState.ImageCache))]] []) -} -- usage: $(additions "MyTypeName" OldTypeName) {- - Add fields to a record type for handling basic UI state for Hieroglyph. Gives you mouse buttons, etcetera -} additions newtypenamestr nm = do TyConI (DataD _ _ _ [RecC _ fielddefs]) <- reify nm let newtypename = mkName newtypenamestr return $ (DataD [] newtypename [] [RecC newtypename [(mkName "mousePositionf",NotStrict,ConT ''Point) ,(mkName "mouseLeftButtonDownf",NotStrict,ConT ''Bool) ,(mkName "mouseRightButtonDownf",NotStrict,ConT ''Bool) ,(mkName "mouseMiddleButtonDownf",NotStrict,ConT ''Bool) ,(mkName "mouseWheelf",NotStrict,ConT ''Int) ,(mkName "keyCtrlf",NotStrict,ConT ''Bool) ,(mkName "keyShiftf",NotStrict,ConT ''Bool) ,(mkName "keyAltf",NotStrict,ConT ''Bool) ,(mkName "keyf",NotStrict,ConT ''Key) ,(mkName "drawingf",NotStrict,AppT (ConT ''Maybe) (ConT ''Widget)) ,(mkName "sizeXf",NotStrict,ConT ''Double) ,(mkName "sizeYf",NotStrict,ConT ''Double) ,(mkName "imageCachef",NotStrict,AppT (ConT ''Maybe) (ConT ''ImageCache))] ++ fielddefs] []) -- | Apply a Binary type constructor to given type: "t" -> "Binary t" appUIState :: Type -> Type appUIState t = AppT (ConT ''UIState) t -- | Generate from list of type names result of types application: -- appType T [a,b] -> "T a b" appType :: Name -> [Name] -> Type --appType t [] = ConT t -- T --appType t [t1] = AppT (ConT t) (VarT t1) -- T a --appType t [t1,t2] = AppT (AppT (ConT t) (VarT t1)) (VarT t2) -- T a b == (T a) b appType t ts = foldl (\a e -> AppT a (VarT e)) (ConT t) ts -- general definition -- | Generate `n` unique variables and return them in form of patterns and expressions genNames :: Int -> Q ([PatQ],[ExpQ]) genNames n = do ids <- replicateM n (newName "x") return (map varP ids, map varE ids) -- usage: $(deriveUIState ''MyTypeWithUIState) {- - Derive an instance of UIState from some type that has had UIState fields added to it. -} deriveUIState tp = do return [InstanceD [] (appUIState $ appType tp []) [FunD 'mousePosition [| mousePositionf |] ,FunD 'mouseLeftButtonDown [| mouseLeftButtonDownf |] ,FunD 'mouseRightButtonDown [| mouseRightButtonDownf |] ,FunD 'mouseMiddleButtonDown [| mouseMiddleButtonDownf |] ,FunD 'mouseWheel [| mouseWheelf |] ,FunD 'keyCtrl [| keyCtrlf |] ,FunD 'keyShift [| keyShiftf |] ,FunD 'keyAlt [| keyAltf |] ,FunD 'key [| keyf |] ,FunD 'drawing [| drawingf |] ,FunD 'sizeX [| sizeXf |] ,FunD 'sizeY [| sizeYf |] ,FunD 'imageCache [| imageCachef |] ,FunD 'setMousePosition [| \b a -> a{ mousePositionf=b } |] ,FunD 'setMouseLeftButtonDown [| \b a -> a{ mouseLeftButtonDownf=b } |] ,FunD 'setMouseRightButtonDown [| \b a -> a{ mouseRightButtonDownf=b } |] ,FunD 'setMouseMiddleButtonDown [| \b a -> a{ mouseMiddleButtonDownf=b } |] ,FunD 'setMouseWheel [| \b a -> a{ mouseWheelf=b } |] ,FunD 'setKeyCtrl [| \b a -> a{ keyCtrlf=b } |] ,FunD 'setKeyShift [| \b a -> a{ keyShiftf=b } |] ,FunD 'setKeyAlt [| \b a -> a{ keyAltf=b } |] ,FunD 'setKey [| \b a -> a{ keyf=b } |] ,FunD 'setDrawing [| \b a -> a{ drawingf=b } |] ,FunD 'setSizeX [| \b a -> a{ sizeXf=b } |] ,FunD 'setSizeY [| \b a -> a{ sizeYf=b } |] ,FunD 'setImageCache] [| \b a -> a{ imageCachef=b } |] ]

6 6

State Monad - using the updated state in an adhoc manner
by Phil 08 Jan '09

08 Jan '09

Hi, I¹m a newbie looking to get my head around using the State Monad for random number generation. I¹ve written non-monad code that achieves this no problem. When attempting to use the state monad I can get what I know to be the correct initial value and state, but can¹t figure out for the life of me how to then increment it without binding more calls there and then. Doing several contiguous calls is not what I want to do here and the examples I¹ve read all show this (using something like liftM2 (,) myRandom myRandom). I want to be able to do: Get_a_random_number < a whole load of other stuff > Get the next number as defined by the updated state in the first call <some more stuff> Get another number, and so on. I get the first number fine, but am lost at how to get the second, third, forth etc without binding there and then. I just want each number one at a time where and when I want it, rather than saying give 1,2,10 or even n¹ numbers now. I¹m sure it¹s blindly obvious! Note: I¹m not using Haskell¹s built in Random functionality (nor is that an option), I¹ll spare the details of the method I¹m using (NRC¹s ranq1) as I know it works for the non-Monad case, and it¹s irrelevent to the question. So the code is: ranq1 :: Word64 -> ( Double, Word64 ) ranq1 state = ( output, newState ) where newState = ranq1Increment state output = convert_to_double newState ranq1Init :: Word64 -> Word64 ranq1Init = convert_to_word64 . ranq1Increment . xor_v_init -- I¹ll leave the detail of how ranq1Increment works out for brevity. I know this bit works fine. Same goes for the init function it¹s just providing an initial state. -- The Monad State Attempt getRanq1 :: State Word64 Double getRanq1 = do state <- get let ( randDouble, newState ) = ranq1 state put newState return randDouble _________ And then in my main _________ -- 124353542542 is just an arbitrary seed main :: IO() main = do let x = evalState getRanq1 (ranq1Init 124353542542) print (x) As I said this works fine; x gives me the correct first value for this sequence, but how do I then get the second and third without writing the giveMeTenRandoms style function? I guess what I want is a next() type function, imperatively speaking. Many thanks for any help, Phil.

2 1

Tying a simple circularly STM linked list
by John Ky 08 Jan '09

08 Jan '09

Hi, I've written a circularly linked list, but there is some code in it I feel is redundant, but don't know how to get rid of: -- Transactional loop. A loop is a circular link list. data Loop a = ItemLink { item :: a , prev :: TVar (Loop a) , next :: TVar (Loop a) } | InitLink -- Create a new empty transactional loop. newLoop :: a -> STM (TVar (Loop a)) newLoop item = do tLoop <- newTVar InitLink writeTVar tLoop (ItemLink item tLoop tLoop) return tLoop In the above, the InitLink value is only ever used in the newLoop function to create a single one element circular linked list. Is there a way to write newLoop to avoid using this value? Thanks -John

2 2

Linking in GMP as a Windows DLL w/ GHC
by Sigbjorn Finne 08 Jan '09

08 Jan '09

Hi, a number of folks have been asking/looking for ways to avoid statically linking in GMP into GHC binaries under Windows. I've written up some notes on how to go about doing this, which are now available from http://haskell.forkio.com/gmpwindows Let me know if it is useful (or works ;-) ) enjoy --sigbjorn

1 0

template haskell
by brian 08 Jan '09

08 Jan '09

Smart constructors sometimes need to generate errors like "Foo.Bar.Baz.create: invalid value, etc". Can TH generate the module and function names?

1 0

ICFP: Child care at conference
by Matthew Fluet (ICFP Publicity Chair) 08 Jan '09

08 Jan '09

Potential ICFP attendees: The ACM-SIGPLAN Executive Committee, which oversees and sponsors ICFP, is considering expanding its travel grants for event participants in need of child care assistance. (Such expansion would also apply to other major SIGPLAN-sponsored conferences, such as PLDI, POPL, OOPSLA.) The purpose of this message is to solicit feedback from potential attendees regarding the desirability for child care assistance and what forms of assistance be most appropriate. If child care assistance at ICFP (or other SIGPLAN events) is of interest to you, please contact me (at icfp.publicity(a)gmail.com -or- fluet(a)tti-c.org) with your comments. Some questions of particular interest: * What age groups of children are most appropriate to cover? * What types of costs might SIGPLAN cover to facilitate your participation in events? Sincerely, -Matthew Fluet (ICFP Publicity Chair)

1 0

Maintaining laziness
by Jan Christiansen 08 Jan '09

08 Jan '09

This is great. I am working on the very same topic for quite a while now. My aim is to develop a tool that tells you whether a function is least strict. My work is based on an idea by Olaf Chitil: http://www.cs.kent.ac.uk/people/staff/oc/ I think he was the first who introduced the idea of least strictness. In a paper in the pre-proceedings of IFL 2006 he introduced the term least strict and presented the idea behind a tool called StrictCheck. This tool was supposed to check whether a function is least strict. The problem with this tool was that it proposes non-sequential function definitions that are not implementable in Haskell. The main example in the StrictCheck paper is the definition of unzip by using foldr. This one is very similar to your partition example as it is too strict if you do not use lazy pattern matching for the tuple structure. unzip2 :: [(a, b)] -> ([a], [b]) unzip2 = foldr (\(x,y) (xs,ys) -> (x:xs,y:ys)) ([],[]) My favorite example right now is the multiplication of Peano numbers. Suppose the definition of a data type for Peano numbers and addition and multiplication. data Natural = Z | S Natural (+) :: Natural -> Natural -> Natural Z + y = y S x + y = S (x + y) (*) :: Natural -> Natural -> Natural Z * _ = Z S x * y = y + (x * y) Now we can define an infinite Peano number as follows infinity :: Natural infinity = S infinity The evaluation of Z * infinity yields Z while infinity * Z does not terminate. The tool I am working on is supposed to yield the following information. Natural> strictCheck2 (*) 5 100 You have to fulfil f (S _|_) Z = Z (_|_) This states that the evaluation of S _|_ * Z yields _|_ but it could yield Z. Therefore we can improve the function as follows (*) :: Natural -> Natural -> Natural Z * _ = Z S _ * Z = Z S x * y = y + (x * y) Now the evaluation of infinity * Z yields Z as we wanted it to. If we test this new implementation using StrictCheck we get *Natural> strictCheck2 (*) 5 100 The function seems to be least strict The non least strict definition of (*) is taken from the module Data.Number.Natural which was published by Lennart Augustsson in the numbers package at hackage. I mention this here because I think it clearly shows that it is even hard for experienced Haskell programmers to define least strict functions. An even more promising area of application are functional logic programs as least strict functions result in smaller search spaces. For example I was able to improve the enumeration of pythagorean triples in Curry by a factor of 4 only by making a function least strict. Cheers, Jan

2 3

Updating doubly linked lists
by S. Günther 07 Jan '09

07 Jan '09

Hello, I was trying to wrap my head around the stuff at http://haskell.org/haskellwiki/Tying_the_Knot (again) and along the way came a question: Is it possible to change a particular node of the doubly linked list? That is to say, that would like to have a function: update :: DList a -> a -> DList a where update node newValue returns a list where only the value at the node which is passed in is set to the new Value and all other values are the same. All this of course in a pure way, that is without using (M/T/TM)Vars or IORefs. I do apologize if the answer is really obvious and am thankfull for any hints. Thanks in advance Stephan Günther

7 15

Haskell good for parallelism/concurrency on manycore?
by Ahn, Ki Yung 07 Jan '09

07 Jan '09

Dear Haskellers, I just got an inquiry from a research department head of a Korean company seriously considering to adopt functional languages because they heard that functional languages can do good at parallel programming on manycore platforms. They want to know what technologies out there implemented in functional languages, such as Erlang or Haskell, that can help them write more maintainable programs that are better at utilizing parallelism and avoiding bugs related to synchronization. They are also very interested in Cilk, as well as Haskell or Erlang. My first thought is that maybe Cilk would work better for them just because it would not be easy to recruit Erlang or Haskell programmers experienced in network, security, or concurrent/parallel programming. I myself can answer basic inquiries such as what libraries to look for to implenent such and so, but not able to give advice on large scale projects such as unified security solution package. So, it wouldn't be practical form them to launch a project without inviting an Erlang or Haskell expert in their domain as a project manager from overseas, which I don't think they are very willing to do. Are there Haskell consultants or Haskell experts on this subject, who believes that Haskell based approach might work better for them, or Haskell can be useful along with other approaches (e.g. DSL, prototyping, formal modeling of policies)? If so, I would like to recommend them trying contact you, and try my best to help communicating with them, if needed. They know English, of course, but may not be familiar with functional programming orlanguage-oriented programming jargons such as DSLs, oops, I mean language middleware :-) For your information: The company is a network security company whose main products are VPN and firewall appliances and their management software. Their research department is in search of better technologies to implement their future UTM (unified threat management) solutions utilizing the manycore platforms. In Korea, there are some research groups and few companies using OCaml, but almost no Erlang or Haskell communities. This company is preferring local researchers or consultants for advice or consulting, but there's no local group using Haskell seriously, as far as I know, even in research yet. That's why this person contacted me, just because I wrote a small tutorial on Haskell Server Programming while ago. P.S. If you happen to be a local Korean expert on this matter, sorry for my ignorance, and I'd be happy to forward their inquiry to you! -- Ahn, Ki Yung

9 13

how out of memory is handled in Haskell
by Manlio Perillo 07 Jan '09

07 Jan '09

Hi. Here: http://damienkatz.net/2008/03/what_sucks_abou.html I found how Erlang (or at least old versions of Erlang) handles out of memory failure: it just calls exit(1). How is this handled in GHC? - exit(1)? - abort()? - IO exception? Thanks Manlio Perillo

2 2

Lack of inlining -> slow parsing with Data.Binary
by Eugene Kirpichov 07 Jan '09

07 Jan '09

Hi, I'm parsing Java classfiles with Data.Binary, the code is here: http://paste.org/index.php?id=4625 The problem is that the resulting code parses rt.jar from JDK6 (about 15K classes, 47Mb zipped) in 15 seconds (run the program with main -mclose rt.jar, for instance), which is 10 times slower than my Java version of the same code. I compile the program with -O2 ; I tried -ddump-inlinings and it turns out that my readByte/readWord16/readWord32 functions don't get inlined, despite being simply aliases for 'get::Get WordXX'; so, in places where my Java version does a pointer access (after being JIT-compiled), the Haskell version does two function calls. What can be the reason of this lack of inlining? Or how do I understand the output of -ddump-inlinings? -- Eugene Kirpichov

3 3

Pattern combinators
by Andrew Wagner 07 Jan '09

07 Jan '09

All, Wadler posted a blog entry the other day about a paper on pattern-matching in Haskell (http://wadler.blogspot.com/) I've taken a first stab at turning it into actual code for hackage (http://hpaste.org/13215) There are two commented-out definitions that don't type-check, though, and the types are too wild for me to grok. Anybody have any suggestions for 1.) How to fix it and/or 2.) How to use data/type/newtype to simplify the types and make it more manageable? Thanks!

4 6

Re: bug in HPDF?
by Roman Cheplyaka 07 Jan '09

07 Jan '09

Thanks for helping! * alpheccar <misc(a)alpheccar.org> [2009-01-06 19:47:21+0100] > Roman, > > Here is the source code to do what you want: > > import Graphics.PDF > import Complex > > main = runPdf "bug.pdf" standardDocInfo (PDFRect 0 0 100 100) pdf > where > pdf = do > p <- addPage Nothing > drawWithPage p $ do > moveto $ 10 :+ 10 > sequence $ replicate 10 $ drawText $ text (PDFFont Helvetica > 10) 0 0 (toPDFString "ABC") > > > It is the Text monad that needs to be replicated. Each copy will start > drawing the text lines at (10,10) as specified by the moveto. > But, inside the text monad, each "text" will create a new line. > > Thanks, > Christophe. > > >> Here is a program which illustrates an unexpected behaviour: >> >> import Graphics.PDF >> >> main = runPdf "bug.pdf" standardDocInfo (PDFRect 0 0 100 100) pdf >> where >> pdf = do >> p <- addPage Nothing >> drawWithPage p $ drawText $ >> sequence $ replicate 10 $ >> text (PDFFont Helvetica 10) 10 10 (toPDFString "ABC") >> >> What I expect here is "ABC" printed 10 times on the same place >> (starting >> at (10,10)). What I see (in Okular) is ABC printed each time in the new >> place like this: >> >> ABC >> ABC >> ABC >> ABC >> >> What's happening here? I'm using HPDF-1.4.1 from Hackage. >> >> -- >> Roman I. Cheplyaka :: http://ro-che.info/ >> "Don't let school get in the way of your education." - Mark Twain > -- Roman I. Cheplyaka :: http://ro-che.info/ "Don't let school get in the way of your education." - Mark Twain

1 0

bug in HPDF?
by Roman Cheplyaka 07 Jan '09

07 Jan '09

Here is a program which illustrates an unexpected behaviour: import Graphics.PDF main = runPdf "bug.pdf" standardDocInfo (PDFRect 0 0 100 100) pdf where pdf = do p <- addPage Nothing drawWithPage p $ drawText $ sequence $ replicate 10 $ text (PDFFont Helvetica 10) 10 10 (toPDFString "ABC") What I expect here is "ABC" printed 10 times on the same place (starting at (10,10)). What I see (in Okular) is ABC printed each time in the new place like this: ABC ABC ABC ABC What's happening here? I'm using HPDF-1.4.1 from Hackage. -- Roman I. Cheplyaka :: http://ro-che.info/ "Don't let school get in the way of your education." - Mark Twain

2 1

HDBC-Sqlite3 "attaching databases"
by Günther Schmidt 07 Jan '09

07 Jan '09

Hi, has anybody here successfully tried to "attach" another database to an Sqlite database with HDBC-Sqlite3? I keep failing, so I'd be grateful for a hint how to do it. Günther

1 0

Threads with high CPU usage
by Günther Schmidt 07 Jan '09

07 Jan '09

Hi, in an application of mine I start a long-running operation in a thread via forkIO so that the UI process doesn't get blocked. It just so happens, that the long-running process also takes the CPU to nearly 100% while it runs. During that time the run-time system does *not* switch back and forth between the UI-process and the long-running task, it seems that the UI process only gets woken up *after* the high CPU thread finishes completely. To the effect of course that it makes no difference at all to the UIs responsiveness whether I use forkIO or not. The long running process is pretty atomic, it's a single query to the database which takes up to a minute to complete so I don't see a chance to squeeze a "mainIteration" in there. What can I do? Günther

16 40