December 2006 - Haskell

type hacking hoas
by jeff p 24 Dec '06

24 Dec '06

Hello, This message is essentially a repost of a message I sent to haskell-cafe which received no replies. I'm trying to mix type hackery with HOAS. More specifically, here is a data type which I would like to use: data Exp a where Lam :: (Exp a -> Exp b) -> Exp (a -> b) App :: Exp (a -> b) -> Exp a -> Exp b Rcd :: (IsRecord (HCons (F l (Exp v)) r) => F l (Exp v) -> Exp r -> Exp (HCons (F l (Exp v)) r) Proj :: (HasLabel l r v) => l -> Exp r -> Exp v where F is a datatype constructor for record fields (as in the HList paper), IsRecord checks that the argument is an HList of F l v and has unique labels (also as in the HList paper); and HasLabel fishes the appropriate type out of a HList of fields. The catch is that I want GHC to infer my types, as well as check ascribed types. Thus I would like to be able to successfully infer a type for: Lam $ \x -> App (Proj l1 x) (Proj l2 x) for some given labels l1 and l2. However, I cannot figure out how to write HasLabel (or if it's even possible). I suspect I need to be able to test for unifiability of type vars in order to pull this off. AFAIK, neither TypeCast nor TypeEq provide this since TypeCast just fails if its two arguments aren't unifiable, and TypeEq doesn't work on uninstantiated type vars; correct me if I'm wrong about these. Does anyone have any thoughts on this? thanks, Jeff

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Job Announcement
by Yaron Minsky 22 Dec '06

22 Dec '06

I'm pleased to announce that Jane Street Capital is yet again looking to hire some top-notch functional programmers. Of particular note is that Jane Street Europe Ltd. now has an office in London, and we are particularly interested in hiring someone for that office with strong systems administration skills in addition to experience with functional programming languages. You can find out more about Jane Street here: http://www.janestcapital.com/tech.html Here's the full job announcement. ------------------------------------------------- Jane Street Capital is a proprietary trading firm that operates around the clock and around the world. We bring a deep understanding of trading, a scientific approach, and innovative technology to bear on the problem of trading profitably on the world's highly-competitive financial markets. We run a small, nimble operation where technology and trading are tightly integrated. At Jane Street, there is room to get deeply involved in a number of areas at the same time. We are actively looking for people interested in software development, system administration, and quantitative research--potentially all on the same day. We have offices in the New York, London and Tokyo. Most of our hiring is for the New York office, but we are looking for applicants in our foreign offices as well. In some circumstances we have hired foreign applicants for the US office through the H1-B and the F1 visa programs. In such cases, we support and fund the visa application process. We are looking for talented candidates across the range. We are also particularly interested in finding someone with skills and interest in systems administration for Jane Street Europe Ltd.'s London office. The ideal candidate has: * A commitment to the practical. One of the big attractions of our work is the opportunity to apply serious ideas to real-world problems. * Experience with functional programming languages (OCaml, SML, Scheme, Haskell, Lisp, F#, Erlang, etc) is important. Applicants should also have experience with UNIX and a deep understanding of computers and technology. * A strong mathematical background. This is a must for candidates interested in research, and includes a good understanding of probability and statistics, calculus, algorithms, etc. We draw on ideas from everywhere we can, so we value interest and experience in a range of scientific fields. * Good second-order knowledge. In trading, understanding the boundary between what you do and don't know is as (or more) important than how much you know. The environment at Jane Street is open, informal, intellectual, and fun. You can wear a t-shirt and jeans to the office, the kitchen is stocked, and discussions are always lively. We encourage a focus on learning, both through formal seminars and classes, as well as through day-to-day conversations with colleagues. Other perks include competitive salaries, rapid advancement for people who do well, excellent benefits, free lunch, and a gym on-site. We are currently looking for full-time positions as well as summer interns (internship positions are only avaliable in the US office, and for people who already have the right to work in the USa). If you are interested, send an application to Yaron Minsky (yminsky(a)janestcapital.com) including a resume, cover letter, and optionally some sample code you've written. If you'd like to know a little bit more about how we came to use OCaml as our primary development language, take a look at these slides from a talk we gave at CUFP 2006: http://www.galois.com/cufp/slides/2006/YaronMinsky.pdf

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Re: [Haskell] How to make sure that a function is evaluated only once?
by Axel Jantsch 22 Dec '06

22 Dec '06

Thanks to all who replied. I still haven't figured out a solution. I thought about memoizing the result, as several of you have suggested, but I am scared by the huge argument space. Lemmih <lemmih(a)gmail.com> wrote: > Do you have a minimal test case showing the undesirable behaviour? gf (actCnt,sndCnt, sizex, sizey, locx, locy) iPack = (newSndCnt sndCnt outPack, outPack) where outPack = fWrapper sizex sizey locx locy actCnt iPack newSndCnt n (Abst,_) = n newSndCnt n (Prst _,_) = n + 1 This is a simplified context for the call to the C function, which is wrapped by fWrapper. I figured out that one reason for double evaluation is that outPack is used twice in the return expression of gf. So my question now is, can I force the evaluation machine to evaluate fWrapper once and use the same result value in both places where outPack is used? I tried the NOINLINE pragma both on outPack and on fWrapper, but to no avail. Again, I am grateful for any hint to a work-around. Cheers, Axel > On 12/21/06, Axel Jantsch <axel(a)kth.se> wrote: > > > > Hello, > > > > I have a function with type > > > > f :: Int -> Int -> Int -> Int -> Int -> NRup -> NRdown > > > > and I want to make sure it is evaluated only once for the same set of > > arguments but I observe that it is always evaluated several times. > > > > f is implemented in C and called via the FFI. The C function may or may > > not be pure. In principle it is but it is convenient to keep some state > > for debugging and monitoring. > > Anyway, even if it is pure, I want it to evaluate only once for a set of > > arguments because of performance. > > > > How can I enforce this? > > Is there a GHC option for that? > > Is there a way to write the Haskell code to achieve this? > > > > > > I have asked the same question in the context of FFI, but now it seems to > > me this question is not directly related to the FFI. > > Haskell implementations are free to evaluate expressions as many times > as they feel like. If you want to be absolutely sure, use the IO > monad. On the other hand, you might get the same effect with a few > cleverly placed {-# NOINLINE #-} pragmas. > Do you have a minimal test case showing the undesirable behaviour? > > -- > Cheers, > Lemmih > _______________________________________________ > Haskell mailing list > Haskell(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell > -- Phone: +46 8 790 4124, Email: axel(a)kth.se, Web: www.it.kth.se/~axel

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Beautiful concurrency with a Christmas flaour
by Simon Peyton-Jones 22 Dec '06

22 Dec '06

Dear Haskellers I've been writing a modest chapter for a book edited by Greg Wilson called "Beautiful code". In the end I decided to write about concurrency and transactional memory. If you'd like to have a look at my first draft, go here http://haskell.org/haskellwiki/Talk:SantaClausProblem I would welcome constructive suggestions for improvement. The book is aimed at a general audience of programmers, not Haskell geeks, so I have tried to explain everything necessary as I go along. So if you are not a Haskell expert, your input would be particularly valuable to me. Happy Christmas Simon

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How to make sure that a function is evaluated only once?
by Axel Jantsch 21 Dec '06

21 Dec '06

Hello, I have a function with type f :: Int -> Int -> Int -> Int -> Int -> NRup -> NRdown and I want to make sure it is evaluated only once for the same set of arguments but I observe that it is always evaluated several times. f is implemented in C and called via the FFI. The C function may or may not be pure. In principle it is but it is convenient to keep some state for debugging and monitoring. Anyway, even if it is pure, I want it to evaluate only once for a set of arguments because of performance. How can I enforce this? Is there a GHC option for that? Is there a way to write the Haskell code to achieve this? I have asked the same question in the context of FFI, but now it seems to me this question is not directly related to the FFI. Any hints? Thanks, Axel -- Phone: +46 8 790 4124, Email: axel(a)kth.se, Web: www.it.kth.se/~axel -- --- Phone: +46 8 790 4124, Email: axel(a)imit.kth.se, Web: www.imit.kth.se/~axel

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ANN: hscolour-1.6
by Malcolm Wallace 21 Dec '06

21 Dec '06

hscolour-1.6 ------------ HsColour is a syntax-highlighter for Haskell code. It can generate ANSI terminal colour codes, HTML, and CSS, and can insert hyperlink anchors for function definitions (useful in conjunction with Haddock). HsColour-1.6 is now available. The major addition is a new LaTeX output mode (contributed by Thomas Sutton). Minor fixes include a buffering change to allow you to use HsColour interactively in co-operation with e.g. ghci, on an ANSI terminal. http://www.cs.york.ac.uk/fp/darcs/hscolour Regards, Malcolm

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Haskell Weekly News: December 20, 2006
by dons＠cse.unsw.edu.au 20 Dec '06

20 Dec '06

--------------------------------------------------------------------------- Haskell Weekly News http://sequence.complete.org/hwn/20061220 Issue 54 - December 20, 2006 --------------------------------------------------------------------------- Welcome to issue 54 of HWN, a weekly newsletter covering developments in the Haskell community. A new release of the Edison data structures library, along with several other new libraries, and some new Haskell articles in the blogspace. Announcements Edison 1.2.1. Robert Dockins [1]announced the 1.2.1 release of [2]Edison. Edison is a famous library of efficient, purely-functional data structures in Haskell. Notable changes from the previous version include: a new sequence implementation based on finger trees; documentation fixes dealing with the licence; added a few methods to EnumSet for wrapping and unwrapping the underlying Word 1. http://article.gmane.org/gmane.comp.lang.haskell.general/14662 2. http://www.eecs.tufts.edu/~rdocki01/ Basic serialisation library using SYB. Stefan O'Rear [3]announced GenericSerialize, [4]a library for serialization using the existing generic-programming framework. It is often advocated that support for serialization should be added to the compiler (e.g. in the form of a deriving(Binary)). With this project Stefan wants to show that the existing infrastructure is sufficient, and has some advantages over a dedicated serialization interface. GenericSerialize supports multiple serialization modes. 3. http://article.gmane.org/gmane.comp.lang.haskell.general/14656 4. http://members.cox.net/stefanor/genericserialize vty 1.0. Stefan O'Rear [5]announced vty 1.0, a simple [6]terminal interface library. It provides: handling of suspend/resume, window resizes, computation of minimal differences, minimizes repaint area, automatically decodes keyboard keys into (key,modifier) tuples, and more! 5. http://article.gmane.org/gmane.comp.lang.haskell.general/14670 6. http://members.cox.net/stefanor/vty Monad.Reader call for copy. Wouter Swierstra [7]reminded us that its still not too late to write something for the next issue of [8]The Monad.Reader! We have a nice issue slowly shaping up, but your contribution is still very welcome. Get in touch with Wouter if you intend to submit something -- the sooner you let him know what you're up to, the better. 7. http://article.gmane.org/gmane.comp.lang.haskell.general/14638 8. http://www.haskell.org/haskellwiki/TheMonadReader Ranged Sets. Paul Johnson [9]announced that Ranged Sets now have a Monoid instance, and singletons (i.e. a range holding a single value), thanks to Jean-Philippe Bernardy. 9. http://article.gmane.org/gmane.comp.lang.haskell.general/14663 CalDims 1.0 (RC 1). schneegloeckchen [10]announced CalDims, a calculator aware of units. Its available from [11]the Haskell wiki. It includes support for user defined basic units and derrived units; user defined functions; work sheets can be modified/saved via shell; (1/3)*3 == 1 (No rounding errors); built-in feature to simplify units and easy unit-conversion. 10. http://article.gmane.org/gmane.comp.lang.haskell.general/14671 11. http://www.haskell.org/haskellwiki/CalDims JoinCabal. Dougal Stanton [12]announced an alternative to mkcabal, for initialising new cabal projects: [13]JoinCabal, available [14]via darcs. JoinCabal will create stub sources files with a license header, and appropriate license for you code, making it easier to set up a valid cabal build system. 12. http://article.gmane.org/gmane.comp.lang.haskell.cafe/17600 13. http://brokenhut.no-ip.org/~dougal/cgi-bin/darcsweb.cgi?r=JoinCabal;a=summa… 14. http://brokenhut.no-ip.org/~dougal/darcs/joincabal/ Haskell Vim plugin. Arthur van Leeuwen [15]announced a new [16]vim plugin for Haskell providing some preliminary folding support, easy insertion of type signatures into programs, and support for handling .hi files. 15. http://article.gmane.org/gmane.comp.lang.haskell.cafe/17675 16. http://www.cs.uu.nl/~arthurvl/haskell.vba Haskell' This section covers the [17]Haskell' standardisation process. * [18]Strictly matching monadic let and overloaded Bool 17. http://hackage.haskell.org/trac/haskell-prime 18. http://thread.gmane.org/gmane.comp.lang.haskell.prime/1956/focus=1974 Discussion Automonadization of code. Adam Megacz [19]asked if there any work on automatic translation of code in some tiny imperative language into Haskell code that uses the ST and/or IO monads (or perhaps even pure functional code)? 19. http://thread.gmane.org/gmane.comp.lang.haskell.cafe/17326/focus=17326 What are the points in pointfree style?. Steve Downey [20]wondered about the origin of the term 'points' in pointfree style. 20. http://thread.gmane.org/gmane.comp.lang.haskell.cafe/17560/focus=17560 Rebinding >>= for restricted monads. David Roundy [21]pondered rebinding the >>= syntax in order to provide a more restricted monad. 21. http://thread.gmane.org/gmane.comp.lang.haskell.cafe/17646/focus=17646 #haskell reaches 300 users. Don Stewart [22]noticed that the [23]#haskell IRC channel has now reached the 300 user mark, making it one of the top 15 channels on the freenode network. 22. http://thread.gmane.org/gmane.comp.lang.haskell.cafe/17705/focus=17705 23. http://haskell.org/haskellwiki/IRC_channel Blog noise [24]Haskell news from the blogosphere. * [25]Do-notation considered harmful * [26]Reddit discussion considered productive * [27]The Dot-Matrix Printhead: a Haskell Toy * [28]The Revised Dot-Matrix Printhead * [29]The monad laws * [30]On documentation * [31]Software requirements do not change * [32]Haskell and the web: some ideas in progress * [33]Using Haskell in Python: haskellembed * [34]Literate programming * [35]The lambda revolution, Episode V, the deb strikes back * [36]On syntax * [37]Syntax: adding Lisp macros * [38]More on syntax * [39]UCBLogo and the Fibonacci sequence * [40]Linguistic comparisons: string reversal * [41]New Software Technology: Blockage On Line * [42]More silver bullets * [43]Preventing NullPointerExceptions, Maybe * [44]Adventures in Haskell: the Parsec magic weapon * [45]Advertising Haskell * [46]Programming Haskell: part 1 * [47]Programming Haskell: part 2 * [48]Programming Haskell: part 3 * [49]Introductory Haskell Programming in the UNIX Environment * [50]On Haskell, Intuition And Expressive Power * [51]Your order is ready sir * [52]It's alive (the French Haskell community) * [53]Maybe monad in Java * [54]Secret Santas in Haskell I: Preliminaries * [55]Secret Santas in Haskell II: Orbits and Lists * [56]Why Closures in Dolphin is a Good Idea * [57]Haskell Stacks : Two Different Ways * [58]Haskell Golf Scores * [59]Languages * [60]Questions on Haskell Style (and Polynomials redux) * [61]A simple RSS aggregator in 50 lines of Haskell using HXT 24. http://planet.haskell.org/ 25. http://syntaxfree.wordpress.com/2006/12/12/do-notation-considered-harmful/ 26. http://syntaxfree.wordpress.com/2006/12/13/reddit-discussion-considered-pro… 27. http://praisecurseandrecurse.blogspot.com/2006/12/dot-matrix-printhead-hask… 28. http://praisecurseandrecurse.blogspot.com/2006/12/revised-dot-matrix-printh… 29. http://cale.yi.org/index.php/The_Monad_Laws 30. http://blog.milkfarmsoft.com/?p=29 31. http://blog.tmorris.net/software-requirements-do-not-change/ 32. http://haskell-web.blogspot.com/2006/12/some-ideas-in-progress_13.html 33. http://www.kuarepoti-dju.net/index.php?p=86 34. http://benjisimon.blogspot.com/2006/12/programs-as-stories.html 35. http://pupeno.com/blog/the-lambda-revolution-episode-v-the-deb-strikes-back 36. http://cgi.cse.unsw.edu.au/~dons/blog/2006/12/14#on-syntax 37. http://nominolo.blogspot.com/2006/12/syntax.html 38. http://www.cse.unsw.edu.au/~dons/blog/2006/12/15#on-syntax-again 39. http://sob.apotheon.org/?p=171 40. http://sob.apotheon.org/?p=170 41. http://cogito.blogthing.com/2006/12/14/new-software-technology-blockage-on-… 42. http://clabs.org/blogki/blogki.cgi?page=/ComputersAndTechnology/AndTheNomin… 43. http://rickyclarkson.blogspot.com/2006/09/using-strong-typing-to-eliminate.… 44. http://jpmoresmau.blogspot.com/2006/12/adventures-in-haskell-parsec-magic.h… 45. http://neilmitchell.blogspot.com/2006/12/advertising-haskell.html 46. http://cgi.cse.unsw.edu.au/~dons/blog/2006/12/16#programming-haskell-intro 47. http://www.cse.unsw.edu.au/~dons/blog/2006/12/17#programming-haskell-part-2 48. http://cgi.cse.unsw.edu.au/~dons/blog/2006/12/18#ph-3 49. http://blog.kfish.org/software/haskell/intro-haskell-unix.html 50. http://www.defmacro.org/ramblings/haskell-productivity.html 51. http://lambda-the-ultimate.org/node/1905#comment-23273 52. http://koweycode.blogspot.com/2006/12/its-alive.html 53. http://blog.tmorris.net/maybe-monad-in-java/ 54. http://mult.ifario.us/articles/2006/12/18/secret-santas-in-haskell-i-prelim… 55. http://mult.ifario.us/articles/2006/12/18/secret-santas-in-haskell-ii-orbit… 56. http://rickyclarkson.blogspot.com/2006/12/why-closures-in-dolphin-is-good-i… 57. http://progexpr.blogspot.com/2006/11/haskell-stacks-two-different-ways.html 58. http://progexpr.blogspot.com/2006/11/haskell-golf-scores.html 59. http://computativestylings.blogspot.com/2006/12/languages.html 60. http://sambangu.blogspot.com/2006/12/questions-on-haskell-style-and 61. http://cale.yi.org/index.php/HRSS Quotes of the Week * Cale: I spent a whole summer being paid to write a nondeterministic search program in the list monad * monochrom: 007 titles for Haskellers: fromForeignPtr withCString; You Only lift Twice; Bottoms Are Forever; The Monad with the Pure Function; For Your Types Only; A view to A seq; Licence to killThread; iterate Never Dies; unsafePerformIO Is Not Enough. * DavidRoundy: ... fun of haskell-cafe is that it's where all the cool people hang out * dons: Coding drunk in Haskell is ok though, its like drinking on a passenger plane flight -- the stewards will prevent you from hurting anyone. However, coding drunk in C is just scary, DUI territory, and should not be allowed. * JulianMorrison: Small startup companies are the expendable sensory and feeding apparatus of the mainstream. * droundy: With a higher-kinded monad and phantom existential witness types, darcs would be very fun... * dons: The alien rulers of the galaxy must surely use a statically typed language with type inference * dibblego: Java programmers prefer referential transparency too, they just call it 'design patterns' or something * tessier: After the last newbies head exploded trying to read everything on that monad link there was a lot of paperwork. We'd like to avoid doing that again. Code Watch Fri Dec 8 21:03:34 PST 2006. Andy Gill. [62]Adding tracing support (HPC) 62. http://article.gmane.org/gmane.comp.lang.haskell.cvs.ghc/18263 Wed Dec 13 08:45:38 PST 2006. simonpj. [63]You can now have an MVar with a polymorphic contents 63. http://article.gmane.org/gmane.comp.lang.haskell.cvs.ghc/18279 About the Haskell Weekly News Each week, new editions are posted to [64]the Haskell mailing list as well as to [65]the Haskell Sequence and [66]Planet Haskell. [67]RSS is also available, and headlines appear on [68]haskell.org. Headlines are available as [69]PDF. The Haskell Weekly News is also [70]available in Spanish translation. To help create new editions of this newsletter, please see the [71]contributing information. Send stories to dons at cse.unsw.edu.au. The darcs repository is available at darcs get [72]http://www.cse.unsw.edu.au/~dons/code/hwn 64. http://www.haskell.org/mailman/listinfo/haskell 65. http://sequence.complete.org/ 66. http://planet.haskell.org/ 67. http://sequence.complete.org/node/feed 68. http://haskell.org/ 69. http://www.cse.unsw.edu.au/~dons/code/hwn/archives/20061220.pdf 70. http://haskell.org/haskellwiki/HWN/es 71. http://haskell.org/haskellwiki/HWN 72. http://www.cse.unsw.edu.au/~dons/code/hwn

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Announce: CalDims 1.0 (RC 1) - A calculator aware of units
by mm 19 Dec '06

19 Dec '06

Hello, i'd like to inform you about the first announced version of CalDims. Its website is in the haskellwiki: http://www.haskell.org/haskellwiki/CalDims CalDims is a calculation tool with the following features: * Support for user defined basic units and derrived units * user defined functions * work sheets can be modified/saved via shell * (1/3)*3 == 1 (No rounding errors) * built-in feature to simplify units * easy unit-conversion Useful hints: * to compile just type `make` and then (optionally) 'make install' as superuser. * to read documentation, start the program and then enter `\?` Future: Short term goal is to provide a stable release so please report all bugs and quirks. For the next major release I am planning to write a worksheet GUI for CalDims + support more datatypes. Here's an example session: /home/schneegloeckchen# caldims CalDims 1.0-rc1 Version: GPL 2 (see the LICENSE file) Type in `\?' to read the inline help. > # A 60 Watt electric light is burning 2 hours a day. How much energy is consumed in one week? > 60 W * 2 h / d * 7 d 3024000.0 J > # What's that in Watt hours? > 60 W * 2 h / d * 7 d | W h 840.0 W h > #How much kWh is it in a year (we derrive `year` and `kW` first) > year := 365 d New unit year:d ok. > kW := 1000 W New unit kW:W ok. > 60 W * 2 h / d * 1 year | kW h 43.8 h kW > #Define new basic dimension for currency + variable for energy price > \u USD Added basic unit `USD'. > energy_price = 0.2 USD / kW / h New function energy_price:USD / kW / h ok. > 43.8 kW h * energy_price 8.76 USD > #A simple function for energy costs > energy_costs (t : s, e:W) = t*e*energy_price New function energy_costs:USD W s / kW / h ok. > energy_costs (20 min, 60 kW) 4.0 USD > #Save everything (units, functions) to a file > \s my_calculation.cal schneegloeckchen

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ANN: vty 1.0: simple terminal interface library
by Stefan O'Rear 18 Dec '06

18 Dec '06

Announcing vty, a very simple terminal interface library. In <150 non-blank non-comment lines of Haskell (and 7 lines of C) vty provides: * Automatic handling of suspend/resume (SIGTSTP+SIGCONT) * Automatic handling of window resizes * Automatic computation of minimal differences * Minimizes repaint area, thus virtually eliminating the flicker problem that plagues ncurses programs * Automatically decodes keyboard keys into (key,[modifier]) tuples * Automatically supports refresh on Ctrl-L. * Automatically supports timeout after 50ms for lone ESC (a barely noticable delay) * Interface is designed for relatively easy compatible extension. Current disadvantages: * No current support for non-ANSI terminals. * Minimal support for special keys on terminals other than the linux-console. (F1-5 and arrow keys should work, but anything shifted isn't likely to.) * Uses the TIOCGWINSZ ioctl to find the current window size, which appears to be limited to Linux and *BSD. darcs get --tag=rel-1.0 http://members.cox.net/stefanor/vty http://members.cox.net/stefanor/vty/dist/vty-1.0.tar.gz To compile the demonstration program: ghc --make Test.hs gwinsz.c

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ANNOUNCE: Visual Haskell 0.2 final
by Krasimir Angelov 18 Dec '06

18 Dec '06

Hello Haskellers, The final version of Visual Haskell 0.2 is ready: http://www.haskell.org/visualhaskell This is the first version that is: - available for both VStudio 2003 and VStudio 2005 - distributed with a stable GHC version (6.6) - the plugin itself is much more stable than its first 0.0 version There were important changes after the prerelease version so it is recommended to upgrade you installation. Many thanks to Simon Marlow, Brian Smith, Wayne Vucenic, Jared Updike and Pepe Iborra that helped me with the preparation of this new release. Cheers, Krasimir

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