May 2007 - Haskell-Cafe

RE: [Haskell] Inaugural London HUG meeting; attendee list suggestion
by Bayley, Alistair 22 May '07

22 May '07

[this time to the list :-] > > Should we start something here, perhaps? > > > http://www.haskell.org/haskellwiki/London_Haskell_User_Group/Events/2007 0426 > > Sounds a plausible idea, but shouldn't it be 20070523? > > Jeremy Yeah... sorry, I totally fished that date out of a body cavity. I think it was the date of the original blog post about the meeting. Corrected URL: http://www.haskell.org/haskellwiki/London_Haskell_User_Group/Events/2007 053 Is that a good way to label the event, though? If it needs to change dates then the URL changes, which isn't so good. An alternative might be a sequence number e.g.: http://www.haskell.org/haskellwiki/London_Haskell_User_Group/Events/LHUG -1 Alistair ***************************************************************** Confidentiality Note: The information contained in this message, and any attachments, may contain confidential and/or privileged material. It is intended solely for the person(s) or entity to which it is addressed. Any review, retransmission, dissemination, or taking of any action in reliance upon this information by persons or entities other than the intended recipient(s) is prohibited. If you received this in error, please contact the sender and delete the material from any computer. *****************************************************************

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Inaugural London HUG meeting; attendee list suggestion
by Bayley, Alistair 22 May '07

22 May '07

Tomorrow evening's London Haskell User Group meeting has a reasonable list of attendees (inferred from the comments): http://www.londonhug.net/2007/04/26/announcement-first-meeting-of-the-lo ndon-haskell-user-group/ I think it'd be a good idea to do what the BCS-SPA does, and maintain a wiki-page where attendees can add/remove themselves e.g. http://bcs-spa.org/cgi-bin/view/SPA/MakingSenseOfInformalInformation (scroll down to bottom of page, above map) This gives a current picture of who's planning to attend, which might be useful if you want to catch up with particular people. Should we start something here, perhaps? http://www.haskell.org/haskellwiki/London_Haskell_User_Group/Events/2007 0426 Alistair ***************************************************************** Confidentiality Note: The information contained in this message, and any attachments, may contain confidential and/or privileged material. It is intended solely for the person(s) or entity to which it is addressed. Any review, retransmission, dissemination, or taking of any action in reliance upon this information by persons or entities other than the intended recipient(s) is prohibited. If you received this in error, please contact the sender and delete the material from any computer. *****************************************************************

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Wanted: extended static checking for xmonad
by dons＠cse.unsw.edu.au 22 May '07

22 May '07

When working on xmonad, we're trying to produce very clean, correct code -- a window manager that just works. To do this, we're looking to employ more static checking tools to the code base. Currently we use: * QuickCheck (checks high level window manager behaviour) * Catch (Neil's pattern match verifier, caught a couple of bugs) * ghc -Wall One other tool we're considering: * HPC. check program coverage of the test suite. But I suspect that there's an awful lot of other small checkers of various kinds out there. So: * Do you know of some other cool code checking tool for Haskell? * That could be used on xmonad (xmonad.org) to reduce code smell? If so, let us know! :-) -- Don P.S. Perhaps it is time for a wiki page documenting the available extended static checking tools for Haskell.

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How Albus Dumbledore would sell Haskell
by Dan Weston 21 May '07

21 May '07

It is unscientific to ask the (highly biased) people on this list how to sell Haskell. A focus group of the target audience is clearly called for. Having said that, I will now violate my own advice. Knowledge of the audience is critical to the success of a presentation. Simon (aka Dumbledore) is speaking to four different Houses: scientists (Ravenclaw), engineers (Hufflepuff), entrepreneurs (Slytherin), and managers (Griffindor). My advice to him is: You have already won over the scientists. Stop priming the pump. Algebraic runes, spells from the Book of Category Theory, bananas, lenses, arrows, cata, ana, hylo, endo, all is vanity. You cannot interest the engineer with new tricks. He has spent too long mastering difficult tricks to work around limitations to want to see them eliminated. His competitive advantage and prized reputation are at stake. Get him to believe that he is on the verge of being "left behind". You cannot win over the entrepreneur with promises of "easier and more robust". This translates to "anyone can do it" and the valuable "trade secret" of arcane wizardry is now devalued. Show something exciting that cannot be done (or would never have been attempted) without FP. Monads, CPS, existential data types, or other high-priestess wizardry are no impediment (less competition from lesser peers). But marginal improvement is unimpressive. A new language brings high risk, and must have high reward. Examples must be powerful (not another prime number generator). Effective complexity management, correctness reasoning in concurrent/distributed processing, and so on will get their interest. Risk is scary for managers. "Great promise" promises the manager only headaches. This translates to "no one can be hired to do it" and "those that can will shake me down for more money". You must promote instead the *inevitability* of Haskell's success: the fact that Haskell has recently dominated the ICFP Programming Contest, the strongly upward trend of Haskell taught at universities (hopefully there is a strongly upward trend?), some anecdotal measure of the predisposition to relatively better "quality" of those who seek to learn Haskell. Allay the unspoken fear that there are always plenty of Haskellers to cause trouble in a company (design and implement great tasks), but never enough more mediocre Haskellers to keep out of trouble (content to do "maintenance" tasks and minor upgrades to existing software) by marketing Haskell as a recruitment tool to attract the best, a motivational tool to reenergize engineers (see above) who've let their skills go stale in larger companies, and a staff-development tool to spur internal friendly competition to modernize their skills (and excuse wage stagnation in those that do not). Since "inevitability" is a hard sell for Haskell right now, avoid mentioning ML or Erlang. There needs to be only one FPL for a manager, or he will fret about VHS vs. Betamax syndrome (and he won't want to have been the one to invest in a Betamax). Don't show any Haskell function with the prefix "unsafe". Even now, I find the name unsettling. The Harry Potter series makes money because we can't do magic but we want to. It is human nature that what is given away will never be thought to have value. Don't give away the magic of Haskell. Let the audience in on the secret that the Haskell wizarding world is doing great things with it for glory and profit, and others will steal the secret for themselves readily enough. If Simon can do all this, then he really is worthy of the name Dumbledore. :) Dan

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Haskell: the Craft of Functional Programming
by PR Stanley 21 May '07

21 May '07

Hi I've acquired a copy of the above title but it requires a lot of transcription work. So, I thought I'd first ensure it's worth the time and effort. This edition was published in 1999. All Opinions on the text, good or bad, would be very welcome. Thanks, Paul

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Profiling, GUIs and libraries
by Anthony Chaumas-Pellet 21 May '07

21 May '07

Hello, I'm currently hacking away a wxhaskell program that uses up 100% CPU even when it should be idle. So, rather than doing blind guesswork, I've thought about using profiling to spot the zealous function. I do not need a very accurate result, though. ghc with -prof -auto(-all) produces the following error message, however: "Could not find module `Graphics.UI.WX': Perhaps you haven't installed the profiling libraries for package wx-0.10.1? Use -v to see a list of the files searched for." So, I guess I need to rebuild wx with profiling, but I'm not sure how to do that exactly; <search engine> and the Wiki did not turn up any pointers, and there are no ready-made options to build wx with profiling. It seems Cabal can build a library with profiling automatically, but I don't think there exists a Cabalized version of wx yet. That, and I'm not sure if recompiling libraries to add profiling is a great idea, as wx itself may depend on other external libraries. So, is there a better way than profiling in my case, or am I missing something obvious here? Thanks! Anthony

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AngloHaskell 2007
by Philippa Cowderoy 21 May '07

21 May '07

I'd like to announce AngloHaskell 2007, or at least the start of the process of organising it! Last year there was a get-together organised around the interviews for the job of GHC maintainer. There were talks, with a largely pragmatic flavour, and there was plenty of socialising. It was fun, and doing it again seems a good idea. In the vein of last year's process, organising an event this year shall take place on the Haskell Wiki page at: http://www.haskell.org/haskellwiki/AngloHaskell and on IRC, in #anglohaskell on irc.freenode.net So far this is almost all the detail available, as neither date nor venue have been set - a summer date seems good though, and by the time we're organised I imagine late July or August looks most likely. Simon Peyton-Jones has offered to ask Microsoft Research for space to give talks in. This worked well last year, but if anyone else wants to offer a venue that's fine! -- flippa(a)flippac.org The task of the academic is not to scale great intellectual mountains, but to flatten them.

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Behavior of groupBy
by Hans van Thiel 21 May '07

21 May '07

Hello All, The standard function groupBy of List.hs doesn't work as I expect in this case: groupBy (\x y -> (last x) == (last y)) ["abc", "bd","cac"] results in: [["abc"],["bd"],["cac"]] where I want: [["abc","cac"], ["bd"]] Am I doing something wrong, or is this a bug? The function is defined (in List.hs in the hugs Data library) as : -- | The 'groupBy' function is the non-overloaded version of 'group'. groupBy :: (a -> a -> Bool) -> [a] -> [[a]] groupBy _ [] = [] groupBy eq (x:xs) = (x:ys) : groupBy eq zs where (ys,zs) = span (eq x) xs Thanks, Hans van Thiel

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how can I select all the 3-element-combination out of a list efficiently
by geniusfat 21 May '07

21 May '07

hi dear haskell lover ;) what I want to do is simply this: select3 :: [a] -> [(a, a, a)] and how can it be done efficiently? thanks in advance! -- View this message in context: http://www.nabble.com/how-can-I-select-all-the-3-element-combination-out-of… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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Lazy HTML parsing with HXT, HaXML/polyparse, what else?
by Henning Thielemann 21 May '07

21 May '07

I want to parse and process HTML lazily. I use HXT because the HTML parser is very liberal. However it uses Parsec and is thus strict. HaXML has a so called lazy parser, but it is not what I consider lazy: *Text.XML.HaXml.Html.ParseLazy> Text.XML.HaXml.Pretty.document $ htmlParse "text" $ "<html><head></head><body>"++undefined++"</body></html>" *** Exception: Prelude.undefined *Text.XML.HaXml.Html.ParseLazy> Text.XML.HaXml.Pretty.document $ htmlParse "text" $ "<html><head></head><body>&</body></html>" *** Exception: Expected "</" but found & at file text at line 1 col 26 If it would be lazy, it would return some HTML code before the error. HaXML uses the Polyparse package for parsing which contains a so called lazy parser. However it has return type (Either String a). That is, for the decision whether the parse was successful, the document has to be parsed completely. *Text.ParserCombinators.PolyLazy> runParser (exactly 4 (satisfy Char.isAlpha)) ("abc104"++undefined) ("*** Exception: Parse.satisfy: failed If it would have return type (String, a) it could return both a partial value of type 'a' and the error message as String. It would be even better if it has some handling for incorrect input texts, and returns ([String], a), where [String] is the type of a list of warnings and error messages and 'a' is the type of a total value of parse output. Is there some parser of this type? Unfortunately http://www.haskell.org/haskellwiki/Applications_and_libraries/Compiler_tools does not compare the laziness of the mentioned parsers.

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