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Re: [Haskell] boilerplate boilerplate
by Jacques Carette 22 May '07

22 May '07

Personally I would like {-# OPTIONS -fglasgow-exts -fgenerics -} module Blog.Types where family Usual = (Eq, Ord, Read, Show, Typeable) data BlogEntry = Entry EpochSeconds Name Email Title Body deriving Usual newtype Name = Name String deriving Usual newtype Title = Title String deriving Usual newtype Body = Body String deriving Usual Of course, if you're doing that, you might as well change those last 3 lines with newtype-schema NamedString = NamedString String deriving Usual instantiate NamedString with Name, Title, Body To me, that is very clear. Syntax-wise, I am sure things can be improved (but I rather like 'family', 'foo-schema' and 'instantiate', but not really the 'with'). Jacques Alex Jacobson wrote: > Consider this module for a blog entry that I will want to put in > various generic collections that require Ord > > {-# OPTIONS -fglasgow-exts #-} > module Blog.Types where > import Data.Typeable > import Data.Generics > > data BlogEntry = Entry EpochSeconds Name Email Title Body > deriving (Eq,Ord,Read,Show,Typeable) > > newtype Name = Name String deriving (Eq,Ord,Read,Show,Typeable) > newtype Title = Title String deriving (Eq,Ord,Read,Show,Typeable) > newtype Body = Body String deriving (Eq,Ord,Read,Show,Typeable) > > > It seems really unnecessarily verbose. Having to add the OPTION > header AND import Data.Typeable and Data.Generics just to derive > Typeable is a beat-down. It is even more of a beat-down to have to > add a deriving clause for every newtype to make this all work nicely. > Is there a way to make all types automatically derive everything > unless there is an explicit instance declaration otherwise? > > {-# OPTIONS -fglasgow-exts -fgenerics -fderiving#-} > module Blog.Types where > > data BlogEntry = Entry EpochSeconds Name Email Title Body > newtype Name = Name String newtype Title = Title String newtype > Body = Body String > Isn't that much nicer? > > -Alex- > > > _______________________________________________ > Haskell mailing list > Haskell(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell

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How do I insert an element in HaXml?
by Jeremy Shaw 22 May '07

22 May '07

Hello, How do I create a HaXml filter that adds a new element as a child of an existing element. For example, let's say I have the XML document: <a> <b/> <c/> </a> How do I add a new element under <a /> so that I have the document: <a> <newElement/> <b/> <c/> </a> The following works, but it seems very hackish because I have to use the underlying HaXml data types. I think I could possibly use the filters in Text.XML.HaXml.Xtract.Combinators, but that seems like overkill for such a seemingly simple (and common?) operation. import Text.XML.HaXml import Text.XML.HaXml.Types import Text.XML.HaXml.Pretty insertNewElement = extract (\(CElem (Elem name _as cs)) -> mkElem name [mkElem "newElement" [] , const cs]) where extract :: (Content -> CFilter) -> CFilter extract f = \c -> f c undefined -- these are just helper functions to invoke my filter. But, I'll -- happily accept improvements to these functions as well ;) main = print $ document $ runFilter "<a><b /><c /></a>" insertNewElement runFilter :: String -> CFilter -> Document runFilter xmlStr cfilter = let (Document p s e m) = xmlParse "" xmlStr in case (cfilter (CElem e)) of [CElem e'] -> Document p s e' m [] -> error "produced no output" _ -> error "produced more than one output" thanks! j.

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List algorithm
by Steffen Mazanek 22 May '07

22 May '07

Hello, is there an efficient algorithm that takes two positive numbers n and m and that computes all lists l of numbers 0<x<=n such that sum l = m? For instance alg 5 1 = [[1]] alg 5 2 = [[1,1],[2]] alg 5 3 = [[1,1,1],[1,2],[2,1],[3]] ... I know that filter (\l->sum l == m) (powerSet [1..n]) would do the job, however I am looking for a more efficient approach. Help is greatly appreciated, even a google search term would be fine :-) I really hope for a polynomial algorithm although I am not very optimistic about this. Ciao, Steffen

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Re: [Haskell-cafe] Ann: Emping 0.2
by Hans van Thiel 22 May '07

22 May '07

On Mon, 2007-05-21 at 14:06 -0700, Scott Cruzen wrote: > Is Emping an implementation of C4.5 or something new? No, it's new. The reduction algorithm is not based on a measure and is not a search either. It's a construction of predicate combinations, starting with singletons, that are not contradictory to known facts. The third step, following hypothesis and then falsification, is verification with observed rules. The complexity appears to be quadratic to the original rule length and also to the number of original rules, but I have yet to prove this. I believe I have proved that the algorithm produces the complete and shortest disjunctive normal form of a logical rule table. The white paper 'Deriving heuristic rules from facts' http://j-van-thiel.speedlinq.nl/dwnl/EmpingWP.pdf (21 pages) discusses the theory from examples, and an earlier wp (also available on the web site) uses some first order predicate logic and set theory. Version 0.2 also shows logical dependencies in the reduced rules and this is just a search in the original rules and in the poset of reductions. So this is not new in a theoretical sense, though I believe no other implementation has ever considered such dependencies. Regards, Hans van Thiel > > * Hans van Thiel <hthiel.char(a)zonnet.nl> [070521 05:45]: > > Hello All, > > > > Version 0.2 of Emping, a utility to derive heuristic rules from a table > > of nominal data, is available. In addition to a reduced normal form > > in .csv format, which can be read by Open Office Calc, it now also > > displays observed implications and equivalences in the reduced rules (if > > present) and ambiguities in original rules (if present). > > A bug in the response to a user typing error has been fixed and some > > code has been simplified, compared to 0.1. An online user guide is at: > > http://j-van-thiel.speedlinq.nl/emp/empug.html and the Cabal package can > > be downloaded from there, or from the HackageDB. > > > > Thanks, > > > > Hans van Thiel > > > > > > _______________________________________________ > > Haskell-Cafe mailing list > > Haskell-Cafe(a)haskell.org > > http://www.haskell.org/mailman/listinfo/haskell-cafe

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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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