August 2008 - Haskell-Cafe

The Italian Haskellers Summer Meeting is Over: What Next?
by Titto Assini 13 Aug '08

13 Aug '08

E cosi' il primo incontro balnear/ricreativo degli Haskeller Italiani si e' felicemente concluso. Come da programma, rigidamente rispettato, abbiamo fatto il bagno e mangiato pizza e gelato ma soprattutto ci siamo conosciuti ed abbiamo discusso di tutto e di piu', prevalentemente Haskell (folds, arrows, happs, sviluppo web) ma non solo (democrazia elettronica, la crisi economica, mercato ed autosufficienza). Non si puo' dire che l'affluenza sia stata oceanica ma non dimentichiamo che con 3 persone si puo' fare una cellula rivoluzionaria e con una cellula rivoluzionaria si puo' cominciare a pensare a fare la rivoluzione :-) Insomma e' stato piacevole ed utile e lo riorganizzeremo certamente il prossimo anno. Ed adesso? E' evidente, dobbiamo organizzare un incontro invernale, sullo stile di AngloHaskell, per presentare e discutere progetti di sviluppo, idee per futuri progetti ed iniziative che possano servire a rafforzare ed estendere la comunita' Haskell Italiana. Interessati? Date uno sguardo a http://www.haskell.org/haskellwiki/ItaloHaskell Saluti, titto -- Pasqualino "Titto" Assini, Ph.D. 25 Heath Road - CO79PT Wivenhoe - Colchester - U.K.

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Dealing with heterogeneously-typed lists?
by Leif Warner 13 Aug '08

13 Aug '08

Hi all, I'm dealing with some datatype, say: data Invoice = Invoice { invoiceNum :: String, dollarAmt :: Currency, printDate :: Date, dueDate :: Date, vendorNum :: Int, vendorName :: String, isStock :: Bool, } that I want to put in or take out of rows in xml spreadsheets, databases, that sort of thing. The elements of the spreadsheet row want type annotations like "String", "Number, "DateTime", etc. Also, each data element is required to be formatted differently, e.g. (show dueDate) ++ "T00:00:00", because the spreadsheet format wants a time tacked on, too. I know in a language like Java or C++ I might do some sort of run-time type identification, which would detect the type of each element in the list, and output it appropriately. As it is, I'm tupling each accessor function with the the type the spreadsheet wants, as a string: e.g. [(invoiceNum, "String"), (dollarAmt, "Number"), (printDate, "DateTime"),...] Is there a better way of doing this? Maybe messing with the "show" function to precede the string representation with a few characters about the type it's supposed to be? Or would looking into something like this be a good idea? It looks a lil old... http://homepages.cwi.nl/~ralf/HList/<http://homepages.cwi.nl/%7Eralf/HList/> --Strongly typed heterogeneous collections Thanks! -Leif

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GLfloat on a Mac
by Conor McBride 12 Aug '08

12 Aug '08

Hi folks I thought I'd try a bit of OpenGL. Perhaps I should send this to the more specific list, but perhaps other people are, like me, trying out a variety of UI technology. I thought I'd give OpenGL a go, because I saw the name whizz by when I upgraded to 6.8.3. I found the tutorial and got copy-paste cracking. It was all going swimmingly while I did almost nothing, but then ghc started dying on me. I chopped down my failing code to this joyous specimen: HelloWorld.lhs contains ---------------------------------- > module Main where > import Graphics.Rendering.OpenGL > import Graphics.UI.GLUT > myFloat :: GLfloat > myFloat = 0.0 > main = return () ---------------------------------- $ ghc -package GLUT HelloWorld.lhs -o HelloWorld Illegal instruction I'm using ghc 6.8.3 on a Mac PowerBook G4. Googling "OpenGL illegal instruction" produced an unending choice of horror stories. What message am I not getting? Is there some crucial manual I'm not reading? Is there something which we should be being told on the relevant wiki page? Distressed Conor

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Re: [Haskell-cafe] Recommended Haskell Books
by haskell＠kudling.de 12 Aug '08

12 Aug '08

Also the Beta Version of "Real World Haskell" looks promising: http://book.realworldhaskell.org/beta/index.html >On Sun, Aug 10, 2008 at 1:29 PM, Warren Aldred <warren(a)live.com> wrote: >> I'm new to Haskell and looking for recommendations on introductory Haskell >> books. Online or offline. Any suggestions? > >I like >_Programming in Haskell_ (Hutton) >_The Haskell School of Expression_ (Hudak) >_Real World Haskell_ (Goerzen, O'Sullivan, Stewart) > >I'd advise against _The Haskell Road to Logic, Maths, and Programming_ >until much later unless you're strongly mathematically inclined.

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my first post
by shoulders 11 Aug '08

11 Aug '08

Thanks -----邮件原件----- 发件人: haskell-cafe-bounces(a)haskell.org [mailto:haskell-cafe-bounces@haskell.org] 发送时间: 2008年8月11日 18:33 收件人: onlyforjob(a)163.com 主题: The results of your email commands The results of your email command are provided below. Attached is your original message. - Results: Ignoring non-text/plain MIME parts You can access your personal options via the following url: http://www.haskell.org/mailman/options/haskell-cafe/onlyforjob%40163.com help Help for Haskell-Cafe mailing list: This is email command help for version 2.1.5 of the "Mailman" list manager. The following describes commands you can send to get information about and control your subscription to Mailman lists at this site. A command can be in the subject line or in the body of the message. Note that much of the following can also be accomplished via the World Wide Web, at: http://www.haskell.org/mailman/listinfo/haskell-cafe In particular, you can use the Web site to have your password sent to your delivery address. List specific commands (subscribe, who, etc) should be sent to the *-request address for the particular list, e.g. for the 'mailman' list, use 'mailman-request@...'. About the descriptions - words in "<>"s signify REQUIRED items and words in "[]" denote OPTIONAL items. Do not include the "<>"s or "[]"s when you use the commands. The following commands are valid: confirm <confirmation-string> Confirm an action. The confirmation-string is required and should be supplied by a mailback confirmation notice. end Stop processing commands. Use this if your mail program automatically adds a signature file. help Print this help message. info Get information about this mailing list. lists See a list of the public mailing lists on this GNU Mailman server. password [<oldpassword> <newpassword>] [address=<address>] Retrieve or change your password. With no arguments, this returns your current password. With arguments <oldpassword> and <newpassword> you can change your password. If you're posting from an address other than your membership address, specify your membership address with `address=<address>' (no brackets around the email address, and no quotes!). Note that in this case the response is always sent to the subscribed address. set ... Set or view your membership options. Use `set help' (without the quotes) to get a more detailed list of the options you can change. Use `set show' (without the quotes) to view your current option settings. subscribe [password] [digest|nodigest] [address=<address>] Subscribe to this mailing list. Your password must be given to unsubscribe or change your options, but if you omit the password, one will be generated for you. You may be periodically reminded of your password. The next argument may be either: `nodigest' or `digest' (no quotes!). If you wish to subscribe an address other than the address you sent this request from, you may specify `address=<address>' (no brackets around the email address, and no quotes!) unsubscribe [password] [address=<address>] Unsubscribe from the mailing list. If given, your password must match your current password. If omitted, a confirmation email will be sent to the unsubscribing address. If you wish to unsubscribe an address other than the address you sent this request from, you may specify `address=<address>' (no brackets around the email address, and no quotes!) who password See everyone who is on this mailing list. The roster is limited to list administrators and moderators only; you must supply the list admin or moderator password to retrieve the roster. Commands should be sent to haskell-cafe-request(a)haskell.org Questions and concerns for the attention of a person should be sent to haskell-cafe-owner(a)haskell.org - Done.

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ANN: BLAS bindings for haskell, version 0.5
by Patrick Perry 11 Aug '08

11 Aug '08

Hey everyone, I've put together a new release of the Haskell BLAS bindings, now available on hackage. Here are the new features: * Add Banded matrix data type, as well as Tri Banded and Herm Banded. * Add support for trapezoidal dense matrices (Tri Matrix (m,n) e, where m is not the same as n). Note that trapezoidal banded matrices are *NOT* supported. * Add Diag matrix data type for diagonal matrices. * Add Perm matrix data type, for permutation matrices. * Enhance the RMatrix and RSolve type classes with an API that allows specifying where to store the result of a computation. * Enhance the IMatrix, RMatrix, ISolve, and RSolve type classes to add "scale and multiply" operations. * Remove the scale parameter for Tri and Herm matrix data types. * Flatten the data types for DVector and DMatrix. * Some inlining and unpacking performance improvements. As far as what to expect in version 0.6, I plan to add support for operations in the ST monad. This is going to require a pretty big code reorganization and will cause quite a few API breakages, but I think it's worth the pain. The next release will also come with a tutorial and examples. After the big code reorganization, I'll get started on LAPACK bindings. Please let me know if you are using the library. I'm really interested in what people like and don't like. If you think that some functionality is missing, let me know. If you think the API is awkward in certain places, let me know that, too. Patrick Cross-posted to: http://quantile95.com/2008/08/11/ann-blas-bindings-for-haskell-version-05/

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Thinking about an unlistN
by Michael Feathers 11 Aug '08

11 Aug '08

I wrote this function the other day, and I was wondering if I'm missing something.. whether there is already a function or idiom around to do this. unlist3 :: (a -> a -> a -> b) -> [a] -> b unlist3 f (x:y:z:xs) = f x y z I was also wondering whether the function can be generalized to N or whether this is just one of those edges in the type system that you can't abstract over. Thanks, Michael

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best strategy for comparing comparisons
by Jason Dusek 11 Aug '08

11 Aug '08

I'd like to perform a micro-benchmark, pitting ByteString comparision against Word comparison. What I'd like to do is: . Generate a bunch of random Word32s. . Generate strict ByteStrings containing those Word32s. . Compare every Word32 with every other Word32 for equality, and see how long it takes. . Ditto with ByteStrings. The first two are easy and I'm done with them already. The latter two seem like they'll run afoul of laziness. I have two ideas about how to do them: . Do every comparison with a list comprehension, compressing the result as I go using `all`. . Do every comparison in a recursive IO procedure. Are there other ways? Will strictness annotations, for example, allow me to run comparisons on larger lists? -- _jsn

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self-referential data
by brian 11 Aug '08

11 Aug '08

>From https://secure.wikimedia.org/wikipedia/en/wiki/Bencoding , I think I should code data BValue = BString String | BInteger Integer | BList [BValue] | BDictionary (M.Map BString BValue) but: Not in scope: type constructor or class `BString' The implementations I've found just implemented BDictionary's map key as String, but I think that's kind of wrong.

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Anglo Haskell 2008
by Matthew Sackman 11 Aug '08

11 Aug '08

Anglo Haskell is a gathering of all people Haskell-related from beginners, to seasoned hackers to academic giants. All and more are welcomed by large fuzzy green lambdas. Anglo Haskell has happened for the last two years and we see no reason why it should not happen again this year. For the last two years they've tended to have talks on Fridays and then Other Things on the Saturday (including Punting and Group Hacking, some or more of which may have happened in Pubs). We're proposing the same general format this year. In contrast to the last two years which have been held at MSR Cambridge (UK), we're this year proposing to hold the event at Imperial College, London. London is probably easier to get to and from (though more tedious to get across) than Cambridge and we hope this will attract people who previously have not been able to get out to Cambridge. The proposed dates are Friday the 8th and Saturday the 9th of August. More details are available on the wikipage: http://www.haskell.org/haskellwiki/AngloHaskell/2008 Please feel free to add to this page. If you're at all interested in coming along then please add your name to the wikipage. If you're interested in giving a talk - on literally anything Haskell related: this is not a solely theory day or solely a practical day; anything goes which is Haskell related - then similarly add yourself to the list of speakers. Schedule and other details will be confirmed soon. Lodging, travel and all other related matters will be discussed on the irc channel (and will eventually migrate to the wikipage) #anglohaskell. If you're free that weekend then please come along! Also, if you'd really like to come but can't make those dates and there are many of you then please shout loudly and things could potentially be moved, though with the lack of time left before the event, we'd prefer to avoid this. Matthew

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