- Haskell-Cafe - Haskell.org

Error in HTTP 4.004 + CouchDB?
by Alex Ott 24 Jan '09

24 Jan '09

Hello (i sent this message to the mail of author, specified at hackage, but it returned to me, so i decided to resend it here) I found strange thing during work with latest versions of CouchDB & HTTP module on my Mac OS X 10.4. The problem is, that when i send document to the couchdb server, it name was cut to the 2 characters. After some investigation i found, that this is problem in combination of the CouchDB + HTTP usage. CouchDB module fills out the Request structure, and specify scheme & authority fields. This lead that i see (in network dumps) following data: PUT http://127.0.0.1:5984/test1/Users_ott_tmp_1_tst HTTP/1.1 Content-Type: ... ... Host: 127.0.0.1:5984 ... instead of proper PUT /test1/Users_ott_tmp_1_tst HTTP/1.1 I can eliminate this by explicitly setting of the scheme & authority into empty values, but this lead to improper creation of the Host header in request: PUT /test1/Users_ott_tmp_1_tst HTTP/1.1 Content-Type: ... ... Host: ... I tried to fix this problem myself, but I'm novice in the industrial Haskell programming ;-) P.S. this code from (CouchDB/HTTP.hs) will restore proper behaviour of the code, but break Host header makeURL path query = CouchMonad $ \conn -> do return ( (ccURI conn) { uriPath = '/':path , uriQuery = if null query then "" else '?':(urlEncodeVars query) , uriScheme = "" , uriAuthority = Nothing } ,conn ) -- With best wishes, Alex Ott, MBA http://alexott.blogspot.com/ http://xtalk.msk.su/~ott/ http://alexott-ru.blogspot.com/

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How to define an operation in terms of itself (but of different type)?
by Olex P 23 Jan '09

23 Jan '09

Hello Haskellers! It's probably a simple question but I can't find a proper solution... Suppose we have a class Vector which overloads (+) operation. I'd like to represent a Matrix data type as a set of vectors: data Matrix3 = M3 !Vector3 !Vector3 !Vector3 In this case (+) for matrices could be implemented as: (M3 r11 r12 r13) + (M3 r21 r22 r23) = M3 (r11 + r21) (r12 + r22) (r13 + r23) But GHC says about ambiguous occurrence. I don't understand why because it's should be pretty clear that rows are vectors and (+) has to be vector type as well. I could use Vector.+ instead but it doesn't look good. Probably I'm missing something. Any ideas? Cheers, Oleksandr.

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Employment
by Andrew Coppin 23 Jan '09

23 Jan '09

Is it possible to earn money using Haskell? Does anybody here actually do this? Inquiring minds want to know... ;-)

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mapM_ -> Monoid.Monad.map
by Henning Thielemann 23 Jan '09

23 Jan '09

I always considered the monad functions with names ending on '_' a concession to the IO monad. Would you need them for any other monad than IO? For self-written monads you would certainly use a monoid instead of monadic action, all returning (), but IO is a monad. (You could however wrap (newtype Output = Output (IO ())) and define a Monoid instance on Output.) However our recent Monoid discussion made me think about mapM_, sequence_, and friends. I think they could be useful for many monads if they would have the type: mapM_ :: (Monoid b) => (a -> m b) -> [a] -> m b I expect that the Monoid instance of () would yield the same efficiency as todays mapM_ and it is also safer since it connects the monadic result types of the atomic and the sequenced actions. There was a recent discussion on the topic: http://neilmitchell.blogspot.com/2008/12/mapm-mapm-and-monadic-statements.h…

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Re: [Haskell-cafe] MySQL and HDBC?
by Duncan Coutts 23 Jan '09

23 Jan '09

On Fri, 2009-01-23 at 03:54 -0600, Galchin, Vasili wrote: > On Fri, Jan 23, 2009 at 3:23 AM, Duncan Coutts > <duncan.coutts(a)worc.ox.ac.uk> wrote: > > This was uploaded to hackage yesterday: > > http://hackage.haskell.org/cgi-bin/hackage-scripts/package/HDBC-mysql-0.1 > > You might like to test it and give feedback to the > author. > ooops ... cabal install HDBC-mysql doesn't work ....?? > > Vasili Vasili, it's really no good telling me this, especially since you did not cc the author or haskell-cafe. I am not the author of the package. And even if I were you have provided no details on what goes wrong. I'm sure it was not your intention, but to some people your response would sound demanding and unhelpful, possibly even rude. Make sure that when you do contact the author that you send them something that they will find helpful. So when you email the author make sure you provide full details about what you did and about the environment you are using, eg platform, ghc version, where your mysql libraries are installed, what version they are etc. I would guess that the most likely problem is that you do not have the mysql development package installed. It should provide the mysql_config program. If that is not the problem then contact the package author with the full details. Duncan

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Haskell mode for Emacs question
by Peter Verswyvelen 23 Jan '09

23 Jan '09

I have a silly problem. I'm using Emacs with the Haskell mode extension on Windows I have a source file in say c:/foo/src/main.hs main.hs is importing some other modules in that same src directory When I invoke GHCi from within Emacs, the first thing it does is :cd c:/foo and then :load "src/main.hs" But of course GHCi won't find the imported modules now, since the current directory is wrong. If I type in GHCi :cd src :load "main.hs" then it compiles fine. Does anyone have an idea why Emacs or the Haskell mode is switching to the parent directory of src instead of src itself, and how to fix this? Thanks a lot, Peter

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Good books on advanced topics
by Mark Spezzano 23 Jan '09

23 Jan '09

Hi, Is there a good book on some of the more advanced or esoteric Haskell topics, like monads, and morphisms, category theory etc. Cheers, Mark No virus found in this outgoing message. Checked by AVG. Version: 7.5.552 / Virus Database: 270.10.12/1910 - Release Date: 22/01/2009 6:28 PM

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Is it possible to model eventually consistent side effects?
by Daryoush Mehrtash 23 Jan '09

23 Jan '09

I am trying to figure out if there is a way to model cloud computing computations in Haskell. My specific problems is that in cloud computing, as in Amazon WebServices, side effects (writes to storage, simple database, queue) follow the eventually consistent model. Which means even if your writes succeeds, it may or may not be immediately available for reads. Each call to the services may also temporarily experience network errors or end-system unavailability. Virtual machines can mount a local disk that does offers consistent storage, but the storage wont survive a reboot (after a crash). I am trying to find out if I can model computation (and its compositions) where I can hide reliability and consistency issues. And also be able to reason about a computation to make sure no combination of errors and inconsistencies would break my logic. Is it possible? If so is there a good starting point? -- Daryoush

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Liskell installation configuration problems in Cygwin on Windows XP Professional, Service Pack 2
by Benjamin L.Russell 23 Jan '09

23 Jan '09

Yesterday, I downloaded Darcs 2.2 and Liskell for Windows XP Professional, Service Pack 2, but the following error message appeared when I ran "./Setup.lhs configure" in Cygwin: Configuring liskell-0.1... Setup.lhs: At least the following dependencies are missing: ghc-paths -any Does anybody know how this issue should be resolved? -- Benjamin L. Russell -- Benjamin L. Russell / DekuDekuplex at Yahoo dot com http://dekudekuplex.wordpress.com/ Translator/Interpreter / Mobile: +011 81 80-3603-6725 "Furuike ya, kawazu tobikomu mizu no oto." -- Matsuo Basho^

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Re: [Haskell-cafe] Employment
by George Pollard 23 Jan '09

23 Jan '09

On Fri, 2009-01-23 at 16:19 +1100, Toby Hutton wrote: > On Fri, Jan 23, 2009 at 11:56 AM, Jonathan Cast > <jonathanccast(a)fastmail.fm> wrote: > > > > Not really. My company *advertises* for Haskell developers, and then > > when they come in to interview, informs them that the code base is > > actually written in Perl. Works, too --- we have several Haskellers > > working here. If all you care about is the quality of the developers, > > and not their productivity once you've got them, you don't actually need > > to let them use Haskell after the interview is over... > > I saw this trick recently for a job advertised locally to me, in > Melbourne. I was initially pretty excited that someone in this city > was actually advertising for Haskell programmers, until I realised > they needed to be good at Javascript and Perl so they could work on > their web apps. Argh, I didn't bother applying. I think I saw the same advertisement; unfortunately it was on a joint Australasian website so I was in the wrong country anyway :)

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