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Hackage Server not reachable
by Stuart Coyle 22 Jun '11

22 Jun '11

I cannot reach the hackage server so cabal can't download packages. Have I the correct address? http://hackage.haskell.org stuart@rumbaba:~# resolveip hackage.haskell.org IP address of hackage.haskell.org is 69.30.63.204 I also cannot access any of the Hackage web pages. I suspect that my ISP may be filtering or that their DNS is giving me the wrong info. -- Stuart Coyle stuart dot coyle at gmail dot com

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yi + cabal-dev
by Alex Rozenshteyn 22 Jun '11

22 Jun '11

I'm trying to run yi. More precisely, I'm trying to run yi in its own sandbox, created by cabal-dev. yi uses dyre to recompile its config file. Unsurprisingly, this fails, since ghc doesn't know anything about the yi install unless pointed to a separate package database. Has anyone gotten a similar setup to work, or does anyone have any suggestions? -- Alex R

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Category theory as a design tool
by Arnaud Bailly 22 Jun '11

22 Jun '11

Hello, I ha

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Using cmake with haskell
by Robert Clausecker 22 Jun '11

22 Jun '11

Hello readers of haskell-cafe! I asked this question on Stackoverflow: http://stackoverflow.com/q/5969616/417501 Because I want to get a good answer, I cross post it here. It may be interesting: Is it possible to use cmake for Haskell projects? I am planning a project written in Haskell, maybe there are some parts in C as well. For the buildsystem I decided against the common choice for Haskell programs cabal, mainly because I want to learn how building programs in other languages works. I heard about CMake and I think it is a pretty cool product. Although I have no knowledge in how to use it, I want use CMake for that project, just to find out how it works. Googleing did not reveal any facts about how to use cmake with haskell, and all tutorials I read were rather confusing. Is it possible, and if yes, how is it possible, to compile a projectwritten in Haskell using CMake? Robert Clausecker

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Re: [Haskell-cafe] Why aren't there anonymous sum types in Haskell?
by wren ng thornton 22 Jun '11

22 Jun '11

On 6/21/11 6:54 PM, Casey McCann wrote: > That said, I don't think either retains the tidy algebraic properties > that disjoint unions and tuples have, so I'm not sure if calling them > sums and products is actually correct. I don't think there are any problems[1]. Categorically speaking, a product of A and B is simply a thing, which we can call Foo[2], along with two projections, f :: Foo -> A and oo :: Foo -> B[3], such that the Foo is the "closest" thing to A and B[4]. I chose the funny names intentionally. Nothing in the definition of products says that there has to be any kind of notion of order between the components, only that the composite has projections for each component and that it has no extra junk. Consequently A*B is isomorphic to B*A, which means they are essentially the same thing[5]. The choice of order is just a convention. For a slightly stronger notion of products we can look to monoidal categories which have that A*(B*C) is isomorphic to (A*B)*C, again meaning that they're essentially the same thing. Outside of category theory, we can look to domain theory (or the associated CPO categories), but domain products and smash products both have the same order independence as categorical products. For coproducts, dually. [1] modulo the A:+:A ~ A issue. [2] More conventionally called A*B, but remember: that's just a name. [3] More traditionally, pi_1 and pi_2, or fst and snd, or... [4] That is, for any other Bar with b :: Bar -> A and ar :: Bar -> B, there is a unique function from Bar to Foo such that everything commutes. [5] The clever reader will notice that they are not *exactly* the same thing, not identical; however, category theorists rarely care about the differences between isomorphic things unless it's mucking up a proof. -- Live well, ~wren

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Getting library documentation for installed packages on Windows
by John Ky 21 Jun '11

21 Jun '11

Hi all, Lately I've been finding the Network module missing from the docs I download from the GHC website, which brings me to the question: Is there any way I could generate libraries for this, the core libraries and all installed cabal packages into one reference so I can work offline? Cheers, -John

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Best Linux Distro for >>=
by Tom Murphy 21 Jun '11

21 Jun '11

Hi List, If my choice of Lunix distro depended 100% on its solidness as a Haskell devel platform (I am), what would you all recommend? Thanks for your time, Tom

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Re: [Haskell-cafe] Choice of Linux distro redux
by John Wagner 21 Jun '11

21 Jun '11

Friends: When you say that support for GHCi is in general best among the Linux distros (unordered) "Gentoo, Arch, Fedora and Debian (I think Testing)", is Ubuntu to be inferred from Debian, or is there some scandalous and well-known to all (but me) malady that lingers around the bellies of Ubuntu distros that interferes with the purity of Glasgow, producing side-effects? My grandfather was a Scot, so I can be trusted: tell me if I should cast off Canonical ways. Thanks, John Wagner -- John Wagner Alhambra and Los Angeles

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Re: [Haskell-cafe] [Haskell-beginners] boomBangs xs = [ if x < 10 then "BOOM!" else "BANG!" | x <- xs, odd x]
by Jack Henahan 21 Jun '11

21 Jun '11

Hi Anthony. What you've got there is a list comprehension. I'll try to break down what it says for you. The basic structure of a list comprehension is function <arguments> = [ description of the list ] In this function, the argument 'xs' is a Haskell idiom. We use 'xs' to mean "a list of x's". 'xs' doesn't *have* to be a list (since you can call anything 'xs'), but conventionally it is. So 'boomBang xs' simply means "the function boomBangs takes a list xs as an argument." Now the odd part, the list comprehension itself. The first part describes what will be in the list, and the second part limits it. What this comprehension does is construct a list of all the odd members of the list 'xs' (in this case, they're numbers) and map "BOOM!" to values under 10, and "BANG!" to anything above 10. So what is says is "if the nth number in my list is less than 10, I want you to make the nth element of the new list "BOOM!", otherwise I want you to make it "BANG!". Furthermore, I only want you to consider odd values of x from my list." So testList xs = [ x | x <- xs, odd x ] called with the list [1,4,8,9,12,15] would return [1,9,15] -- That is, all the odd members of the list xs Now, if 'x' is the expression 'if x < 10 then "BOOM!" else "BANG!"', the list returned is instead ["BOOM!","BOOM!","BANG!"] Decent explanation? Or have I just muddled the issue? On Jun 21, 2011, at 3:58 PM, anthony niro wrote: > Hello, > > My name is Anthony i have few question about Haskell. > > I was reading a tutorial everything goes well. but now i have few things that i dont understang. > > HERE: > > boomBangs xs = [ if x < 10 then "BOOM!" else "BANG!" | x <- xs, odd x] > > > > I dont understand this line except the "if" "then" "else". > What is xs? > what is the | ? > and why doing this " | x <- xs, odd x]" > > why x <- xs????? what is that > > and what is odd x? > > thx everyone for answer me. > _______________________________________________ > Beginners mailing list > Beginners(a)haskell.org > http://www.haskell.org/mailman/listinfo/beginners

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[ANN] mysql-simple - your go-to package for talking to MySQL
by Bryan O'Sullivan 21 Jun '11

21 Jun '11

Hi, folks - Over the past few days, I've released two MySQL-related packages on Hackage that I think should be pretty useful. The first is mysql-simple: http://hackage.haskell.org/package/mysql-simple This is a mid-level binding to the MySQL client API. I aimed it squarely at being both fast and easy to use, and I'm very pleased with the results so far. - Performance: compared to HDBC-mysql, mysql-simple yields a 60% performance improvement on my real-world application. - Ease of use: compared again to HDBC, my real-world application's DB-specific code shrunk by 50% while achieving the above performance increase. - Type safety: it's intentionally hard to construct SQL queries by string concatenation, but of course I provide some nice safe APIs for formatting queries and converting results. That safety does not come at the expense of performance or expressive bloat, as the above results indicate. Because I know that some people favour interacting with their databases via a model such as iteratees, the mysql-simple library is built on top of a very lightweight library. That lower-level library is named mysql: http://hackage.haskell.org/package/mysql This is a low-level binding to the MySQL client API. It is aimed at high performance and simplicity, but more specifically for consumption by authors of higher-level database libraries. It is bare enough of features that it doesn't even perform conversion between Haskell and SQL types, but at the same time it uses bytestrings sensibly, cheaply avoids some nasty signal interruption problems with the MySQL client library, and abstracts some of the tiresome details of memory management. It gives a higher-level library complete control over result conversion, how to fetch results, and all that, so you can focus purely on building iteratees and not the lower-level gunk. Enjoy! B.

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