- Haskell-Cafe - Haskell.org

Functions of type foo :: f a -> g a
by Gordon J. Uszkay 11 May '10

11 May '10

I would like to build a class that includes a method to move data from one arbitrary functor to another, a natural transformation. The structures might be more than just functors, but I can start with that. I ran into some practical issues with resolving the type variables for my multiparameter type class, which I can resolve with functional dependencies. I can also not isolate the natural transformation from my overall operation, muddling it with the element transformation. I was wondering if anyone had any words of advice, example or warning about this kind of function or method in general? Class (Functor f, Functor g) => Foo f g a where foo :: f a -> g a bar :: (a->b) -> g a -> g b Thanks, Gordon J. Uszkay, McMaster University uszkaygj(a)mcmaster.ca

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How efficient is read?
by Tom Hawkins 11 May '10

11 May '10

I have a lot of structured data in a program written in a different language, which I would like to read in and analyze with Haskell. And I'm free to format this data in any shape or form from the other language. Could I define a Haskell type for this data that derives the default Read, then simply print out Haskell code from the program and 'read' it in? Would this be horribly inefficient? It would save me some time of writing a parser. -Tom

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Haskell meeting in Leipzig, Germany, June 4
by Johannes Waldmann 11 May '10

11 May '10

The Fifth Haskell in Leipzig meeting will be held on June 4, see http://www.iba-cg.de/hal5.html . You can still submit proposals (and wishes) for tutorials and talks (deadline: May 14). For instance, we'd love to hear about recent advances in Haskell IDEs (eclipsefp? leksah?), and parallel/multicore programming is always a hot topic as well. The primary language is German but we can switch to English if requested/agreed upon by audience and speaker. Best regards, Johannes Waldmann, HTWK Leipzig.

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Why cannot ghc find a existng module ?
by zaxis 11 May '10

11 May '10

%pacman -Q|grep xmonad xmonad 0.9.1-4 xmonad-contrib 0.9.1-2 %%xmonad --recompile Error detected while loading xmonad configuration file: /home/sw2wolf/.xmonad/xmonad.hs xmonad.hs:20:7: Could not find module `XMonad.Layout.LayoutHints': Use -v to see a list of the files searched for. %ghc-pkg list|grep xmonad xmonad-0.9.1 xmonad-contrib-0.9.1 %pacman -Ql xmonad-contrib|grep LayoutHints xmonad-contrib /usr/lib/ghc-6.12.1/site-local/xmonad-contrib-0.9.1/XMonad/Layout/LayoutHints.hi Sincerely! ----- e^(π⋅i) + 1 = 0 -- View this message in context: http://old.nabble.com/Why-cannot-ghc-find-a-existng-module---tp28507704p285… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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ANNOUNCE: hamlet 0.2.0
by Michael Snoyman 10 May '10

10 May '10

Hi all, I'm happy to announce the second major release of Hamlet[1]. Hamlet is a HTML templating library which works via quasi-quoting, giving you compile-time assurances, type safety and very efficient HTML generation. I wrote a blog post[2] describing the changes in this version of Hamlet. The two most major are reversing of identifier order in references and dropping the template argument. Combined, these changes make a Hamlet template fit in better with Haskell code. All the other changes are noted in that same blog post. The Hamlet documentation[3] has been updated to reflect all changes. Please let me know if the documentation needs clarification on any points. Cheers, Michael [1] http://hackage.haskell.org/package/hamlet-0.2.0 [2] http://www.snoyman.com/blog/entry/hamlet-version-0-2/ [3] http://docs.yesodweb.com/hamlet/

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Well-Typed are hiring
by Duncan Coutts 10 May '10

10 May '10

We are looking to hire a Haskell expert to work with us at Well-Typed as a Haskell consultant. We are seeing an increasing demand for our services, and are thus seeking to expand our capacity. This is an exciting opportunity for someone who is passionate about Haskell and who is keen to improve and promote Haskell in a professional context. The role is quite general and could cover any of the projects and activities that we are involved in as a company. The tasks may involve: * working on the Haskell compilers, libraries and tools; * Haskell application development; * working directly with clients to solve their problems. Well-Typed has a variety of clients. For some we do proprietary Haskell development and consulting. For others, much of the work involves open-source development and cooperating with the rest of the Haskell community: the commercial, open-source and academic users. The position is initially as a contractor for one year with a salary comprised of a base level of 150 GBP per day, plus a profit-dependent component. We offer flexible hours and work from home. Living in England is not required. In the longer term there is the opportunity to become a member of the partnership with a full stake in the business: being involved in business decisions, and fully sharing the risks and rewards. If you are interested, please apply via info(a)well-typed.com. Send us your CV, and tell us why you are interested and why you would be a good fit for the job. We are more than happy to answer informal enquiries. Contact Duncan Coutts or Ian Lynagh for further information, either by email or IRC. The deadline for applications is Friday 11 June 2010. -- Duncan Coutts, Haskell Consultant Well-Typed LLP, http://www.well-typed.com/

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polyvariadic function for Applicative instances
by Xiao-Yong Jin 10 May '10

10 May '10

Hi, Is it possible to have a function accept variable number of arguments, such that 'f' can be instantiated to different concrete types as f :: Applicative a => (e1 -> f) -> a e1 -> A f f g a = pure g <*> a f :: Applicative a => (e1 -> e2 -> f) -> a e1 -> a e2 -> A f f g a b = pure g <*> a <*> b Thanks, Xiao-Yong -- J c/* __o/* X <\ * (__ Y */\ <

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Re: [Haskell-cafe] accents
by Dupont Corentin 10 May '10

10 May '10

Thank you Daniel and Ivan, with firefox i finded out that my text file was encoded in WINDOWS-1252. So a commande line such as: iconv -f WINDOWS-1252 -t ISO-8859-1 liste.txt > liste2.txt did the trick. Alternatively, i modified my code with: myReadFile a = do h <- openFile a ReadMode te <- mkTextEncoding "CP1252" hSetEncoding h te hGetContents h Corentin On 5/7/10, Daniel Fischer <daniel.is.fischer(a)web.de> wrote: > On Friday 07 May 2010 17:05:08 you wrote: >> I don't know the encoding of my file, how to deduce it? > > What happens if you open it in a text editor? > When it looks right, can you find out from the editor what encoding it > used? > > You could try opening the file in Firefox (or some other browser), go to > View -> Character Encoding > and see what it thinks it is (click Auto-Detect if you have selected some > default encoding) if it looks right. If it looks garbled, try selecting > different encodings to see if one looks right. > > You could send me [a part of] the file and I could try and find something > out (play a little with iconv). >

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darcs to mercurial migration
by Günther Schmidt 10 May '10

10 May '10

Hello, I'm switching from darcs to mercurial with some of my projects. I'd like to retain as much of the history as possible, what tools are there available for this? Günther

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Type of (>>= f) where f :: a -> m b
by Milind Patil 10 May '10

10 May '10

For a function f :: a -> m b f = undefined I am having trouble understanding how the type of (>>= f) is (>>= f) :: m a -> m b where, by definition, type of (>>=) is (>>=) :: (Monad m) => m a -> (a -> m b) -> m b I do not see how (>>= f) even unifies. I mean if I code a function with the same type as (>>=) ie. tt :: (Monad m) => m a -> (a -> m b) -> m b tt = undefined type of (tt f) does not infer to the same type as (>>= f), from ghc ... (tt f) :: (Monad ((->) b)) => (m a -> b -> b1) -> b -> b1 There seems to something special about (>>=) apart from its type. And whats (Monad ((->) b))? I am new to Haskell and I may have gaps in my understanding of type inference in Haskell. regards, Milind Patil

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