May 2010 - Haskell-Cafe

Tsuru Capital is hiring
by Simon Cranshaw 11 May '10

11 May '10

We are looking to hire a programmer that has experience programming in a functional language, and is familiar with Haskell. There is a wide range of tools and applications that we want to build over time, including graphical interfaces and profiling tools for our trading system, parsers for new market feeds, and algorithms for trading strategies. If you have experience in any one of these areas it's a definite bonus. You will be working with a small team of developers, and from time to time will be working along side Traders to assist in tool development. Annual compensation will be in the region of USD150k with potential for growth. The team is based in Tokyo but there are projects that could be done remotely so feel free to apply for this type of work also. http://tsurucapital.com/en/jobs.html#programmer

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comprehending monad execution
by rick＠rickmurphy.org 11 May '10

11 May '10

All: I've been analyzing monads for a while now and have a question on execution. I suspect I've missed something basic. In the snippet below the ghc debugger shows that in just one single step Discriminate has been substituted for g. Even with :trace turned on in the debugger there is no evidence of an execution step to carry out the substitution. This leads me to assume a compile time operation identifies Discriminate as a substitute for g. Could someone correct my assumption or explain how this works? Note: this snippet was extracted from [1]. > {-# LANGUAGE EmptyDataDecls, Rank2Types #-} > module LEM where > import Control.Monad.Identity > > data F -- no constructors > > data Discriminate a b = L a | R b > instance Monad (Discriminate e) where > return = R > R x >>= f = f x > L x >>= f = L x > > h :: (forall m. Monad m => ((a -> m F) -> m F)) -> a > h g = case g (\x -> L x) of > R x -> error "Not reachable" > L x -> x > > -- A proof term for a -> NOT NOT a > lift :: forall a m. Monad m => a -> ((a -> m F) -> m F) > lift x = \k -> k x > > t = h $ lift True > ghci trace ... C:\haskell\lem>ghci lem.lhs GHCi, version 6.8.3: http://www.haskell.org/ghc/ :? for help Loading package base ... linking ... done. [1 of 1] Compiling LEM ( lem.lhs, interpreted ) Ok, modules loaded: LEM. *LEM> :b h Breakpoint 0 activated at lem.lhs:(14,2)-(16,17) *LEM> :trace t Loading package mtl-1.1.0.1 ... linking ... done. Stopped at lem.lhs:(14,2)-(16,17) _result :: a = _ [lem.lhs:(14,2)-(16,17)] *LEM> :step Stopped at lem.lhs:(14,8)-(16,17) _result :: a = _ g :: (a -> Discriminate a F) -> Discriminate a F = _ [lem.lhs:(14,8)-(16,17)] *LEM> :hist -1 : h (lem.lhs:(14,2)-(16,17)) -2 : t (lem.lhs:22:6-18) <end of history> [lem.lhs:(14,8)-(16,17)] *LEM> My previous analysis includes running single step execution on Wadler's I monad here [2], where I am able to observe execution of the I monad. [essence.lhs:32:33-37] *Essence> :hist -1 : unitI (essence.lhs:7:34) -2 : unitI (essence.lhs:7:2-34) -3 : interp (essence.lhs:32:26-38) -4 : interp (essence.lhs:(31,2)-(39,44)) -5 : test (essence.lhs:54:41-51) -6 : showval (essence.lhs:(26,2)-(28,45)) -7 : showI (essence.lhs:9:34-42) -8 : showI (essence.lhs:9:2-42) -9 : test (essence.lhs:54:34-52) -10 : test (essence.lhs:54:2-52) <end of history> [1] http://okmij.org/ftp/Computation/lem.html [2] http://homepages.inf.ed.ac.uk/wadler/papers/essence/essence.ps Thanks much in advance for your help. Rick

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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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