October 2010 - Haskell-Cafe

Web development work
by Michael Snoyman 10 Oct '10

10 Oct '10

Hi all, Often times when trying to pitch Haskell to potential clients the concern is the lack of qualified developers willing to take on projects. As I'm sure many of you are familiar with, clients prefer not to be locked in to a single programmer: an errant bus can significantly reduce the value of their investment. So I'd like to get an idea of how many people out there would be interested in Haskell web development work. For the moment I'm just interested in doing this informally via email, though perhaps in the future it would be beneficial to the community to have this information centralized on a website. I think it would be useful to have some basic skills and experience information, including system administration abilities. I might also have one or two Yesod projects that I'll need to pass off in the future, though it's still unclear if this will happen. If you're interested in that, please let me know, I can give some more details privately. Michael

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Layered maps
by Alex Rozenshteyn 10 Oct '10

10 Oct '10

Does there exist a library which allows me to have maps whose elements are maps whose elements ... with a convenient syntax. Alternatively, does there exist a library like Data.Tree where forests are sets rather than lists? -- Alex R

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[Off-topic]Functional parsing theory
by Maurício CA 09 Oct '10

09 Oct '10

Hi, all, I've been working in a tool that reads a grammar with associated actions and act on input based on that grammar. I would like to rewrite it in a functional style, but I've not been able to find a theory that would handle any possible grammar with cyclicity and empty productions, and flexibility is more important for this tool than performance. Do you have a suggestion on that? What I'm using now is this (non-functional) article on Earley method: http://www.springerlink.com/content/602270808666074p Thanks, Maurício

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Ordering vs. Order
by Christian Sternagel 09 Oct '10

09 Oct '10

Hi all, I'm not a native English speaker and recently I was wondering about the two words "order" and "ordering" (the main reason why I write this to the Haskell mailing list, is that the type class "Ordering" does exist). My dictionaries tell me that "order" (besides other meanings) denotes an ordered structure on elements and "ordering" (as only meaning) denotes some request that I made at some entity. So, to me it seems that calling the type class "Ordering" is wrong ;) However, I do know that there are many publications about "ordered structures" which use the word "ordering" (most of which I'm aware of, not by native speakers). What do native speakers have to say about that? best regards chris

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Re: [Haskell-cafe] Re: Bulletproof resource management
by Brandon Moore 08 Oct '10

08 Oct '10

On Oct 8, 2010, at 2:18 PM, Florian Weimer <fw(a)deneb.enyo.de> wrote: * Ben Franksen: You might be interested in Lightweight Monadic Regions http://okmij.org/ftp/Haskell/regions.html#light-weight which solve the problem (IMHO) in a much cleaner way, i.e. w/o explicit closing and also w/o using finalizers. Is this approach composeable in the sense that you can combine code written in this code with code from another library? I don't think so. The other library might provide something like IORef, and then it's impossible to uphold static guarantees. IMHO, this rules out such an approach (until it's part of the standard library and forced upon everyone, which seems unlikely). If you are worried about a handle being saved in the IORef to keep it outside the region, that's precisely what the library prevents. The typed handle itself can be carried outside the region, but the types make it impossible to perform any operations on it, so the resources can be released on exit from the region anyway. It's quite clever.

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Haskell Helper
by Eduardo Ribeiro 08 Oct '10

08 Oct '10

Hello, I 'm developing a new language in haskell and I need someone to help me. Anyone would like to help?

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Associated data types and contexts
by Eric Walkingshaw 08 Oct '10

08 Oct '10

The following code compiles happily in GHC: > {-# LANGUAGE TypeFamilies #-} > > class C a where > data D a > m :: D a -> Bool > > test :: C a => D a -> Bool > test = m My question is why do I need the context in the function "test"? It seems like since "D" is associated with class "C", the compiler can safely assume that any time I have a "D a", "a" must be an instance of C. But GHC complains if the context is removed. At first I thought that maybe the associated type was just syntactic sugar for a non-associated data type family. But this doesn't seem to be the case since I cannot instantiate D outside of an instance of C. Google and the type family documentation provided no other leads. Am I missing something here? Is the context assumption invalid? Or is it just an assumption that GHC doesn't make (yet)? Thanks in advance for any replies. -Eric

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Reminder: BelHac: A Hackathon in Belgium, 5-7 November
by Jasper Van der Jeugt 08 Oct '10

08 Oct '10

Hello all, We would like to remind you of BelHac, an international Hackathon taking place in Ghent, Belgium next month. All details are available on this wiki page [1]. You can register here [2]. WHEN Friday November 5: 2pm - 7pm Saturday November 6: 10am - 6pm Sunday November 7: 10am - 6pm WHERE In “The Therminal”, in Ghent, Belgium. Please see the wiki [1] for more details! SPONSORS Well-Typed: http://well-typed.com/ O’Reilly: http://oreilly.com/ Incubaid: http://incubaid.com/ We hope to see you in Belgium! [1]: http://www.haskell.org/haskellwiki/Ghent_Functional_Programming_Group/BelHac [2]: http://www.haskell.org/haskellwiki/Ghent_Functional_Programming_Group/BelHa… On behalf of the GhentFPG organizing committee, Kind regards, Jasper Van der Jeugt

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Re: [Haskell-cafe] Re: Re: Lambda-case / lambda-if
by Jonathan Geddes 08 Oct '10

08 Oct '10

I can honestly say that I haven't felt much pain from the status quo regarding this. Most of the time my code is structured so that case statements don't appear in do blocks. When they do, I don't see it as a big issue. The special case for operator - is a bigger wart on haskell syntax than this, imo. I would vote in favor of keeping the language simple. I do like the idea of generalizing lambda functions to include multiple cases, however. On the other hand, I almost never use lambdas now since named functions yield better "self-documenting" code. --jonathan On Oct 8, 2010 8:09 AM, "Peter Wortmann" <scpmw(a)leeds.ac.uk> wrote: On Fri, 2010-10-08 at 01:13 +0300, Lauri Alanko wrote: > Your "general" rule doesn't subsume your ... Yes, that's what I meant. Thanks for describing it properly. On Fri, 2010-10-08 at 05:41 -0700, Nicolas Pouillard wrote: > Imagine find this code: > > do s1 > ... This is roughly what I meant with "abused": Where "C" is very complex, it might become non-obvious where exactly the monad actions are supposed to happen. Hence such traps when refactoring. Also of note: Just moving sub-expressions around isn't something that is guaranteed to be save. Introducing new names and using them in "s2" would be problematic, for example: do map (\x -> (<- putStrLn x)) ["a", "b"] Obviously can't be made to work. You might have to check for this - or maybe even disallow the shorthand inside lamdbas and lets. Might be less satisfying to have such special cases, but it is still a good bit more general than what is available right now. Greetings, Peter Wortmann _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe(a)haskell.org...

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I'm still having challenges to get a Haskell GUI to work under Windows 7
by caseyh＠istar.ca 08 Oct '10

08 Oct '10

I'm still having challenges to get a Haskell GUI to work under Windows 7; even after various instructions on the web. e.g. Haskell Platform 2010.2.0.0, wxWidgets-2.9.1, wxHaskell 0.12.1.6 Similar challenges with GTK+ and gtk2hs.

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