March 2009 - Haskell-Cafe

Hashing over equivalence classes
by Roman Cheplyaka 16 Mar '09

16 Mar '09

Are there some known ways to define hashing (or any other) functions over equivalence classes? I.e. a ~ b => hash(a) == hash(b) where (~) is some equivalence relation. For example, you might want to hash lambda terms modulo alpha-equivalence or hash logical terms with respect to commutativity of (&&) and (||). Often we can choose 'canonical' element from each class and hash it. But (at least, in theory) it's not necessary. So, are there (practical) ways to define hash function without it? -- Roman I. Cheplyaka :: http://ro-che.info/ "Don't let school get in the way of your education." - Mark Twain

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ANN: The Monad.Reader (13)
by Wouter Swierstra 16 Mar '09

16 Mar '09

I am pleased to announce that a new issue of The Monad.Reader is now available: http://www.haskell.org/haskellwiki/The_Monad.Reader The Monad.Reader is a quarterly magazine about functional programming. Issue 13 consists of the following four articles: * Stephen Hicks Rapid Prototyping in TEX * Brent Yorgey The Typeclassopedia * Chris Eidhof, Eelco Lempsink Book Review: "Real World Haskell" * Derek Elkins Calculating Monads with Category Theory Special thanks to Ashley Yakeley for his help with publishing The Monad.Reader on the Haskell wiki. If you'd like to write something for the next issue of The Monad.Reader, please get in touch. I haven't fixed the deadline for the next issue, but it should be mid-May or thereabouts. Wouter

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Uploading files to the wiki
by Wouter Swierstra 16 Mar '09

16 Mar '09

I can't manage to upload files to the Haskell wiki. I've tried different browsers, different internet connections, different machines, different operating systems, and different user accounts - all without success. Is this a new anti-spam measure? This is slightly annoying. I was looking to release the next Monad.Reader on the wiki. Thanks for any advice, Wouter

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Re: [Haskell-cafe] Dynamically typing TH.Exp at runtime
by Martin Hofmann 16 Mar '09

16 Mar '09

Sorry, maybe it I didn't made it clear enough. > Perhaps I'm misunderstanding something, but since one can infer types > in GHCI, that implies one can infer types in the GHC API; since Hint > wraps the GHC API, that implies one can infer types in Hint, doesn't > it? And indeed, there are functions to infer the type of a String; > iirc I've even used them myself in mueval. > That's true, inferring the type of a string is not the problem. However, I am not only interested in the top-level type, but also in the type of any arbitrary subexpression. Consider for example following recursive definition: foo [] = [] foo (x:xs) = (:)(x+1)(foo xs) Typing only the first rule one would says its type is '[a] -> [a]'. However, the second rule specialises the type to 'Num a => [a] -> [a]'. So, if I am interested in the type of the 'x' in the pattern of the second rule, I need to be aware of its binding on the right-hand side. Consequently the type of 'x' is 'Num a'. AFAIK, I need Hendley-Milner type inference for that (please correct me if I am wrong) and I was wondering if I really need to implement it by myself, when it is already somewhere inside GHC. Thanks, Martin

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examples for error handling in takusen?
by Gü?nther Schmidt 16 Mar '09

16 Mar '09

Hi, can someone please point me to error handling examples with takusen? I try to run a piece of code with takusen but just get the very sparse "Database.InternalEnumerator.DBException

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A new haskell tools project
by L.Guo 16 Mar '09

16 Mar '09

Hi all: I have just created an haskell work toolset project in google code. Currently there are only a few tools to process YUV image files I wrote before. Welcome you attend in to improve my code or to add in your tools. And also pleasure to get your advices. http://code.google.com/p/haskellworkingtools/ Regards -------------- L.Guo 2009-03-16

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Re: is there a version of hsffig that builds on a recent versin of haskell?
by Dmitry Golubovsky 16 Mar '09

16 Mar '09

On Mar 15, 4:03 am, Anatoly Yakovenko <aeyakove...(a)gmail.com> wrote: > is there a version of hsffig that builds on a recent versin of haskell? > _______________________________________________ > Haskell-Cafe mailing list > Haskell-C...@haskell.orghttp://www.haskell.org/mailman/listinfo/haskell-cafe Unfortunately no. There have been too many changes in GCC and GHC since, and too low interest to the project from other developers, so I did not maintain it. Dmitry.

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Most elegant funciton for removing adjacent duplicates from a list using foldl and foldr
by R J 15 Mar '09

15 Mar '09

I need to write an implementation using foldl, and a separate implementation using foldr, of a function, "remdups xs", that removes adjacent duplicate items from the list xs. For example, remdups [1,2,2,3,3,3,1,1]= [1,2,3,1]. My approach is first to write a direct recursion, as follows: remdups :: (Eq a) => [a] -> [a] remdups [] = [] remdups (x : []) = [x] remdups (x : xx : xs) = if x == xx then remdups (x : xs) else x : remdups (xx : xs) This code works, but it has three cases, not usual two, namely [] and (x : xs). What, if any, is the implementation using only two cases? Also, if three cases are required, then how can it be implemented using foldr, and how using foldl? Thanks. _________________________________________________________________ Express your personality in color! Preview and select themes for Hotmail®. http://www.windowslive-hotmail.com/LearnMore/personalize.aspx?ocid=TXT_MSGT…

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Help with Bird problem 4.5.6: sequence of successive maxima
by R J 15 Mar '09

15 Mar '09

This Bird problem vexes me, in the first instance because it doesn't seem to specify a unique solution: Given a list xs = [x_1, x_2, . . . , x_n], the sequence of successive maxima "ssm xs" is the longest subsequence [x_j1, x_j2, x_j3..x_jk] such that j_1 = 1 and j_m < j_n => x_jm < x_jn. For example, xs = [3, 1, 3, 4, 9, 2, 10, 7] => ssm xs = [3, 4, 9, 10]. Define "ssm" in terms of "foldl". >From this specification, I infer: ssm [] = [] ssm [1] = [1] ssm [1, 2, 3] = [1, 2, 3] ssm [1, 0, 3, 2] = [1, 3] However, what is ssm [1,0,100,2,3,4,5]? Is it [1, 100] or [1, 2, 3, 4, 5]? I think the latter, but am not certain. Whichever it is, what's the solution? Thanks. _________________________________________________________________ Windows Live™ Groups: Create an online spot for your favorite groups to meet. http://windowslive.com/online/groups?ocid=TXT_TAGLM_WL_groups_032009

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State monad is missing Applicative instance
by Peter Verswyvelen 15 Mar '09

15 Mar '09

I think. Or is it defined in some other package?

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