- Haskell-Cafe - Haskell.org

Visualising the Hierarchical Namespace
by Don Stewart 16 Mar '09

16 Mar '09

I've just finished a post (and quick tool) for graphing the complete module namespace of Haskell, taken from the core libraries and all of Hackage. It's quite large: http://donsbot.wordpress.com/2009/03/16/visualising-the-haskell-universe/

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Re: Haskell-Cafe Digest, Vol 67, Issue 41
by abdullah abdul Khadir 16 Mar '09

16 Mar '09

> > > On Mon, 16 Mar 2009 08:05:37 -0700 Don Stewart <dons(a)galois.com> wrote : > Subject: [Haskell-cafe] Visualising the Hierarchical Namespace > > I've just finished a post (and quick tool) for graphing the complete > module namespace of Haskell, taken from the core libraries and all of > Hackage. > > It's quite large: > > > http://donsbot.wordpress.com/2009/03/16/visualising-the-haskell-universe/ Hi Don, That was really good. Amazing stuff there. In fact I just (re) joined the haskell and lisp mailing lists. Course work was keeping me at bay from following my passion for functional programming paradigms except that one of my courses was in fact haskell last semester. Well, it seems as if haskell is taking the functional world by storm. Even in the lisp mailing lists I hear of people reading haskell books, "Real World Haskell" was one of them and in fact they seem to be appreciating the type system offered by haskell. Now, that really means something. You know how hard it is to impress the lisp folks. All in all an impressive run so far. Hats off to all those developers and the Haskell community at large.

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Re: [Haskell-cafe] Map in Terms of Fold in terms of Map
by Neil Mitchell 16 Mar '09

16 Mar '09

Hi Mark, What's the definition of foldr in terms of map? As far as I was aware, its not possible. And as it happens, map is (or is sometimes) defined in term of foldr :-) While you can use mutual recursion, you can't define a in terms of b and b in terms of a unless one of them actually does something at the end - so it does depend exactly what the definitions are. Thanks Neil 2009/3/16 Mark Spezzano <mark.spezzano(a)chariot.net.au>: > Hi, > > > > I’ve noticed that it’s possible to define map as foldr and foldr as a map. > Would this be sensible to define both in terms of each other in the Prelude? > i.e mutually recursive? > > > > Cheers, > > > > Mark > > > > No virus found in this outgoing message. > Checked by AVG. > Version: 7.5.557 / Virus Database: 270.11.15/2003 - Release Date: 15/03/2009 > 2:07 PM > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe > >

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Map in Terms of Fold in terms of Map
by Mark Spezzano 16 Mar '09

16 Mar '09

Hi, I’ve noticed that it’s possible to define map as foldr and foldr as a map. Would this be sensible to define both in terms of each other in the Prelude? i.e mutually recursive? Cheers, Mark No virus found in this outgoing message. Checked by AVG. Version: 7.5.557 / Virus Database: 270.11.15/2003 - Release Date: 15/03/2009 2:07 PM

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Re: [Haskell-cafe] Natural Numbers: Best implementation?
by Alexander Dunlap 16 Mar '09

16 Mar '09

2009/3/12 Mark Spezzano <mark.spezzano(a)chariot.net.au>: > Hi, > > > > I was wondering what the best way to implement Natural number would be. Is > there a package which already does this? > > > > Here are some options: > > > > 1. Don’t bother. Just use Integer. > > 2. Use the type > > data Natural = Zero | Succ !Natural > > 3. Use the following definition taken from the Gentle Introduction to > Haskell 98 > > newtype Natural = MakeNatural Integer > > toNatural ::Integer-> Integer > > toNatural x | x < 0 = error “Can’t create negative naturals!” > > | otherwise = MakeNatural x > > fromNatural :: Natural -> Integer > > fromNatural (MakeNatural i) = i > > > > and then... > > > > instance Num Natural where > > fromInteger = toNAtural > > x + y = toNatural (fromNatural x + fromNatural y) > > x – y = etc.. > > x * y = etc... > > > > Which method is best? So far, I’ve been picking option #1 – just leaving > things as they are and using Integer to keep things simple. > > > > I’ve got that feeling that [2] would be fast and [3] would be slow. Comment > appreciated on the merits of each. > > > > Cheers, > > > > Mark Spezzano > I would tend to use option (1) unless there's a compelling reason not to. Since naturals aren't closed under subtraction, you would in practice probably have just as many non-total functions as you would with the regular Int{,eger} type. Also, a lot of functions just take Integers so it would be more of a pain to use. In terms of speed, I think that [3] would be reasonably fast (unless you do a ton of subtraction with bounds-checking) and [2] would probably be quite slow, because you don't get the speed-boost from doing computations right on the processor. Alex

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Packet analysis framework for Haskell.
by Vimal 16 Mar '09

16 Mar '09

Hi all, Is there a packet analysis framework in Haskell? The framework should abstract away the features of packet analysis in the form of a library that people can use and write extensions to perform computation on the packets flowing through the network. What are the "features" of packet analysis? 1. Support for packet packet selection, matching a particular criteria. Example: (protocol = tcp && destination_port = 80). 2. Multi-threaded. Since the process of computation on packets is inherently data-parallel, the framework should abstract away the parallelism and make use of different CPU cores if available. I believe the features in Haskell would be a great boon to easily achieve this. 3. Support for statistics. Packet analysis is usually done for a purpose. Say, I would like to collect statistics about the rate of connections to a webserver. I would want to: (a) Filter packets, using features in (1). (b) Count the number of packets that pass through after applying operation (a). (c) Generate reports if a particular condition is flagged in (b). Example: (a) protocol = tcp && SYN set in flags && destination_port = 80, (b) count the number of SYN packets, (c) report if d(count)/dt > 100. 4. Availability of efficient data structures for achieving real time performance. Example: The user might want to check if ARP spoofing is taking place. In which case, one has to cache the "already seen" mapping between protocol and hardware addresses learnt from the packets that flowed, and constantly checking new addresses against this structure for duplicates. The above are some of the features which I believe are necessary for packet analysis (or, analytics maybe?). There could be more. I was wondering if Haskell would be a good language to achieve these things. I had a brief idea and started writing an application in C and I realised the need for such a framework. The ease of parallelism offered by Haskell prompted me to think about using it. I would like to hear your comments. Also, would this qualify as a project that someone in the Haskell community would be willing to mentor for GSoC 2009? (Assuming Haskell would be on the list of mentoring organisations, although I have no reason to doubt it wouldn't.) Thanks, Regards, -- Vimal

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A non-inductive Haskell proof?
by R J 16 Mar '09

16 Mar '09

The following theorem is obviously true, but how is it proved (most cleanly and simply) in Haskell? Theorem: (nondecreasing xs) => nondecreasing (insert x xs), where: nondecreasing :: (Ord a) => [a] -> Bool nondecreasing [] = True nondecreasing xxs@(x : xs) = and [a <= b | (a, b) <- zip xxs xs] insert :: (Ord a) => a -> [a] -> [a] insert x xs = takeWhile (<= x) xs ++ [x] ++ dropWhile (<= x) xs Thanks. _________________________________________________________________ Hotmail® is up to 70% faster. Now good news travels really fast. http://windowslive.com/online/hotmail?ocid=TXT_TAGLM_WL_HM_70faster_032009

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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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