March 2009 - Haskell-Cafe

Lambda-Cube 3D render engine
by Csaba Hruska 16 Mar '09

16 Mar '09

Hi! I've created a wiki page for my project. Currently only some text and screenshot are available. Additional documentation and source code are coming soon.. :) http://www.haskell.org/haskellwiki/LambdaCubeEngine Cheers, Csaba Hruska

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Incompatibility of different (monad transformer) libraries
by ariep 16 Mar '09

16 Mar '09

Hi, ==== Problem instance ==== In my code, I use some monad transformers. I used to use the "mtl" package, but I recently switched to the combination "transformers"/"monads-tf" (mainly for the Applicative instances). The same code also uses the "haskeline" library, for line reading. Haskeline allows to edit the user's history (previously entered lines), but only through a Control.Monad.State interface, and that happens to be the Control.Monad.State from "mtl". So now, my package also needs to import "mtl". This combination of package dependencies gives import conflicts: whenever I import some Control.Monad.{State,Error,Cont,...} module, there is more than one package providing such a module. ==== Solution? ==== For now, I resolve the ambiguity by using "Package imports", i.e. annotate the ambiguous imports by the right package name. Is this the best solution? The ghc manual states, that the package import extension should not be necessary. I can think of another solution, by creating a dedicated package, which imports mtl:Control.Monad.* and re-exports them under another name, but that's even less elegant than package imports. What is the recommended way to deal with module name conflicts, such as this one? Regards, Arie BTW: how can I let the user edit a line of text, from a non-empty starting value for the text? (In other words, actually *edit* a line, not enter a new one.) I currently do this by adding the starting value to the user's history, so he/she can get it by pressing the up arrow key. This is clearly not optimal.

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Re: Has anybody replicated =~ s/../../ or even something more basic for doing replacements with pcre haskell regexen?
by Thomas Hartman 16 Mar '09

16 Mar '09

So, I tweaked Text.Regex to have the behavior I need. http://patch-tag.com/repo/haskell-learning/browse/regexStuff/pcreReplace.hs FWIW, the problem I was trying to solve was deleting single newlines but not strings of newlines in a document. Dead simple for pcre-regex with lookaround. But, I think, impossible with posix regex. -- replace single newlines, but not strings of newlines (requires pcre look-around (lookaround, lookahead, lookbehind, for googlebot)) http://perldoc.perl.org/perlre.html testPcre = ( subRegex (mkRegex "(?<!\n)\n(?!\n)") "asdf\n \n\n\nadsf" "" ) == "asdf \n\n\nadsf" Can I lobby for this to make its way into the Regex distribution? Really, I would argue that every regex flavor should have all the functions that Text.Regex get, not just posix. (subRegex is just the most important, to my mind) Otherwise I'll make my own RegexHelpers hackage package or something. Hard for me to see how to do this in an elegant way since the pcre packages are so polymorphic-manic. I'm sure there is a way though. Or if you point me to the darcs head of regex I'll patch that directly. 2009/3/14 Thomas Hartman <tphyahoo(a)gmail.com>: > Right, I'm just saying that a "subRegex" that worked on pcre regex > matches would be great for people used to perl regexen and unused to > posix -- even it only allowed a string replacement, and didn't have > all the bells and whistles of =~ s../../../ in perl. > > 2009/3/12 ChrisK <haskell(a)list.mightyreason.com> >> Thomas Hartman wrote: >>> >>> Is there something like subRegex... something like =~ s/.../.../ in >>> perl... for haskell pcre Regexen? >>> >>> I mean, subRegex from Text.Regex of course: >>> http://hackage.haskell.org/cgi-bin/hackage-scripts/package/regex-compat >>> >>> Thanks for any advice, >>> >>> thomas. >> >> Short answer: No. >> >> This is a FAQ. The usual answer to your follow up "Why not?" is that the >> design space is rather huge. Rather than justify this statement, I will >> point at the complicated module: >> >> http://hackage.haskell.org/packages/archive/split/0.1.1/doc/html/Data-List-… >> >> The above module is "a wide range of strategies for splitting lists", which >> is a much simpler problem than your subRegex request, and only works on >> lists. A subRegex library should also work on bytestrings (and Seq). >> >> At the cost of writing your own routine you get exactly what you want in a >> screen or less of code, see >> http://hackage.haskell.org/packages/archive/regex-compat/0.92/doc/html/src/… >> for "subRegex" which is 30 lines of code. >> >> Cheers, >> Chris >> >

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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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