- Haskell-Cafe - Haskell.org

random shuffle and random list partition
by Manlio Perillo 21 Mar '09

21 Mar '09

Hi. For my Netflix Prize project I have implemented two reusable modules. The first module implements a random shuffle on immutable lists. It uses http://okmij.org/ftp/Haskell/perfect-shuffle.txt, with an additional "wrapper" function, having a more friendly interface. The second module implements a function used to partition a list into n sublists of random length. I have pasted the modules here: http://hpaste.org/fastcgi/hpaste.fcgi/view?id=2483 http://hpaste.org/fastcgi/hpaste.fcgi/view?id=2485 If someone is interested (and if Oleg give me permission), I can release them as a package on Hackage. I need to improve documentation, however. In future I can add an implementation of the random shuffle algorithm on mutable arrays in the ST monad. Manlio

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least fixed points above something
by Jens Blanck 21 Mar '09

21 Mar '09

Hi, I found myself writing the following leastFixedPoint :: (Eq a) => (a -> a) -> a -> a leastFixedPoint f x = fst . head . dropWhile (uncurry (/=)) $ zip l (tail l) where l = iterate f x and was a bit surprised that I couldn't get any matches on hoogle for the type above. The closest one is fix :: (a -> a) -> a but that sort of assumes that we're starting the fixed point search from the bottom element (undefined). Anyway, is there a nicer way of doing the above? Jens

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Grokking zippers
by Nathan Bloomfield 20 Mar '09

20 Mar '09

Hello all- I'm trying to understand the categorical guts underlying zippers. In the Haskell wikibook (and other places) I've seen zippers described roughly as the derivatives of functors. However, I haven't been able to find any references that develop this idea rigorously. For instance, what exactly is the "derivative" of a functor at an object, in the direction of some arrow? I'm interested in studying this concept in more depth, but I can't find a definition to start with. Any pointers to good books or papers would be greatly appreciated. :) Thank you all- Nathan Bloomfield

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Re: [Haskell-cafe] Re: categories and monoids
by Kalman Noel 20 Mar '09

20 Mar '09

Wolfgang Jeltsch schrieb: > Okay. Well, a monoid with many objects isn’t a monoid anymore since a monoid > has only one object. It’s the same as with: “A ring is a field whose > multiplication has no inverse.” One usually knows what is meant with this but > it’s actually wrong. Wrong for two reasons: First, because the multiplication > of a field has an inverse. Second, because the multiplication of a ring is > not forced to have no inverse but may have one. “A ring is like a field, but without a multiplicative inverse” is, in my eyes, an acceptable formulation. We just have to agree that “without” here refers to the definition, rather than to the definitum.

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what does "atomically#" mean?
by Daryoush Mehrtash 20 Mar '09

20 Mar '09

Any idea was the atomically# mean in the following code? atomically :: STM a -> IO a atomically (STM m) = IO (\s -> (*atomically# *m) s ) Code is from GHC.Conc module http://www.haskell.org/ghc/docs/6.6/html/libraries/base/GHC-Conc.html daryoush

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Re: [Haskell-cafe] Ease of Haskell development on OS X?
by Jason Dusek 20 Mar '09

20 Mar '09

2009/03/20 Mark Spezzano <mark.spezzano(a)chariot.net.au>: > Question: Does the iMac have good support for Haskell > development? Yes. I generally use the Mac binary of GHC and then install C libs with MacPorts. > Question: What environment setups do people commonly use (e.g. > Eclipse Xcode etc)? I use screen and vim. In particular, I use MacVim, which provides both a vim console application and a Mac native GUI. > Question: Are there any caveats I should be aware of before > changing systems (i.e. unpleasant surprises). Mac laptops get very warm. You'll have a powerful urge to buy an iPhone and pimp Macs to your friends. There is really no 'home' or 'end' key. To move back and forth on a line in the Terminal application, use fn-shift-leftarrow and fn-shift-rightarrow (annoyed me for days). To move back and forth on a line in grapical application -- for example, the search bar in Firefox -- use cmd-leftarrow and cmd-rightarrow. MacPorts is not as robust as comparable systems on other operating systems: Gentoo's Portage and FreeBSD's Ports. It corrupts its registry and terrible things happen. The remedy is usually a total reinstall of all packages. This has happened to me only once on one Mac in a year and a half of administering several Macs; but it's not something I've ever dealt with on other source-based systems. -- Jason Dusek

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Re: [Haskell-cafe] Ease of Haskell development on OS X?
by Cristiano Paris 20 Mar '09

20 Mar '09

2009/3/20 Mark Spezzano <mark.spezzano(a)chariot.net.au>: > Hi, > > I’ve been thinking of changing over to an iMac from my crappy old PC running > Windows Vista. > > Question: Does the iMac have good support for Haskell development? > > Question: What environment setups do people commonly use (e.g. Eclipse Xcode > etc)? > > Question: Are there any caveats I should be aware of before changing systems > (i.e. unpleasant surprises). > > I want to be able to use the machine for Haskell OpenGL programming. Take a look at this (rather long) thread on Haskell-cafè: http://www.haskell.org/pipermail/haskell-cafe/2009-February/056545.html Cristiano

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Editing Haskell wiki pages
by Colin Paul Adams 20 Mar '09

20 Mar '09

How am I supposed to edit a page on the Haskell wiki? If I click on an "Edit this page" link, then Firefox prompts me to choose a tool to open an application/x-external-editor. When i just choose the default (emacs) I get the following opened in emacs: [Process] Type=Edit text Engine=MediaWiki Script=http://www.haskell.org/haskellwiki/ Server=http://www.haskell.org Path=/haskellwiki Special namespace=Special [File] Extension=wiki URL=http://www.haskell.org/haskellwiki/?title=Applications_and_libraries/Ga… which isn't much use (and further, it's write-protected!). -- Colin Adams Preston Lancashire

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Ease of Haskell development on OS X?
by Mark Spezzano 20 Mar '09

20 Mar '09

Hi, I’ve been thinking of changing over to an iMac from my crappy old PC running Windows Vista. Question: Does the iMac have good support for Haskell development? Question: What environment setups do people commonly use (e.g. Eclipse Xcode etc)? Question: Are there any caveats I should be aware of before changing systems (i.e. unpleasant surprises). I want to be able to use the machine for Haskell OpenGL programming. Cheers, Mark Spezzano No virus found in this outgoing message. Checked by AVG. Version: 7.5.557 / Virus Database: 270.11.20/2012 - Release Date: 19/03/2009 12:26 PM

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Forcing evaluation when seq or strictness annotation are not possible [was: guess on stack-overflows - thunks build-up and tail recursion]
by Gü?nther Schmidt 20 Mar '09

20 Mar '09

Hi all, here I am again with my all time favorite unsolved problem: stack overflows. The advice I have received so far from the Haskell community (this list and #haskell) was to use strictness annotation or seq in most cases. And indeed it did help. It certainly helped when I used a data structure that I designed myself and a filling routine that I also wrote myself, thanks guys (and girls possibly, I mean I don't know). But sometimes neither of the 2 helps. Especially when you are using a brilliantly designed data structure such as IxSet, from happstack, originally HAppS. Since it's someone else code it's too late to add strictness annotation. Or the "core" insert that you have to use in your filling routine is lazy. In such cases you're just out of a luck. I have read about alternatives to the above mentioned techniques, CPS, Codensity-Monad and this http://wwwtcs.inf.tu-dresden.de/~voigt/mpc08.pdf in Blogs, Wikis and Papers, also on this mailing list a post called "stupid question #852". Well when I say alternatives I mean I *perceived* these techniques to be *meant* as alternatives, as a way to control evaluation order, or *force* evaluation. And this is my question: Am I barking up the wrong tree, is the purpose of these techniques actually not to force evaluation order? Günther

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