- Haskell-Cafe - Haskell.org

Re: [Haskell-cafe] length defined with foldr
by PR Stanley 20 Sep '07

20 Sep '07

>>Still fighting with the botched OCR? >>I am, as a matter of fact! Is that okay with you? *smile* > > > >>length = foldr (\_ n -> 1 + n) 0 >> >>-- >>brandon s. allbery [solaris,freebsd,perl,pugs,haskell] allbery(a)kf8nh.com >>system administrator [openafs,heimdal,too many hats] allbery(a)ece.cmu.edu >>electrical and computer engineering, carnegie mellon university KF8NH

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Why isn't pattern matching lazy by default?
by Peter Verswyvelen 19 Sep '07

19 Sep '07

I'm experimenting with functional reactive programming for creating simple 2D/3D video games and interactive apps, trying to develop my own version of it from scratch, for learning Haskell. I got stuck with an endless loop when trying to split a stream into a pair of two streams (a kind of reactive if/then/else). Luckily I first read the Haskell School of Expression so I remembered that pattern matching is not lazy and this could be the cause, which it was (I had to replace (x:xs) by ~(x:xs)) I could also fix the problem by not using pattern matching at all, using head/tail calls instead. Now why isn't pattern matching lazy by default? This seems odd for a newbie since everything else is lazy by default. Thanks, Peter

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Re: [Haskell-cafe] Missing Symbol (2)
by PR Stanley 19 Sep '07

19 Sep '07

The book is very well written. I did try to follow the wikibook thingummajig but found the English a bit trying. Cheers Paul t 07:43 19/09/2007, you wrote: >Paul, > >The Hutton book has gotten great reviews, but pouring over a faulty >OCRd version can only be counterproductive. I highly recommend Yet >Another Haskell Tutorial[1]. After that you can check out all the >other wonderful online resources (starting here [2]). At that point a >few OCR errors won't be able to stand in your way! > >Best wishes on you Haskell explorations, >Joel > >[1] http://www.cs.utah.edu/~hal/docs/daume02yaht.pdf >[2] http://haskell.org/haskellwiki/Learning_Haskell >_______________________________________________ >Haskell-Cafe mailing list >Haskell-Cafe(a)haskell.org >http://www.haskell.org/mailman/listinfo/haskell-cafe

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Re: [Haskell] Functor ((,) a)
by Ross Paterson 19 Sep '07

19 Sep '07

On Wed, Sep 19, 2007 at 10:24:50AM +0200, Janis Voigtlaender wrote: > BTW, what would have been the easiest way for me to find this out on my > own? Somehow, I would have hoped to be able to navigate to the appropriate > point from the mentioned place in the online docs, where it is stated that > the instance in question exists. Usually the instance for C (T a1 ... an) should be in either the module defining C or the one defining T (so if you import both you have it); if T is from the Prelude, then the instance should be together with C. But Control.Monad.Instances is a special case, because exists to hold instances of Haskell 98 classes for H98 types that are not defined in H98. Putting them in the proper place would break compatibility with H98.

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Missing Symbol (2)
by PR Stanley 19 Sep '07

19 Sep '07

Hi Here's another one: f [] = v f (x:xs) = x . f xs The "." is supposed to denote a generic operator - f [] maps to some value v and applied to non-empty list applies head to ? operator. Thanks, Paul

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Re: [Haskell] downtime for monk ({darcs, hackage, cvs}.haskell.org) Monday from 3pm UTC
by Ian Lynagh 19 Sep '07

19 Sep '07

On Wed, Sep 19, 2007 at 09:16:56AM +0100, Bayley, Alistair wrote: > > From: haskell-bounces(a)haskell.org > > [mailto:haskell-bounces@haskell.org] On Behalf Of Ian Lynagh > > > > The machine called monk ... > > will be down from 3pm UTC on Monday 24th for an OS and RAM upgrade. > > Do you have any estimate for how long the upgrades will take? That depends on how many things go wrong. I'll send an e-mail when we believe it's all up and working again. Thanks Ian

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(win)hipe for Haskell?
by Peter Verswyvelen 19 Sep '07

19 Sep '07

During a googling session, I can across (Win)HIPE, a visualization program for the functional language HOPE. See http://dalila.sip.ucm.es/~cpareja/winhipe IMHO a similar tool would be a nice for learning/teaching Haskell; does that exist, or something else that comes close? Thanks, Peter

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Re: [Haskell-cafe] Missing Symbol (2)
by PR Stanley 19 Sep '07

19 Sep '07

It's probably \oplus then. The font size according to my trusty screen reader is 11 point which is not particularly large. Still, if anybody else finds it annoying I'll reduce it to 9pt. Paul At 13:21 19/09/2007, you wrote: >This looks like foldr, specifically: >f = foldr (.) v > >so really, there's no way of telling what the operator could be, other >than that: >(.) :: a -> b -> b >where >f :: [a] -> b >v :: b > >BTW, I find that most of your emails are in a very large font size. Do >other people notice this? It's not very nice to read. Gmail's 'Show >Original' feature tells me that the HTML version of your message >contains <font size=3> near the start. > >On 19/09/2007, PR Stanley <prstanley(a)ntlworld.com> wrote: > > Hi > > Here's another one: > > f [] = v > > f (x:xs) = x . f xs > > The "." is supposed to denote a generic operator - f [] maps to some > > value v and applied to non-empty list applies head to ? operator. > > Thanks, Paul > > > > _______________________________________________ > > Haskell-Cafe mailing list > > Haskell-Cafe(a)haskell.org > > http://www.haskell.org/mailman/listinfo/haskell-cafe > >

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transparent parallelization
by Thomas Girod 19 Sep '07

19 Sep '07

Hi there. Beeing rather new to the realm of Haskell and functional programming, I've been reading about "how is easier it is to parallelize code in a purely functional language" (am I right saying that ?). My knowledge of parallelization is also very weak, but I've been thinking about this and I have a question. Let's say we take a small piece of code, like the quicksort algorithm. > qsort [] = [] > qsort (x:xs) = qsort lesser ++ [x] ++ qsort greater > where lesser = filter (<x) xs greater = filter (>=x) xs (actually I didn't test this code, this is just for the example) Here, computing "lesser" and "greater" are two disjoint actions - we don't need the result from one to compute the other, and haskell does not allow one to alter data so that would change the behaviour of the other. So I guess those could be run in parallel. Would it be fairly simple to automatically determine parts of code that can be run in parallel, and parts that cannot (at least in non-monadic code) ? So the final question is : if it is possible to automatically define segments of code that can be run in parallel, is [insert compiler name here] compiling this code as a one thread thing, or as a multithreaded version ? I guess on single processor machines, it is more efficient to do it as a single thread - but with those "many-cores" processors that are coming, it probably won't be true anymore. Sorry if this post is blowing off open doors (I guess this doesn't mean anything in english but it sounds cool) ... Regards Thomas Girod

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Re: [Haskell] Functor ((,) a)
by Bas van Dijk 19 Sep '07

19 Sep '07

On 9/19/07, Janis Voigtlaender <voigt(a)tcs.inf.tu-dresden.de> wrote: > BTW, what would have been the easiest way for me to find this out on my > own? The following is probably not the easiest way: I keep a copy of the sources of GHC and the libraries [1] on my disk. When I want to search for something I simply do a grep. In emacs you also have a grep-find function which performs a grep in the background and shows the result in a new buffer. The results are hyperlinked so you can just click on a result and emacs will open the respected file and jump to the found string. In your case you do a 'M-x grep-find' followed by the search string: "instance Functor ((,)". This will result in: ./Control/Monad/Instances.hs:19:instance Functor ((->) r) where ./_darcs/pristine/Control/Monad/Instances.hs:19:instance Functor ((->) r) where I think you can get rid of the annoying _darcs... by following: [3]. I haven't tried that though. (Note I'm on Linux. I don't know if this works in Windows. It probably does with Cygwin.) Of course the disadvantage of this approach is that you have to know exactly what you are looking for. Then there's also Hoogle [3] with which you can search for names or types. However in your case this wouldn't have helped. regards, Bas [1] http://hackage.haskell.org/trac/ghc/wiki/Building/GettingTheSources [2] http://wiki.darcs.net/DarcsWiki/HintsAndTips#head-c189bd259a3d141796ebb4f8d… [3] http://haskell.org/hoogle

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