December 2006 - Haskell-Cafe

Re: Re: [Haskell-cafe] Writing "Haskell For Dummies Or At Least For People Who Feel Like Dummies When They See The Word 'Monad'"
by Andrew Wagner 11 Dec '06

11 Dec '06

Well, perhaps if nothing else, we could use a wikibook to collaboratively work on the structure of such a book, and then from that you could publish a "real" book. I don't really know the legal issues, though. I am thinking of several books though which have been written and released both as full paper books, and as free digital books. Could we do something similar?

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Re: Re: [Haskell-cafe] Writing "Haskell For Dummies Or At Least For People Who Feel Like Dummies When They See The Word 'Monad'"
by Andrew Wagner 11 Dec '06

11 Dec '06

Well, I'm not opposed at all to a written final form. I guess I just don't see that and using a wikibook to assist in our collaboration as mutually exclusive. Anyway, I'd love to help in any such project. By the way, I seem to be messing up the threads. What is considered the "correct" way to reply to a particular thread? I've been copying and pasting the subject line and writing to haskell-cafe(a)haskell.org.

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Num is such a fat and greedy class
by Dan Weston 11 Dec '06

11 Dec '06

The following observations are not new, insightful, or gracious, but I was lusting after the innocent +,-,* operators for my own evil ends and was mildly curious why... Num is such a fat and greedy class. If you want to marry Cinderella, you have to take her ugly stepsisters too. 1) Groups may only want to define addition. Why can't they use + (instead of <+>, >++++?&**+>, or other such perversion)? 2) Affine spaces have a (-) but no (+). Worse, the signature might be (-) :: Point -> Point -> Vector, which doesn't unify with (a -> a -> a). Wouldn't the following be more useful/general? class Subtraction a b | a -> b where (-) :: a -> a -> b Or would this require needless type annotation for the common subset of (a -> a -> a) instances? 3) Quaternions have no signum, unit quaternions have (*), (/) but no (+) or (-), abs would have a different signatures (Quaternion -> Double) which doesn't unify with (a -> a), and fields cannot be scaled with (*) as in (*) :: (Field f) => Double -> f -> f Would it not make sense to put each of these operators (division too) into their own individual superclasses that Num inherits? My (obviously naive) philosophy about type classes is that operations should be bundled only when they are mutually recursive (i.e. there is more than one useful minimal definition). If there is just one minimal set of operations, they can be in their own parent class too. Then again, I should get over my lust and stick with my own operators <+++++++++>, <---------------------->, and <*******>. Not too pretty, but they have a wonderful personality all their own! Dan

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IFF reader and writer
by Henning Thielemann 11 Dec '06

11 Dec '06

On AmigaOS there is a library called iffparse.library, which is used for processing the Interchange File Format, which is a binary container format developed by Electronic Arts for any kind of data. http://en.wikipedia.org/wiki/Interchange_File_Format The best known instances of this format are certainly the AIFF sampled sound format and WAV (which is RIFF, that is little endian IFF). Short question: Is there some Haskell library for parsing and constructing files of this format?

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Re: Haskell-Cafe Digest, Vol 40, Issue 27
by Kirsten Chevalier 11 Dec '06

11 Dec '06

On 12/11/06, haskell-cafe-request(a)haskell.org <haskell-cafe-request(a)haskell.org> wrote: > Message: 2 > Date: Mon, 11 Dec 2006 08:18:44 +0100 > From: Andy Georges <andy.georges(a)elis.ugent.be> > Subject: Re: [Haskell-cafe] Aim Of Haskell > To: Haskell-cafe <haskell-cafe(a)haskell.org> > Message-ID: <CD73F4B7-2B38-4958-9803-D1AE7CF5FC0B(a)elis.ugent.be> > Content-Type: text/plain; charset=US-ASCII; delsp=yes; format=flowed > > Hi, > > > one particular thing that we still lack is something like book > > "Haskell in > > real world" > > We need a 'Dive into Haskell' book. > "Head First Haskell"? Or perhaps it should be "unsafeHead First Haskell". Cheers, Kirsten -- Kirsten Chevalier* chevalier(a)alum.wellesley.edu *Often in error, never in doubt "The miracle of life is that, despite the best grip we can get on reality, it continuously manages to surprise us. The beauty of science is that, notwithstanding all our tacit assumptions, these surprises can get through." --Evelyn Fox Keller

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laziness
by Ranjan Bagchi 10 Dec '06

10 Dec '06

Hi -- I'm using ghc 6.6, and I had some questions about the extend of laziness and foldr. I've noticed the following: >x = foldr (:) [] [1..] >(take 10) x yields [1..10] Which is great.. however, what I'd like to fold the list over a tuple: >foo x (l,payload) = ((x:l), payload) >(x,_) = foldr foo ([], Nothing) [1..] >(take 10) x Doesn't terminate, until the stack overflows So I'm clearly expecting ghc to be more psychic than it's currently capable. But what I'd like to do is pass an infinite list through a processing function which maintains some state (foo could easily be a pseudo random number generator), and take the first bit of it. Any help? Thanks, Ranjan Bagchi

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One question...
by Waldemar Biernacki 10 Dec '06

10 Dec '06

Hello to everyone! Two days ago I have found Haskell in Internet. It sounds very nice. I have read some articles, few examples, ... yes it sounds nice. Now my problem is connected with the "non-update" object feature. I can't write "variable" instead object because - from the meaning of the word ( variable ) - it has the possibility to CHANGE its value. Yes that my problem :-( Let's assume that I've got to write the application that should works as follows: 1. At the beginning the user should write her/his name (for instance as the application parameter). 2. Let's assume that the application has many various functions defined inside, and - after printing each of the outputs - the name has to be printout. How should I think "properly" in Haskell to get described action? cheers Waldemar

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Re: [Haskell-cafe] One question...
by Waldemar Biernacki 10 Dec '06

10 Dec '06

apflemus, Thank for your answer: > Counter question: can you make quicksort shorter than three lines > > qsort :: Ord a => [a] -> [a] > qsort [] = [] > qsort (x:xs) = [y | y <- xs, y <= x] ++ [x] ++ [y | y <- xs, x < y] The classical Perl qsort has 5 lines - more but still short :-) (look how similar they are, I think it is because they're recursive and have sub-list selectors functions ) : ###################### #!/usr/bin/perl -w my @a = ('a',1,'c','z','h','oik','e','t','77','b'); my @b = &qqsort ( @a ); while ( @b ) { print shift @b,"\n" } sub qqsort { @_ || return (); my $p = shift; return (qqsort(grep $_ lt $p, @_), $p, qqsort(grep $_ ge $p, @_)); } ###################### however, I've questioned about client - server applications - (remote SQL database server (postgres for instance) + users GUI + remote resources (file, document etc ) - that is quite different world to compare with the "algorithm" one. Are there some people who perform such applications? What is your experienced in them? cheers Waldemar

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Designing an object model in Haskell
by John Ky 10 Dec '06

10 Dec '06

Hi, I've got an object model that I have a difficult time conceptualising how it might look like in Haskell: class Element { } class Inline : Element { } class ParentInline : Inline { List<Inline> children; } class Bold : ParentInline { } class Underline : ParentInline { } class Link : ParentInline { String link; } class Text : Inline { String text; } class Block : Element { } class Paragraph : Block { List<Inline> paragraph; } class Heading : Block { List<Inline> heading; } class Document : Element { List<Block> blocks; } How best to represent this OO data model in Haskell? Thanks -John

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SMP support in GHC 6.6 on PPC Macs
by Ivan Tomac 09 Dec '06

09 Dec '06

I've noticed that code compiled with -threaded doesn't like +RTS - N<thread count> as an argument. This seems to be the case with both the binary distribution of GHC 6.6 at www.haskell.org/ghc as well as the version I built from sources. I got rid of all the old versions of GHC on my system and ghc -- version reports 6.6. So I'm guessing this is a bug? Ivan

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