February 2002 - Haskell

RE: Simpler Fibonacci function
by Brian Berns 10 Feb '02

10 Feb '02

> The only reason the first version of fib was used in the Gentle Intro was to > demonstrate recursive stream processing... Thank you for the explanation. For reference, the fib example occurs in a section (3.4) titled "'Infinite' Data Structures", of which I believe the simpler function is also an example. There is no reference in this section to recursive stream processing (this occurs in section 4.4). In any case, as a newbie, I can tell you that I found the fib function puzzling as stated. > ...and not to show a "canonical" version of Fibonacci Nonetheless, it seems to have become the canonical version. For example, see the list of references to this version on Google: http://www.google.com/search?q=%22zip+fib+%28tail+fib%22. This is perhaps unfortunate since fib is a kind of "hello world" for Haskell and other FP languages. -- Brian

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(no subject)
by Patrick McDonnell 10 Feb '02

10 Feb '02

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what does fixST do?
by Hal Daume III 10 Feb '02

10 Feb '02

subject says it all... -- Hal Daume III "Computer science is no more about computers | hdaume(a)isi.edu than astronomy is about telescopes." -Dijkstra | www.isi.edu/~hdaume

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Re: [Newbie] Programming with MArray
by Jay Cox 10 Feb '02

10 Feb '02

> Date: Fri, 8 Feb 2002 18:35:14 -0200 > From: =?iso-8859-1?Q?Jos=E9_Romildo_Malaquias?= <romildo(a)uber.com.br> > To: haskell(a)haskell.org > Subject: [Newbie] Programming with MArray > > > --82I3+IH0IqGh5yIs > Content-Type: text/plain; charset=iso-8859-1 > Content-Disposition: inline > Content-Transfer-Encoding: 8bit > > Hello. > > To learn how to program with muttable arrays in Haskell, I have done > a very simple program to sum two arrays. I am submitting it to this > group so that it can be reviewd and commented. I have not find > examples on how to program with muttable arrays. > > I would like for instance to see comments on the way the > iteration over the array indices was done: using a list > of indices. I looked for a way of incrementting an > index, starting from the lower bound towards the upper > bound, but failed in finding it. Is there other ways of > iterating over the muttable array other then using > its list of indices? > > Regards, > > Romildo in short, I dont see why you couldn't have some helper function defined in your "where" clause like: update' i = if (i > some_max_bound) then return () else do x1<- readArray v1 i x2<- readArray v2 i writeArray v3 (x1+x2) update' (succ i) I do know of papers where they write monadic functions this way so I presume it is efficient. indeed you might consider using the Bounded class for this purpose --you can do this right?? newtype Foo = F Int deriving (Eq,Enum,Ord,...) instance Bounded Foo where maxBound = F 9 minBound = F 1 In terms of arbitrary indices? If I've read the current Haskell reports right then if your type "a" doesnt have "Enum a" and (Eq "a" or Ord "a") instance, it looks like iterating over a list is the only solution. Jay Cox

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Re: Re: XSLT, Perl, Haskell, & a word on language design
by Dimitre Novatchev 09 Feb '02

09 Feb '02

On Tuesday 15 January 2002, 21:58:06, Eray Ozkural (exa) wrote: > On Wednesday 16 January 2002 04:35, Manuel M. T. Chakravarty wrote: > > > > The really bad thing is that the designers of XSLT [1.0] > > made the language free of side effects but then failed to > > include fundamental support for basic functional > > programming idioms. > > > > So, there are actually people who seriously take XSLT to be a > programming language? Interesting, as I think it is just overdoing an > already overdone > concept (hint: it's a poor ascii tree). Yes, there are such people. Actually, all the examples from John Hughes' article "Why functional programming matters" can be implemented in XSLT 1.0, as shown in the recently published article: "The Functional Programming Language XSLT" at http://www.topxml.com/xsl/articles/fp Cheers, Dimitre Novatchev. __________________________________________________ Do You Yahoo!? Send FREE Valentine eCards with Yahoo! Greetings! http://greetings.yahoo.com

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Kinds Workaround
by Ashley Yakeley 09 Feb '02

09 Feb '02

At 2002-02-08 03:36, Koen Claessen wrote: >Here, you might do the following trick: > > type HasKind_Help x dummy = x > type HasKind_Star_To_Star x = HasKind_Help x (x Int) > > type C c x y = c (HasKind_Star_To_Star x) > (HasKind_Star_To_Star y) > > type Composer c = > forall x y z . C c y z -> C c x y -> C c x z Yuck... I discovered a much simpler workaround, a class which has a parameter of kind '*': class T a where { undef :: a; -- force kind '*' }; instance T a where -- true for all types { undef = undefined; }; data (T (p Bool),T (q Bool)) => CMap0 p q = MkCMap0; type Composer c = forall x y z. (T (x Bool)) => (c y z) -> (c x y) -> (c x z); Neat, huh? Finally, a reason for allowing contexts in data type declarations! -- Ashley Yakeley, Seattle WA

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RE: efficiency question
by Konst Sushenko 09 Feb '02

09 Feb '02

> > On Friday 08 February 2002 22:14, you wrote: > > define > > > > test1 l = > > let s1 = foldr (+) 1 l > > s2 = foldr (-) 1 l > > in (s1, s2) > > > > test2 l = > > let s = foldr (\x (a,b) -> (x+a,x-b)) (1,1) l > > in s > > > > why is test1 so much faster than test2 for long lists l (eg > > [1..1000000])? replacing foldr with foldl makes it faster > (of course), > > but test2 is still much slower. > > > > i *expected* test2 to be much faster because you're only > traversing the > > list once. presumably the two elements "a" and "b" in > test2 could be put > > in registers and i'd imagine test2 should be faster (it > certainly would be > > if written in c). > > I'd say that's because in the second case you also got to > apply the (,), > besides the (+)/(-) constructor during the transversing... > Am I right? > > J.A. My guess is that it is due to the laziness of the addition/subtraction in (,)

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RE: efficiency question
by Konst Sushenko 09 Feb '02

09 Feb '02

Did you try strict +/-? In (,). I am just curious. > -----Original Message----- > From: Hal Daume III [mailto:hdaume@ISI.EDU] > Sent: Friday, February 08, 2002 3:53 PM > To: Konst Sushenko > Cc: Jorge Adriano; haskell(a)haskell.org > Subject: RE: efficiency question > > > I've tried using a strict fold: > > foldl' f a [] = a > foldl' f a (x:xs) = (foldl' f $! f a x) xs > > but that has no effect (or minimal effect). > > -- > Hal Daume III > > "Computer science is no more about computers | hdaume(a)isi.edu > than astronomy is about telescopes." -Dijkstra | www.isi.edu/~hdaume > > On Fri, 8 Feb 2002, Konst Sushenko wrote: > > > > > > > On Friday 08 February 2002 22:14, you wrote: > > > > define > > > > > > > > test1 l = > > > > let s1 = foldr (+) 1 l > > > > s2 = foldr (-) 1 l > > > > in (s1, s2) > > > > > > > > test2 l = > > > > let s = foldr (\x (a,b) -> (x+a,x-b)) (1,1) l > > > > in s > > > > > > > > why is test1 so much faster than test2 for long lists l (eg > > > > [1..1000000])? replacing foldr with foldl makes it faster > > > (of course), > > > > but test2 is still much slower. > > > > > > > > i *expected* test2 to be much faster because you're only > > > traversing the > > > > list once. presumably the two elements "a" and "b" in > > > test2 could be put > > > > in registers and i'd imagine test2 should be faster (it > > > certainly would be > > > > if written in c). > > > > > > I'd say that's because in the second case you also got to > > > apply the (,), > > > besides the (+)/(-) constructor during the transversing... > > > Am I right? > > > > > > J.A. > > > > My guess is that it is due to the laziness of the > addition/subtraction > > in (,) > > _______________________________________________ > > Haskell mailing list > > Haskell(a)haskell.org > > http://www.haskell.org/mailman/listinfo/haskell > > > >

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efficiency question
by Hal Daume III 08 Feb '02

08 Feb '02

define test1 l = let s1 = foldr (+) 1 l s2 = foldr (-) 1 l in (s1, s2) test2 l = let s = foldr (\x (a,b) -> (x+a,x-b)) (1,1) l in s why is test1 so much faster than test2 for long lists l (eg [1..1000000])? replacing foldr with foldl makes it faster (of course), but test2 is still much slower. i *expected* test2 to be much faster because you're only traversing the list once. presumably the two elements "a" and "b" in test2 could be put in registers and i'd imagine test2 should be faster (it certainly would be if written in c). - hal -- Hal Daume III "Computer science is no more about computers | hdaume(a)isi.edu than astronomy is about telescopes." -Dijkstra | www.isi.edu/~hdaume

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[Newbie] Programming with MArray
by José Romildo Malaquias 08 Feb '02

08 Feb '02

Hello. To learn how to program with muttable arrays in Haskell, I have done a very simple program to sum two arrays. I am submitting it to this group so that it can be reviewd and commented. I have not find examples on how to program with muttable arrays. I would like for instance to see comments on the way the iteration over the array indices was done: using a list of indices. I looked for a way of incrementting an index, starting from the lower bound towards the upper bound, but failed in finding it. Is there other ways of iterating over the muttable array other then using its list of indices? Regards, Romildo -- Prof. José Romildo Malaquias Departamento de Computação http://iceb.ufop.br/~romildo Universidade Federal de Ouro Preto romildo(a)iceb.ufop.br Brasil romildo(a)uber.com.br

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