December 2006 - Haskell-Cafe

Re: what are the points in pointsfree?
by Chad Scherrer 15 Dec '06

15 Dec '06

> so pointsfree is a step beyond leaving the domain unspecified. Actually, the domain is specified - a function written as f = g . h has the same domain as h has. > my reading knowledge of haskell at this point far exceeds my ability > to write haskell. but so far, it has seemed to me that functions > written in the pf style are the most reuseable. > > from what you just told me, it's not an artifact of the pf style, but > that maximally reusable functions will be expressible in a pointsfree > style. that those functions embody a pattern of computation, without > concern for the details. > I don't see where reusability is affected either way. Many (all?) functions can be written in either style, and the definitions are equivalent. There are a few advantages I've seen to pf style: 1. Function definitions are shorter, and sometimes clearer. 2. It saves you from having to give points in the domain a name. 3. It can make reasoning about programs simpler. For example, if we know that reverse . reverse == id then anywhere we see reverse . reverse, we can replace it with id, without having to track any other variables. In particular, Richard Bird's book and articles use this to great effect for program transformation and derivation from specification ("Bird-Meertens formalism") The only disadvantage I know of is that it can lead to obfuscation, especially if Haskell hasn't twisted your brain yet (in a good way). -- Chad Scherrer "Time flies like an arrow; fruit flies like a banana" -- Groucho Marx

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On improving libraries: wanted list
by dons＠cse.unsw.edu.au 14 Dec '06

14 Dec '06

With the upcoming hackathon[1], and its timely focus on real world libraries, infrastructure and tools support, I've started a list of truly missing libraries, for particular problem domains. If you have a problem, for which you think there *should* be a Haskell library, please add it to this page: http://haskell.org/haskellwiki/Wanted_libraries Or, if you see a 'missing' entry there, but happen to know of a real library that solves the problem, add it to the list. If you feel an existing library isn't suitable for a task, note this in the 'could be improved section'. A good constructive list here will help focus developer efforts, and someone may just step up with the code! -- Don The future only happens if we write the code. 1. http://haskell.org/haskellwiki/Hac_2007

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Dynamic types through unsafeCoerce
by Alfonso Acosta 14 Dec '06

14 Dec '06

Hi, I've been using Data.Dynamic but the Typeable requirement doesn't go well with FFI declarations (which don't accept type contexts). This is a little example of what I would like to do: data MyDyn = MyDyn myToDyn :: a -> MyDyn myToDyn = unsafeCoerce myFromDyn :: MyDyn -> a myFromDyn = unsafeCoerce -- this is a type I want to transform back and forth from dynamic -- and store into lists etc etc ... data MyType a b = MyType a b -- This it's "abstract" Generic type (it's contructor should be hidden) data MyGenType = MyGenType MyDyn mt2mgt :: MyType a b -> MyGenType mt2mgt = MyGenType.myToDyn mgt2mt :: MyGentype -> MyType a b mgt2mt (MyGenType dyn) = myfromDyn dyn The question is, ¿if only mt2mgt and mgt2mt are used by the user, would the use of unsafeCoerce be dangerous? Thanks in advance, Alfonso Acosta

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Reversing a string of words: C# v Perl V Ruby v Haskell
by Andrew Savige 14 Dec '06

14 Dec '06

Recently, the C# programmers at work have been asking job applicants to write a function to reverse a string of words. For example, given an input string of: " one two three four " the function should produce: "four three two one" That is, the input string reversed word by word, with a single space between each word and with no leading or trailing space. (You may assume that the input string consists only of alphabetic characters, spaces and tabs). A popular C# approach goes something like: private static string reverseWords(string str) { string[] words = Array.FindAll<string>(str.Split( new char[] {' ','\t'}), delegate(string s) { return !String.IsNullOrEmpty(s); }); int i = words.Length - 1; if (i < 0) return String.Empty; StringBuilder sb = new StringBuilder(words[i]); while (--i >= 0) sb.Append(' ').Append(words[i]); return sb.ToString(); } Though the C# programmers seem happy enough with this solution, it's a bit too verbose for my personal taste. For cheap thrills, I'd like to show them some solutions to their little problem in other, er, less verbose languages. In Perl, I have: sub reverseWords { join ' ', reverse split(' ', shift) } In Ruby: def reverseWords(str) str.split(' ').reverse.join(' ') end Finally, and please don't laugh, in Haskell (GHC): reverseWords :: String -> String reverseWords xs = (concat (intersperse " " (reverse (words xs)))) which does appear to work correctly. However, I would like to do justice to Haskell as a language -- and given that my clumsy Haskell is not written in expert style -- I'm hoping that someone on this list might offer a better, faster, or more idiomatic Haskell solution to this little problem. Thanks, /-\ Send instant messages to your online friends http://au.messenger.yahoo.com

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Interoperating with Java
by mark＠ixod.org 14 Dec '06

14 Dec '06

I was wondering, what's the status of the various means of two-way interoperation with Java? Are things actively maintained? According to the WWW, HaskellDirect's (I have Lambada in mind) current version was released Jan 2004 and compiles with 'recent' versions of GHC, such as ghc-6.2, and the latest released JVM Bridge isn't much newer. We were looking at using the Standard Widget Toolkit (SWT) but don't want to rely on driving it from Haskell if the connectors are going to rot away. (We could still use FP, after all - e.g. Scheme's Kawa and Bigloo have good Java interoperation.) -- Mark

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Interpreter output in color
by dons＠cse.unsw.edu.au 14 Dec '06

14 Dec '06

Forward to haskell-cafe(a)haskell.org for further discussion. ----- Forwarded message from Walter Moreira <gwallter(a)gmail.com> ----- Date: Thu, 14 Dec 2006 06:46:44 +0000 From: Walter Moreira <gwallter(a)gmail.com> To: haskell(a)haskell.org Subject: [Haskell] interpreter ouput in color Hi all. I was wondering if some people miss the colored output of some applications, such like the IPython enhanced shell. I've been googling for similar options for Haskell but I found nothing. I feel that the type error messages, for example, would be a lot more readable (or at least quicker to read). Is there a way to write plugins for ghci to get this?, or do you people have some suggestion about what would be the easiest component to hack for applying colors? Thank you. Cheers, Walter ------------------------------------------------------------------------ Walter, I think this is a great idea. Currently, there is an existing tool, HsColour: http://www.cs.york.ac.uk/fp/darcs/hscolour/ which does a great job of marking up source files to ansi, html and css output. I think with only a little bit of effort, it could be integrated into ghci (or a ghci wrapper written), that runs each expression in ghci through HsColour. Here's a mockup (the result of dumping ghci's output through HsColour): http://www.cse.unsw.edu.au/~dons/tmp/colour-ghci.png Is someone interested in writing a wrapper over ghci that pipes input and output through HsColour? We might have to modify HsColour slightly to take input from stdin. I've put up an initial wiki page for this here: http://haskell.org/haskellwiki/GHCi_in_colour If you've thoughts on how to implement this- -- or want to just go ahead and do it, please dive in! -- Don

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Optimization again.
by szefirov＠ot.ru 14 Dec '06

14 Dec '06

From GHC documentation: "Once profiling has thrown the spotlight on the guilty time-consumer(s), it may be better to re-think your program than to try all the tweaks listed below." So, how should I rethink my program? Which way to take?

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haskell.org memory upgrade
by Paul Hudak 14 Dec '06

14 Dec '06

Dear Haskellers -- Haskell.org will go down today at 1500 EST for about 10 minutes for a memory upgrade. Sorry for the inconvenience, -Paul Hudak

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typeclass constraint question
by Jeff Polakow 14 Dec '06

14 Dec '06

Hello, I'm trying to understand what happens when a typeclass constraint with an uninstantiated type variable gets duplicated. Does the type checker treat the separate copies of the constraint as two constraints (with the same type variable) which are checked separately? Or does the type checker only consider one constraint which is checked (simultaneously) in different contexts? Also does it matter if the constraints are on datatype, or even gadt, constructors rather than functions? thanks, Jeff --- This e-mail may contain confidential and/or privileged information. If you are not the intended recipient (or have received this e-mail in error) please notify the sender immediately and destroy this e-mail. Any unauthorized copying, disclosure or distribution of the material in this e-mail is strictly forbidden.

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Re: Designing an object model in Haskell (RESOLVED)
by John Ky 14 Dec '06

14 Dec '06

Hi all, I'm now using existential types. I avoided learning about them because the name sounded so highly technical and obscure it did not occur to me they could be related to OO. Thanks, -John On 12/7/06, John Ky <newhoggy(a)gmail.com> wrote: > > 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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