April 2008 - Haskell-Cafe

[Newbie] Problem with Data.Map (or something else ?)
by Bruno Carnazzi 01 Apr '08

01 Apr '08

Dears Haskellers, As an Haskell newbie, I'm learning Haskell by trying to resolve Euler Project problems (http://projecteuler.net/ ). I'm hanging on problem 14 (Collatz problem). I've written the following program... Which does not end in a reasonable time :( My algorithm seems ok to me but I see that memory consumption is gigantic... Is this a memory problem with Data.Map ? Or an infinite loop ? (Where ?) In a more general way, how can I troubleshoot these kind of problem ? Here's the code : import qualified Data.List as List import qualified Data.Map as Map f n | even n = n `div` 2 | otherwise = 3 * n + 1 chain m n = let chain' cn cm | Map.member cn m = Map.map (+ (m Map.! cn)) cm | otherwise = chain' (f cn) $! Map.insert cn 1 (Map.map (+1) cm) in chain' n Map.empty chains n = List.foldl' (\m i -> Map.union m (chain m i)) (Map.singleton 1 1) [2..n] maxCollatz c1@(_,l1) c2@(_,l2) | l1 < l2 = c2 | otherwise = c1 maxChain = List.foldl' maxCollatz (0,0) . Map.toList . chains main = let n = 1000000 in putStrLn $ show $ maxChain n Hope someone can help me, I really don't see what is th problem... Best regards, Bruno.

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How can I represent 4x4 map in haskell
by iliali16 01 Apr '08

01 Apr '08

Hi can you please tell me how can I represent 4x4 map in haskell.Should I show it as a list of lines and each line to be a list of char or I am mistaken? Thanks in advance! -- View this message in context: http://www.nabble.com/How-can-I-represent-4x4-map-in-haskell-tp16396273p163… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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unix support
by Galchin Vasili 01 Apr '08

01 Apr '08

Hello, On my personal computer, I added some functionality to the unix package and now I want to test this functionality. Basically I did a cabal install to the global env on my laptop. I just ran a session of ghci: vigalchin@ubuntu:~$ ghci GHCi, version 6.8.2: http://www.haskell.org/ghc/ :? for help Loading package base ... linking ... done. Prelude> :m System.Posix Prelude System.Posix> :t openFd openFd :: FilePath -> OpenMode -> Maybe FileMode -> OpenFileFlags -> IO Fd Prelude System.Posix> :t mqOpen <interactive>:1:0: Not in scope: `mqOpen' There is a signature for openFd (existing functionality) but not mqOpen (new functionality!) The new functionality got some warnings vis-a-vis Storage because I have defined "alignment" and something else yet. However, when I did "runhaskell Setup.hs install" everything seemed to get installed. ??? Thanks, B.

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function type def
by PR Stanley 01 Apr '08

01 Apr '08

HI It's one of those things - I know sort of instinctively why it is so but can't think of the formal rationale for it: f g x = g (g x) :: (t -> t) -> (t -> t) Why not (t -> t) -> t -> (t -> t) to take account of the argument x for g? Cheers Paul

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Haskell vs Scheme
by Greg Meredith 01 Apr '08

01 Apr '08

Douglas, Excellent questions you posed to Simon P-J -- who then forwarded them to the Haskell Cafe list. By way of answering i should say i was a Schemer from the get-go; it was really the first programming language i studied as an undergraduate majoring in maths at Oberlin in the early 80's. Eventually, i went on to design and build my own language (at MCC with Christine Tomlinson, the principal investigator) called Rosette. While Scheme was Sussman and Abelson's way of making sense of Hewitt's ideas in a sequential setting Rosette was our way of doing the full banana -- including the actor-based form of concurrency as well as both structural and 3-Lisp-style procedural reflection and a whole host of other advanced features. So, i was naturally profoundly frustrated when the world at large turned to languages like C, C++ and even Java. i have been waiting more than 20 years for the industry to catch up to the joys of advanced language design. Now that the industry has taken a shine to functional languages again i have been spending more time with the various modern flavors and have to say that while each of the major contenders (ML, OCaml, Erlang, Scala, Haskell) have something to be said for them, Haskell stands out among them. Haskell enjoys a particular form of mental hygiene that few other languages enjoy. Its syntax, by comparison with Scheme, is remarkably concise -- and, the importance of syntax is almost impossible to gauge because at the end of the day it is code one is staring at ;-). The chief semantic differences that make a difference to my mind may be classified as follows. - types - monads - meta-programming In order, then: at its outset Haskell made a strong commitment to a potent (static) typing scheme. Even if types can be layered on Scheme, the two language design vectors are remarkably different and give programming a different feel. As a result of my academic and professional training i have come to rely heavily on types as a development discipline. In fact, if i cannot devise a sensible type "algebra" for a given (application) domain then i feel i don't really have a good understanding of the domain. One way of seeing this from the Schemer point if view is that the deep sensibility embodied in the Sussman and Abelson book of designing a DSL to solve a problem is further refined by types. Types express an essential part of the grammar of that language. In Haskell the close connection between typing and features like pattern-matching are also part of getting a certain kind of coherence between data and control. Again, this can be seen as taking the coherence between data and control -- already much more evident in Scheme than say C or C++ or even Java -- up a notch. Haskell's language-level and library support for monads are really what set this language apart. i feel pretty confident when i voice my opinion that the most important contribution to computing (and science) that functional programming has made in the last 15 years has been to work out practical presentations of the efficacy of the notion of monad. As a Schemer i'm sure you understand the critical importance of composition and compositional design. The monad provides an important next step in our understanding of composition effectively by making the notion parametric along certain dimensions. This allows a programmer to capture very general container patterns and control patterns (as well as other phenomena) with a very concise abstraction. Again, this could be layered onto Scheme, but Haskell embraced monad as a central abstraction at the language design level and this leads to very different approaches to programming. Now, the place where Haskell and the other statically typed functional languages have some catching up to do is meta-programming. Scheme, Lisp and other languages deriving from the McCarthy branch of the investigation of lambda-calculus-based programming languages enjoy a much longer and deeper investigation of meta-programming constructs. While MetaOCaml stands out as a notable exception i think it safe to say that 3-Lisp and Brown are pretty strong evidence of the long history and much richer investigation of meta-programming notions along the McCarthy branch than along the Milner branch. The industry as a whole, i think, has embraced the value of meta-programming -- witness (structural) reflection in such mainstream languages as Java and C#. And the Milner branch family of languages are moving rapidly to catch up -- see the efforts on generic programming like S P-J's SYB or TemplateHaskell -- but the deep coherence evident in the simplicity of the monadic abstraction has not met up with the deep coherence of 3-Lisp, yet. Anyway, that's my two cents... but i note that US currency is not worth what it used to be. Best wishes, --greg Message: 21 Date: Tue, 1 Apr 2008 11:18:25 +0100 From: Simon Peyton-Jones <simonpj(a)microsoft.com> Subject: [Haskell-cafe] FW: Haskell To: Haskell Cafe <haskell-cafe(a)haskell.org> Cc: "kerrangster(a)gmail.com" <kerrangster(a)gmail.com> Message-ID: < 638ABD0A29C8884A91BC5FB5C349B1C32AB9FA8B7B(a)EA-EXMSG-C334.europe.corp.microsoft.com > Content-Type: text/plain; charset="us-ascii" Dear Haskell Cafe members Here's an open-ended question about Haskell vs Scheme. Don't forget to cc Douglas in your replies; he may not be on this list (yet)! Simon -----Original Message----- From: D. Gregor [mailto:kerrangster@gmail.com] Sent: 30 March 2008 07:58 To: Simon Peyton-Jones Subject: Haskell Hello, In your most humble opinion, what's the difference between Haskell and Scheme? What does Haskell achieve that Scheme does not? Is the choice less to do with the language, and more to do with the compiler? Haskell is a pure functional programming language; whereas Scheme is a functional language, does the word "pure" set Haskell that much apart from Scheme? I enjoy Haskell. I enjoy reading your papers on parallelism using Haskell. How can one answer the question--why choose Haskell over Scheme? Regards, Douglas -- L.G. Meredith Managing Partner Biosimilarity LLC 806 55th St NE Seattle, WA 98105 +1 206.650.3740 http://biosimilarity.blogspot.com

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[GSoC] Student applications deadline extended one week
by Don Stewart 01 Apr '08

01 Apr '08

Hey all, As you might know, the Google Summer of Code application submission period is open. Google has decided to extend the deadline for initial student applications by one week, http://groups.google.com/group/google-summer-of-code-announce/browse_thread… If you've thought about submitting, and are interested in hacking on Haskell for 3 months this summer, and bootstrapping into the Haskell developer community, check our project wiki, http://hackage.haskell.org/trac/summer-of-code/wiki/SoC2008 and get your applications into google this week! -- Don

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all threads are blocked by recvFrom
by Vitaliy Akimov 01 Apr '08

01 Apr '08

Hello, I have a problem with building multithreaded UDP server. If main thread is waiting for new request in recvFrom all other threads are blocked too. I've checked every variant with forkIO,forkOS,-threaded etc, nothing's helped. After reading GHC docs I've understood this is happened becouse foreign function call from recvFrom (network library) is marked to be unsefe, so it's execution blocks every other thread. How can I resolve it? Thank you. Vitaliy.

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Unescaping with HaXmL (or anything else!)
by Anton van Straaten 01 Apr '08

01 Apr '08

I want to unescape an encoded XML or HTML string, e.g. converting " to the quote character, etc. Since I'm using HaXml anyway, I tried using xmlUnEscapeContent with no luck, e.g. with HaXml 1.19.1: let (CString _ s _) = head $ xmlUnEscapeContent stdXmlEscaper $ [CString False "This is a "quoted string"" ()] in s The result is unchanged, i.e. "This is a "quoted string"". Am I doing something wrong, or are my expectations wrong, or is this a bug? Or, is there any other library that includes a simple unescape function for XML or HTML? (The Network.URI module includes an unescape function, but that's specific to URIs, naturally.) Anton

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ANN: wxHaskell 0.10.3
by Jeremy O'Donoghue 01 Apr '08

01 Apr '08

The wxHaskell development team is pleased to announce the release of wxHaskell 0.10.3, a Haskell binding for the wxWidgets GUI library. The Haskell support is built on a reasonably complete C language binding, which could be used as the basis for wxWidgets support on other languages/platforms which do not have easy mechanisms for linking with C++ code. The feature set is the same as wxHaskell 0.10.3 rc1 and rc2, with a number of additional bugfixes. This is the first full release since June 2005, and is the result of a great deal of work by a new team of contributors. Highlights of 0.10.3 include: - Support for Unicode builds of wxWidgets - Support for additional widgets including calendar, toolbar divider, styled text control (wxScintilla), media control - Support for clipboard, drag and drop - Support for 64bit (Linux) targets - Support for wxWidgets 2.6.x (support for wxWidgets 2.4.2 if you compile from source). wxWidgets 2.8 is not yet supported - Support for building with GHC 6.6.x and 6.8.x - Parts of wxHaskell are now built with Cabal - New test cases - Removed support GHC version < 6.4 - Profiling support - Smaller generated binary sizes (using --split-objs) Binary packages are available from the wxHaskell download site at http://sourceforge.net/project/showfiles.php?group_id=73133, for the following platforms: - Debian - Windows - OS X (Intel and PPC platforms) - Source code .tar.gz and .zip - Documentation (cross-platform) The wxHaskell libraries (wxcore and wx) are also available from Hackage (http://hackage.haskell.org) About wxHaskell --------------- wxHaskell is a Haskell binding to the wxWidgets GUI library for recent versions of the Glasgow Haskell Compiler. It provides a native look and feel on Windows, OS X and Linux, and a medium level programming interface. The main project page for wxHaskell is at http://wxhaskell.sourceforge.net. The latest source code for wxHaskell can always be obtained from http://darcs.haskell.org/wxhaskell. There are developer (wxhaskell-devel(a)lists.sourceforge.net and user (wxhaskell-users(a)lists.sourceforge.net) mailing lists, and a wiki page at http://haskell.org/haskellwiki/WxHaskell which can provide more information to those interested. wxHaskell was originally created by Daan Leijen. The contributors to this new release include: - Eric Kow - shelarcy - Arie Middelkoop - Mads Lindstroem - Jeremy O'Donoghue - Lennart Augustson The C language binding for wxHaskell was derived from an original C language binding created for the Eiffel programming language by the ELJ project (http://elj.sourceforge.net) -- Jeremy O'Donoghue jeremy.odonoghue(a)gmail.com

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