- Haskell-Cafe - Haskell.org

Google Summer of Code
by Malcolm Wallace 19 Mar '09

19 Mar '09

I am pleased to announce that haskell.org has once again been accepted as a mentoring organisation, for the 2009 Google Summer of Code. Student applications open on Monday (23rd March) at 1900 UTC, for a period of 12 days (until Fri 3rd April, also at 1900 UTC). Students applicants are encouraged to interact with the community via mailing lists, prior, during, and after the submission of their ideas for projects. In general terms, project ideas that benefit the whole community (e.g. infrastructure like ghc, cabal, libraries) will be preferred over things of more marginal interest (e.g. games). Unless you make a _really_ good case, of course! Because (sadly) the darcs community did not get accepted as a separate organisation this year, haskell.org will be willing to accept proposals relating to darcs (as it has done in previous years), but of course they will be competing in the general pool, alongside all the other ideas. Also, we are still accepting mentors. Regards, Malcolm

1 0

.hi inconsistency bug.
by Joe Fredette 19 Mar '09

19 Mar '09

List, I've got this project, source on patch-tag here[1] It's a nice little project, I've got the whole thing roughly working, it compiles okay, everything seems to work, until I try to run it, specifically when I run it in ghci, or when I run the main executable (which uses hint), and look at any type involving my "Email" type, it gives me the following error: Type syonym HackMail.Data.MainTypes.Filter: Can't find interface-file declaration for type constructor or class HackMail.Data.ParseEmail.Email Probable cause: bug in .hi-boot file, or inconsistent .hi file Use -ddump-if-trace to get an idea of which file caused the error As far as I understand, it wants to find the interface-file declaration for a specific type (Email) exported by the ParseEmail module, all of the exports (I think) are in order. I've tried mucking around with it a bit, but I don't fully understand what the error even means, much less how to fix it. Other relevant info, Email is exported in a roundabout way, namely by importing a module MainTypes, which exports a module Email, which exports a the ParseEmail Module, which exports the datatype Email. The "Filter" delcaration it _actually_ complains about (it's just the first place the email type is invoked) is: type Filter a = ReaderT (Config, Email) IO a nothing particularly special. Any help fixing this is greatly appreciated, I did find this bug report[2] which seems like it might be relevant. But trying to unregister -> cabal clean -> cabal install doesn't fix it. I've also tried manually removing the dist/ folder, and also unregistering the package. Thanks again. /Joe [1] http://patch-tag.com/repo/Hackmail/browse [2] http://hackage.haskell.org/trac/ghc/ticket/2057

2 6

Re: [Haskell-cafe] Who generates Haskell code and uses type information at runtime? (fwd)
by Chris Brown 19 Mar '09

19 Mar '09

I'm not sure if you got my previous message, as I was having some problems posting to the list... > Putting in a nutshell, I generalize an extensional defined function > definition into a recursive one. This is done in a number of steps by > modifying expressions and exploiting type information of > sub-expressions. For example: > > rev [] = [] > rev [a] = [a] > rev [a,b] = [b,a] > rev [a,b,c] = [c,b,a] > > ~~> > > rev x = y > > ~~> > > rev [] = [] > rev (x:xs) = (y:ys) > > ~~> > > rev [] = [] > rev (x:xs) = (last (x:xs)) : (reverse (x:xs)) > > > The initial set of rules is given by the user (in a file, via IO, ...). > The problem later is to infer the type of an intermediate variable, e.g. > 'y'. I'm still not entirely sure what you want to do here. But it sounds like HaRe could already do most of this for you via a sequence of folds, unfolds, introduce pattern matching and generative folding. HaRe already has built-in support for some symbolic evaluation, which is already used in the generative fold refactoring, and has type checking support too. http://www.cs.kent.ac.uk/projects/refactor-fp/hare.html If it doesn't do exactly what you want out of the tin, it does have a large API for designing transformations over Haskell code. http://www.cs.kent.ac.uk/projects/refactor-fp/hare/haddock/RefacUtils.html Chris. > > Thanks, > > Martin > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe > -- Chris Brown Visualization Software Engineer, Peiriannydd Meddalwedd Delweddu. Cast Ltd., Technium CAST, Ffordd Penlan, Parc Menai, Bangor, Gwynedd UK. LL57 4HJ. Tel: +44 (0)1248 675038 Fax: +44 (0)1248 675012 Mobile: +44 (0)7917 763712 -- Centre for Advanced Software Technology Limited is a limited company registered in England and Wales. Registered Number: 04473521, Registered Office: Finance Office, Bangor University, College Road, Bangor, Gwynedd. LL57 2DG.

2 1

encoding for least fixpoint
by ben 19 Mar '09

19 Mar '09

Hello, I am trying to understand the definition of (co)inductive types in Haskell. After reading the beginning of Vene's thesis [1] I was happy, because I understood the definition of the least fixpoint: newtype Mu f = In (f (Mu f)). But this definition coincides with his definition of the greatest fixpoint newtype Nu f = Wrap (f (Nu f)), which in reality is not true (e. g. for f x = a * x -- the 'stream functor' the type Mu f should be empty, isn't it?). Then I stumbled over a blog entry of Shin-Cheng Mu [2] and from there over an article of Wadler [3], where the least fixpoint is encoded as Lfix X. F X = All X. (F X -> X) -> X. and the greatest fixpoint as Gfix X. F X = Exists X. (X -> F X) * X. I would like to understand these definitions, or get an intuition about their meaning. Could somebody give some explanation or a pointer to a place where I can find one? Thanks a lot in advance, ben [1] http://www.cs.ut.ee/~varmo/papers/thesis.pdf [2] http://www.iis.sinica.edu.tw/~scm/2007/encoding-inductive-and-coinductive-t… [3]http://homepages.inf.ed.ac.uk/wadler/papers/free-rectypes/free-rectypes.txt

7 11

Need an overview of FP-related research topics
by Yitzchak Gale 19 Mar '09

19 Mar '09

I spoke to a faculty member in a decent Computer Science department in which no one has ever done anything related to FP. (You may say that is an inherent contradiction, but what can I do, the department does have a good reputation. I am withholding names to protect the innocent.) This faculty member happens to be the one who was forced to teach the single programming languages course offered by the department, due to his having the least seniority in the department. As such, he is now actually starting to become a bit interested in FP and Haskell. I gave him a few links to Haskell resources, which he appreciated. But coming from more of a pragmatic non-research point of view, I didn't really know what to say. The best I could do was to point him to the Haskell Wikibook, and to the "Why FP Matters" paper. I would like some links that would give such a person a nice overview of the various active areas of FP-related research these days, leaning towards Haskell. I want to give him a fairly broad view of what is interesting and exciting, why various topics are important, where to find ideas for collaboration and applications to other areas, etc. I actually know about a few departments like that. This could be a good strategy for drumming up more research interest in Haskell. In some cases, the person in question has already been influenced somewhat by the lisp fanatics, so I would like some help in how to deal with that also. Thanks, Yitz

4 4

libgmp for GHC 6.10.1 on Mac OS X 10.5
by Dean Herington 19 Mar '09

19 Mar '09

I'm trying to install GHC 6.10.1 on Mac OS X 10.5 (PowerPC). I installed Xcode 3.1.2. I built libgmp 4.2.4 and installed it in /usr/local/lib. When I do "./configure" in GHC's dist directory, however, I get: bash-3.2$ ./configure checking build system type... powerpc-apple-darwin9.6.0 checking host system type... powerpc-apple-darwin9.6.0 checking target system type... powerpc-apple-darwin9.6.0 Which we'll further canonicalise into: powerpc-apple-darwin checking for path to top of build tree... dyld: Library not loaded: /usr/local/lib/libgmp.3.dylib Referenced from: /Users/family/Desktop/Downloads/ghc-6.10.1/dist/utils/pwd/pwd Reason: no suitable image found. Did find: /usr/local/lib/libgmp.3.dylib: mach-o, but wrong architecture /usr/local/lib/libgmp.3.dylib: mach-o, but wrong architecture configure: error: cannot determine current directory Any ideas what I'm doing wrong? Thanks.

3 4

UNPACK multiple constructors
by Louis Wasserman 19 Mar '09

19 Mar '09

{-# UNPACK #-} is specified to work on single-constructor types, but would there be advantages to, for instance, compiling data BinTree e = Node e {-# UNPACK #-} !(Maybe (BinTree e)) {-# UNPACK #-} !(Maybe (BinTree e)) into four constructors, one for each combination of Maybe constructors? Thoughts? Louis Wasserman wasserman.louis(a)gmail.com

1 0

symbolic evaluator for Haskell?
by Tim Newsham 19 Mar '09

19 Mar '09

Is there a symbolic evaluator for Haskell that will perform all applications except on specified functions? Ie. I would love something that would take foldr (+) (6 `div` 5) [1,2,3*4] and "(+) (*)" and return 1 + (2 + (3*4 + 1)) by performing all the applications except for (+) and (*). (Something that supports ghc extensions is preferred :) Tim Newsham http://www.thenewsh.com/~newsham/

4 4

Re: [Haskell-cafe] OpenGL and Cabal installation
by Henk-Jan van Tuyl 19 Mar '09

19 Mar '09

On Wed, 18 Mar 2009 04:34:29 +0100, Mark Spezzano <mark.spezzano(a)chariot.net.au> wrote: > > configure: error: no GLUT header found, so this package cannot be built > > See `config.log' for more details. > > > Where is it looking for these glut.h files? I’ve tried putting them > everywhere in my PATH but msys won’t find them. Add an environment variable C_INCLUDE_PATH and assign it the value of the directory where you keep the header files; for instance, if the glut.h file is in: C:\usr\local\include\GLUT then do Set C_INCLUDE_PATH=C:\usr\local\include before compiling, or set this variable in Windows with System properties. Similarly, set LIBRARY_PATH for the libraries to link. -- Regards, Henk-Jan van Tuyl -- http://functor.bamikanarie.com http://Van.Tuyl.eu/ --

1 0

Re: [Haskell-cafe] Re: Query on list comprehension
by Kalman Noel 18 Mar '09

18 Mar '09

Jon Fairbairn schrieb: > Melanie_Green <jac_legend_sas(a)hotmail.com> writes: >> What are the limitations of list comprehension. [...] >> a >> aa >> aaa > > I'm not clear what you mean by the question. Why do you want > to use list comprehensions? What if they aren't the best way > of getting the result you want? > > You can write > > [a | b <- [replicate n 'a' | n <- [1..]], a <- b ++ "\n"] > > but does that "replicate" fail to meet your specification? Since we're »holfing« again: fix $ \xs -> [ 'a':xs | xs <- []:xs ]

1 0

ANN: regex-tdfa-1.1.0
by ChrisK 18 Mar '09

18 Mar '09

I have just uploaded the new regex-tdfa-1.1.0 to hackage. This version is a small performance update to the old regex-tdfa-1.0.0 version. Previously all text (e.g. ByteString) being search was converted to String and sent through a single engine. The new version uses a type class and SPECIALIZE pragmas to avoid converting to String. This should make adding support for searching other Char containers easy to do. The new version includes six specialized engine loops to take advantage of obvious optimizations of the traversal. The previous version had only a couple of such engines. The new code paths have been tested for correctness and no performance degradations have shown up. -- Chris

1 0

Re: [Haskell-cafe] Design Patterns by Gamma or equivalent
by wren ng thornton 18 Mar '09

18 Mar '09

Mark Spezzano wrote: > Because Haskell is not OO, it is functional, I was wondering if there is > some kind of analogous “design pattern”/”template” type concept that > describe commonly used functions that can be “factored out” in a general > sense to provide the same kind of usefulness that Design Patterns do for > OOP. Basically I’m asking if there are any kinds of “common denominator” > function compositions that are used again and again to solve problems. If > so, what are they called? Most of the (particular) problems OO design patterns solve are non-issues in Haskell because the language is more expressive. The issues giving rise to patterns like Visitor and Factory are non-issues, so their "solutions" are trivial. A number of other patterns can actually be written down once and for all (in higher-order functions like foldr, map,...) instead of needing repetition. But that's not to say we don't have our own expressiveness deficiencies ;) The analogous term for the genre is "functional pearl", though the individual pearls don't tend to be as codified as in OOP. One example is using coproducts for open unions[1] which solves the dual problem to Visitor. Another is using open recursion[2]. A third example along the same track is using two-level types[3]. But again a lot of the patterns, once discovered, get turned into libraries that can be used off the shelf: e.g. the two-continuation solution for efficient and fair backtracking search[4], and list-fusion techniques[5]. There also a number of "idioms" which are similar in scope to the idioms that arise in other languages: using tail recursion, accumulators, continuation-passing transformations, closures over recursion[6], Schwartzian transforms, etc. And then there are some things like monoids which fall somewhere between idiom and pearl. Some specific OO patterns have direct analogues in "defunctionalization"[7]. For the most part defunctionalization is something best left to the compiler, but on occasion we want to be explicit about it and this too is an idiom/pearl border case IMO. And, of course, there's the entire cottage industry of using category theory for fun and profit. [1] http://www.cs.nott.ac.uk/~wss/Publications/DataTypesALaCarte.pdf [2] http://www.cs.utexas.edu/~wcook/Drafts/2006/MemoMixins.pdf [3a] http://web.cecs.pdx.edu/~sheard/papers/generic.ps [3b] http://web.cecs.pdx.edu/~sheard/papers/JfpPearl.ps [4a] http://okmij.org/ftp/papers/LogicT.pdf [4b] http://hackage.haskell.org/cgi-bin/hackage-scripts/package/logict [5a] http://www.cse.unsw.edu.au/~dons/papers/CSL06.html [5b] http://hackage.haskell.org/cgi-bin/hackage-scripts/package/bytestring [5c] http://www.cse.unsw.edu.au/~dons/papers/CLS07.html [5d] http://hackage.haskell.org/cgi-bin/hackage-scripts/package/stream-fusion [5e] http://homepages.inf.ed.ac.uk/wadler/papers/vanish/vanish.pdf [6] For lack of a better name. I mean doing this: > -- Here the a,b,c,d variables are captured in the closure for go > recursive a b c d = go > where > go 0 = ...a...b... > go n = ...c...d...go (n-1) instead of this: > -- ...instead of needing to be passed through the recursion each time > recursive a b c d 0 = ...a...b... > recursive a b c d n = ...c...d...recursive a b c d (n-1) [7a] http://blog.plover.com/prog/defunctionalization.html -- Live well, ~wren

8 13

Re: [Haskell-cafe] Re: Haskell Logo Voting has started!
by Daniel Schüssler 18 Mar '09

18 Mar '09

Hi, > Even worse, the buttons for moving items up and down are buggy - at > least on my browser (Firefox 3.1 beta 2 on Linux). They sometimes > reorder my other votes! Even assuming that the list box code is not > buggy (which I now doubt), not being able to use the buttons makes this > form almost unusable! I guess it works like this: "Move down" on an element X will make the /smallest/ change (with respect to the abstract ordering) that makes X worse than it's current value. This may require changing more than one number, for example if we have: (105: ) (106: A) (107: X,B) (108: C,D) (109: E ) (110: ) moving down X will result in either (105: A) (106: B) (107: X ) (108: C,D) (109: D ) (110: ) or equivalently (105: ) (106: A) (107: B ) (108: X ) (109: C,D) (110: D) of course, maybe you already know that but you're saying that it is doing this wrongly ;) Btw, I really think the logos should be on the same page as the voting form. Hotkeys for the buttons would be good too. It's usable though if you (incorrectly) consider the numbers as a sort of reverse "star rating" to make a O(n) pass, possibly finetuning later. Greetings, Daniel

5 12

Re: [Haskell-cafe] Who generates Haskell code and uses type information at runtime?
by Martin Hofmann 18 Mar '09

18 Mar '09

> GHC is the current state of the art, which will give you a full > Haskell AST enriched with type information. You can then just modify the > AST directly. I experimented with the AST from HscTypes.CoreModule in the ghc api. However, it seems that this representation is too far away from my theory. I would prefer something similar as Template Haskell. > The ghc-api is somewhat sparsely documented That might be the reason, why I have not seen how it can help me. > It really depends on what you are trying to do... Putting in a nutshell, I generalize an extensional defined function definition into a recursive one. This is done in a number of steps by modifying expressions and exploiting type information of sub-expressions. For example: rev [] = [] rev [a] = [a] rev [a,b] = [b,a] rev [a,b,c] = [c,b,a] ~~> rev x = y ~~> rev [] = [] rev (x:xs) = (y:ys) ~~> rev [] = [] rev (x:xs) = (last (x:xs)) : (reverse (x:xs)) The initial set of rules is given by the user (in a file, via IO, ...). The problem later is to infer the type of an intermediate variable, e.g. 'y'. Thanks, Martin

1 0

Who generates Haskell code and uses type information at runtime?
by Martin Hofmann 18 Mar '09

18 Mar '09

I try to modify Haskell code, parsed from an external source, at runtime. Therefore I need type information about this code. Sometimes to this is referred to as treating "data as code" and "code as data". In homoiconic languages such as Lisp this would be less cumbersome, but in Haskell this seems to be not trivial. Therefore I would like to know how other people solved this. Do you, for example, * use ghc-as-api and manipulate the code snippet in GHC's core language? * use Language.Haskell.Syntax or Language.Haskell.TH and get your type information via IO (from Hint or ghc) as a String? * use your own language representation for code and type and use a tailored type inference/unification algorithm? * use your own language representation combined with Data.Typeable and Data.Dynamics, but what about polymorphic types? * ...? Thanks a lot, Martin

1 0

Re: [Haskell-cafe] OpenGL Linking Issue
by Sean Lee 18 Mar '09

18 Mar '09

Hi Mark, >From what I remember -- I haven't used OpenGL for about a year -- you need something like ghc --make -package GLUT -lglut Test1.hs assuming that you have both HOpenGL haskell package and OpenGL c library. Cheers, Sean 2009/3/18 Mark Spezzano <mark.spezzano(a)chariot.net.au>: > Hi, > > > > I hope I’m posting to the right forum. > > > > I’ve got OpenGL up and running on my Windows Vista machine (finally!) and it > runs perfectly well under Eclipse, bringing up a window with a rendered > image as expected. > > > > The only issue starts when I try to compile the program without Eclipse > (using GHC rather than GHCi like Eclipse does). > > > > I type: > > > > C:\Users\Mark\workspace2\OpenGLPractice\src>ghc --make Test1.hs > > > > And get the following output > > > > Linking Test1.exe ... > > C:\Program > Files\Haskell\GLUT-2.1.1.2\ghc-6.10.1/libHSGLUT-2.1.1.2.a(Extensions. > > o):fake:(.text+0xcc): undefined reference to `glutGetProcAddress@4' > > collect2: ld returned 1 exit status > > > > What’s happening here? Obviously it’s a linking issue, but I was wondering > whether I need to include some library or file or option to ghc so that it > links correctly. > > > > Cheers, > > > > Mark Spezzano > > > > No virus found in this outgoing message. > Checked by AVG. > Version: 7.5.557 / Virus Database: 270.11.18/2008 - Release Date: 17/03/2009 > 4:25 PM > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe > > -- Sean Lee PhD Student Programming Language and Systems Research Group School of Computer Science and Engineering University of New South Wales

1 0

OpenGL Linking Issue
by Mark Spezzano 18 Mar '09

18 Mar '09

Hi, I hope I’m posting to the right forum. I’ve got OpenGL up and running on my Windows Vista machine (finally!) and it runs perfectly well under Eclipse, bringing up a window with a rendered image as expected. The only issue starts when I try to compile the program without Eclipse (using GHC rather than GHCi like Eclipse does). I type: C:\Users\Mark\workspace2\OpenGLPractice\src>ghc --make Test1.hs And get the following output Linking Test1.exe ... C:\Program Files\Haskell\GLUT-2.1.1.2\ghc-6.10.1/libHSGLUT-2.1.1.2.a(Extensions. o):fake:(.text+0xcc): undefined reference to `glutGetProcAddress@4' collect2: ld returned 1 exit status What’s happening here? Obviously it’s a linking issue, but I was wondering whether I need to include some library or file or option to ghc so that it links correctly. Cheers, Mark Spezzano No virus found in this outgoing message. Checked by AVG. Version: 7.5.557 / Virus Database: 270.11.18/2008 - Release Date: 17/03/2009 4:25 PM

1 0

Haskell on your system? Information wanted!
by Don Stewart 18 Mar '09

18 Mar '09

Hey all, I noticed we didn't have an easy page to find out how to get hold of the Haskell toolchain for various systems. So there's now a link from haskell.org to (existing) pages on how to obtain Haskell on windows, mac osx and linux and bsd. If you're a distro maintainer for these systems, please consider adding relevant pointers to the pages, so that users of these systems can find all the info they need. http://haskell.org/haskellwiki/Windows http://haskell.org/haskellwiki/OSX http://haskell.org/haskellwiki/GNU/Linux http://haskell.org/haskellwiki/BSD -- Don

1 0

Re: [Haskell-cafe] Design Patterns by Gamma or equivalent
by Gregg Reynolds 18 Mar '09

18 Mar '09

2009/3/11 Mark Spezzano <mark.spezzano(a)chariot.net.au>: > I’m very familiar with the concept of Design Patterns for OOP in Java and > C++. They’re basically a way of fitting components of a program so that > objects/classes fit together nicely like Lego blocks and it’s useful because > it also provides a common “language” to talk about concepts, like Abstract > Factory, or an Observer to other programmers. In this way one programmer can > instantly get a feel what another programmer is talking about even though > the concepts are fundamentally abstract. > > Because Haskell is not OO, it is functional, I was wondering if there is > some kind of analogous “design pattern”/”template” type concept that > describe commonly used functions that can be “factored out” in a general > sense to provide the same kind of usefulness that Design Patterns do for > OOP. Basically I’m asking if there are any kinds of “common denominator” > function compositions that are used again and again to solve problems. If > so, what are they called? > You might find it useful to read the original works upon which the whole "design pattern" economy is based, namely Christopher Alexander's books (http://www.patternlanguage.com/leveltwo/booksframe.htm?/leveltwo/../booksto…) I liked "Notes on the Synthesis of Form", which predates his pattern language stuff. My $0.02: design patterns became popular in the imperative language community precisely because such languages lack the formal discipline of functional languages. Alexander came up with the notion of a pattern language in order to bring some kind of discipline and abstraction to observed regularities in architecture, urban design, etc. By definition such a language cannot have anything close to formal semantics, any more than a natural language lexicon can have formal semantics. Alexander's writing is very interesting and thought provoking, but I'm not sure it has much to offer the world of functional programming. Programmers used Alexander's ideas to try to bring some order to both specifications and programs. A pattern language is a natural for trying to describe a real-world problem one is trying to solve. Imperative programmers also used it to describe programming patterns. Implementations of things like Observer/VIsitor etc. are ad-hoc, informal constructions; the equivalent in a functional language is a mathematical structure (feel free to fix my terminology). I don't think one needs design patterns for Haskell; it has mathematics and more-or-less formal semantics. Who needs "Visitor" when you have "For All"? Although design patterns may still be useful for describing a real-world problems I suspect one would be better off dumping the notion of design pattern, the better to avoid cognitive dissonance when trying to think in purely functional terms. Cheers, -gregg

3 2

How can I check which thunks are piling up on the stack?
by Gü?nther Schmidt 18 Mar '09

18 Mar '09

Hi, How can I check which thunks are piling up on the stack? Günther

2 1

OpenGL and Cabal installation
by Mark Spezzano 18 Mar '09

18 Mar '09

Hi all, I’m trying desperately to get OpenGL up and running. I type >cabal.exe configure and get the following ... checking GL/glut.h usability... no checking GL/glut.h presence... no checking for GL/glut.h... no checking for GLUT library... no checking for GL/glut.h... (cached) no checking GLUT/glut.h usability... no checking GLUT/glut.h presence... no checking for GLUT/glut.h... no configure: error: no GLUT header found, so this package cannot be built See `config.log' for more details. Where is it looking for these glut.h files? I’ve tried putting them everywhere in my PATH but msys won’t find them. Cheers, Mark Spezzano No virus found in this outgoing message. Checked by AVG. Version: 7.5.557 / Virus Database: 270.11.18/2008 - Release Date: 17/03/2009 4:25 PM

1 0

a newbies confusion with repositories - darcs or git
by Gü?nther Schmidt 18 Mar '09

18 Mar '09

Hi, now that I'm finished with my haskell app I take the opportunity to follow up on some things I hadn't quite understood in my so far short venture into the haskell world. Means: Prepare for a number of naive posts to follow. I had the impression that darcs was the preferred repository for haskellers, but there also is git. So far I've just been using the goodol' svn. But since I have the ambition to become a real haskeller I was gonna make myself acquainted with darcs. Should I skip that and head straight for git? Günther

11 10

Haskell and the Cell Processor
by Galchin, Vasili 18 Mar '09

18 Mar '09

Hello, http://www.power.org/resources/devcorner/cellcorner .... Is there project to port GHC to the Cell? Seems like a really cool challenge. Regards, Vasili

2 1

Has anybody replicated =~ s/../../ or even something more basic for doing replacements with pcre haskell regexen?
by Thomas Hartman 18 Mar '09

18 Mar '09

Is there something like subRegex... something like =~ s/.../.../ in perl... for haskell pcre Regexen? I mean, subRegex from Text.Regex of course: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/regex-compat Thanks for any advice, thomas.

4 5

can GHC build an executable from a C source file?
by Manlio Perillo 18 Mar '09

18 Mar '09

Hi. I'm checking if it possible to build an executable from C source files only. As an example: #include <stdio.h> int main () { printf("hello world\n"); return 0; } $ghc --make foo.c However this only produces the object file, foo.o; it does not build the executable file. What is the reason for this behaviour? I have tested with GHC 6.8.2. Thanks Manlio

4 5

Data.Binary
by Galchin, Vasili 18 Mar '09

18 Mar '09

Hello, I installed ghci on my work Windows machine. If I do a ":m +Data.Word", everything is OK. If I a ":m +Data.Binary", can't be found. Why? Regards, Vasili

2 1

RE: SQL Parsers in Haskell
by Larry Coleman 17 Mar '09

17 Mar '09

> Hi Haskelleers > > Has anybody written a SQL parser in Haskell (and is willing to share the > code) ? > > Greetings, > > Mads Lindstrøm I have a mostly-working SQL parser that I wrote to help with DBA tasks at work. I'm under one of the standard employment IP agreements, however, so my employer owns the code. I have asked about having it released, but that process will take some time. I'll post to the group if/when it ever happens. Larry Coleman

1 0

Apologies for spamming Planet Haskell
by Bjorn Buckwalter 17 Mar '09

17 Mar '09

Dear Planet Haskell readers, Just want to apologize for spamming Planet Haskell. I did some minor formatting edits of old posts and it seems Planet Haskell picked them up and republished them. I didn't expect this to happen (Google Reader doesn't do that) and will be more careful about gratuitously editing old posts in the future. Thanks, Bjorn Buckwalter

1 0

SQL Parsers in Haskell
by Mads Lindstrøm 17 Mar '09

17 Mar '09

Hi Haskelleers Has anybody written a SQL parser in Haskell (and is willing to share the code) ? Greetings, Mads Lindstrøm

1 0

ANNOUNCE: libffi 0.1 released
by Remi Turk 17 Mar '09

17 Mar '09

I am happy to announce libffi 0.1, binding to the C library libffi, allowing C functions to be called whose types are not known before run-time. Why? Sometimes you can't use the haskell foreign function interface because you parse the type of the function from somewhere else, i.e. you're writing an interpreter for a language that has an FFI itself. What? The main function it exports is: callFFI :: FunPtr a -> RetType b -> [Arg] -> IO b And because code is worth a thousand words, here a small program that uses C to write a 1Gb buffer of random garbage to a file: > import System.Posix.DynamicLinker > import Foreign.LibFFI > > main = do > malloc <- dlsym Default "malloc" > creat <- dlsym Default "creat" > write <- dlsym Default "write" > let sz = 2 ^ 30 > buf <- callFFI malloc (retPtr retVoid) [argCSize sz] > fd <- callFFI creat retCInt [argString "/tmp/test", argCUInt 0o644] > n <- callFFI write retCSize [argCInt fd, argPtr buf, argCSize sz] > putStrLn $ show n ++ " bytes written" It should work on any 32/64bits machine on which libffi works, but has been primarily tested on linux x86_64. The current libffi is not exception-safe (exception = memory leak) and callFFI has quite some overhead that would be unnecessary with another api. It is, however, very easy to use :) More interesting examples are included in examples/ in the package. Where? Hackage: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/libffi Module docs: http://www.science.uva.nl/~rturk/doc/libffi-0.1 Cheers, Remi

1 0

DLL Hell: OpenGL
by Mark Spezzano 17 Mar '09

17 Mar '09

Hi all, I’m trying to make the Teapot OpenGL example in Haskell on Windows. It keeps coming up with this: C:\Users\Mark\workspace2\OpenGLPractice\src>ghc --make Teapot Linking Teapot.exe ... C:\ghc\ghc-6.10.1\gcc-lib\ld.exe: cannot find -lSM collect2: ld returned 1 exit status As far as I know, -lSM is to do with XWindows (not windows Vista).How do I fix this ? What’s going on? I’ve been on this problem now for hours, and all I wanted to do was get a window up on the screen. Sigh Mark Spezzano No virus found in this outgoing message. Checked by AVG. Version: 7.5.557 / Virus Database: 270.11.15/2004 - Release Date: 16/03/2009 7:04 AM

1 0

GHC Core - technical document
by Peter Verswyvelen 17 Mar '09

17 Mar '09

Does an up-to-date technical document exist describing GHC Core? Google found http://www.haskell.org/ghc/docs/papers/core.ps.gz I'm not sure this document reflects the current state of art.

2 1

Haskell, OpenGL on Windows
by Mark Spezzano 17 Mar '09

17 Mar '09

Hi, Can someone PLEASE help me to understand exactly how to configure OpenGL on Window using Haskell. I'm using Eclipse IDE. When I try to run the file Test1.hs in GHCi then I get the following error: Loading package syb ... linking ... done. Loading package base-3.0.3.0 ... linking ... done. Loading package array-0.2.0.0 ... linking ... done. Loading package containers-0.2.0.0 ... linking ... done. Loading package OpenGL-2.2.1.1 ... can't load .so/.DLL for: GL (addDLL: could not load DLL) Prelude Main> I've copied the GLUT dll into Windows/System32. Is there some flag or option that I need to enable to tell Haskell where to go to link in the library. What do I do with the other .h and .lib files from GLUT? Thanks Mark

2 1

Re: [Haskell-cafe] Polynomial and VectorSpace
by Henning Thielemann 17 Mar '09

17 Mar '09

On Mon, 16 Mar 2009, Matthew Donadio wrote: > Thy polynomial and matrix libraries weren't really developed to be stand > alone libraries. I was developing some DSP libraries that required > polynomial and matrix math, so I implemented what I needed so I could test > the DSP. Both libraries work for lower orders, but I suspect they are buggy > with big inputs. My long term goal was to interface with LAPACK and/or GSL > for the numeric heavy lifting, but that never happened. > > The same goes for the FFT library. That was written in support of the DSP > libraries, and also as a learning exercise for me (to better understand > how/why the FFT works). FFTW was the long term goal. In the meantime fftw and gsl-matrix interfaces exist on Hackage.

1 0

using Typeable with STRefs
by Michael Vanier 17 Mar '09

17 Mar '09

Hi, I'm having a problem using Typeable with STRefs. Basically, I want to store STRefs (among other things) in a universal type. STRef is an instance of Typeable2, which means that STRef s a is Typeable if s and a are both Typeable. The problem is that the state type s is opaque and I can see no way to make it Typeable (other than making it RealWorld, and I don't want to use IO for this). If this is the case, then AFAICT there is no point in having STRefs be instances of Typeable2. Am I missing something? Here's the code I'd like to write: import Data.Typeable import Data.STRef import Control.Monad.ST data Value = forall a . Typeable a => V a deriving Typeable getValue :: Typeable a => Value -> Maybe a getValue (V v) = cast v -- I need the Typeable s constraint for the code to compile, but I'd rather leave it out. test :: Typeable s => ST s Integer test = do ref <- newSTRef (10 :: Integer) let refVal = V ref case getValue refVal of Nothing -> error "BAD" Just r -> readSTRef r -- This doesn't compile, because s is not Typeable. test2 :: Integer test2 = runST test Thanks in advance, Mike

3 4

darcs fundraising drive - only $720 left to go!
by Eric Kow 17 Mar '09

17 Mar '09

Hi everybody, Just a quick update on the fundraising drive. We are trying to raise $1000 to help pay for travel to the upcoming Haskell hackathon (the second of our biannual darcs hacking sprints). So far we have raised $280, so we're almost a third of the way there. Think you can help? Details on this page http://www.darcs.net/donations.html Many thanks! Eric PS. Ultimately, what we are trying to buy is *time*. So if you have a few spare moments in your week, the most generous donation you can make is by offering us a little of your time. Come join us in Utrecht, 17-19 April or on #darcs! Otherwise, we accept Paypal :-) -- Eric Kow <http://www.nltg.brighton.ac.uk/home/Eric.Kow> PGP Key ID: 08AC04F9

3 3

Re: [Haskell-cafe] Lambda-Cube 3D render engine
by Csaba Hruska 17 Mar '09

17 Mar '09

2009/3/16 Duane Johnson <duane.johnson(a)gmail.com> > Looks great! > By any chance, is the mesh loading functionality separate from the texture > loading? > Yes, it is completly separated from material, it only refers to material's name. You can define the material writing a nice human readable script: http://www.ogre3d.org/docs/manual/manual_14.html#SEC23 > > -- Duane Johnson > > On Mar 16, 2009, at 1:01 PM, Csaba Hruska wrote: > > Hi! > > I've created a wiki page for my project. > Currently only some text and screenshot are available. > Additional documentation and source code are coming soon.. :) > > http://www.haskell.org/haskellwiki/LambdaCubeEngine > > Cheers, > Csaba Hruska > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe > > >

1 0

Polynomial and VectorSpace
by Peter Verswyvelen 17 Mar '09

17 Mar '09

The DSP package on Hackage seems to contain complex polynomial functions. It seems these functions are completely independent of the other DSP functions, so this package could be split of? Also, polynomials form a vector space, so could be made an instance of VectorSpace class?

3 3

Lambda-Cube 3D render engine
by Csaba Hruska 17 Mar '09

17 Mar '09

Hi! I've created a wiki page for my project. Currently only some text and screenshot are available. Additional documentation and source code are coming soon.. :) http://www.haskell.org/haskellwiki/LambdaCubeEngine Cheers, Csaba Hruska

3 2

Incompatibility of different (monad transformer) libraries
by ariep 17 Mar '09

17 Mar '09

Hi, ==== Problem instance ==== In my code, I use some monad transformers. I used to use the "mtl" package, but I recently switched to the combination "transformers"/"monads-tf" (mainly for the Applicative instances). The same code also uses the "haskeline" library, for line reading. Haskeline allows to edit the user's history (previously entered lines), but only through a Control.Monad.State interface, and that happens to be the Control.Monad.State from "mtl". So now, my package also needs to import "mtl". This combination of package dependencies gives import conflicts: whenever I import some Control.Monad.{State,Error,Cont,...} module, there is more than one package providing such a module. ==== Solution? ==== For now, I resolve the ambiguity by using "Package imports", i.e. annotate the ambiguous imports by the right package name. Is this the best solution? The ghc manual states, that the package import extension should not be necessary. I can think of another solution, by creating a dedicated package, which imports mtl:Control.Monad.* and re-exports them under another name, but that's even less elegant than package imports. What is the recommended way to deal with module name conflicts, such as this one? Regards, Arie BTW: how can I let the user edit a line of text, from a non-empty starting value for the text? (In other words, actually *edit* a line, not enter a new one.) I currently do this by adding the starting value to the user's history, so he/she can get it by pressing the up arrow key. This is clearly not optimal.

4 3

Re: Has anybody replicated =~ s/../../ or even something more basic for doing replacements with pcre haskell regexen?
by Thomas Hartman 17 Mar '09

17 Mar '09

So, I tweaked Text.Regex to have the behavior I need. http://patch-tag.com/repo/haskell-learning/browse/regexStuff/pcreReplace.hs FWIW, the problem I was trying to solve was deleting single newlines but not strings of newlines in a document. Dead simple for pcre-regex with lookaround. But, I think, impossible with posix regex. -- replace single newlines, but not strings of newlines (requires pcre look-around (lookaround, lookahead, lookbehind, for googlebot)) http://perldoc.perl.org/perlre.html testPcre = ( subRegex (mkRegex "(?<!\n)\n(?!\n)") "asdf\n \n\n\nadsf" "" ) == "asdf \n\n\nadsf" Can I lobby for this to make its way into the Regex distribution? Really, I would argue that every regex flavor should have all the functions that Text.Regex get, not just posix. (subRegex is just the most important, to my mind) Otherwise I'll make my own RegexHelpers hackage package or something. Hard for me to see how to do this in an elegant way since the pcre packages are so polymorphic-manic. I'm sure there is a way though. Or if you point me to the darcs head of regex I'll patch that directly. 2009/3/14 Thomas Hartman <tphyahoo(a)gmail.com>: > Right, I'm just saying that a "subRegex" that worked on pcre regex > matches would be great for people used to perl regexen and unused to > posix -- even it only allowed a string replacement, and didn't have > all the bells and whistles of =~ s../../../ in perl. > > 2009/3/12 ChrisK <haskell(a)list.mightyreason.com> >> Thomas Hartman wrote: >>> >>> Is there something like subRegex... something like =~ s/.../.../ in >>> perl... for haskell pcre Regexen? >>> >>> I mean, subRegex from Text.Regex of course: >>> http://hackage.haskell.org/cgi-bin/hackage-scripts/package/regex-compat >>> >>> Thanks for any advice, >>> >>> thomas. >> >> Short answer: No. >> >> This is a FAQ. The usual answer to your follow up "Why not?" is that the >> design space is rather huge. Rather than justify this statement, I will >> point at the complicated module: >> >> http://hackage.haskell.org/packages/archive/split/0.1.1/doc/html/Data-List-… >> >> The above module is "a wide range of strategies for splitting lists", which >> is a much simpler problem than your subRegex request, and only works on >> lists. A subRegex library should also work on bytestrings (and Seq). >> >> At the cost of writing your own routine you get exactly what you want in a >> screen or less of code, see >> http://hackage.haskell.org/packages/archive/regex-compat/0.92/doc/html/src/… >> for "subRegex" which is 30 lines of code. >> >> Cheers, >> Chris >> >

4 6

Visualising the Hierarchical Namespace
by Don Stewart 17 Mar '09

17 Mar '09

I've just finished a post (and quick tool) for graphing the complete module namespace of Haskell, taken from the core libraries and all of Hackage. It's quite large: http://donsbot.wordpress.com/2009/03/16/visualising-the-haskell-universe/

5 9

Re: Haskell-Cafe Digest, Vol 67, Issue 41
by abdullah abdul Khadir 17 Mar '09

17 Mar '09

> > > On Mon, 16 Mar 2009 08:05:37 -0700 Don Stewart <dons(a)galois.com> wrote : > Subject: [Haskell-cafe] Visualising the Hierarchical Namespace > > I've just finished a post (and quick tool) for graphing the complete > module namespace of Haskell, taken from the core libraries and all of > Hackage. > > It's quite large: > > > http://donsbot.wordpress.com/2009/03/16/visualising-the-haskell-universe/ Hi Don, That was really good. Amazing stuff there. In fact I just (re) joined the haskell and lisp mailing lists. Course work was keeping me at bay from following my passion for functional programming paradigms except that one of my courses was in fact haskell last semester. Well, it seems as if haskell is taking the functional world by storm. Even in the lisp mailing lists I hear of people reading haskell books, "Real World Haskell" was one of them and in fact they seem to be appreciating the type system offered by haskell. Now, that really means something. You know how hard it is to impress the lisp folks. All in all an impressive run so far. Hats off to all those developers and the Haskell community at large.

1 0

Re: [Haskell-cafe] Map in Terms of Fold in terms of Map
by Neil Mitchell 17 Mar '09

17 Mar '09

Hi Mark, What's the definition of foldr in terms of map? As far as I was aware, its not possible. And as it happens, map is (or is sometimes) defined in term of foldr :-) While you can use mutual recursion, you can't define a in terms of b and b in terms of a unless one of them actually does something at the end - so it does depend exactly what the definitions are. Thanks Neil 2009/3/16 Mark Spezzano <mark.spezzano(a)chariot.net.au>: > Hi, > > > > I’ve noticed that it’s possible to define map as foldr and foldr as a map. > Would this be sensible to define both in terms of each other in the Prelude? > i.e mutually recursive? > > > > Cheers, > > > > Mark > > > > No virus found in this outgoing message. > Checked by AVG. > Version: 7.5.557 / Virus Database: 270.11.15/2003 - Release Date: 15/03/2009 > 2:07 PM > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe > >

1 0

Map in Terms of Fold in terms of Map
by Mark Spezzano 17 Mar '09

17 Mar '09

Hi, I’ve noticed that it’s possible to define map as foldr and foldr as a map. Would this be sensible to define both in terms of each other in the Prelude? i.e mutually recursive? Cheers, Mark No virus found in this outgoing message. Checked by AVG. Version: 7.5.557 / Virus Database: 270.11.15/2003 - Release Date: 15/03/2009 2:07 PM

1 0

Re: [Haskell-cafe] Natural Numbers: Best implementation?
by Alexander Dunlap 16 Mar '09

16 Mar '09

2009/3/12 Mark Spezzano <mark.spezzano(a)chariot.net.au>: > Hi, > > > > I was wondering what the best way to implement Natural number would be. Is > there a package which already does this? > > > > Here are some options: > > > > 1. Don’t bother. Just use Integer. > > 2. Use the type > > data Natural = Zero | Succ !Natural > > 3. Use the following definition taken from the Gentle Introduction to > Haskell 98 > > newtype Natural = MakeNatural Integer > > toNatural ::Integer-> Integer > > toNatural x | x < 0 = error “Can’t create negative naturals!” > > | otherwise = MakeNatural x > > fromNatural :: Natural -> Integer > > fromNatural (MakeNatural i) = i > > > > and then... > > > > instance Num Natural where > > fromInteger = toNAtural > > x + y = toNatural (fromNatural x + fromNatural y) > > x – y = etc.. > > x * y = etc... > > > > Which method is best? So far, I’ve been picking option #1 – just leaving > things as they are and using Integer to keep things simple. > > > > I’ve got that feeling that [2] would be fast and [3] would be slow. Comment > appreciated on the merits of each. > > > > Cheers, > > > > Mark Spezzano > I would tend to use option (1) unless there's a compelling reason not to. Since naturals aren't closed under subtraction, you would in practice probably have just as many non-total functions as you would with the regular Int{,eger} type. Also, a lot of functions just take Integers so it would be more of a pain to use. In terms of speed, I think that [3] would be reasonably fast (unless you do a ton of subtraction with bounds-checking) and [2] would probably be quite slow, because you don't get the speed-boost from doing computations right on the processor. Alex

4 5

Packet analysis framework for Haskell.
by Vimal 16 Mar '09

16 Mar '09

Hi all, Is there a packet analysis framework in Haskell? The framework should abstract away the features of packet analysis in the form of a library that people can use and write extensions to perform computation on the packets flowing through the network. What are the "features" of packet analysis? 1. Support for packet packet selection, matching a particular criteria. Example: (protocol = tcp && destination_port = 80). 2. Multi-threaded. Since the process of computation on packets is inherently data-parallel, the framework should abstract away the parallelism and make use of different CPU cores if available. I believe the features in Haskell would be a great boon to easily achieve this. 3. Support for statistics. Packet analysis is usually done for a purpose. Say, I would like to collect statistics about the rate of connections to a webserver. I would want to: (a) Filter packets, using features in (1). (b) Count the number of packets that pass through after applying operation (a). (c) Generate reports if a particular condition is flagged in (b). Example: (a) protocol = tcp && SYN set in flags && destination_port = 80, (b) count the number of SYN packets, (c) report if d(count)/dt > 100. 4. Availability of efficient data structures for achieving real time performance. Example: The user might want to check if ARP spoofing is taking place. In which case, one has to cache the "already seen" mapping between protocol and hardware addresses learnt from the packets that flowed, and constantly checking new addresses against this structure for duplicates. The above are some of the features which I believe are necessary for packet analysis (or, analytics maybe?). There could be more. I was wondering if Haskell would be a good language to achieve these things. I had a brief idea and started writing an application in C and I realised the need for such a framework. The ease of parallelism offered by Haskell prompted me to think about using it. I would like to hear your comments. Also, would this qualify as a project that someone in the Haskell community would be willing to mentor for GSoC 2009? (Assuming Haskell would be on the list of mentoring organisations, although I have no reason to doubt it wouldn't.) Thanks, Regards, -- Vimal

3 2

A non-inductive Haskell proof?
by R J 16 Mar '09

16 Mar '09

The following theorem is obviously true, but how is it proved (most cleanly and simply) in Haskell? Theorem: (nondecreasing xs) => nondecreasing (insert x xs), where: nondecreasing :: (Ord a) => [a] -> Bool nondecreasing [] = True nondecreasing xxs@(x : xs) = and [a <= b | (a, b) <- zip xxs xs] insert :: (Ord a) => a -> [a] -> [a] insert x xs = takeWhile (<= x) xs ++ [x] ++ dropWhile (<= x) xs Thanks. _________________________________________________________________ Hotmail® is up to 70% faster. Now good news travels really fast. http://windowslive.com/online/hotmail?ocid=TXT_TAGLM_WL_HM_70faster_032009

2 1

Hashing over equivalence classes
by Roman Cheplyaka 16 Mar '09

16 Mar '09

Are there some known ways to define hashing (or any other) functions over equivalence classes? I.e. a ~ b => hash(a) == hash(b) where (~) is some equivalence relation. For example, you might want to hash lambda terms modulo alpha-equivalence or hash logical terms with respect to commutativity of (&&) and (||). Often we can choose 'canonical' element from each class and hash it. But (at least, in theory) it's not necessary. So, are there (practical) ways to define hash function without it? -- Roman I. Cheplyaka :: http://ro-che.info/ "Don't let school get in the way of your education." - Mark Twain

2 2

ANN: The Monad.Reader (13)
by Wouter Swierstra 16 Mar '09

16 Mar '09

I am pleased to announce that a new issue of The Monad.Reader is now available: http://www.haskell.org/haskellwiki/The_Monad.Reader The Monad.Reader is a quarterly magazine about functional programming. Issue 13 consists of the following four articles: * Stephen Hicks Rapid Prototyping in TEX * Brent Yorgey The Typeclassopedia * Chris Eidhof, Eelco Lempsink Book Review: "Real World Haskell" * Derek Elkins Calculating Monads with Category Theory Special thanks to Ashley Yakeley for his help with publishing The Monad.Reader on the Haskell wiki. If you'd like to write something for the next issue of The Monad.Reader, please get in touch. I haven't fixed the deadline for the next issue, but it should be mid-May or thereabouts. Wouter

1 0