March 2008 - Haskell-Cafe

RE: How to implement Read instance for user defined type
by Verma Anurag-VNF673 24 Mar '08

24 Mar '08

That works, but is probably not what you want. You can use the lex function to parse identifiers not enclosed in quotes: > instance Read Mark where > readsPrec _ str = [(Mark x, t') | ("mark",t) <- lex str, > (x,t') <- reads t I played a bit around with lex function and it seems that under certain circumstances it doesn't read the string of token properly For e.g. -- IP address *Mark> lex "192.168.0.1" [("192.168",".0.1")] -- digit + char Also lex "8021p" output is *Mark> lex "8021p" [("8021","p")] Is there any other lex variant available? Or should I switch to using Parsec library, in that case, will I still be able to use it with Read instances? If so, how? Anurag

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HFuse: ls fails in HelloFS
by Georg Neis 24 Mar '08

24 Mar '08

Hello, I've installed the HFuse package from hackage and am playing with the HelloFS example in the System/Posix/HFuse directory. The problem that I encounter is that listing the directory doesn't work: % ghc --make HelloFS.hs [1 of 1] Compiling Main ( HelloFS.hs, HelloFS.o ) Linking HelloFS ... % mkdir bla % ./HelloFS bla % cd bla % cat hello Hello World, HFuse! % ls ls: cannot open directory .: Function not implemented Here's the relevant output from strace: open(".", O_RDONLY|O_NONBLOCK|O_LARGEFILE|O_DIRECTORY|0x80000) = -1 ENOSYS (Function not implemented) write(2, "ls: ", 4ls: ) = 4 write(2, "cannot open directory .", 23cannot open directory .) = 23 write(2, ": Function not implemented", 26: Function not implemented) = 26 Some system information: - Debian unstable - Linux 2.6.24 - Fuse 2.7.3 - GHC 6.8.2 - HFuse 0.1 Any ideas? Thanks, Georg

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Any ideas for "->" (function type) in a simply-typed lambda calculus version of Brett Victor's Alligator Eggs game?
by Benjamin L. Russell 24 Mar '08

24 Mar '08

Brett Victor has invented an Alligator Eggs game (very loosely resembling Conway's Game of Life), using formal rules to determine feeding and breeding patterns for alligators and their eggs, but the game represents the *untyped* lambda calculus: Alligator Eggs! http://worrydream.com/AlligatorEggs/ I am interested in adapting this to the *simply typed* lambda calculus with Curry-style typing, to aid students of Haskell. Brett Victor's game currently aids students of LISP, and I think that it would be helpful to have a Haskell counterpart (see http://wadler.blogspot.com/2007/05/oh-no-alligators.html for a comment on how the Alligator Eggs game has been used by EE students at Caltech in learning LISP): Simply typed lambda calculus - Wikipedia, the free encyclopedia http://en.wikipedia.org/wiki/Simply_typed_lambda_calculus Thereafter, I would like to create a new entry for this Haskell version of Alligator Eggs on HaskellWiki, and create a link to it under a new subsection, "Board Games," under the current "Learning Haskell" category: http://www.haskell.org/haskellwiki/Learning_Haskell In Brett Victor's version, the following representations hold: * a hungry alligator: lambda abstraction * an old alligator: a pair of parentheses * an egg: a variable * the Eating Rule: beta-reduction * the Color Rule: (over-cautious) alpha-conversion * the Old Age Rule: the rule that a pair of parentheses containing a single term may be removed However, in thinking about how to adapt the game, I am not quite sure how to incorporate the representation of "->" (function type): * ???: "->" (function type) What ideas, if any, would anybody have on how "->" (function type) could be represented in a simply-typed lambda calculus version of Brett Victor's Alligator Eggs? Benjamin L. Russell

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hoisting as well as lifting and returning
by Matthew Pocock 23 Mar '08

23 Mar '08

Hi, Happy Easter! I've been using monad transformers for the first time in anger this week. They certainly do the job. However, there's an operation that I keep defining over and over again. It is sort of like lift and return in that it's glue to get operations and values into the monad from outside it. Here are two examples. hoistList :: (Monad m) => [a] -> ListT m a hoistList = foldr (mplus . return) mzero hoistMaybe :: (Monad m) => Maybe a -> MaybeT m a hoistMaybe = maybe mzero return The general idea is that you have some legacy, non-transform operation in some monad MonadFoo, and you are writing an operation in MonadFooT. You want to get the monadFoo value into MonadFooT. So, you say something like: do vInMFT <- hoist vInMF Is this a common enough operation on a monad transformer and it's 'raw' monad to warrant a class of its own? I have 'hoist' methods for several monad transformers, including RamdomMonadT, although I've probably defined them in ways that are overly eager or eat stack/heap, so I'd prefer knowledgable people to write them :) There must be other operations that link the base monad with its transformed version, other than 'hoist' - the runFooT method for example. Perhaps 'hoist' already exists and I'm re-inventing the wheel? Matthew

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Type checking of partial programs
by ac 23 Mar '08

23 Mar '08

Is anybody interested in working on this? This is a project I've been interested in for some time, but recognize I probably need some guidance before I go off and start hacking on it. As dcoutts pointed out on #haskell-soc, this may be of particular interest to people working on yi and HaRe. Other interesting and related projects include parsing partial programs to insert "placeholders" in appropriate places. An example of a partial program could be: foo :: [Foo] -> <placeholder 1> foo xs = map <placeholder 2> xs What are the possible type signatures for placeholder 1 and the possible expressions for placeholder 2? I would like to stir up a discussion about this, and eventually write some useful code. -Abram

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"doctest" for haskell -- a good project?
by Shaun Cutts 23 Mar '08

23 Mar '08

Hello, I am an experienced programmer, currently learning Haskell. Currently I write many things in python. I use both the "doctest" and "unittest" modules extensively. As I write code, I simultaneously write doctest code in the doc strings to explain/set out the "typical narrative" of how the code is used. Then finishing off a module I write unittests for boundary conditions, more complex test cases, and generally code that would be annoying to write & read in doctests. I note that there is a unit testing framework for Haskell, but I don't see any doctest module. Might this be a good project? If so, suggestions as to resources would be greatly appreciated. I believe I can't just "introspect" Haskell modules to get at documentation/comments, like I can in python? (Why not? :)) I notice that there are a few "documentation generators". Should I try to write an extension of one of these? Haddock, for instance? Are there any Haddock developers hanging out on this list, to encourage or dissuade me? :) (And where is the Haddock doc for Haddock?) In any case, thanks in advance for any comments & advice. - Shaun Cutts

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Re: Haskell-Cafe Digest, Vol 55, Issue 30
by Deng Chao 23 Mar '08

23 Mar '08

Hi Sebastian Sylvan, Thank you very much! I thick the last column return a string because of the Haskell "Show" function, for when I made test, I got such an error message: Show ([SqlValue] -> IO [[SqlValue]]). Any way, thank you very much! Deng Chao 在 2008-03-22六的 13:55 -0400，haskell-cafe-request(a)haskell.xn--org:-7z8fk634a > Send Haskell-Cafe mailing list submissions to > haskell-cafe(a)haskell.org > > To subscribe or unsubscribe via the World Wide Web, visit > http://www.haskell.org/mailman/listinfo/haskell-cafe > or, via email, send a message with subject or body 'help' to > haskell-cafe-request(a)haskell.org > > You can reach the person managing the list at > haskell-cafe-owner(a)haskell.org > > When replying, please edit your Subject line so it is more specific > than "Re: Contents of Haskell-Cafe digest..." > > > Today's Topics: > > 1. compile error with FFI code (Galchin Vasili) > 2. "doctest" for haskell -- a good project? (Shaun Cutts) > 3. Re: [GSoC] Plugins for GHC (Bulat Ziganshin) > 4. Re: "doctest" for haskell -- a good project? (Roman Cheplyaka) > 5. Re: "doctest" for haskell -- a good project? (Don Stewart) > 6. Re: [GSoC] Plugins for GHC (Neil Mitchell) > 7. Re: "doctest" for haskell -- a good project? (Neil Mitchell) > 8. Re: An ugly zip3 problem.. (Michael Feathers) > 9. Re: An ugly zip3 problem.. (Michael Feathers) > 10. How to program with sqlite? (Deng Chao) > 11. Re: "doctest" for haskell -- a good project? (Thomas Schilling) > 12. Re: How to program with sqlite? (Sebastian Sylvan) > 13. Re: "doctest" for haskell -- a good project? (Sebastian Sylvan) > 14. HXT and types in Control.Arrow.ArrowTree (Robert Vollmert) > 15. Opening Windows .lnk Files (Dominic Steinitz) > 16. RE: "doctest" for haskell -- a good project? (Shaun Cutts) > 17. Re: An ugly zip3 problem.. (Bryan O'Sullivan) > 18. Re: "doctest" for haskell -- a good project? (Paul Johnson) > 19. Re: AMQP framing layer: design help requested. (Paul Johnson) > 20. Re: Problem with OpenAL (Antoine Latter) > 21. Re: AMQP framing layer: design help requested. (Derek Elkins) > 22. Re: Opening Windows .lnk Files (Neil Mitchell) > 23. Re: "doctest" for haskell -- a good project? (Neil Mitchell) > 24. Re: Opening Windows .lnk Files (Dominic Steinitz) > 25. RE: "doctest" for haskell -- a good project? (Shaun Cutts) > > > ---------------------------------------------------------------------- > > Message: 1 > Date: Fri, 21 Mar 2008 23:21:26 -0500 > From: "Galchin Vasili" <vigalchin(a)gmail.com> > Subject: [Haskell-cafe] compile error with FFI code > To: Haskell-cafe <haskell-cafe(a)haskell.org> > Cc: Galchin Vasili <vigalchin(a)gmail.com> > Message-ID: > <5ae4f2ba0803212121w17fe482ek105c13cf6999e603(a)mail.gmail.com> > Content-Type: text/plain; charset="iso-8859-1" > > Hello, > > In my blah.hsc, I have "allocbytes (#const (struct bozo)) .. where > > struct bozo is a bunch of "long int" ,,, In the "runhaskell Setup.hs build" > step I get a nasty error message about an "incomplete type". I have look at > many times but this error doesn't make sense to me. ?? > > Kind regards, Vasili >

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ANNOUNCE: wxHaskell 0.10.3 rc1
by Jeremy O'Donoghue 23 Mar '08

23 Mar '08

We are pleased to announce the first release candidate of wxHaskell 0.10.3. This is the first update with binary packages available since June 2005, and is the result of a great deal of work by a new team of contributors. We are hoping to make a full release shortly, and issues and bug reports either through the wxHaskell user mailing list (wxhaskell-users(a)sourceforge.net) or via the Sourceforge bug tracker (http://sourceforge.net/tracker/?group_id=73133&atid=536845) Highlights of 0.10.3 rc1 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 retained if you compile from source) - 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) About wxHaskell wxHaskell is a Haskell binding to the wxWidgets GUI library. 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 -- Jeremy O'Donoghue jeremy.odonoghue(a)gmail.com

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compile error with FFI code
by Galchin Vasili 23 Mar '08

23 Mar '08

Hello, In my blah.hsc, I have "allocbytes (#const (struct bozo)) .. where struct bozo is a bunch of "long int" ,,, In the "runhaskell Setup.hs build" step I get a nasty error message about an "incomplete type". I have look at many times but this error doesn't make sense to me. ?? Kind regards, Vasili

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Using HaskellDb to connect to PostgresSql
by Marc Mertens 22 Mar '08

22 Mar '08

Hello, I'm trying to learn to use HaskellDb. I have managed to finally compile and install it on my linux box (I have ghc 6.8.2). But when I try to create a database description (as described in http://haskelldb.sourceforge.net/getting-started.html) (using DBDirect-dynamic instead of DBDirect) I'm stuck. I have tried different names for the driver but I always get the message 'DBDirect-dynamic: user error (Missing field driver)'. Has someone get this working for PostgresSQL and if so, can he/she help me out with some instructions. Also if someone has some pointers to more recent documentation of HaskellDB, I would really be pleased if I could get these. Marc Mertens

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