- Haskell-Cafe - Haskell.org

ANN: darcswatch, a way to track your contributions
by Joachim Breitner 15 Apr '08

15 Apr '08

Hi Haskellers, today I scrached an itch that was icking for a while: When I submit patches to some project or person, I’m never sure that I won’t forget checking that the patch will actually be applied. But if I forget, and the maitainer forgets (or decides against), my patch would be lost. Therefore I wrote a little haskell program darcswatch, which has a bunch of patches to track, and a bunch of repositories to watch, and shows you what patches have to be applied where. You see it in action here: http://darcswatch.nomeata.de/ To use it conveniently, I have put this in my ~/.darcs/defaults: send sign send cc darcswatch(a)nomeata.de so all patches I send will be signed and copied to that address. For security reasons, only mails signed by allowed gpg keys are accepted and the webpage is updated. It is also updated every hour by a cronjob. As you can see the program is ready to track patches by several authors. If you want to try it out and help debug it, just drop me a (signed) note with your gpg key-id, also listing all repositories you want to be tracked. I would also appreciate any help with programming darcswatch. There is a darcs repository on http://darcs.nomeata.de/darcswatch/ (with DarcsWeb at http://darcs.nomeata.de/cgi-bin/darcsweb.cgi?r=darcswatch;a=summary) There is a lot to do yet, I quote the TODO list on http://darcs.nomeata.de/darcswatch/README : * Support for tagged repositories (fetching older inventories) * Cache the inventories and only update when needed, by issuing a HEAD request. * Add a download link to each pach * Add the diff to each patch, with some javascript hiding/unhiding. * Cabalize the program. * Detect inverted patches and treat specially (e.g. an unapplied patch where the user also submitted an inverted patch can be considered obsolete) * Nicer output (CSS magic!) * (Maybe) Add repositores per gpg signed command Haskell projects could also make use of darcswatch by tracking patches that were submitted to a bugtracker (e.g. for xmonad) or via the mailing list, to make sure no contribution is lost. I could also imagine that it might be a service that, if proven reliable, could be run on community.haskell.org or similar. As always, comments are appreciated. Enjoy, Joachim -- Joachim "nomeata" Breitner mail: mail(a)joachim-breitner.de | ICQ# 74513189 | GPG-Key: 4743206C JID: nomeata(a)joachim-breitner.de | http://www.joachim-breitner.de/ Debian Developer: nomeata(a)debian.org

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Re: RFC: A standardized interface between web servers and applications or frameworks (ala WSGI)
by Adam Langley 15 Apr '08

15 Apr '08

On Sun, Apr 13, 2008 at 6:32 PM, Chris Smith <cdsmith(a)twu.net> wrote: > Does old code that handled these headers stop working, just because it > was looking in the "other" section, but now needs to check a field > dedicated to that header? Yes, but it would be very sad if we couldn't do common header parsing because of this. I'd suggest that all the headers given in RFC 2616 be parsed and nothing else. That leaves the question of how we would handle the addition of any extra ones in the future. Firstly, packages could depend on a given version of this interface and we declare that the set of handled headers doesn't change within a major version. Better would be some static assertion that the interface doesn't handle some set of headers. Maybe there's a type trick to do this, but I can't think of one, so we might have to settle for a non static: checkUnparsedHeaders :: [String] -> IO () Which can be put in 'main' (or equivalent) and can call error if there's a mismatch. AGL -- Adam Langley agl(a)imperialviolet.org http://www.imperialviolet.org

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Re: [Haskell-cafe] Re: Embedding newlines into a string?
by Abhay Parvate 14 Apr '08

14 Apr '08

Yes, they are. That's what perhaps Neil Mitchell means by mapM_ putStrLn == putStr . unlines And whether the trailing newline is to be called the last blank line depends upon the convention; The string that is output in both the cases contains a single newline character. Are you calling that a blank line at end? And I actually meant to reply to haskell-cafe as well; omitted haskell-cafe by mistake. Anyway, you got the same answer from two people! This time I am including haskell-cafe in the recipients. Regards, Abhay On Mon, Apr 14, 2008 at 4:05 PM, Benjamin L. Russell <dekudekuplex(a)yahoo.com> wrote: > Abhay Parvate, > > Thank you; that answered my question. > > Then, the following two lines of code should be > equivalent: > > In hanoi.hs: > hanoi n = mapM_ putStrLn (hanoi_helper 'a' 'b' 'c' n) > > In hanoi_unlines.hs: > hanoi n = putStr (unlines(hanoi_helper 'a' 'b' 'c' n)) > > I tested them both out on WinHugs (Version Sep 2006); > they both generated one blank line at the end. > > Benjamin L. Russell > > --- Abhay Parvate <abhay.parvate(a)gmail.com> wrote: > > > unlines puts newline after each string; putStrLn > > puts newline after the > > given string. As a result, the output contains two > > newlines in the end. You > > can use putStr instead, since the resultant string > > from 'unlines' will have > > a newline at the end. > > > > On Mon, Apr 14, 2008 at 2:12 PM, Benjamin L. Russell > > <dekudekuplex(a)yahoo.com> > > wrote: > > > > > Ok; much better. Here's my new type signature and > > > definition: > > > > > > hanoi.hs: > > > hanoi :: Int -> IO () > > > hanoi n = mapM_ putStrLn (hanoi_helper 'a' 'b' 'c' > > n) > > > > > > hanoi_helper :: Char -> Char -> Char -> Int -> > > > [String] > > > hanoi_helper source using dest n > > > | n == 1 = ["Move " ++ show source ++ " to " ++ > > > show dest ++ "."] > > > | otherwise = hanoi_helper source dest using > > (n-1) > > > > > > ++ hanoi_helper source using dest > > 1 > > > ++ hanoi_helper using > > source > > > dest (n-1) > > > > > > Then in WinHugs (Version Sep 2006): > > > > > > Hugs> :load hanoi.hs > > > Main> hanoi 2 > > > Move 'a' to 'b'. > > > Move 'a' to 'c'. > > > Move 'b' to 'c'. > > > > > > Great! > > > > > > One minor question: I tried out both of your > > > following suggestions: > > > > > > > > mapM_ putStrLn (hanoi 2) -- outputs each > > move > > > > in a new line > > > > > putStrLn (unlines (hanoi 2)) -- same as > > > > previous line > > > > > > and discovered that putStrLn with unlines (the > > lower > > > option) in fact generates one extra blank line at > > the > > > end. Just curious as to why.... > > > > > > Benjamin L. Russell > > > > > > --- Tillmann Rendel <rendel(a)daimi.au.dk> wrote: > > > > > > > Benjamin L. Russell wrote: > > > > > but got stuck on outputting newlines as part > > of > > > > the string; > > > > > > > > quoting is done by the show function in Haskell, > > so > > > > you have to take > > > > care to avoid calling show. your code calls show > > at > > > > two positions: > > > > (1) when you insert the newline into the string > > > > (2) when you output the string > > > > > > > > with respect to (1): > > > > > > > > you use (show '\n') to create a newline-only > > string, > > > > which produces a > > > > machine-readable (!) textual representation of > > '\n'. > > > > try the difference > > > > between > > > > > > > > > '\n' > > > > > > > > and > > > > > > > > > show '\n' > > > > > > > > to see what I mean. instead of using (show > > '\n'), > > > > you should simply use > > > > "\n" to encode the string of length 1 containing > > a > > > > newline character. > > > > > > > > with respect to (2): > > > > > > > > the type of your top-level expression is String, > > > > which is automatically > > > > print'ed by the interpreter. but print x = > > putStrLn > > > > (show x), so there > > > > is another call to show at this point. to avoid > > this > > > > call, write an IO > > > > action yourself. try the difference between > > > > > > > > putStrLn (hanoi ...) > > > > > > > > and > > > > > > > > print (hanoi ...) > > > > > > > > to see what I mean. > > > > > > > > Last, but not least, I would like to point out a > > > > different aproach to > > > > multiline output which is often used by Haskell > > > > programmers: The worker > > > > functions in this aproach produces a list of > > > > strings, which is joined > > > > together with newlines by the unlines function. > > In > > > > your case: > > > > > > > > hanoi_helper :: ... -> [String] > > > > | ... = ["Move " ++ ...] > > > > | otherwise = hanoi_helper ... ++ > > hanoi_helper > > > > ... > > > > > > > > hanoi n = hanoi_helper 'a' 'b' 'c' n > > > > > > > > and in the interpreter one of these: > > > > > > > > > hanoi 2 -- outputs a list > > > > > mapM_ putStrLn (hanoi 2) -- outputs each > > move > > > > in a new line > > > > > putStrLn (unlines (hanoi 2)) -- same as > > > > previous line > > > > > > > > Tillmann > > > > _______________________________________________ > > > > Haskell-Cafe mailing list > > > > Haskell-Cafe(a)haskell.org > > > > > > http://www.haskell.org/mailman/listinfo/haskell-cafe > > > > > > > > > > _______________________________________________ > > > Haskell-Cafe mailing list > > > Haskell-Cafe(a)haskell.org > > > > > http://www.haskell.org/mailman/listinfo/haskell-cafe > > > > > > >

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ANNOUNCE: Generic Haskell 1.80 (Emerald)
by Thomas van Noort 14 Apr '08

14 Apr '08

Generic Haskell version 1.80 (Emerald) ====================================== We are happy to announce the fifth release of Generic Haskell, an extension of Haskell that facilitates generic programming. Generic Haskell includes the following features: * type-indexed values -- generic functions that can be instantiated on all Haskell data types. * type-indexed types -- types which are indexed over the type constructors underlying Haskell datatypes. The Generic Haskell compiler takes Generic Haskell source and produces Haskell code. Changes since 1.62 (Diamond) ---------------------------- * Generic views for generic types [1] are now supported. * The implementation of type-indexed types is improved. Download -------- The Generic Haskell compiler is available in source and binary distributions. Binaries for Linux, Windows, and MacOSX are available. These are available from: http://www.generic-haskell.org/compiler.html The documentation is also available separately from that page. For more general information, point your browser to: http://www.generic-haskell.org Why Generic Haskell? -------------------- Software development often consists of designing datatypes, around which functionality is added. Some functionality is datatype specific, whereas other functionality is defined on almost all datatypes in such a way that it depends only on the structure of the datatype. A function that works on many datatypes in this way is called a generic function. Examples of generic functionality include editing, pretty-printing or storing a value in a database, and comparing two values for equality. Since datatypes often change and new datatypes are introduced, we have developed Generic Haskell, an extension of the functional programming language Haskell that supports generic definitions, to save the programmer from (re)writing instances of generic functions. The original design of Generic Haskell is based on work by Ralf Hinze. Pleasant programming, The Generic Haskell Team at Utrecht University info(a)generic-haskell.org [1] Thomas van Noort. Generic views for generic types. Master's thesis, Utrecht University, 2008.

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Type families and GADTs in 6.9
by Dan Doel 14 Apr '08

14 Apr '08

Hello, I've been playing around with type families off and on in 6.8, but, what with the implementation therein being reportedly incomplete, it's hard to know what I'm getting right and wrong, or what should work but doesn't and so on. So, I finally decided to take the plunge and install 6.9 (which, perhaps, isn't yet safe in that regard either, but, such is life :)). But, when I loaded up one of my programs, I got a type error. The subject is inductively defined tuples: ------------------------------------- {-# LANGUAGE GADTs, TypeFamilies, EmptyDataDecls, TypeOperators #-} data Zero data Succ n data Nat n where Zero :: Nat Zero Succ :: Nat n -> Nat (Succ n) data FZ data FS a data Fin fn n where FZ :: Fin FZ (Succ n) FS :: Fin fn n -> Fin (FS fn) (Succ n) f0 = FZ f1 = FS f0 f2 = FS f1 -- ... etc. data Nil data t ::: ts type family Length ts :: * type instance Length Nil = Zero type instance Length (t ::: ts) = Succ (Length ts) type family Lookup ts fn :: * type instance Lookup (t ::: ts) FZ = t type instance Lookup (t ::: ts) (FS fn) = Lookup ts fn infixr 4 ::: data Tuple ts where ZT :: Tuple Nil (:::) :: t -> !(Tuple ts) -> Tuple (t ::: ts) {- This type signature gets complained about in 6.8, but it seems like a sensible one: proj :: (Length ts ~ n) => Fin fn n -> Tuple ts -> Lookup ts fn Indexing the tuple by its length is also an option (which works). In any case, the code doesn't even work with the lenient 6.8 signature: -} proj :: Fin fn n -> Tuple ts -> Lookup ts fn proj FZ (v ::: vs) = v proj (FS fn) (v ::: vs) = proj fn vs ------------------------------------- The overall goal being Haskell-alike tuples with a single projection function that works for all of them, without having to use template haskell for instance (fst = proj f0, snd = proj f1, etc.). However, proj results in a type error: Occurs check: cannot construct the infinite type: t = Lookup (t ::: ts) fn In the pattern: v ::: vs In the definition of `proj': proj FZ (v ::: vs) = v Oddly (to me), if I reverse the clauses, the compiler doesn't complain about the FS case, still complaining about FZ. Now, my thought process here is that the pattern match against FZ (the value) requires fn to be FZ (the type), which should tell the compiler to solve "t ~ Lookup (t ::: ts) FZ" which is, of course, the first instance above. Am I doing something wrong, or have I bumped into as-yet-unimplemented functionality? Thanks for your time and help. -- Dan

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Re: RFC: A standardized interface between web servers and applications or frameworks (ala WSGI)
by Daniel McAllansmith 14 Apr '08

14 Apr '08

On Mon, 14 Apr 2008 13:32:07 Chris Smith wrote: > On Sun, 13 Apr 2008 16:06:43 -0700, Adam Langley wrote: > > On Sun, Apr 13, 2008 at 4:59 AM, Johan Tibell <johan.tibell(a)gmail.com> > > > > wrote: > >> * Using a different set of data types would work better. > > > > Give that this is Haskell, I'd suggest more types ;) > > > > HTTP headers aren't just strings and, at the risk of tooting my own > > horn, I'll point to the Headers structure in [1]. > > Wait, I'm not sure I agree here. How are headers not just strings? Headers, at least their values, aren't strings. The specification says so. I think headers should be represented by something more specific than a string. > By > assuming that, are we guaranteeing that anything using this interface > cannot respond gracefully to a client that writes malformed headers? Having explicit types for headers doesn't preclude trying to handle messages with malformed headers. Soldiering on in the face of malformed messages as a general strategy is pretty dubious in my opinion. In the specific cases where you've determined it is necessary you want to be able to register a work-around parser for that section of the message, and be able to tell that it has been used. A decent framework can supply a catalogue of commonly required work-arounds. > > Another perspective: there is unnecessary variation there in how > interfaces are represented. If I'm looking for a header, and I know its > name as a string, how do I look for it? Well, apparently it's either a > named field (if it's known to the interface) or in the "other" section > (if not). So I write a gigantic case analysis? But then suppose the > interface is updated later to include some headers that popped up > unofficially but then are standardized in a future RFC. (This is not too > odd; lots of REST web services invent new headers every day, many of > which do things that make sense outside of the particular application.) > Does old code that handled these headers stop working, just because it > was looking in the "other" section, but now needs to check a field > dedicated to that header? I don't like the idea of having a fixed enumeration of methods or headers. You need to be able to define new methods and headers at will, and ideally have the usage of headers constrained to valid contexts. This suggests to me type classes that establish a 'can occur in' relationship between request/response, method and a given general/request/response/entity header. By importing new method or header data type, appropriate type class instances and registering an appropriate message parser extension you can mix and match which headers and methods you support. GET and HEAD are the only ones that MUST be supported after all. Daniel

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install trouble with SDL on win32+cygwin
by Conal Elliott 14 Apr '08

14 Apr '08

I'm trying to install the SDL package (on win32+cygwin), and configure isn't able to find my SDL.dll, though it's on my PATH. Any ideas? - Conal bash-3.2$ cabal install SDL 'SDL-0.5.3' is cached. [1 of 1] Compiling Main ( Setup.lhs, dist\setup/Main.o ) Linking dist\setup\setup.exe ... Configuring SDL-0.5.3... checking for sdl-config... no checking for sdl11-config... no configure: error: *** SDL not found! Get SDL from www.libsdl.org. If you already installed it, check it's in the path. If problem remains, please send a mail to the address that appears in ./configure --version indicating your platform, the version of configure script and the problem. cabal.exe: Error: some packages failed to install: SDL-0.5.3 failed during the configure step. The exception was: exit: ExitFailure 1 bash-3.2$ which SDL.dll /cygdrive/c/WINDOWS/system32/SDL.dll bash-3.2$

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more on my ghc package issues
by Galchin, Vasili 13 Apr '08

13 Apr '08

Hello, I "unregistered" unix-2.3.0.0. Now I cannot rebuild the Unix package. Please see the below log: vigalchin@ubuntu:~$ ghc-pkg latest unix unix-2.2.0.0 vigalchin@ubuntu:~$ ghc-pkg describe unix name: unix version: 2.2.0.0 license: BSD3 copyright: maintainer: libraries(a)haskell.org stability: homepage: package-url: description: This package gives you access to the set of operating system services standardised by POSIX 1003.1b (or the IEEE Portable Operating System Interface for Computing Environments - IEEE Std. 1003.1). . The package is not supported under Windows (except under Cygwin). category: author: exposed: True exposed-modules: System.Posix System.Posix.DynamicLinker.Module System.Posix.DynamicLinker.Prim System.Posix.Directory System.Posix.DynamicLinker System.Posix.Env System.Posix.Error System.Posix.Files System.Posix.IO System.Posix.Process System.Posix.Process.Internals System.Posix.Resource System.Posix.Temp System.Posix.Terminal System.Posix.Time System.Posix.Unistd System.Posix.User System.Posix.Signals System.Posix.Signals.Exts System.Posix.Semaphore System.Posix.SharedMem System.Posix.MQueue hidden-modules: import-dirs: /usr/local/lib/unix-2.2.0.0/ghc-6.8.2 library-dirs: /usr/local/lib/unix-2.2.0.0/ghc-6.8.2 hs-libraries: HSunix-2.2.0.0 extra-libraries: util dl extra-ghci-libraries: include-dirs: /usr/local/lib/unix-2.2.0.0/ghc-6.8.2/include includes: HsUnix.h execvpe.h depends: base-3.0.1.0 directory-1.0.0.0 hugs-options: cc-options: ld-options: framework-dirs: frameworks: haddock-interfaces: /usr/local/share/doc/unix-2.2.0.0/html/unix.haddock haddock-html: /usr/local/share/doc/unix-2.2.0.0/html vigalchin@ubuntu:~$ vigalchin@ubuntu:~/FTP/Haskell/unix-2.2.0.0$ runhaskell Setup.hs configure unknown package: unix-2.3.0.0 In my .cabal file, I have specified the version as 2.2.0.0, 2.3.0.0 and 2.4.0.0 all with the same results: "unknown package: unix-2.3.0.0" Vasili

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help understanding pj-lester-book
by minh thu 13 Apr '08

13 Apr '08

Hi! I don't understand something in there : pj-lester-book : Implementing functional languages: a tutorial by Simon Peyton Jones and David Lester, available at http://research.microsoft.com/~simonpj/Papers/pj-lester-book/ eval/apply : Making a Fast Curry: Push/Enter vs. Eval/Apply for Higher-order Languages by Simon Marlow and Simon Peyton Jones, available at http://www.haskell.org/~simonmar/papers/ The introduction in eval/apply states the arity of a function and the number of arguments in a call can match or differs (and it should be dealt with). In pj-lester-book (bottom of page 46 and top of page 47), it is stated that if the program has been type-cheched, the underflow check is unnecessary. When continuing to the G-machine then to the TIM-machine chapters, I haven't found discussion of an arity-mismatch. What am I missing ? Thanks a lot, Thu

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library version problem
by Galchin, Vasili 12 Apr '08

12 Apr '08

Hello, I doing work using Linux. The wrong version (for me) of the unix package seems to be visible. I see possibilities to use ghc-pkg to suppress the unix package that I don't want(2.3.0.0) but that seems dangerious. Details are below . What should I do? Regards, vasili When I do: > ghci :m System.Posix I am getting the wrong version of the Unix package. I know this to be true because I did ghc-pkg latest unix and got unix-2.3.0.0 I want unix.2.2.0.0 because this version has changes that I made to the unix package. libHSunix-2.2.0.0.a is installed under /usr/local/lib/unix-2.2.0.0/ghc-6.8.2 I did a "nm -a libHSunix-2.2.0.0.a" and found symbols that I added.

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do construct in wxhaskell
by Jodi-Ann Prince 12 Apr '08

12 Apr '08

hi, im working ona project, and im having problem loading some code in wxhaskell: onOpen :: Frame a -> TextCtrl b -> MenuItem c -> StatusField -> IO () onOpen f sw mclose status = do mbfname <- fileOpenDialog f False True "Open image" fileTypes "" "" case (mbfname) of (Nothing) -> return () (Just (fname)) -> do fileContent <- readFile fname set sw [text := fileContent] set mclose [enabled := True] set status [text := fname] return () i keep getting the error : "the last statement in a 'do' construct must be an expression." ive tried rearranging it many times, but i still get the same error. any help would be greatly appreciated. _________________________________________________________________ Connect to the next generation of MSN Messenger http://imagine-msn.com/messenger/launch80/default.aspx?locale=en-us&source=…

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Longest increasing subsequence
by Matt Amos 12 Apr '08

12 Apr '08

I'm trying to break out of my imperative mind-set by learning Haskell in small snippets. After a few successes I've hit a bit of a roadblock with one of the classic dynamic programming problems, the longest increasing subsequence: http://en.wikipedia.org/wiki/Longest_increasing_subsequence The most efficient algorithm relies on destructive updates, so a simple translation doesn't seem possible. I've been trying to think of a way of removing the need for destructive updates while keeping the efficiency, but so far without much luck. Ideally, I'd like to avoid threading the whole thing with a state monad, since then it would just be an imperative algorithm in disguise. Any suggestions would be very gratefully appreciated. Cheers, Matt

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Control.Parallel.Strategies
by Sebastian Sylvan 11 Apr '08

11 Apr '08

Hi, I was toying with the Control.Parallel.Strategies library, but can't seem to get it to actually do anything in parallel! Here's the code: import System.Random import Control.Parallel.Strategies fib :: Int -> Int fib 0 = 1 fib 1 = 1 fib n = fib (n-1) + fib (n-2) main = print $ parMap rnf fib $ take 80 $ randomRs (30,35) (mkStdGen 123) I compile with "-threaded", and run with +RTS -N2 -RTS, but yet it stays at 50% on my dual core machine! I should point out that any manual threading with forkIO does indeed use 100% of the CPU, and this problem happens on my work computer too (Vista on my home computer, XP on the work one). Anything I'm missing here? Thanks, -- Sebastian Sylvan +44(0)7857-300802 UIN: 44640862

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Haskell IDE: Someone else interested in enhancing Shim ? / How to contact author Benedikt Schmidt ?
by Marc Weber 11 Apr '08

11 Apr '08

Hi, I've added some features to shim. You can get my patches here: http://mawercer.de/~publicrepos/shim/ The Vim interface now supports :GrepScope regex Which greps the whole scope of all used packages by regex. Very useful to find all functions taking/ returning some type etc :ModulesExporting identifier returns all modules exporting identifiers. Can be used to add imports more automatically. Completion: camelCaseMatching Use pSL<c-x><c-o> to get putStrLn If you want to join and add features contact me via irc or email. I'd like to talk to the author about my patches. I've send him two mails but I haven't received any response yet. Sincerly Marc Weber

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Re: [Haskell-cafe] library version problem
by Galchin, Vasili 11 Apr '08

11 Apr '08

Hi Philip, Before I got your email, I deregistered unix-2.3.0.0 which made my unix-2.2.0.0 namespace changes visible. However, deregistering seems to made other things worse, e.g. runhaskell Setup.hs configure gives an error message "unknown parameter package: unix-2.3.0.0". Sigh .. how do I get back to where i was in order to do a "hide"? Kind regards, Vasili On Thu, Apr 10, 2008 at 8:22 PM, Philip Weaver <philip.weaver(a)gmail.com> wrote: > 2008/4/10 Galchin, Vasili <vigalchin(a)gmail.com>: > > Hello, > > > > I doing work using Linux. The wrong version (for me) of the unix > > package seems to be visible. I see possibilities to use ghc-pkg to > suppress > > the unix package that I don't want(2.3.0.0) but that seems dangerious. > > Details are below . What should I do? > > If you don't want to use it, then it's safe to hide it: > > ghc-pkg hide unix-2.3.0.0 > > You can always unhide it later. > > You can also tell ghc to use a specific version of a package: > > ghc -package unix-2.2.0.0 > > Of course, you'll need to make sure unix-2.2.0.0 is registered with > ghc-pkg. > > > > > Regards, vasili > > > > > > > > When I do: > > > > > ghci > > :m System.Posix > > > > I am getting the wrong version of the Unix package. I know this to be > true > > because I did > > ghc-pkg latest unix > > > > and got unix-2.3.0.0 > > > > I want unix.2.2.0.0 because this version has changes that I made to the > unix > > package. libHSunix-2.2.0.0.a is installed under > > /usr/local/lib/unix-2.2.0.0/ghc-6.8.2 I did a "nm -a > libHSunix-2.2.0.0.a" > > and found symbols that I added. > > > > _______________________________________________ > > Haskell-Cafe mailing list > > Haskell-Cafe(a)haskell.org > > http://www.haskell.org/mailman/listinfo/haskell-cafe > > > > >

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type families and type signatures
by Ganesh Sittampalam 11 Apr '08

11 Apr '08

Hi, The following program doesn't compile in latest GHC HEAD, although it does if I remove the signature on foo'. Is this expected? Cheers, Ganesh {-# LANGUAGE TypeFamilies #-} module Test7 where type family Id a type instance Id Int = Int foo :: Id a -> Id a foo = id foo' :: Id a -> Id a foo' = foo

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RE: [Haskell-cafe] type families and type signatures
by Sittampalam, Ganesh 11 Apr '08

11 Apr '08

Manuel Chakravarty wrote: >Ganesh Sittampalam: >> On Mon, 7 Apr 2008, Manuel M T Chakravarty wrote: >> >>> Ganesh Sittampalam: >>>> The following program doesn't compile in latest GHC HEAD, although >>>> it does if I remove the signature on foo'. Is this expected? >>> >>> Yes, unfortunately, this is expected, although it is very >>> unintuitive. This is for the following reason. >>> >>> Let's alpha-rename the signatures and use explicit foralls for >>> clarity: >>> >>> foo :: forall a. Id a -> Id a >>> foo' :: forall b. Id b -> Id b >>> >>> GHC will try to match (Id a) against (Id b). As Id is a type synonym >>> family, it would *not* be valid to derive (a ~ b) from this. After >>> all, Id could have the same result for different argument types. >>> (That's not the case for your one instance, but maybe in another >>> module, there are additional instances for Id, where that is the >>> case.) >> >> Can't it derive (Id a ~ Id b), though? > That's what it does derive as a proof obligation and finds it can't prove. > The error message you are seeing is GHC's way of saying, I cannot assert that > (Id a ~ Id b) holds. No, I meant can't it derive that equality when matching (Id a) against (Id b)? As you say, it can't derive (a ~ b) at that point, but (Id a ~ Id b) is known, surely? > > Generally speaking, is there any way to give a signature to foo'? > Sorry, but in the heat of explaining what GHC does, I missed the > probably crucial point. Your function foo is useless, as is foo'. > Not only can't you rename foo (to foo'), but you generally can't use > it. It's signature is ambiguous. Try evaluating (foo (1::Int)). The > problem is related to the infamous (show . read). My real code is somewhat analogous to (foo :: (Id Int -> Id Int)) (1::Int). Isn't that unambiguous in the same way as (show.read) is if I give show or read a signature? Cheers, Ganesh ============================================================================== Please access the attached hyperlink for an important electronic communications disclaimer: http://www.credit-suisse.com/legal/en/disclaimer_email_ib.html ==============================================================================

6 21

Re: HXT 7.4/7.5 leaking TCP connections
by Paul Brown 11 Apr '08

11 Apr '08

Hi, Bjorn and Uwe -- > > this is a known problem with HTTP package (version 3001.0.4). > > Paul Brown has described this somewere in his blog. > > (http://mult.ifario.us/t/haskell) but my firefox only shows > > an incomplete page of this blog, the solution is missing. > > Paul promissed in his blog to send a patch to Björn Bringert. Here's the link to the article about SimpleHTTP; for some reason, it wasn't showing up. (Guess I have a bug to fix there...) http://mult.ifario.us/p/a-short-adventure-with-simplehttp > > P.S. to Björn: It would be nice, if you could include this change into the > > HTTP module and upload a new version to hackage. > Thank you Daniel and Uwe for reporting this (again). Strangely, I > thought that I had already applied Paul's patch, but it seems I > haven't. Paul: I seem to recall us talking about this, but can't > remember the conclusion, nor find any e-mail about it. Do you remember > anything about this? I can't remember either; I'm sure that the darcs patch is lurking on one of my boxes. I'll track it down and send it your way so we can squash this one. Best. -- Paul -- paulrbrown(a)gmail.com http://mult.ifario.us/

2 2

Fighting the monad stack, MonadIO
by Adam Smyczek 11 Apr '08

11 Apr '08

For a small webapi binding I try to implement a session like monad by building a stack including BrowserAction from Network.Browser module as following: newtype RBAction a = RBAction { exec :: ErrorT String (StateT RBState BrowserAction) a } deriving (Functor, Monad, MonadState RBState) I would like the RBAction to implement MonadIO as well, but fight with the liftIO function for hours now, without success. Any idea how the implementation of liftIO could look like? Thanks for help, Adam

7 10

ghc
by Cetin Sert 11 Apr '08

11 Apr '08

Hi, Is GHC required to be installed on the target OS I compile Haskell binaries for in order for these binaries to run? I need a quick answer on that! By the way there is no computer in the 4 or so networks I have online access to on which ghc is not installed, which might be a sign people like haskell here in Heidelberg/Mannheim, Germany. Best Regards, Cetin Sert

11 11

Pattern match failure
by Jackm139 11 Apr '08

11 Apr '08

I'm trying to write a function to recognize a context free grammar, but I keep getting pattern match failure errors. This is what I have: data Grammar c = Brule c c c | Rule c c gez = [(Brule 'S' 'p' 'D'),(Brule 'D' 't' 'E'),(Rule 'E' 'j')] recog :: String -> String -> [Grammar Char] -> Bool recog a b list = case list of [Brule x y z] -> if a == [x] then recog [z] b list else recog a b list [Rule x y] -> True how can I solve this pattern matching error? -- View this message in context: http://www.nabble.com/Pattern-match-failure-tp16600643p16600643.html Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

4 3

instance Monad m => Functor m
by Hans Aberg 11 Apr '08

11 Apr '08

Using 'hugs -98', I noticed it accepts: instance Monad m => Functor m where fmap f x = x >>= return.f Has this been considered (say) as a part of the upcoming Haskell Prime? Hans Aberg

4 13

Haskell library install question
by Galchin, Vasili 11 Apr '08

11 Apr '08

Hello, I have configured, built and installed a library. When I "runhaskell Setup.hs install", I noticed the message Registering unix-2.2.0.0... In what sense is it being registered? Can I query this registry information? Thanks, Vasili

2 2

PhD or PostDoc opening in Dresden, Germany
by Janis Voigtlaender 10 Apr '08

10 Apr '08

There is a research position on offer in the project described at: http://wwwtcs.inf.tu-dresden.de/~voigt/project/ The official job advertisement (in German) can be found at: http://www.verw.tu-dresden.de/StellAus/einzelstelle.asp?id=783 The most important details are: - full-time faculty position - payment according to German public sector scale E 13 TV-L Ost (gross monthly salary expected to start around 2680 Euro) - no teaching duties - initial appointment for up to 30 months - no knowledge of German required (but English is) Please contact me directly with any further questions. Please also forward to potentially interested students. The closing date for applications is 15th May 2008. Best wishes, Janis Voigtlaender -- Dr. Janis Voigtlaender http://wwwtcs.inf.tu-dresden.de/~voigt/ mailto:voigt@tcs.inf.tu-dresden.de

1 0

Lock-Free Data Structures using STMs in Haskell
by Pete Kazmier 10 Apr '08

10 Apr '08

I recently read the STM paper on lock-free data structures [1] which I found very informative in my quest to learn how to use STM. However, there are a few things I do not fully understand and was hoping someone might be able to explain further. In the STM version of the ArrayBlockingQueue, the following type is defined: data ArrayBlockingQueueSTM e = ArrayBlockingQueueSTM { shead :: TVar Int, stail :: TVar Int, sused :: TVar Int, slen :: Int, sa :: Array Int (TVar e) } It's unclear to me why the Array's elements must be wrapped in TVars. Why aren't the TVars on shead, stail, and sused sufficient? Here is the only function that reads from the queue: readHeadElementSTM :: ArrayBlockingQueueSTM e -> Bool -> Bool -> STM (Maybe e) readHeadElementSTM abq remove block = do u <- readTVar (sused abq) if u == 0 then if block then retry else return Nothing else do h <- readTVar (ihead abq) let tv = sa abq ! h -- Why are the array elements wrapped in TVars? e <- readTVar tv if remove then do let len = slen abq let newh = h `mod` len writeTVar (shead abq) $! newh writeTVar (sused abq) $! (u-1) else return () return (Just e) It is not immediately obvious to me why the elements need to be wrapped in TVars. Could someone help elaborate? The other question is in regards to section 2 where STM is introduced. The authors define the following: decT :: TVar Int -> IO () decT v = atomically (do x <- readTVar v if x == 0 then retry else return () writeTVar v (x-1)) And then go on to show how easy it is to compose STM types with this function: decPair v1 v1 :: TVar Int -> TVar Int -> IO () decPair v1 v2 = atomically (decT v1 `orElse` decT v2) Will this actually compile? I was under the impression that 'orElse' could only combine STM types, not IO () types. Thank you, Pete [1] Anthony Discolo, Tim Harris, Simon Marlow, Simon Peyton Jones, and Satnam Singh. Lock-free data structures using STMs in Haskell. In Eighth International Symposium on Functional and Logic Programming (FLOPS.06), April 2006.

3 3

problems building hpodder
by Karl Hasselström 10 Apr '08

10 Apr '08

I'm trying to build hpodder 1.1.2 with ghc 6.8.2. I successfully downloaded, built, and installed (the latest versions of) all its dependencies, but when building hpodder itself, I get FeedParser.hs:146:26: `Content' is not applied to enough type arguments Expected kind `??', but `Content' has kind `* -> *' In the type `Content -> Either String String' In the type `String -> Content -> Either String String' In the type signature for `strof_either': strof_either :: String -> Content -> Either String String I don't know enough Haskell to even determine what kind of problem this is -- wrong version of some dependency? Wrong version of ghc? Help appreciated. :-) ( FWIW, the hackage auto-builder thingie seems to have the exact same problem I have: http://hackage.haskell.org/packages/archive/hpodder/1.1.2/logs/failure/ghc-… ) -- Karl Hasselström, kha(a)treskal.com www.treskal.com/kalle

4 6

What's the difference?
by PR Stanley 10 Apr '08

10 Apr '08

Hi What is the difference between data T0 f a = MkT0 a instance Eq (T0 f a) where ... and data T0 f a = MkT0 a instance Eq a => Eq (T0 f a) where ... I've only seen the "=>" operator used for declaring extended classes but never with class instances. By the way, what is the correct terms for the "=>" and the "->"? Cheers Paul

6 7

RE: [Haskell-cafe] type families and type signatures
by Sittampalam, Ganesh 09 Apr '08

09 Apr '08

OK, thanks. I think I'm finally understanding :-) Cheers, Ganesh -----Original Message----- From: haskell-cafe-bounces(a)haskell.org [mailto:haskell-cafe-bounces@haskell.org] On Behalf Of Martin Sulzmann Sent: 09 April 2008 07:21 To: Ganesh Sittampalam Cc: Manuel M T Chakravarty; haskell-cafe(a)haskell.org Subject: Re: [Haskell-cafe] type families and type signatures Manuel said earlier that the source of the problem here is foo's ambiguous type signature (I'm switching back to the original, simplified example). Type checking with ambiguous type signatures is hard because the type checker has to guess types and this guessing step may lead to too many (ambiguous) choices. But this doesn't mean that this worst case scenario always happens. Consider your example again type family Id a type instance Id Int = Int foo :: Id a -> Id a foo = id foo' :: Id a -> Id a foo' = foo The type checking problem for foo' boils down to verifying the formula forall a. exists b. Id a ~ Id b Of course for any a we can pick b=a to make the type equation statement hold. Fairly easy here but the point is that the GHC type checker doesn't do any guessing at all. The only option you have (at the moment, there's still lots of room for improving GHC's type checking process) is to provide some hints, for example mimicking System F style type application by introducing a type proxy argument in combination with lexically scoped type variables. foo :: a -> Id a -> Id a foo _ = id foo' :: Id a -> Id a foo' = foo (undefined :: a) Martin Ganesh Sittampalam wrote: > On Wed, 9 Apr 2008, Manuel M T Chakravarty wrote: > >> Sittampalam, Ganesh: > >>> No, I meant can't it derive that equality when matching (Id a) >>> against (Id b)? As you say, it can't derive (a ~ b) at that point, >>> but (Id a ~ Id b) is known, surely? >> >> No, it is not know. Why do you think it is? > > Well, if the types of foo and foo' were forall a . a -> a and forall b > . b -> b, I would expect the type-checker to unify a and b in the > argument position and then discover that this equality made the result > position unify too. So why can't the same happen but with Id a and Id > b instead? > >> The problem is really with foo and its signature, not with any use of >> foo. The function foo is (due to its type) unusable. Can't you >> change foo? > > Here's a cut-down version of my real code. The type family Apply is > very important because it allows me to write class instances for > things that might be its first parameter, like Id and Comp SqlExpr > Maybe, without paying the syntactic overhead of forcing client code to > use Id/unId and Comp/unComp. It also squishes nested Maybes which is > important to my application (since SQL doesn't have them). > > castNum is the simplest example of a general problem - the whole point > is to allow clients to write code that is overloaded over the first > parameter to Apply using primitives like castNum. I'm not really sure > how I could get away from the ambiguity problem, given that desire. > > Cheers, > > Ganesh > > {-# LANGUAGE TypeFamilies, GADTs, UndecidableInstances, > NoMonomorphismRestriction #-} > > newtype Id a = Id { unId :: a } > newtype Comp f g x = Comp { unComp :: f (g x) } > > type family Apply (f :: * -> *) a > > type instance Apply Id a = a > type instance Apply (Comp f g) a = Apply f (Apply g a) type instance > Apply SqlExpr a = SqlExpr a type instance Apply Maybe Int = Maybe Int > type instance Apply Maybe Double = Maybe Double type instance Apply > Maybe (Maybe a) = Apply Maybe a > > class DoubleToInt s where > castNum :: Apply s Double -> Apply s Int > > instance DoubleToInt Id where > castNum = round > > instance DoubleToInt SqlExpr where > castNum = SECastNum > > data SqlExpr a where > SECastNum :: SqlExpr Double -> SqlExpr Int > > castNum' :: (DoubleToInt s) => Apply s Double -> Apply s Int castNum' > = castNum > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe(a)haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ============================================================================== Please access the attached hyperlink for an important electronic communications disclaimer: http://www.credit-suisse.com/legal/en/disclaimer_email_ib.html ==============================================================================

1 0

translating from fundeps into type families
by Ganesh Sittampalam 09 Apr '08

09 Apr '08

Hi, Can I have some advice on translating the attached Test1.hs into type families? My attempt at doing so is in Test1a.hs, but firstly it requires FlexibleInstances where Test1.hs didn't, and secondly it fails because it can't infer the instance for Bar (Either Int Int) whereas the fundeps version had no problems there. (And yes, this is another cut down example of something I really want to do :-) Cheers, Ganesh

3 3

deriving
by PR Stanley 09 Apr '08

09 Apr '08

Hi data Bool = False | True deriving (Eq, Or, Show, Read) Bool is an instance of Eq, Ord, Show and Read. It is derived form these classes. If that is the meaning of the keyword deriving then wouldn't a different keyword such as from or derivative or even derivation be closer to the semantics of the concept? Discuss :-) Cheers, Paul

11 23

ANN: graphics-drawingcombinators 0.1
by Luke Palmer 09 Apr '08

09 Apr '08

graphics-drawingcombinators 0.1 has just been uploaded to hackage: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/graphics-drawing… Have you ever tried to write a simple 2D interface or game but found: * wxHaskell and gtk2hs are hard to install and learn * SDL does not provide enough by itself * OpenGL is gross and imperative? Then you're in luck, with the release of this very immature new module, Graphics.DrawingCombinators! This module is a wrapper around OpenGL with a functional interface, handling a small subset but still common set of 2D OpenGL functions. It currently handles some simple geometric primitives and transformations, loading images from files and SDL.Surfaces, and drawing text using SDL.TTF. I plan to add more support for lots of stuff in the future, the next priorities being: * A clean "picking" interface. * Ability to draw non-regular polygons (yeah, that's kinda important isn't it...) If there are any features which you would find useful (as long as it's in the 2D realm, I'm still contemplating how to do 3D stuff), I'd be glad to hear it and implement it. Look at example.hs in the distro for an example of how to use this with SDL. Enjoy! Luke

1 0

ANN: Hayoo! beta 0.1
by Timo B. Hübel 09 Apr '08

09 Apr '08

Hello, we are pleased to announce the first beta release of Hayoo!, a Haskell API search engine providing advanced features like suggestions, find-as-you-type, fuzzy queries and much more. Visit Hayoo! here: http://holumbus.fh-wedel.de/hayoo Please bear in mind that this is still a beta release and we are continuously working on further improvements. Our plans for the future include: - Covering all documentation available at Hackage. - Compatibility with non-JavaScript enabled browsers. - Providing a web interface where people can point Hayoo! to an URI linking to Haddock documentation which will be automatically included in Hayoo!. Hayoo! was developed as a use-case for the Holumbus framework, which aims to help at the creation of very flexible and highly specialized search engines. Although Holumbus is still under heavy development and we have no official release yet, some informations and a Darcs repository are available at the Holumbus homepage: http://holumbus.fh-wedel.de Any suggestions and feedback is highly welcomed. Cheers, Timo B. Hübel & Sebastian M. Schlatt

8 14

Re: [Haskell-cafe] problems building hpodder
by gwern0＠gmail.com 09 Apr '08

09 Apr '08

On 2008.04.08 15:19:12 -0500, John Goerzen <jgoerzen(a)complete.org> scribbled 1.1K characters: > On Mon April 7 2008 9:31:04 pm gwern0(a)gmail.com wrote: > > > > Well, changing the deps at least would be a good idea. > > Right. I've uploaded a new version of hpodder to hackage that will require > the correct HaXml versions. Great. > > And actually, there is a method - Cabal supports the 'stability:' field, > > but I don't believe it's enforced by cabal-install or anything. > > I think part of the problem is visibility in Hackage. Unless I'm wrong, > Hackage always takes you to the release with the highest version number. > There is no "latest stable version" list and "latest development version" > list, which would be helpful. Something akin to, say, > http://packages.debian.org/hpodder Yes, that's true. There is no good way of doing in Hackage that doesn't involve self-discipline or messing with release schedules. I do have a thought about the 'stability:' field, though. Feel free to weigh in at <http://hackage.haskell.org/trac/hackage/ticket/265>. > > (Personally, I'd update it, not just because of issues like this, but > > because there looked to be a number of changes necessary when I commented > > all the type sigs, and the longer you wait... &etc.) > > The thing is -- I don't want a stable hpodder to jump to a development > version of HaXml. The released version is working just fine, and when there > is a new stable release, I'll update then. XML parsing is quite central to > hpodder. > > -- John Sure, but there are costs to not transitioning just as there are costs to so doing. I was just saying which cost I found preferable. -- gwern JANET Salsa GSA TRANSEC LASINT A/B DJC Al BIOLWPN Xandros

1 0

testing for same characters in lists of strings
by Jackm139 09 Apr '08

09 Apr '08

I'm new to Haskell, and I'm finding it is very different from any other language I have worked with. I have an assignment to make a program to test whether two lists use the same characters for each string. e.g. sameCharacter ["rock", "cab"] ["cork", "abc"] True My plan to tackle this was to use: nub to eliminate duplications, sort to put the characters in order, and map to put characters together.. and then somehow check to see if these characters are the same. My problem right now is just figuring out how to make a function that uses these three functions to give me a list of tuples. I know this is an awfully newbish question, and I'm not trying to get an answer to my homework problem. I just need some sort of working example syntax to off of. Any help would be appreciated! Thanks. -- View this message in context: http://www.nabble.com/testing-for-same-characters-in-lists-of-strings-tp165… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

6 7

Function Precedence
by PR Stanley 09 Apr '08

09 Apr '08

Hi If f x = x and g y = y then f g x returns an error because f takes only one argument. Why can't we have function application implemented outwardly (inside-out). So f g x would be applied with gx first followed by its return value passed to f instead of putting g x in brackets. Cheers, Paul

12 28

announcing the darcs 2.0.0 release
by David Roundy 09 Apr '08

09 Apr '08

Hello darcs users, I am pleased to announce the release of darcs 2.0.0! It's been a long time coming, and hopefully you will be pleased with the result. Notable new features include (in no particular order): * New ssh-connnection mode that dramatically improves connection times when darcs 2.0.0 is present on the server as well as the client. * New "hashed" format that improves darcs' robustness with regard to network or filesystem errors, and also allows for efficient (and safe) caching of patch data, greatly improving access times to a repository that you have had previous contact with--which is the common case of pulling, pushing or sending. This format allows patches to move back and forth with darcs1-format repositories, and is the recommended format (with an old-format mirror) if you do not wish to require darcs 2.0.0, but want to benefit from most of its improvements. * New "darcs-2" format, which features improved conflict handling, and also has all the benefits of the hashed format. Because this format features new and different semantics, it is not possible to interact with darcs 1.x, and care must be taken in switching to this format. It is the recommended format for new projects (although we haven't made it the default). * The --partial get is now replaced with a --lazy get when using hashed formats, and in fact if you forget to provide one of these flags on the command line, you can acheive a lazy get simply by hitting control-C after darcs prints a message suggesting you do this. Lazy repositories are identical to full repositories so long as you have a working network connection and the original repository is accessible, with patches downloaded as they are needed. When the original repository is no longer accessible, a lazy repository will behave like the old --partial repositories. * We have added support for pipelined http downloading via either libwww or a very recent version of libcurl. Neither of these is yet the default, but it is recommended that you try one of them if possible, as they greatly improve download times. * There is now a configurable global cache, which can help reduce network traffic if you have multiple local copies of the same--or similar--remote repositories. * I'm sure there are other new features, but this is all that comes to mind at the moment. BUGS There are numerous issues with the --darcs-2 repository semantics (see a discussion at http://wiki.darcs.net/DarcsWiki/DarcsTwo) In general, I believe the status is that anything you could do with darcs 1.x you can do with the darcs-2 semantics, but the high-level code hasn't been updated to take into account the new possibilities opened up by the new semantics. In particular, patch dependencies are much more complex. Alas, there has not been enough time (or sufficient contributors) to make the code take these complexities into account, and if you're clever you can reveal bugs. You have been warned. Darcs 2.0.0 contains some performance regressions when running with large repositories. It is possible that these regressions will be sufficient to prevent its effective use in some projects. If this describes your project--and the only way to know is to try it--then I recommend considering either switching to a different revision control system, or helping us to improve darcs. The former is certainly the easier path. If I knew how to easily make darcs dramatically faster, I would have done so. FUTURE DIRECTIONS FOR DARCS The existing model for darcs maintenance has been the use of two branches, a darcs-stable branch for well-tested changes, and a darcs-unstable branch for more experimental work. Each branch has a separate maintainer, so most patches will be reviewed by two maintainers before making it into a darcs release. This is quite a nice system. Unfortunately, it is also a rather labor-intensive process, and due to a lack of contributors, we've moving to a more streamlined process. Starting with the darcs 2.0.0 release, there will be just one central branch of darcs and only one maintainer: for now this is me, David Roundy. Moreover, I will be attempting to act as a much lower-profile maintainer than we've had previously. I will not be reading bug reports, reading the darcs-users email list or even the darcs-devel mailing list. I will only be reviewing patches that are contributed. I will not write up long user-friendly release announcements like this for future darcs releases. I will only be reviewing and applying contributed patches. What does this mean? It means that if darcs is to continue to improve, we need new contributors. If bug reports are to be read, responded to and bugs are to be fixed, we need new contributors. If darcs is to be made more efficient, we need new contributors. The new URL for the darcs repository is simply http://darcs.net. (Yes, this is a lame ending to the email, but I'm sending it anyhow, because I'm tired, and I've got to get this done.)

6 6

Problem building HXT
by ln 09 Apr '08

09 Apr '08

Hi, I would like to try HXT, but I can't manage to build it. I resolved all the dependencies, but I get the following error: > > ln@igemyvuci:~/Desktop/hxt$ make all > > make -C src all VERSION=7.5 > > make[1]: Entering directory `/home/ln/Desktop/hxt/src' > > make install_local_hxt > > make[2]: Entering directory `/home/ln/Desktop/hxt/src' > > ghc -Wall -O2 -fglasgow-exts -ignore-package hxt -package-name hxt-7.5 > > -package parsec -package HTTP -package tagsoup --make HXT.hs > > [ 1 of 143] Compiling Text.XML.HXT.Validator.RE ( > > Text/XML/HXT/Validator/RE.hs, Text/XML/HXT/Validator/RE.o ) > > (.....) > > [100 of 143] Compiling Text.XML.HXT.Parser.TagSoup ( > > Text/XML/HXT/Parser/TagSoup.hs, Text/XML/HXT/Parser/TagSoup.o ) > > Text/XML/HXT/Parser/TagSoup.hs:305:21: Not in scope: `options' > > make[2]: *** [libHShxt.a] Error 1 The alternative installation method with cabal leads to the same result. Using Ubuntu 8.04 Beta and GHC6.8. Maybe there are incompabilities with newer versions of the dependencies? Would appreciate any help. Regards Lukas

4 4

Parsec expressions with alphaNum operators
by Paul Keir 08 Apr '08

08 Apr '08

Hi, I'm using buildExpressionParser, and I'd like to use alphanumeric operator characters. I get an (unexpected "a") error though. With a test string like "-a" if "a" is used in any of the "reservedOpNames". I'm aiming for the Fortran operators like ".and.". The listing below may be helpful. It's taken from the Haskell wiki's "Parsing expressions and statements" article (minus the statement part).I've added an ":a:" operator. The article uses "~" as a unary operator (I'm heading for +/-). It can be tested with: $ parseTest exprparser "~a" -- code begins module Main where import Control.Monad(liftM) import Text.ParserCombinators.Parsec import Text.ParserCombinators.Parsec.Expr import Text.ParserCombinators.Parsec.Token import Text.ParserCombinators.Parsec.Language data Expr = Var String | Con Bool | Uno Unop Expr | Duo Duop Expr Expr deriving Show data Unop = Not deriving Show data Duop = And | Iff deriving Show data Stmt = Nop | String := Expr | If Expr Stmt Stmt | While Expr Stmt | Seq [Stmt] deriving Show def = emptyDef{ commentStart = "{-" , commentEnd = "-}" , identStart = letter , identLetter = alphaNum , opStart = oneOf "~&=:" , opLetter = oneOf "~&=:a" , reservedOpNames = ["~", "&", "=", ":=", ":a:"] , reservedNames = ["true", "false", "nop", "if", "then", "else", "fi", "while", "do", "od"] } TokenParser{ parens = m_parens , identifier = m_identifier , reservedOp = m_reservedOp , reserved = m_reserved , semiSep1 = m_semiSep1 , whiteSpace = m_whiteSpace } = makeTokenParser def exprparser :: Parser Expr exprparser = buildExpressionParser table term <?> "expression" table = [ [Prefix (m_reservedOp "~" >> return (Uno Not))] , [Infix (m_reservedOp "&" >> return (Duo And)) AssocLeft] , [Infix (m_reservedOp "=" >> return (Duo Iff)) AssocLeft] , [Infix (m_reservedOp ":a:" >> return (Duo Iff)) AssocLeft] ] term = m_parens exprparser <|> liftM Var m_identifier <|> (m_reserved "true" >> return (Con True)) <|> (m_reserved "false" >> return (Con False)) play :: String -> IO () play inp = case parse exprparser "" inp of { Left err -> print err ; Right ans -> print ans } -- code ends Cheers, Paul Keir Research Student University of Glasgow Department of Computing Science pkeir(a)dcs.gla.ac.uk

2 3

Type Families: infinite compile process?
by Hugo Pacheco 08 Apr '08

08 Apr '08

Hi guys, I have been experimenting some weird stuff (risky, yes I know) but the behaviour was certainly not the one I expected: {-# OPTIONS -fglasgow-exts -fallow-undecidable-instances #-} module Nest where data Nest a = Nil | Cons a (Nest (a,a)) type family F a x :: * type instance F (Nest a) x = Either () (a,F (Nest (a,a)) x) fnn :: F (Nest Int) (Nest Int) fnn = Left () The following module fails to compile (or better, compilation never ends). Maybe there is something very bad going on due to the undecidable-instances extension? Any clue? hugo

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Re: HXT 7.4/7.5 leaking TCP connections
by Uwe Schmidt 08 Apr '08

08 Apr '08

Daniel McAllansmith wrote: > I'm having some problems with connections leaking when using 7.4. I've > just checked and it happens with 7.5 off of hackage as well. > > It appears that readDocument doesn't close the connection when using the > haskell http library, it works ok when curl is used though. > > Following is a program which demonstrates the problem. /tmp/list.txt just > has the name of files available from my local webserver, more than 1024 > files is enough to exhaust my open file limit. > > I think the garbage collector does collect the connections, but they don't > end up fully closed... that's another issue I'm planning on looking at. > > If you need any more information please ask. I may get a chance to try and > write a patch but using curl is a work around while I'm busy. > .... this is a known problem with HTTP package (version 3001.0.4). Paul Brown has described this somewere in his blog. (http://mult.ifario.us/t/haskell) but my firefox only shows an incomplete page of this blog, the solution is missing. Paul promissed in his blog to send a patch to Björn Bringert. Here is the solution: edit the close function in HTTP/TCP.hs as follows: ---------------------------------------------------------------- close ref = do { c <- readIORef (getRef ref) ; closeConn c ; writeIORef (getRef ref) ConnClosed } where -- Be kind to peer & close gracefully. closeConn (ConnClosed) = return () closeConn (MkConn sk addr [] _) = mapM_ (flip Exception.catch $ \_ -> return ()) [ shutdown sk ShutdownSend , suck ref , shutdown sk ShutdownReceive , sClose sk ] suck :: Connection -> IO () suck cn = readLine cn >>= either (\_ -> return ()) -- catch errors & ignore (\x -> if null x then return () else suck cn) --------------------------------------------------------------------- I hope this will help. Cheers Uwe P.S. to Björn: It would be nice, if you could include this change into the HTTP module and upload a new version to hackage.

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ANN: The Monad.Reader Issue 10
by Wouter Swierstra 08 Apr '08

08 Apr '08

I am pleased to announce that a new issue of The Monad.Reader is now available: http://www.haskell.org/haskellwiki/The_Monad.Reader Issue 10 consists of the following two articles: * Bernie Pope Step inside the GHCi debugger * Matthew Naylor Evaluating Haskell in Haskell The Monad.Reader is a quarterly magazine about functional programming. It is less formal than a journal, but somehow more enduring than a wiki page or blog post. 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 just yet. I hope to send out an official call for copy fairly soon. Wouter This message has been checked for viruses but the contents of an attachment may still contain software viruses, which could damage your computer system: you are advised to perform your own checks. Email communications with the University of Nottingham may be monitored as permitted by UK legislation.

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Quick check?
by Galchin, Vasili 08 Apr '08

08 Apr '08

Hello, What are some examples of libraries in HackageDB that use either Quick Check (pure side) or Monad testing? Is there a way to use "darcs" to answer my question? Thanks, vasili

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Re: [Haskell-cafe] deriving
by PR Stanley 08 Apr '08

08 Apr '08

Hi, Thank you very much for your helpful replies. So, we define (or is it declare) a class with a polymorphic value(s which will be used in defining function types or functions associated with that class. The data types are then used for deriving instances of that class specific to each data type. The function defined in the general class can be redefined if necessary. It sort of reminds me of specialisation in OO. Get thee behind me beast. :-) Any comments? Cheers, Paul At 20:05 07/04/2008, you wrote: >Paul, > > > No, sorry. I'm not sure how this differs from my definition. > > Could you elaborate please? > >Gladly. I haven't tested any of the code below, so pardon any typos >and thinkos. > >It sounded like you were trying to say that Bool derives from Eq, Ord, >Show, and Read, and thus the keyword "from" would apply. Of course, >I may have misunderstood your intent. > >I was countering by saying that the "deriving" keyword indicates something >different. I'll try to describe what I meant in a little more detail. > >A type class definition defines properties of the type class. Eg., > >class Example a where > foo :: a > bar :: a -> a > >The class is then related to specific types by giving them instance >declarations. The instance declarations can be explicit, as in > >data Xyzzy = Xy | Zzy >instance Example Xyzzy where > foo = Xy > bar = const Zzy > >Sometimes the instance declaration can be automatically derived instead, >which looks like: > >data Xyzzy = Xy | Zzy > deriving (Example) > >This lets the compiler derive the "obvious" class instance declaration. >But it won't work for just any class, because the compiler needs to know >how to derive "obvious" instances. A few classes have deriving >machinery "built-in": Eq, Ord, Enum, Bounded, Show, Read. > >"deriving (Example)" won't work, since that machinery doesn't exist. > >With effort, you could implement this "automatic derivation" machinery >for your own types, at least in GHC. I've never needed it, so I've >never learned exactly how to do it. > >In short, so "data Foo [...] deriving (Eq)" means: "define a new type Foo, >and automatically derive its class instance for Eq". > >Does this help? > >Regards, >John

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(forw) Re: [Haskell-cafe] deriving
by John Dorsey 08 Apr '08

08 Apr '08

I meant to send this reply to the cafe. ----- Forwarded message ----- Date: Mon, 7 Apr 2008 15:05:00 -0400 To: PR Stanley <prstanley(a)ntlworld.com> Subject: Re: [Haskell-cafe] deriving Paul, > No, sorry. I'm not sure how this differs from my definition. > Could you elaborate please? Gladly. I haven't tested any of the code below, so pardon any typos and thinkos. It sounded like you were trying to say that Bool derives from Eq, Ord, Show, and Read, and thus the keyword "from" would apply. Of course, I may have misunderstood your intent. I was countering by saying that the "deriving" keyword indicates something different. I'll try to describe what I meant in a little more detail. A type class definition defines properties of the type class. Eg., class Example a where foo :: a bar :: a -> a The class is then related to specific types by giving them instance declarations. The instance declarations can be explicit, as in data Xyzzy = Xy | Zzy instance Example Xyzzy where foo = Xy bar = const Zzy Sometimes the instance declaration can be automatically derived instead, which looks like: -- non-working code, see below data Xyzzy = Xy | Zzy deriving (Example) This lets the compiler derive the "obvious" class instance declaration. But it won't work for just any class, because the compiler needs to know how to derive "obvious" instances. A few classes have deriving machinery "built-in": Eq, Ord, Enum, Bounded, Show, Read. "deriving (Example)" won't work, since that machinery doesn't exist. With effort, you could implement this "automatic derivation" machinery for your own types, at least in GHC. I've never needed it, so I've never learned exactly how to do it. In short, so "data Foo [...] deriving (Eq)" means: "define a new type Foo, and automatically derive its class instance for Eq". Does this help? Regards, John ----- End forwarded message -----

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Re: [Haskell-cafe] deriving
by PR Stanley 08 Apr '08

08 Apr '08

Hi >data Bool = False | True > deriving (Eq, Or, Show, Read) > >Bool is an instance of Eq, Ord, Show and Read. It is derived form >these classes. If that is the meaning of the keyword deriving then >wouldn't a different keyword such as from or derivative or even >derivation be closer to the semantics of the concept? >Discuss :-) > >Cheers, >Paul >I'm not getting it. Are you thinking that Bool itself "is derived >from these classes", rather than those four instances for Bool? - Conal >Paul: >I am aware that Bool is an instance of Eq, Ord, ... What I'm not >sure about is the terminology used here. Deriving, deriving what >from Eq, Ord, ...? How about data Bool = False | True from (Eq, Ord, Show, Read) although I have a feeling that "from" is already used somewhere in Haskell. Paul

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ANNOUNCE: Well-Typed LLP - The Haskell Consultants
by Ian Lynagh 07 Apr '08

07 Apr '08

Fellow Haskellers, We (Björn Bringert, Duncan Coutts and Ian Lynagh) are pleased to announce that we have recently set up a Haskell consultancy company, Well-Typed LLP (http://www.well-typed.com/) Our services include application development, library and tool maintenance, project advice, and training. Please see our website or drop us an e-mail for more details. We look forward to hearing from you in the future! -- Björn Bringert, Duncan Coutts, Ian Lynagh http://www.well-typed.com/ info(a)well-typed.com

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[GSoC] WWW::Mechanize-like package for Haskell
by Max Desyatov 07 Apr '08

07 Apr '08

Hi, I'm interested in working on a library for a stateful web browsing in Haskell during Google Summer of Code. The basic idea is described at http://hackage.haskell.org/trac/summer-of-code/ticket/1107. WWW::Mechanize is a ready to use library written in Perl, though I used python's mechanize when I wrote some simple scripts (http://wwwsearch.sourceforge.net/mechanize/) which provides much cleaner interface. Anyway, it gives simple and convenient way to retreive web-sites, to handle cookies, history and to process retrieved content and forms. There are basics of it Network.Browser module from Haskell's HTTP library (http://hackage.haskell.org/packages/archive/HTTP/3001.0.4/doc/html/Network-…) but it's ugly (uses unsafePerformIO for error reporting) and lacks a greater part of needed functionality. My aim is to greatly improve Network.Browser module and to make coding small scripts with it in more functional way possible. At this moment it uses BrowserAction state monad. Though, the deadline is approaching, I still seek some ways to improve my proposal. So here are the questions: are there any other data structures that will make programming with this library more convenient, besides simple state monad? Should we contrive more sophisticated system with other other separate data structures? What other improvements you'd like to see? Thanks in advance for any advice. -- Max.

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Parsec Expected Type
by Paul Keir 07 Apr '08

07 Apr '08

Hi, Does anyone know why this reduced Parsec production stops compilation, and how I can fix it? tester = reserved "parameter" <|> do { reserved "dimension"; symbol ":" } Thanks, Paul

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Shouldn't this loop indefinitely => take (last [0..]) [0..]
by Olivier Boudry 07 Apr '08

07 Apr '08

Hi all, If you compile and run this: main = do putStrLn $ show $ take (last [0..]) [0..] or simply run: take (last [0..]) [0..] in ghci, it first hang for about one minute and then starts to generate an infinite list. I was expecting "last [0..]" to never produce a value and the "take" function to never take from the [0..] list. I found that line of code in a friend's "Skype Message", lauched it in ghci and forgot it. When I came back to my ghci window a couple minutes later it was listing numbers, which was really unexpected. If I just run "last [0..]" it doesn't return a value and my computer will quickly start to paginate to death. Am I missing something? Some laziness magic? Rewrite rules? Thanks, Olivier.

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Re: [Haskell-cafe] Memory allocation in extractor function (newbie question)
by Alexander Kireyev 07 Apr '08

07 Apr '08

On Mon, Apr 7, 2008 at 4:16 PM, Yitzchak Gale <gale(a)sefer.org> wrote: > You didn't show us the code for countForPoints. I'll bet you wrote > something like > > countForPoints area ls count points = > sum $ map (countPathsFrom area (count + 1) ls) points > > Unfortunately, the standard sum function is too lazy in many situations. > And if you wrote out the recursion for countForPoints by hand, you > may have run into the same problem. In either case, you'll be accumulating > huge amounts of unevaluated (+) thunks instead of adding up the > count as you go along. > > If this is the problem, you can fix it by using foldl', a strict version > of foldl from Data.List, instead of sum: > > countForPoints area ls count points = > foldl' (+) 0 $ map (countPathsFrom area (count + 1) ls) points Thanks for the pointer. In fact my version was already using the fold function, albeit the lazy version of it: countForPoints :: Area -> String -> Int -> [Point] -> Int countForPoints area goal count points = let newCount = foldl (countSubPaths area goal) count points in if countExceeded newCount then -1 else newCount countExceeded :: Int -> Bool countExceeded count = (count < 0) || (count > countLimit) countSubPaths :: Area -> String -> Int -> Point -> Int countSubPaths area goal count point = if (countExceeded count) then -1 else countPathsFrom area count goal point Switching to the strict version reduced the amount of garbage in that function and in countSubPaths, but the problem with 'letterAt' still shines. I was thinking that adding strictness to that function might help, but no amount of exclamation marks helped it so far. -Alexander // copying to the mailing list, because first reply didn't get through.

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