- Haskell-Cafe - Haskell.org

GADTs and instances
by Tony Hannan 07 Sep '08

07 Sep '08

Hello haskellers, Anyone know the trick for making a Binary instance of a GADT. See sample code below followed by the type error reported by ghc version 6.8.3 Thanks, Tony ---- {-# LANGUAGE GADTs #-} module GADTTest where import Data.Binary import Control.Monad (liftM) data Query a where Lookup :: String -> Query (Maybe Int) Fetch :: [String] -> Query [Int] instance (Binary a) => Binary (Query a) where put (Lookup x) = putWord8 0 >> put x put (Fetch x) = putWord8 1 >> put x get = getWord8 >>= \tag -> case tag of 0 -> liftM Lookup get 1 -> liftM Fetch get ----- GADTTest.hs:12:0: Couldn't match expected type `Maybe Int' against inferred type `[Int]' When trying to generalise the type inferred for `get' Signature type: forall a. (Binary a) => Get (Query a) Type to generalise: Get (Query a) In the instance declaration for `Binary (Query a)'

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Haskell Weekly News: Issue 84 - September 6, 2008
by Brent Yorgey 07 Sep '08

07 Sep '08

--------------------------------------------------------------------------- Haskell Weekly News http://sequence.complete.org/hwn/20080906 Issue 84 - September 06, 2008 --------------------------------------------------------------------------- Welcome to issue 84 of HWN, a newsletter covering developments in the [1]Haskell community. This is the "This issue is not late since the HWN will henceforth be published on Saturday now that I have real work to do" edition. Featured this week: darcs hacking sprint plans solidify, xmonad 0.8 released, typed sprintf and sscanf, and tons of discussion about everything from functional references to splitting up the base library to the future direction of Haskell. Announcements unicode-properties 3.2.0.0, unicode-names 3.2.0.0. Ashley Yakeley [2]announced the release of the [3]unicode-properties 3.2.0.0 and [4]unicode-names 3.2.0.0 packages, which are representations in Haskell of various data in the Unicode 3.2.0 Character Database. experimental static blog engine in Haskell. jinjing [5]announced the initial release of [6]Panda, an experimental static blog engine written in Haskell. darcs hacking sprint, venues confirmed! (25-26 October). Eric Y. Kow [7]announced that two venues (Brighton, UK and Portland, Oregon, USA) have been confirmed for the [8]darcs hacking sprint on 25-26 October. darcs weekly news #2. Eric Y. Kow The second weekly issue of the [9]darcs weekly news has been published. xmonad 0.8 released!. Don Stewart [10]announced the release of [11]xmonad 0.8, featuring a general purpose "gaps" replacement, locale support, the ability to create your own configuration parsers, and various other [12]enhancements and fixes. POPL logo design contest. Don Stewart [13]forwarded a message announcing a logo design contest for POPL 2009. Dust off your magic markers/photoshop skills and get designing! ICFP09 Announcement. Matthew Fluet [14]announced [15]ICFP 2009, to be held 31st August to 2nd September 2009 in Edinburgh. ICFP provides a forum for researchers and developers to hear about the latest work on the design, implementations, principles, and uses of functional programming. Fast parallel binary-trees for the shootout: Control.Parallel.Strategies FTW!. Don Stewart [16]announced that the Computer Language Shootout recently got a quad core 64 bit machine, and outlined a plan and some initial results for porting the Haskell entries to take advantage of the available parallelism. The initial view on typed sprintf and sscanf. oleg [17]announced an implementation of typed sprintf and sscanf functions sharing the same formatting specifications, which also led to some interesting discussion and alternative proposals. Discussion Generalize groupBy in a useful way?. Bart Massey [18]proposed changing the implementation of groupBy to extend its usefulness for predicates which are not equivalence relations. Splitting SYB from the base package in GHC 6.10. JosÃ© Pedro MagalhÃ£es [19]initiated a discussion regarding splitting the SYB libraries out of the base package for GHC 6.10. The base library and GHC 6.10. Ian Lynagh initiated a [20]discussion on further splitting up the base package for GHC 6.10. Functional references. Tim Newsham began a [21]discussion on functional references and the possibility of merging the existing four or five implementations into something more standard. Types and Trees. Matt Morrow [22]wrote something about types and type representations, involving some commutative diagrams and some code. I haven't read it yet but it looks neat! Research language vs. professional language. Ryan Ingram started an interesting [23]discussion on the future direction(s) of the Haskell language. language proposal: ad-hoc overloading. Ryan Ingram [24]proposed adding ad-hoc name overloading to Haskell, prompting quite a bit of discussion. Top Level <-. Ashley Yakeley originally [25]asked whether there is any interest in implementing a top level "<-" to run monadic code. This set off a cascade of discussion the likes of which have been rarely seen on the Cafe. I would tell you what the discussion has been about but I must confess that I haven't read it. Blog noise [26]Haskell news from the [27]blogosphere. * Eric Kow (kowey): [28]darcs hacking sprint (25-26 October 2008). * Magnus Therning: [29]Confusion with HaskellDB. * Xmonad: [30]xmonad 0.8 released!. * Real-World Haskell: [31]Speaking in Silicon Valley next week. * Luke Palmer: [32]Composable Input for Fruit. * Luke Palmer: [33]Slicing open the belly of the IO monad in an alternate universe. * "FP Lunch": [34]Modular Monad Transformers. * Douglas M. Auclair (geophf): [35]Fuzzy unification parser in Haskell. * Bjoern Edstroem: [36]Speeding up Haskell with C -- a very short introduction. * Eric Kow (kowey): [37]darcs weekly news #2. * Roman Cheplyaka: [38]Physics and polyhedra. * Well-Typed.Com: [39]The new haskell.org community SPARC server is online. * Douglas M. Auclair (geophf): [40]A stream of primes as Comonad. * Eric Kidd: [41]Ubiquitous Hoogle. Eric shows how to integrate a [42]Hoogle search command into [43]Ubiquity. * Dan Piponi (sigfpe): [44]Untangling with Continued Fractions: Part 3. The vector space monad and rational tangles. Quotes of the Week * dons: maybe we should do a "recommend RWH to a java programmer" campaign :) * gwern: we fill fight them in the registers, we will fight them in the caches; we shall fight them in the core, and even in the backing store. And we shall never surrender! About the Haskell Weekly News New editions are posted to [45]the Haskell mailing list as well as to [46]the Haskell Sequence and [47]Planet Haskell. [48]RSS is also available, and headlines appear on [49]haskell.org. To help create new editions of this newsletter, please see the information on [50]how to contribute. Send stories to byorgey at cis dot upenn dot edu. The darcs repository is available at darcs get [51]http://code.haskell.org/~byorgey/code/hwn/ . References 1. http://haskell.org/ 2. http://article.gmane.org/gmane.comp.lang.haskell.libraries/9973 3. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/unicode-properti… 4. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/unicode-names 5. http://article.gmane.org/gmane.comp.lang.haskell.cafe/44181 6. http://jinjing.blog.easymic.com/ 7. http://article.gmane.org/gmane.comp.lang.haskell.cafe/44164 8. http://wiki.darcs.net/index.html/Sprints 9. http://code.haskell.org/darcs/darcs-news 10. http://article.gmane.org/gmane.comp.lang.haskell.cafe/44157 11. http://xmonad.org/ 12. http://haskell.org/haskellwiki/Xmonad/Notable_changes_since_0.7 13. http://article.gmane.org/gmane.comp.lang.haskell.cafe/44069 14. http://article.gmane.org/gmane.comp.lang.haskell.general/16413 15. http://www.cs.nott.ac.uk/~gmh/icfp09.html 16. http://article.gmane.org/gmane.comp.lang.haskell.cafe/43968 17. http://article.gmane.org/gmane.comp.lang.haskell.general/16405 18. http://thread.gmane.org/gmane.comp.lang.haskell.libraries/10016 19. http://thread.gmane.org/gmane.comp.lang.haskell.libraries/9962 20. http://thread.gmane.org/gmane.comp.lang.haskell.libraries/9929 21. http://thread.gmane.org/gmane.comp.lang.haskell.cafe/44098 22. http://thread.gmane.org/gmane.comp.lang.haskell.cafe/44028 23. http://thread.gmane.org/gmane.comp.lang.haskell.cafe/43933 24. http://thread.gmane.org/gmane.comp.lang.haskell.cafe/43897 25. http://thread.gmane.org/gmane.comp.lang.haskell.general/16393 26. http://planet.haskell.org/ 27. http://haskell.org/haskellwiki/Blog_articles 28. http://koweycode.blogspot.com/2008/09/darcs-hacking-sprint-25-26-october-20… 29. http://therning.org/magnus/archives/375 30. http://xmonad.wordpress.com/2008/09/05/xmonad-08-released/ 31. http://www.realworldhaskell.org/blog/2008/09/05/speaking-in-silicon-valley-… 32. http://luqui.org/blog/archives/2008/08/09/composable-input-for-fruit/ 33. http://luqui.org/blog/archives/2008/08/14/slicing-open-the-belly-of-the-io-… 34. http://sneezy.cs.nott.ac.uk/fplunch/weblog/?p=111 35. http://logicaltypes.blogspot.com/2008/09/fuzzy-unification-parser-in-haskel… 36. http://blog.bjrn.se/2008/09/speeding-up-haskell-with-c-very-short.html 37. http://koweycode.blogspot.com/2008/09/darcs-weekly-news-2.html 38. http://physics-dph.blogspot.com/2008/08/physics-and-polyhedra.html 39. http://blog.well-typed.com/2008/09/the-new-haskellorg-community-sparc-serve… 40. http://logicaltypes.blogspot.com/2008/09/stream-of-primes-as-comonad.html 41. http://www.randomhacks.net/articles/2008/09/01/ubiquitous-hoogle 42. http://www.haskell.org/hoogle/ 43. https://wiki.mozilla.org/Labs/Ubiquity 44. http://sigfpe.blogspot.com/2008/08/untangling-with-continued-fractions_31.h… 45. http://www.haskell.org/mailman/listinfo/haskell 46. http://sequence.complete.org/ 47. http://planet.haskell.org/ 48. http://sequence.complete.org/node/feed 49. http://haskell.org/ 50. http://haskell.org/haskellwiki/HWN 51. http://code.haskell.org/~byorgey/code/hwn/

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What's the best way to give up?
by David F. Place 07 Sep '08

07 Sep '08

Hi, all. Say I have a function solve which is a constraint solver. It reconfigures its input to be a solution. If there is no solution, it returns the input. solve :: a -> Either a a solve input output = maybe (Left input) Right $ solve' input If there is a solution, it finds it in a few seconds. If there is no solution, it goes away for days proving that. So, I'd like to give up on it if it doesn't return in a few seconds. I can think of several ways of doing that. I could keep a tally of the number of variable assignments and give up when it reaches an impossibly huge number. I could change the type to a -> IO (Either a a ) and use getCPUTime. Is there a standard way to do this? Can you think of another way to do it? Cheers, David

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Monad and kinds
by Ramin 06 Sep '08

06 Sep '08

Hello, I'm new here, but in the short time I have known Haskell, I can already say it's my favorite computer language. Except for monads, and no matter how many tutorials I read, I find the only kind of monad I can write is the monad that I copied and pasted from the tutorial, i.e. I still don't get it even though I thought I understood the tutorial, and I'm stuck using monads others have already written. My project is this: I am writing a mini-database of sorts. I thought I would make a query a monad, so I could chain multiple queries together using the "do" notation, and run more complex queries on the list in one shot. The query itself is stateful because it contains information that changes as the database is traversed. The query may also make updates to the records. I got the program to work perfectly using purely functional techniques, but complex queries resulted in that stair-step looking code. So I thought this would be the perfect opportunity to try my hand at monads. The query monad I wrote looks something like this (much simplified): data Query state rec = Query !(state, rec) Where "state" is the type of state, so a query can contain any information relevant to the search and can be updated as the search progresses. Then, "rec" is the type of records in the database, which isn't determined inside the database module, so I can't just declare "rec" to be of any type I choose. But I just cannot write the instance declaration for my Query data type no matter what I try. If I write: instance Monad Query where return (initState, someRecord) = Query (initState, someRecord) {- code for (>>=) -} GHC gives an error, "Expected kind `* -> *', but `Scanlist_ctrl' has kind `* -> * -> *' ". If I try this: instance Monad (Query state) where return (initState, someRecord) = Query (initState, someRecord) {- code for (>>=) -} GHC give an error, "Occurs check: cannot construct the infinite type: a = (s, a) when trying to generalise the type inferred for `return' ". I get the sense I am trying to shove a square peg into a round hole. I was thinking of trying some other things, like implementing the monad in a higher-level module where I knew the type of the records I would be using, but I don't like being told where to implement things. I also thought of trying to re-write my query algorithm to somehow use "Control.Monad.State.Strict" instead of my own query type, but then I wouldn't get to write my own monad! Though the information given in this e-mail is limited, is there anyone who can clearly see there is something about monads that I just don't get and tell me what it is? Anyone who took the time to read this, I am very appreciative. Ramin Honary

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ANNOUNCE: xmonad 0.8 released!
by Don Stewart 06 Sep '08

06 Sep '08

http://xmonad.org The xmonad dev team is pleased to announce xmonad 0.8! The headlines: * A general purpose replacement for "gaps" has been adopted. * Floating windows pop up on the current screen by default * Locale support * Rock solid code: No new crash-inducing bugs reported in this release cycle * Many new extensions, including the ability to write your own configuration parsers. * The 1000th commit was made to the project Change logs: http://haskell.org/haskellwiki/Xmonad/Notable_changes_since_0.7 About: xmonad is a tiling window manager for X. Windows are arranged automatically to tile the screen without gaps or overlap, maximising screen use. Window manager features are accessible from the keyboard: a mouse is optional. xmonad is extensible in Haskell, allowing for powerful customisation. Custom layout algorithms, key bindings and other extensions may be written by the user in config files. Layouts are applied dynamically, and different layouts may be used on each workspace. Xinerama is fully supported, allowing windows to be tiled on several physical screens. Features: * Very stable, fast, small and simple. * Automatic window tiling and management * First class keyboard support: a mouse is unnecessary * Full support for tiling windows on multi-head displays * Full support for floating, tabbing and decorated windows * Full support for Gnome and KDE utilities * XRandR support to rotate, add or remove monitors * Per-workspace layout algorithms * Per-screens custom status bars * Compositing support * Powerful, stable customisation and reconfiguration * Large extension library * Excellent, extensive documentation * Large, active development team, support and community Get it! Information, screenshots, documentation, tutorials and community resources are available from the xmonad home page: http://xmonad.org The 0.8 release, and its dependencies, are available from hackage.haskell.org: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/xmonad xmonad packages are available in the package systems of at least: Debian, Gentoo, Arch, Ubuntu, OpenBSD, NetBSD, FreeBSD, Gobo, NixOS, Source Mage, Slackware and 0.8 packages will appear in coming days (some are already available). On the fly updating to xmonad 0.8 is supported. You can even use cabal-install: $ cabal update $ cabal install xmonad-0.8 $ cabal install xmonad-contrib-0.8 $ xmonad --recompile mod-q Extensions: xmonad comes with a huge library of extensions (now around 9 times the size of xmonad itself), contributed by viewers like you. Extensions enable pretty much arbitrary window manager behaviour to be implemented by users, in Haskell, in the config files. For more information on using and writing extensions see the webpage. The library of extensions is available from hackage: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/xmonad-contrib Full documentation for using and writing your own extensions: http://xmonad.org/contrib.html This release brought to you by the xmonad dev team: Spencer Janssen Don Stewart Jason Creighton David Roundy Brent Yorgey Devin Mullins Braden Shepherdson Roman Cheplyaka Lukas Mai Featuring code contributions from over 60 developers: Aaron Denney Adam Vogt Alec Berryman Alex Tarkovsky Alexandre Buisse Andrea Rossato Austin Seipp Bas van Dijk Ben Voui Brandon Allbery Chris Mears Christian Thiemann Clemens Fruhwirth Daniel Neri Daniel Wagner Dave Harrison David Glasser David Lazar Dmitry Kurochkin Dominik Bruhn Dougal Stanton Eric Mertens Ferenc Wagner Gwern Branwen Hans Philipp Annen Ivan Tarasov Ivan Veselov Jamie Webb Jeremy Apthorp Malebria Joachim Breitner Joachim Fasting Joe Thornber Joel Suovaniemi Juraj Hercek Justin Bogner Kai Grossjohann Karsten Schoelzel Klaus Weidner Mathias Stearn Mats Jansborg Matsuyama Tomohiro Michael Fellinger Michael Sloan Miikka Koskinen Neil Mitchell Nelson Elhage Nick Burlett Nicolas Pouillard Nils Anders Danielsson Peter De Wachter Robert Marlow Sam Hughes Shachaf Ben-Kiki Shae Erisson Simon Peyton Jones Stefan O'Rear Tom Rauchenwald Valery V. Vorotyntsev Will Farrington Yaakov Nemoy timthelion Rickard Gustafson Trevor Elliott Ian Zerny Ivan Miljenovic Marco e Silva Michal Janeczek As well as the support of many others on the #xmonad and #haskell IRC channels, and the wider Haskell and window manager communities. Thanks to everyone for their support!

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2 modules in one file
by Maurício 06 Sep '08

06 Sep '08

Hi, Is it allowed to write two different modules in a single file? Something like: module Mod1 (...) where { ... } module Mod2 (...) where { import Mod1; ... } I tried, and got an error, but would like to confirm that there's no way to do that. Thanks, Maurício

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Re: [Haskell-cafe] Reducing code for efficient ShowS
by wren ng thornton 06 Sep '08

06 Sep '08

Sean Leather wrote: >> Why not use the dlist library: >> >> http://hackage.haskell.org/cgi-bin/hackage-scripts/package/dlist >> >> With something like (untested code): >> >> > xs +++ ys = shows xs `append` shows ys >> > x .++ ys = showChar x `cons` shows ys >> > xs ++. y = shows xs `snoc` showChar y >> > >> > ext3' = toList $ '(' .++ n +++ ' ' .++ s ++. ')' >> > > I think you're missing the fromList parts among other things. Ah yes, dlist uses a newtype instead of an alias so `shows` isn't enough. There's also the question of where exactly to hide the (Show a => a -> DList Char) conversions if you want to be more succinct than the (++) route. > That's an interesting idea. It appears to use the same idea as ShowS, but > more generally with lists and not just strings. The difference-list approach to solving the appending problem is classic. There's a variant for unification-based logic languages as well. Both are functional takes on the imperative approach of keeping a tail pointer. Dons just took the time to package it up for us all :) > I think there's an added benefit to not having to remember the the type of > the value being appended. It's one of the more convenient things about many > dynamically typed languages. So, I would still vote for the class-based > method, so that I can use (.+.) for both Char and everything else. Sure. If you're planning on releasing the code, I'd suggest a different spelling of (.+.) though. The (.X.) pattern for a family of operators is pretty common, so it's good to avoid it for modules that want to be used in combination with many others. YMMV and all that. -- Live well, ~wren

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Re: [Haskell-cafe] Reducing code for efficient ShowS
by Sean Leather 06 Sep '08

06 Sep '08

> Why not use the dlist library: > > http://hackage.haskell.org/cgi-bin/hackage-scripts/package/dlist > > With something like (untested code): > > > xs +++ ys = shows xs `append` shows ys > > x .++ ys = showChar x `cons` shows ys > > xs ++. y = shows xs `snoc` showChar y > > > > ext3' = toList $ '(' .++ n +++ ' ' .++ s ++. ')' > I think you're missing the fromList parts among other things. That's an interesting idea. It appears to use the same idea as ShowS, but more generally with lists and not just strings. I think there's an added benefit to not having to remember the the type of the value being appended. It's one of the more convenient things about many dynamically typed languages. So, I would still vote for the class-based method, so that I can use (.+.) for both Char and everything else. Sean

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Reducing code for efficient ShowS
by Sean Leather 06 Sep '08

06 Sep '08

It happens very often that I want to convert a number of values to strings and concatenate those strings into one. No surprise there, of course. Well, I'd prefer to do it efficiently and with as little code as necessary. > {-# LANGUAGE TypeSynonymInstances #-} > module ShowsDemo where Let's say I want to generate the string "(42 abc)" starting with a number and a string stored in variables. > n = 42 :: Int > s = "abc" What are my options? There's the obvious approach that's described in every tutorial, book, and research paper (for didactic purposes, of course). > ex1 = "(" ++ show n ++ " " ++ s ++ ")" It's pretty concise, but it's horribly inefficient due to the use of (++). Then, there's the ShowS approach. > ex2 = showChar '(' . shows n . showChar ' ' . showString s . showChar ')' $ "" This is more efficient, but now the code has bloated up a lot. Why can't I have my cake and eat it, too? I want to write with as little code as |ex1| (or less if possible), and I want it to be as efficient as |ex2|. I propose this example as an improvement. > ex3 = '(' .+. n .+. ' ' .+. s .$. ')' It uses a class I'm calling |Shows|. The class has one method that simply converts a value to the type |ShowS|, where |ShowS| is a type synonym for |String -> String| and is defined in the Prelude. > class Shows a where > toShows :: a -> ShowS Notice the lack of context involving the |Show| class. That's important, because it allows us to create more instances than we could if we were restricted by |(Show a) => ...|, esp. the |ShowS| instance below. The instances for types are all very simple. Most will appear like the instance for |Int|. > instance Shows Int where > toShows = shows Since we don't have |Show| in the class context above, we can't make this a default method. We need a few special instances for |Char| and |String| to make these types convenient to use in the expected way. > instance Shows Char where > toShows = showChar > instance Shows String where > toShows = showString We also need an instance for |ShowS| in order to facilitate concatenation. > instance Shows ShowS where > toShows = id Now, we define a few operators that use |toShows| and make our lives easier and our code more concise. The |(.+.)| replaces list appending, |(++)|, in |ex1| and function composition, |.|, in |ex2|. > infixl 5 .+. > (.+.) :: (Shows a, Shows b) => a -> b -> ShowS > a .+. b = toShows a . toShows b The |(.$.)| replaces the need for |($)| in |ex2|. > infixl 4 .$. > (.$.) :: (Shows a, Shows b) => a -> b -> String > a .$. b = (a .+. b) "" I would find something like this very useful. I'm guessing the idea can be applied to |ByteString| as well. Does it exist in some other form? Sean

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Re: [Haskell-cafe] Consequences of implementing a library in Haskell for C consumption?
by Justin Bailey 05 Sep '08

05 Sep '08

On Thu, Sep 4, 2008 at 2:52 PM, Philippa Cowderoy <flippa(a)flippac.org> wrote: > Would writing Haskell to generate the C via Language.C be an option? > Effectively you'd be using Haskell as a typeful macro system. Interesting idea, and I've done similar things with haskelldb (generating SQL queries). Looking at the package, I think would be pretty painful though. It seems I'd have to build the AST by hand, and it doesn't seem to incorporate any type information in the AST items. In haskelldb, every expression carries its type around in a phantom type, which protects you from trying to compare a bool and a string, for intstance. I don't see anything similar in Language.C. I may be mistaken though! Justin

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Is it usual to read a Maybe (IORef a) ?
by minh thu 05 Sep '08

05 Sep '08

Hi, I'd like to write a data structure to be used inside the IO monad. The structure has some handles of type Maybe (IORef a), i.e. IORef are pointers and the Maybe is like null pointers. So I came up with the following functions : readHandle :: Maybe (IORef a) -> IO (Maybe a) readField :: (a -> b) -> Maybe (IORef a) -> IO (Maybe b) readHandle Nothing = do return Nothing readHandle (Just r) = do v <- readIORef r return $ Just v readField f h = do m <- readHandle h return $ fmap f m Is it something usual ? Are there any related functions in the standard libraries ? Thanks, Thu

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Re: [Haskell-cafe] Re: [Haskell] Top Level <-
by Ganesh Sittampalam 05 Sep '08

05 Sep '08

On Sun, 31 Aug 2008, Adrian Hey wrote: > Ganesh Sittampalam wrote: >> On Sun, 31 Aug 2008, Adrian Hey wrote: >> >>> Thanks for taking the time to do this Dan. I think the safety requirement >>> has been met, but I think it fails on the improved API. The main complaint >>> would be what I see as loss of modularity, in that somehow what should be >>> a small irrelevant detail of the implementation of some obscure module >>> somewhere has propogated it's way all the way upto main. >> >> That's the key point, as I see it - they aren't "irrelevant details of the >> implementation", they are requirements the implementation places on its >> context in order for that implementation to be correct. So they should be >> communicated appropriately. > > Eh? Please illustrate your point with Data.Unique. What requirements > does it place on it's context? (whatever that might mean :-) It requires that its context initialises it precisely once. Data.Unique is actually a poor example, as it is actually fine to initialise it multiple times as long as the resulting Unique values aren't treated as coming from the same datatype. But equally it can be implemented with IORefs, so it's not a good advert for the need for global variables. > The real irony of your remark is that making APIs this robust is > practically impossible *without* using global variables, and you're > now saying that because they've done this work to eliminate these > constraints they now have to be held to account for this with > an absurd API. I think there are two cases to consider here. A Data.Unique style library, which requires genuinely *internal* state, and which is agnostic to having multiple copies of itself loaded simultaneously. In that case, there is no requirement for a process-level scope for <-, just that each instance of the library is only initialised once - the RTS can do this, as can any dynamic loader. The other is some library that really cannot be safely loaded multiple times, because it depends on some lower-level shared resource. Such a library simply cannot be made safe without cooperation from the thing that controls that shared resource, because you cannot prevent a second copy of it being loaded by something you have no control over. If the <- proposal were only about supporting the first of these applications, I would have far fewer objections to it. But it would have nothing to do with process-level scope, then. Ganesh

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Categorical language
by Maurício 05 Sep '08

05 Sep '08

Hi, I’ve been thinking about this and would like your opinion: what could be a “categorical language”? Something like a language where categories and functors are first class; does that makes sense at all? (Sorry if this is insane, I'm a begginer in categories, I've read just the first chapter of Mac Lane's book.) Thanks, Maurício

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Linux version of ghc-6.8.3 won't intall or run for me: "Floating point exception".
by Alan Mackenzie 05 Sep '08

05 Sep '08

Hi, Haskell! I've downloaded the ghc-6.8.3-i386-unknown-linux.tar.bz2 tarball, which I expected to work on my GNU/Linux box (1.2 GHz Athlon, Debian Sarge). I'm new at this game, though I'm thoroughly experienced with (and bear deep battle scars from) GNU software in general. ./configure fails with "checking for path to top of build tree... configure: error: cannot determine current directory". This has been reported before in this list, but I don't think anybody ever explained the fault. Further delving finds that the following statement in the configure script triggers it: hardtop=`utils/pwd/pwd forwardslash` . utils/pwd/pwd, no matter how I call it, always crashes with "Floating point exception". Surely real numbers aren't involved in determining a configuration - probably some floating point library or other is missing from my system. What is utils/pwd/pwd? What's its purpose? It's 460kB large (compared with 14kb for /bin/pwd). Is it a haskell implementation of Unix's pwd, primarily intented for non-unixy OSs? configure is supposed to ascertain my machine's config, so it's particularly disappointing that it crashes because my config isn't what it expects. Should I report this as a bug? ######################################################################### Anyhow, I hacked the configure script by replacing the above line by a hard-coded value, thusly: hardtop=/home/acm/haskell/ghc-6.8.3 , and the configure script finished. ;-) So I did "make install", and the turgid messages rolled up my screen until it crashed, the last few lines being: nstall/lib/ghc-6.8.3"' -DPKG_DATADIR='"/home/acm/haskell/ghc-install/share/ghc-6.8.3"' package.conf.in \ | grep -v '^#pragma GCC' \ | sed -e 's/""//g' -e 's/:[ ]*,/: /g' \ | /home/acm/haskell/ghc-6.8.3/utils/ghc-pkg/ghc-pkg.bin --global-conf /home/acm/haskell/ghc-install/lib/ghc-6.8.3/package.conf update - --force make[1]: *** [install] Error 136 make[1]: Leaving directory `/home/acm/haskell/ghc-6.8.3/rts' make: *** [install] Error 2 Ah yes! Error 136. ;-) Don't we just love GNU bread and butter software? It's not documented on the install man page either - or the info page. Neither is Error 8. Hey, you've all been through this too: enthusiasm -> 0 as t -> infinity. :-( ######################################################################### So, I thought, who needs to install? It's a binary package, so somewhere there's going to be a top-level callable program, probably called something like "ghc". Hey, yes, compile/Stage3/ghc-6.8.3. Guess what: "Floating point exception". :-( I hate binary packages - I utterly loathe them. They never work. At least they never work for me. Or the distro package manager stuffs your cron config with resource hoggers you really don't want, and forgets to ask you first. I suppose they might work if your OS has been installed or updated within the last few months; otherwise, forget it. Source distributions are so much faster and easier to install because the cranial clutter you have to cope with is that much less. Trouble is, to build haskell from source, you need a working haskell to build it with. Presumably, sometime in the recent past (as measured on archaeological time scales), ghc was bootstrapped from C (or some other lowest common denomiator language). Is this route still available? Some proto-proto haskell written in C, sufficiently powerful to build a proto-haskell, sufficiently powerful to build the compiler? ######################################################################### I'd really love to play with haskell. But the time I've wasted so far, trying to get it working, is almost at the stage where I just can't be bothered any more. I've been here before, and I can see that I've got 1 - 3 days x 8 hours of grinding drudgery before I finally get ghc-6.8.3 working. I suppose that's just the way GNU systems are. If anybody could give me some tips as to getting a decent haskell running on my system in a reasonable, predictable amount of time, in a way which doesn't involve diagnosing and fixing problems, I'd be very grateful indeed. Thanks! -- Alan Mackenzie (Nuremberg, Germany).

5 7

Consequences of implementing a library in Haskell for C consumption?
by Justin Bailey 05 Sep '08

05 Sep '08

I am thinking of writing a simple library in Haskell which would be useful in a number of different scenarios, and not always with programs written in Haskell. That makes me think the library should be C-compatible and able to link with C programs. Reading over chapter 9 of the GHC manual ("Foreign function interface"), it seems simple to do. However, what are the consequences of such an implementation? Would any program using my library need to link the entire GHC runtime? The library will not expose any higher-order functions or other business. It would be straightforward but tedious to write in C, which is why I'd rather use Haskell. Thanks in advance for any thoughts. Justin

2 1

Parsec 3 Description
by John Van Enk 05 Sep '08

05 Sep '08

I'm looking for a document describing the differences between Parsec 3 and Parsec 2. My google-foo must be off because I can't seem to find one. Does any one know where to find that information? -- /jve

3 4

Compiler's bane
by Andrew Coppin 05 Sep '08

05 Sep '08

Hi guys. I'm writing a simple interpretter for a small extended-lambda-calculus sort of language. And I'd just like to say... RECURSIVE LET-BINDS! GAAAAH!!! >_< No other part of the program has consumed nearly as much brain power as me trying to figure out when it is and isn't safe to replace a variable with its RHS. To illustrate: let x = f x; y = 5 in x y A simple-minded interpretter might try to replace every occurrance of "x" with "f x". This yields let y = 5 in (f x) y ...and x is now a free variable. OOPS! Trying to tease out exactly under which conditions you can and cannot perform the substitution is utterly maddening. Since this is a Haskell mailing list and as such it is populated by vast numbers of people with PhDs and so forth... does anybody happen to know the *correct* solution to this conundrum? Before I become clinically insane...? o_O By the way... To all those people who work on projects like GHC and so on, who have to get this stuff right "for real": you have my infinite respect!

10 29

[POPL logo design contest]
by Don Stewart 05 Sep '08

05 Sep '08

This sounds relevant to our interests... ----- Forwarded message from Swarat Chaudhuri <swarat(a)cse.psu.edu> ----- Date: Thu, 04 Sep 2008 09:13:55 -0400 From: Swarat Chaudhuri <swarat(a)cse.psu.edu> To: types-list(a)lists.seas.upenn.edu Subject: [TYPES] POPL logo design contest [ The Types Forum, http://lists.seas.upenn.edu/mailman/listinfo/types-list ] Dear POPL community members: We know that you are all holding your breath for POPL 2009; here's some diversion to take your mind off the wait. We are planning to select a logo for POPL---after all, if the Olympics can have one, why not us? Accordingly, we announce the first-ever Contest on POPL Logo Design and Implementation (not to be confused with the other, gentler PLDI). Participants should send submissions in jpg, gif or png formats to Swarat Chaudhuri (swarat at cse.psu.edu) by November 20, 2008. The submission should contain: 1) A logo that doesn't mention the year (e.g., 2010) of the conference. 2) Logos mentioning the years 2010, 2011, and 2012. The contest will be judged by: * Luca Cardelli (Microsoft Cambridge) * Benjamin Pierce (Penn) * Shriram Krishnamurthi (Brown) We do not currently need high-resolution images, but will request them from the winner(s). From 2010 on, the logo will appear on POPL's webpage---in addition to the winning logo, the POPL 2010 webpage will also link to any other logo the judges find noteworthy. The winner(s) and runners-up will be presented t-shirts and cookies during the PC Chair's presentation at POPL 2009. So here's to citius, altius, etc. Get your design juices flowing! Best, Swarat Chaudhuri (Penn State) Publicity chair POPL 2010-2012 ----- End forwarded message -----

1 0

Haskell and i18n (aka gettext) support
by Gour 04 Sep '08

04 Sep '08

Dear Haskellers, Some time ago the question was asked on gtk2hs mailing list about how to work with *.po files, i.e. how to make Haskell program i18n-aware. (see http://thread.gmane.org/gmane.comp.lang.haskell.gtk2hs/592) The answer was that Gtk2hs does not have gettext support (http://article.gmane.org/gmane.comp.lang.haskell.gtk2hs/593) but that it is something which is needed (http://article.gmane.org/gmane.comp.lang.haskell.gtk2hs/598) but it's not clear how to do it. As potential 'solution the 'i18n' package was mentioned (http://hackage.haskell.org/cgi-bin/hackage-scripts/package/i18n) with the remark that its API is not the most beautiful. Moreover, by looking at i18n's *.cabal file one can see it is labelled as 'experimental'. Moreover, if one inspects GNU gettext manual one can see the list of languages with the explanations how to make the program i18n-ready. The list is, unfortunately, without Haskell: * C: C, C++, Objective C * sh: sh - Shell Script * bash: bash - Bourne-Again Shell Script * Python: Python * Common Lisp: GNU clisp - Common Lisp * clisp C: GNU clisp C sources * Emacs Lisp: Emacs Lisp * librep: librep * Scheme: GNU guile - Scheme * Smalltalk: GNU Smalltalk * Java: Java * C#: C# * gawk: GNU awk * Pascal: Pascal - Free Pascal Compiler * wxWidgets: wxWidgets library * YCP: YCP - YaST2 scripting language * Tcl: Tcl - Tk's scripting language * Perl: Perl * PHP: PHP Hypertext Preprocessor * Pike: Pike * GCC-source: GNU Compiler Collection sources Real World Haskell book is at the door and it will bring army of new Haskell programmers to find out that Haskell misses one of the important items in its "battery set". :-( So, we think that i18n issue or proper gettext support should have a stable and well documented implementation and has to be part of "batteries included" or Haskell Platform (http://www.haskell.org/haskellwiki/Haskell_Platform) However, don't ask me 'how' :-D We got some hints on the IRC yesterday, but considering it is important issue for the whole Haskell community, we are bringing it here so that smarter and more skillful (than ourselves) souls can discuss it. I hope you are not seeing it as critique of the Haskell, but rather as attempt to improve our beloved language with the feature necessary to be included in the arsenal of every general programming language. Sincerely, Gour -- Gour | Zagreb, Croatia | GPG key: C6E7162D ----------------------------------------------------------------

2 1

Research language vs. professional language
by Ryan Ingram 04 Sep '08

04 Sep '08

Jonathan, I think we are going to end up just disagreeing on this subject, but I'd like to point out the reasons why we disagree. On Sun, Aug 31, 2008 at 7:27 PM, Jonathan Cast <jonathanccast(a)fastmail.fm> wrote: > This concept of `day-to-day work' is a curious one. Haskell is not a > mature language, and probably shouldn't ever be one. I see where you are coming from here, but I think that train has already started and can't be stopped. I find Haskell interesting as a professional programmer for these four reasons: it's pure, it's functional, it's lazy, and it's got a great compiler. I don't know of any other "mature" language that can fill these shoes; Haskell is the most mature of them. If I'm missing another language that fits my requirements and would be better for day-to-day programming, let me know! The serious competitors I can see are Clean (Hackage blows away anything I've seen from that community) and O'CaML, which is neither pure nor lazy. Meanwhile, Haskell is getting more suitable for regular "professional" use every day. Look at the success of Hackage and the soon-to-be-published "Real World Haskell" book for evidence. Haskell solves many of the problems I have with existing languages today, and makes me a better programmer at the same time. > There will always be new discoveries in purely functional programming, > and as the art advances, features like this ad-hoc overloading hack > (and ACIO) will become obsolete and have to be thrown over-board. This is a good point. However, it seems to me that the pure programming language research is moving towards dependently typed languages, and that progress in Haskell has been more application-side; transactional memory and data-parallel, along with research on various fusion techniques, for example. I also think ACIO is a good theoretical starting point for research on what initialization means, and has potential for work to influence the commutative monads problem in general which could improve the syntax and number of lines of code devoted to working around the verbosity of effectful computations in Haskell. > I'd rather (much rather!) people concerned with day-to-day programming > for writing programs people actually use incorporate Haskell's features > into other, more practical, languages (as those who *actually* care about > such things are) rather than incorporating features from day-to-day > production languages into Haskell. This is already happening, of course; I think ML is going to become pure soon, although I don't think it will ever be non-strict. And I predict that within the next few years, lots of languages will be leaning heavily on the concurrency research that is going on in Haskell right now. But Haskell's community is also growing and becoming more results-oriented. If you want to blame someone for this, Don Stewart is probably your guy :) I'm just one of the johnny-come-latelies who is realizing what a great tool Simon and the rest of the ghc team has built. So, my question for you is: if Haskell is a research language, what direction should it be taking? What changes should be happening at the language (not the library) level that are in the interests of improving the state-of-the-art in functional programming? My goal is to make it not be a joke when I go to work and suggest that we use Haskell for part of our next project. What is yours? -- ryan

7 7

Haskell audio libraries & audio formats
by Eric Kidd 04 Sep '08

04 Sep '08

Greetings, Haskell folks! I'm working on a toy program for drum synthesis. This has lead me to explore the various sound-related libraries in Hackage. Along the way, I've learned several things: 1. There's a lot of Haskell sound libraries, but no agreement on how to represent buffers of audio data. 2. Some of the most useful sound libraries aren't listed in Hackage's "Sound" section, including HCodecs, SDL-mixer and hogg. 3. The Haskell OpenAL bindings are great, but ALUT has been removed from MacOS 10.5. Of course, a pure-Haskell version of ALUT would be pretty easy to write, and it could be used as a fallback. 4. '[[a]]' is _not_ a good representation for multi-channel audio data. Manipulating even 10 seconds of audio is noticeably slow. 5. HCodecs is probably the best library for reading WAVE files. It uses 'DiffUArray Int a' to represent audio data, and not '[[a]]'. It also supports MIDI and SoundFont, which is quite handy. 6. YampaSynth looks really cool, and it might be possible to turn it into a reusable MIDI synthesizer library for use with HCodecs. This would be extremely useful: it would provide a basis for all kinds of crazy MIDI-based programming in Haskell. At the end of this e-mail, you can find a list of Haskell sound libraries and their supported audio formats. What would the ideal Haskell sound API look like? Personally, I would love to see: a. Something like HCodecs's Data.Audio as the basic sound format. Data.Audio is an array-based API, and it supports converting between a wide range of common sample formats. I don't know how fast this is, but it already exists, and it's used by YampaSynth. b. OpenAL for sound playback, with a portable version of ALUT. This may require writing a pure-Haskell version of ALUT. c. Utility functions to convert Data.Audio to OpenAL.AL.Buffer. d. An easy-to-use API for audio processing. It might be possible to start with something like Jack's 'mainMono' and 'mainStereo' functions, which map a function over an audio stream. Other nice-to-have features might include: e. A standard MIDI format, based on either the HCodecs package or the midi package. (HCodecs is used by YampaSynth, and the midi package is used by alsa-midi.) f. A modular version of YampaSynth which can convert MIDI data structures into Data.Audio values. It looks like Haskell could be a really sweet audio programming environment with just a bit of integration work. What do folks think? Are there other libraries I should look at more closely? Other features that should be included in an ideal audio API? Thank you for your advice and feedback! Cheers, Eric Haskell audio libraries Reading and writing sound files: HCodecs: (Audible a) => DiffUArray Int a hsndfile: MArray with Float and Double HSoundFile: [[Double]] ALUT: OpenAL.AL.Buffer WAVE: [[Int32]] Playing sounds: OpenAL: Ptr UInt8, Ptr Int16, 1-N channels SDL-mixer: ForeignPtr ChunkStruct, ForeignPtr MusicStruct Sound processing libraries: dsp: Array a, [a] jack: Operates via mapping functions mainMono :: (CFloat -> IO CFloat) -> IO () mainStereo :: ((CFloat, CFloat) -> IO (CFloat, CFloat)) -> IO () MIDI-based: HCodecs: Reads and writes MIDI files midi: Reads and writes MIDI files alsa-midi: Uses midi library YampaSynth: Stand-alone program Special-purpose APIs only (FFIs, etc.): hCsound: CsoundPtr hsc3: UGen No public sound-buffer API: hbeat: The relevant source files are missing! hogg: Very low-level API for Ogg internals only libmpd: No sound buffer API sonic-visualizer: No sound buffer API truelevel: Stand-alone program (uses WAVE) wavconvert: Stand-alone program

6 14

analyzing modules import/export
by Han Joosten 04 Sep '08

04 Sep '08

I am currently involved with a Haskell project to create a toolset for a formal language. The toolset contains a parser, a typechecker and s range of other features. The Haskell code currently consists of about 20 modules. I am new in the project, and my impression is that a lot of code does not reside in the proper module. I would like to be able to get a good feeling in how the modules are structured, and how they are consumed by other modules. This would enable me to clean up a little bit. I use eclipsefp. Do you know of any way to get insight of the structure of modules? is any tooling available? Any suggestions would help a lot! Thanx for reading. Han Joosten -- View this message in context: http://www.nabble.com/analyzing-modules-import-export-tp19297359p19297359.h… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

2 1

joyent, solaris and ghc
by Jason Dusek 04 Sep '08

04 Sep '08

Is anyone on the list using GHC on Joyent's Solaris (x86_64) setup? If so, I would love to know whether it was easy/hard and what the process is. -- _jsn

2 1

Re: [Haskell-cafe] Is it usual to read a Maybe (IORef a) ?
by minh thu 03 Sep '08

03 Sep '08

2008/9/3 Lennart Augustsson <lennart.augustsson(a)gmail.com>: > I think you should think about why your application needs IORef at all. Do > you really need mutation? And why isn't STRef good enough. Using IORef is > really the last resort. This is for an interactive application (3D mesh editing, the data structure is the half edge structure). So the user can add points, edges, faces, delete them, ... Thu

1 0

Types and Trees
by Matt Morrow 03 Sep '08

03 Sep '08

I really learned a lot from writing the below code, and thought I'd share it with the group. I'm slightly at a loss for words, having just spent the last two hours on this when I most certainly should have been doing other work, but these are two hours I won't regret. I'm very interested in hearing others' thoughts on "this", where "this" is "whatever comes to mind". Regards, Matt {-# LANGUAGE MultiParamTypeClasses, FlexibleInstances #-} module TT where import Data.Tree import Data.Typeable (Typeable(..),TypeRep(..),TyCon(..) ,typeRepTyCon,typeRepArgs,tyConString) import Language.Haskell.TH(Type(..),mkName) -------------------------------------------------------- class ToType a where toType :: a -> Type class ToTree a b where toTree :: a -> Tree b {- Typeable a | typeOf | (0) | v toType TypeRep - - - - - - - - > Type | (4) | toTree | (1) (2) | toTree | | v (3) v Tree TyCon ---------------> Tree Type toTree -} -- (0) typeableToTypeRep :: (Typeable a) => a -> TypeRep typeableToTypeRep = typeOf -- (1) instance ToTree TypeRep TyCon where toTree ty = Node (typeRepTyCon ty) (fmap toTree . typeRepArgs $ ty) -- (2) instance ToTree Type Type where toTree (AppT t1 t2) = let (Node x xs) = toTree t1 in Node x (xs ++ [toTree t2]) toTree t = Node t [] -- (3.a) instance ToType TyCon where toType tyC = let tyS = tyConString tyC in case tyS of "->" -> ArrowT "[]" -> ListT "()" -> TupleT 0 ('(':',':rest) -> let n = length (takeWhile (==',') rest) in TupleT (n+2) _ -> ConT . mkName $ tyS -- (3.b) instance ToType (Tree TyCon) where toType (Node x xs) = foldl AppT (toType x) (fmap toType xs) -- (3) instance ToTree (Tree TyCon) Type where toTree = toTree . toType -- (4) instance ToType TypeRep where toType = toType . (toTree::TypeRep->Tree TyCon) -- (0) typeOf -- (1) toTree -- (2) toTree -- (3) toTree -- (4) toType -- (0) -> (1) tyConTree :: (Typeable a) => a -> Tree TyCon tyConTree = toTree . typeOf -- (0) -> (1) -> (3) typeTree_a :: (Typeable a) => a -> Tree Type typeTree_a = (toTree::Tree TyCon->Tree Type) . (toTree::TypeRep->Tree TyCon) . typeOf -- (0) -> (4) -> (2) typeTree_b :: (Typeable a) => a -> Tree Type typeTree_b = (toTree::Type->Tree Type) . (toType::TypeRep->Type) . typeOf diagram_commutes :: (Typeable a) => a -> Bool diagram_commutes a = typeTree_a a == typeTree_b a -- ghci> diagram_commutes x0 -- True x0 :: (Num a) => ((a,(a,((a,a),a))),(a,(a,a))) x0 = ((0,(0,((0,0),0))),(0,(0,0))) -------------------------------------------------------- printTree :: (Show a) => Tree a -> IO () printTree = putStr . drawTree . fmap show printForest :: (Show a) => Forest a -> IO () printForest = putStr . drawForest . (fmap . fmap) show -------------------------------------------------------- {- ghci> printTree $ tyConTree x0 (,) | +- (,) | | | +- Integer | | | `- (,) | | | +- Integer | | | `- (,) | | | +- (,) | | | | | +- Integer | | | | | `- Integer | | | `- Integer | `- (,) | +- Integer | `- (,) | +- Integer | `- Integer ghci> printTree $ typeTree_a x0 TupleT 2 | +- TupleT 2 | | | +- ConT Integer | | | `- TupleT 2 | | | +- ConT Integer | | | `- TupleT 2 | | | +- TupleT 2 | | | | | +- ConT Integer | | | | | `- ConT Integer | | | `- ConT Integer | `- TupleT 2 | +- ConT Integer | `- TupleT 2 | +- ConT Integer | `- ConT Integer ghci> printTree $ typeTree_b x0 TupleT 2 | +- TupleT 2 | | | +- ConT Integer | | | `- TupleT 2 | | | +- ConT Integer | | | `- TupleT 2 | | | +- TupleT 2 | | | | | +- ConT Integer | | | | | `- ConT Integer | | | `- ConT Integer | `- TupleT 2 | +- ConT Integer | `- TupleT 2 | +- ConT Integer | `- ConT Integer -} --------------------------------------------------------

1 0

What monad am I in?
by Henry Laxen 03 Sep '08

03 Sep '08

Dear Group, When I fire up ghci and define: increment x = return (x+1) I can say: Main> increment 1 and ghci dutifully replies 2. Also as expected, the type signature of increment is: (Num a, Monad m) => a -> m a However, if I say: Main> let a = increment 1 I get: <interactive>:1:8: Ambiguous type variable `m' in the constraint: `Monad m' arising from a use of `increment' at <interactive>:1:8-18 Probable fix: add a type signature that fixes these type variable(s) Have I, like Monsier Jourdain, been running in the IO monad all my life, and didn't even know it? Thanks, Henry Laxen

6 5

ANN: zip-archive 0.0
by John MacFarlane 03 Sep '08

03 Sep '08

I've written a library, zip-archive, for dealing with zip archives. Haddock documentation (with links to source code): http://johnmacfarlane.net/zip-archive/ Darcs repository: http://johnmacfarlane.net/repos/zip-archive/ It comes with an example program that duplicates some of the functionality of 'zip' (configure with '-fexecutable' to build it). I intend to put it on HackageDB, but I thought I'd get some feedback first. Bug reports, patches, and suggestions on the API are all welcome. John

3 3

Arrow without `>>>'
by Valery V. Vorotyntsev 03 Sep '08

03 Sep '08

Hi, friends, I've built GHC from darcs, and... Could anybody tell me, what's the purpose of Arrow[1] not having `>>>' method? 1. http://darcs.haskell.org/packages/base/Control/Arrow.hs $ ghci GHCi, version 6.9.20080104: http://www.haskell.org/ghc/ :? for help Loading package base ... linking ... done. Prelude> :m + Control.Arrow Prelude Control.Arrow> :i Arrow class (Control.Category.Category a) => Arrow a where arr :: (b -> c) -> a b c pure :: (b -> c) -> a b c first :: a b c -> a (b, d) (c, d) second :: a b c -> a (d, b) (d, c) (***) :: a b c -> a b' c' -> a (b, b') (c, c') (&&&) :: a b c -> a b c' -> a b (c, c') -- Defined in Control.Arrow instance Arrow (->) -- Defined in Control.Arrow instance (Monad m) => Arrow (Kleisli m) -- Defined in Control.Arrow Prelude Control.Arrow> Leaving GHCi. I can't build[2] arrows-0.3 package without `>>>' in Arrow. 2. vvv@fun:~/src/arrows$ darcs w { hunk ./Control/Arrow/Operations.hs 36 +import Control.Category ((>>>)) hunk ./Control/Arrow/Transformer/CoState.hs 23 +import Control.Category ((>>>)) } vvv@fun:~/src/arrows$ runhaskell Setup build Preprocessing library arrows-0.3... Building arrows-0.3... [ 3 of 12] Compiling Control.Arrow.Transformer.CoState ( Control/Arrow/Transformer/CoState.hs, dist/build/Control/Arrow/Transformer/CoState.o ) Control/Arrow/Transformer/CoState.hs:29:7: `>>>' is not a (visible) method of class `Arrow' The question arises "should I?", but this is one of lambdabot's[3] depencies. 3. http://code.haskell.org/lambdabot/ Thanks a lot! -- vvv

3 3

ICFP09 Announcement
by Matthew Fluet (ICFP Publicity Chair) 02 Sep '08

02 Sep '08

+--------------------------------------------------------------------+ ANNOUNCEMENT The 14th ACM SIGPLAN International Conference on Functional Programming ICFP 2009 31st August - 2nd September 2009 Edinburgh, United Kingdom ICFP provides a forum for researchers and developers to hear about the latest work on the design, implementations, principles, and uses of functional programming. ICFP 2009 will be held in Scotland's historic capital city of Edinburgh, during the final week of the Edinburgh International Festival. Further information is available from: http://www.cs.nott.ac.uk/~gmh/icfp09.html Graham Hutton General Chair, ICFP 2009 +--------------------------------------------------------------------+ | Dr Graham Hutton Email : gmh(a)cs.nott.ac.uk | | Functional Programming Lab | | School of Computer Science Web : www.cs.nott.ac.uk/~gmh | | University of Nottingham | | Jubilee Campus, Wollaton Road Phone : +44 (0)115 951 4220 | | Nottingham NG8 1BB, UK | +--------------------------------------------------------------------+

1 0

REMINDER: OpenSPARC project application deadline this Friday
by Duncan Coutts 02 Sep '08

02 Sep '08

http://haskell.org/opensparc/ The deadline for applications for the Haskell OpenSPARC project is rapidly approaching. Applications have to be sent to: opensparc(a)community.haskell.org by the end of this week, Friday the 5th September. If you want any comments on your application before you submit it then contact me directly. Duncan (project coordinator) In other news, the server that Sun Microsystems donated to the community has just gone online at Chalmers: http://blog.well-typed.com/2008/09/the-new-haskellorg-community-sparc-serve…

1 0

Re: [Haskell] The initial view on typed sprintf and sscanf
by Matthew Brecknell 02 Sep '08

02 Sep '08

oleg [1]: > We demonstrate typed sprintf and typed sscanf sharing the same > formatting specification. [1]http://www.haskell.org/pipermail/haskell/2008-August/020605.html Reading Oleg's post, I noticed that it is quite straightforward to generalise printing to arbitrary output types. > class Monoid p => Printf p where > printChar :: Char -> p > --etc > > intp :: Printf p => F a b -> (p -> a) -> b > intp FChr k = \c -> k (printChar c) > intp (a :^ b) k = intp a (\sa -> intp b (\sb -> k (mappend sa sb))) > --etc > > printf :: Printf p => F p b -> b > printf fmt = intp fmt id The Printf instances for String, ShowS, ByteStrings, and IO () are all trivial. Printing directly to a file (without building an intermediate string) is slightly more interesting: > instance Monad m => Monoid (ReaderT r m ()) where > mempty = return () > mappend = (>>) > > instance Printf (ReaderT Handle IO ()) where > printChar c = ask >>= \h -> liftIO (hPutChar h c) > --etc > > (<<) :: Handle -> ReaderT Handle IO () -> IO () > (<<) = flip runReaderT > > test_fprintf h = h << printf (lit "Hello " ^ lit "World" ^ chr) '!' Unfortunately, I don't seem to be able to make the expected fprintf function, because printf's format-dependent parameter list makes it impossible to find a place to pass the handle. Hence the C++-like (<<) ugliness.

5 5

Fast parallel binary-trees for the shootout: Control.Parallel.Strategies FTW!
by Don Stewart 02 Sep '08

02 Sep '08

The Computer Language Benchmarks Game recently got a quad core 64 bit machine. Initial results just porting the single-threaded Haskell benchmarks to 64 bit look rather pleasing, http://shootout.alioth.debian.org/u64q/benchmark.php?test=all&lang=all Note that C++, D, Clean et al aren't ported to the 64 bit machine yet (and perhaps some implementations won't survive the 64 bit switch). Now, none of the Haskell programs are annotated for parallelism yet, despite that quad core sitting there ready to go. I've a conjecture that fast, smp capable, concise code, of the kind GHC produces, could dominate the shootout for a while to come, if we can effectively utilise those cores. This is a chance to make an impression! So, the first parallel benchmark has been submitted by Tom Davies and me, a parallel implementation of the binary-trees benchmark, using the Control.Parallel.Strategies lib to set up some top level parallel tasks. Initial results look promising, with the old entry: * Single core, no heap hints: 26.9s $ ghc -O2 -fasm Original.hs --make $ time ./Original 20 ./Original 20 26.79s user 0.16s system 100% cpu 26.933 total Looking at the GC stats (-sstderr) we see its spending too much time doing GC, 57.8%, so we can improve that by setting a default heap size: * Single core, with heap hints: 13.7s $ time ./Original 20 -A350M ... ./Original 20 +RTS -A350M 13.54s user 0.27s system 100% cpu 13.791 total Halves runtime in comparison to the current entry, and reduces GC to 3.8%. * Now, the parallel version, changing one line to use a parMap strategy, and using the N+1 capabilities rule, $ ghc -O2 -fasm Parallel2.hs --make -threaded $ time ./Parallel2 20 +RTS -N5 -A350M ... ./Parallel2 20 +RTS -N5 -A350M 15.38s user 1.57s system 305% cpu 5.684 total So 305% productive (not too bad for this memory-heavy program), and a good speedup. Observations: * check GC stats with -sstderr, then use -AxxxM to set a default heap size * parallelise the top level control using speculative strategies * Use -N+1 capabilities. -- Don

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getting Quickcheck 2.0
by Wolfgang Jeltsch 02 Sep '08

02 Sep '08

Hello, is there a reason why Quickcheck 2.0 isn’t released via Hackage? Does anybody know where to get it? The darcs repository already has 2.1 as the version number in the Cabal file but Hackage still only has 1.1.0.0. Best wishes, Wolfgang

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Haskell Propeganda
by Niels Aan de Brugh 02 Sep '08

02 Sep '08

On Sun, Aug 24, 2008 at 5:44 AM, Thomas M. DuBuisson < thomas.dubuisson(a)gmail.com> wrote: > wrt head [], Niels said: > > So now what? Action plan = [] > > Oh come now. Between ghci, hpc, and manual analysis I've never hit a > Haskell error and thrown my hands up, "I can't go any further, I'm at a > complete loss!" Also it helps that I run into this extremely rarely - I > have a larger habit of hidding black holes :-( I've never used Haskell in an engineering environment, but I have encountered problems that would've been easier to analyze with a step function (almost without exception in a stateful computation, e.g. functions using recursion or some state monad). For example, a graphical frond-end to the GHCi debugger would've been really helpful, especially for new users. C/C++ programmers on Linux have gdb but I know nobody that uses the tool directly on the bare command line. Niels said: > > Thank you for the URL, but I'm aware of the work in GHC(i). > > It might interest you to know some people actually use hpc for debugging > with reasonble success - it colors the unevaluated sections and this can > be very helpful in determining where a program stopped and threw an > exception. I haven't used HPC myself (I use -fwarn-incomplete-patterns but that's about it for "coverage" analysis) but it looks like a powerful tool. Statement and branch coverage are very imporant metrics e.g. to measure test coverage. However, I feel that using a profiler to do a debuggers job is really a poor man's solution. Just like the trace function (tracing/logging itself is useful, but not as a debugger tool). > If you have your eyes on the future you should see Dana Xu's work on > static contract checking (check out her Cambridge page). Thanks for the suggestion, I'll be sure to check that out. I use design by contract on a daily basis, but in a language like C++ it's never more than a couple of glorified asserts. My opinion is that if you want to do contracts good (i.e. not with some macro kludge) it needs to be embedded into the language. I've been looking at Spec#, that looks like a very powerful extension to C#. I think contracts could fit even better in a pure functional language. Checking contracts statically would be awesome, but doing dynamic DbC well is already harder than it seems (e.g. in concurrent code that uses locks). Regards, Niels

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cabal update : getHostByName
by minh thu 01 Sep '08

01 Sep '08

Hi, I've followed the instruction from the upcoming book Real World Haskell to install cabal. Now, when I enter "cabal update", it answers this: Downloading package list from server 'http://hackage.haskell.org/packages/archive' cabal: getHostByName: does not exist (no such host entry) What's the problem ? Thanks a lot, Thu

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Types in bounds
by Brettschneider, Matthias 01 Sep '08

01 Sep '08

Hey there, i was just thinking about types in Haskell and I asked myself, Is there a mechanism, that allows me to define a new type out of an existing one with some restrictions? So that the new type is a subset of the existing one. Lets imagine I want a type for a point like: type Point = (Int, Int) But what, if I can predict that the X and Y values are in a range between 0 and 100? If someone defines a new point like: p :: Point p = (200, 400) that is totally fine with the type definition above, but not correct in my context. Is it possible, to put this information into the type? So that the compiler is able to recognize my mistake? -- Matthias

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Re: [Haskell-cafe] Re: Research language vs. professional language
by Sean Leather 01 Sep '08

01 Sep '08

> >> > This concept of `day-to-day work' is a curious one. Haskell is not a > >> > mature language, and probably shouldn't ever be one. > >> > >> I see where you are coming from here, but I think that train has > >> already started and can't be stopped. > > > > Yeah, it's too late. Too many people have their pay checks riding on > > GHC, the Hackage library set (now up to 740 libraries and tools!), and > > the continued development of the language in general. > > > > If Haskell's not "mature" yet, then perhaps it has reached its early > > twenties, with an reliable heavy duty optimizing compiler, fast runtime, > > large library set, standard documentation, testing, debugging and > > packaging tools, and large community. > > > > We're serious about this thing. > > So, what fills its shoes as a great research language with great tools? > I think the great thing about Haskell is that it works so well now and yet there's room for growth as well. That means that not only is research with the language more efficient, but also that the research tends to be more applicative to people using it. Consider, for example, the fact that the Haskell Workshop has grown into a symposium [1] and that there are so many relevant workshops affiliated with ICFP [2]. [1] http://www.haskell.org/haskell-symposium/ [2] http://www.icfpconference.org/#Affiliated Sean

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Re: [Haskell-cafe] language proposal: ad-hoc overloading
by Philippa Cowderoy 01 Sep '08

01 Sep '08

Oops, forgot to send to list. On Mon, 2008-09-01 at 01:27 +0100, Philippa Cowderoy wrote: > On Mon, 2008-09-01 at 01:11 +0100, David House wrote: > > 2008/8/31 Ryan Ingram <ryani.spam(a)gmail.com>: > > > My proposal is to allow "ad-hoc" overloading of names; if a name is > > > ambiguous in a scope, attempt to type-check the expression against > > > each name. It is only an error if type-checking against all names > > > fails. If type-checking succeeds for more than one then the > > > expression is ambiguous and this is also an error. > > > > -1, at least for now. > > > > Haskell already has one method of overloading: type classes. What you > > propose is a seemingly innocent extension that I now doubt has > > extremely far-reaching consequences into the language. Such a feature > > should be properly researched before it is added to the language. > > Here's an example of such a concern: you write the following: > > > > import Data.Map > > foo = map > > > > What is the type of `foo'? > > If I've understood the proposal, that wouldn't check unless foo was used > in a way (within the module) that makes it clear which of Prelude.map or > Data.Map.map to use and uses it consistently. So if that's the entire > module, it wouldn't check. > > I'm still not keen, but in a sense the idea here is to avoid this > variety of overloading leaking into the type system itself: it could be > implemented (extremely naively) as "typecheck with each possible > permutation for resolving the overloading, use any unique solution or > fail if there isn't one". -- Philippa Cowderoy <flippa(a)flippac.org>

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ANN: wavconvert 0.1.1
by Tim Chevalier 01 Sep '08

01 Sep '08

Hi all, I uploaded to Hackage a little program I wrote to organize my music file collection in the hopes that someone else might find it useful too: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/wavconvert I had a directory tree full of .wav files that I ripped from CDs at times when I didn't want to wait for an .mp3 or .ogg encoder to run. The trouble is that .wav files don't have metadata (ID3 tags) that specify artist, album and track names. So, wavconvert takes a directory tree as an argument, and converts any .wav files in it to .ogg (using an external OGG encoder) while filling in the ID3 tags based on the directory names. The name is a bit misleading, since this is not an OGG encoder written in Haskell; it just handles invoking an existing encoder with the right flags to set the metadata. I'd welcome any patches to make this more generally useful (as it is, it assumes a particular file structure and OGG encoder.) Cheers, Tim -- Tim Chevalier * http://cs.pdx.edu/~tjc * Often in error, never in doubt "Research is what I'm doing when I don't know what I'm doing."--Wernher von Braun

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Re: [Haskell] Compiler Construction course using Haskell?
by Krasimir Angelov 01 Sep '08

01 Sep '08

Hi Johannes, There is a similar course in Chalmers. The home page is here: http://www.cs.chalmers.se/Cs/Grundutb/Kurser/progs/current/ There students were required to implement full parser, partial typechecker and partial interpreter for C++ in either C++, Java or Haskell. We used BNFC for the parser: http://www.cs.chalmers.se/Cs/Research/Language-technology/BNFC/ which was easier for most students I think. In any way I would recomend Happy+Alex or BNFC instead of Parsec but this is only my personal preference. Best Regards, Krasimir 2008/8/20 Johannes Waldmann <waldmann(a)imn.htwk-leipzig.de>: > Hello. > > I plan to give a course in compiler construction, > using Haskell as the implementation language > (not as source or target language). > > Something along these lines: > 1. combinator parsers (Parsec), > 2. simple interpreter (arithmetical expressions) > 3. add algebraic data types, functions > 4. type checker > 5. code generator. > Ideally, 2..5 would be using the very same tree traversal code > and just change the monad for evaluation. > > Any comments appreciated. Have you given such a course? Taken? > > If I really decide to do it, > then slides (in German) will be made available. > > Best regards, J.W. > > > _______________________________________________ > Haskell mailing list > Haskell(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell > >

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Re: [Haskell-cafe] Re: [Haskell] Top Level <-
by Adrian Hey 31 Aug '08

31 Aug '08

Ganesh Sittampalam wrote: > On Sun, 31 Aug 2008, Adrian Hey wrote: > >> Thanks for taking the time to do this Dan. I think the safety >> requirement has been met, but I think it fails on the improved API. >> The main complaint would be what I see as loss of modularity, in that >> somehow what should be a small irrelevant detail of the implementation >> of some obscure module somewhere has propogated it's way all the way >> upto main. > > That's the key point, as I see it - they aren't "irrelevant details of > the implementation", they are requirements the implementation places on > its context in order for that implementation to be correct. So they > should be communicated appropriately. Eh? Please illustrate your point with Data.Unique. What requirements does it place on it's context? (whatever that might mean :-) It just does what it says on the tin AFAICS. There are no requirements it places on "clients" (to use an OO term), as should any halfway decent API IMO. >> To me this seems completely at odds with what I thought was generally >> accepted wisdom of how to write good maintainable, modular software. >> Namely hiding as much implemention detail possible and keeping APIs >> as simple and stable as they can be. I don't know if I'm alone in >> that view nowadays. > > It's no problem to hide implementation detail, but I don't think you > should hide the *requirement* of the implementation that it has > constraints on how it is called, namely that it requires once-only > initialisation or whatever. No decent API should require this. Data.Unique certainly doesn't. In fact is debatable whether any API should requre an initalisation call at all before other stuff should be called (the other stuff check initialisation has occured and do it itself if necessary). The real irony of your remark is that making APIs this robust is practically impossible *without* using global variables, and you're now saying that because they've done this work to eliminate these constraints they now have to be held to account for this with an absurd API. Seems like a clear case of "no good deed going unpunished" :-) Regards -- Adrian Hey

1 0

Calling Lockheed, Indra, Thales, Raytheon
by Paul Johnson 31 Aug '08

31 Aug '08

This is a strange question, I know, but is there anyone working in any of the above companies on this mailing list? Everyone will no doubt be wondering what they have in common. I'm afraid I can't discuss that. Paul.

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idea: TH calculating type level function results / binary types?
by Marc Weber 31 Aug '08

31 Aug '08

Hi, Maybe you've noted that I've started writing an XML library which validates the generated XML against a given DTD at compile time. Not everything is implemented right now and it would be quite usable if it didn't take that much time. The problem: To see some details have look at my small announcement on haskellcafe. type checking 12 lines of XML code has taken > 3hourse using some TypeToNat type equality comparison > 35sec using an implementation proposed by Oleg > 30sec no longer using HLists to check wether an attribute may be added but an AttrOk tag attr class (proposed by Oleg) > ?sec after finishing a state transformation implementation propably generating thausands of intermediate state or a mixture of this and the parser like apporach. Even if the last approach works it's a pity because: * it took much time to write * hard to read code * no error messages.. In the 30sec apprach I've used some instances such as class Consume elType state state' | elType state -> state' class FailHelper state' state'' | state' -> state'' instance FailHelper (Fail a) => FailHelper (F a) () -- F indicating sub element could not be consumed, failure instance FailHelper a a -- So I got error messages telling me no instance for MoreElementsExpected (Or (E Html_T) (E Body_T)) or such.. that's nice and usable. I can't think of a nice way reporting errors when transforming a dtd line (a,b,c) (= tag sequence a b c) into data State1 data State2 data State3 instance Conseme State1 A_T State2 -- consume tag a instance Conseme State2 B_T State3 -- consume tag b instance Conseme State3 C_T ConsumedEnd -- consume tag c in a convinient way without adding much bloat. The XHMTL spec has about 150 ! tags.. So even if creating this kind of state transforming instances this will result in a lot of bloat. So if the compiler has to load a some MB large .hi file it will spend unnecessary time just loading them. an idea: (new?) solution: What about enhancing ghc so that a) you can define type level functions and calculate the result using template haskell functions: benefits: * speed (it can even be compiled! ) * nice error messages on failure * mantainable code * reusing known source (such as parser combinator libraries) syntax could look like this: class Bar a b c d | a b -> c, a b -> d instance (Maybe x) z $(calculateC) $(calculateD) calculateC :: Type -> Type -> Type calculateD :: Type -> Type -> Type an efficient implementation for TypeEq a b typebool | a b -> typebool could be done by everyone with ease without reading HList source or Olegs papers.. (you should do so anyway ..) b) add some binary serialization support so that you don't have to do expensive data -> type level -> data (de)serializations. .hi files wouldn't blow much this way as well. So a parse specification such as (1) could really be included within types and could become even more complex. Than an exisiting parser combinator library could be used and a validating XML library could be written within some hours.. I think ghc is already a superiour compiler because it allows such advanced technics thereby making users such as me even ask for more :-) I consider this a good sign. So has anyone thought about this before? Would someone help / guide me implementing this extension in the near future? Of course it will clash with some instance -X extensions.. but I think that in everyday use you'd probably use either this template haskell approach or use normal class instance declarations so it I guess it could be handled. Maybe this does already exist in one or the other way? Sincerly Marc Weber (1) (Seq (Elem Title_T) (Seq (Star (Or (Elem Script_T) (Or (Elem Style_T) (Or (Elem Meta_T) (Or (Elem Link_T) (Elem Object_T)))))) (Query (Seq (Elem Base_T) (Star (Or (Elem Script_T) (Or (Elem Style_T) (Or (Elem Meta_T) (Or (Elem Link_T) (Elem Object_T))))))))))

1 0

Discussing FFI
by Maurício 31 Aug '08

31 Aug '08

Hi, I saw the (a?) page about FFI on Haskell: http://www.cse.unsw.edu.au/~chak/haskell/ffi It shows a link to a mailing list but, checking the list archives at: http://www.haskell.org/pipermail/ffi , it seems it's dead since 2007. Is there a new list about FFI? Where should I discuss my begginer issues about it? Thanks, Maurício

2 1

Parsec and network data
by brian 31 Aug '08

31 Aug '08

Hi, I've been struggling with this problem for days and I'm dying. Please help. I want to use Parsec to parse NNTP data coming to me from a handle I get from connectTo. One unworkable approach I tried is to get a lazy String from the handle with hGetContents. The problem: suppose the first message from the NNTP server is "200 OK\r\n". Parsec parses it beautifully. Now I need to discard the parsed part so that Parsec will parse whatever the server sends next, so I use Parsec's getInput to get the remaining data. But there isn't any, so it blocks. Deadlock: the client is inappropriately waiting for server data and the server is waiting for my first command. Another approach that doesn't quite work is to create an instance of Parsec's Stream with timeout functionality: instance Stream Handle IO Char where uncons h = do r <- hWaitForInput h ms if r then liftM (\c -> Just (c, h)) (hGetChar h) else return Nothing where ms = 5000 It's probably obvious to you why it doesn't work, but it wasn't to me at first. The problem: suppose you tell parsec you're looking for (many digit) followed by (string "\r\n"). "123\r\n" won't match; "123\n" will. My Stream has no backtracking. Even if you don't need 'try', it won't work for even basic stuff. Here's another way: http://www.mail-archive.com/haskell-cafe@haskell.org/msg22385.html The OP had the same problem I did, so he made a variant of hGetContents with timeout support. The problem: he used something from unsafe*. I came to Haskell for rigor and reliability and it would make me really sad to have to use a function with 'unsafe' in its name that has a lot of wacky caveats about inlining, etc. In that same thread, Bulat says a timeout-enabled Stream could help. But I can't tell what library that is. 'cabal list stream' shows me 3 libraries none of which seems to be the one in question. Is Streams a going concern? Should I be checking that out? I'm not doing anything with hGetLine because 1) there's no way to specify a maximum number of characters to read 2) what is meant by a "line" is not specified 3) there is no way to tell if it read a line or just got to the end of the data. Even using something like hGetLine that worked better would make the parsing more obscure. Thank you very very much for *any* help.

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FunGEn
by Henk-Jan van Tuyl 31 Aug '08

31 Aug '08

L.S., I found the Functional Game Engine FunGEn on the web: http://www.cin.ufpe.br/~haskell/fungen/download.html It looks useful, but since it hasn't been maintained for a long time, it doesn't compile. Is there a newer version I can download? -- Regards, Henk-Jan van Tuyl -- http://functor.bamikanarie.com http://Van.Tuyl.eu/ --

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Haskell Weekly News: Issue 83 - August 30, 2008
by Brent Yorgey 31 Aug '08

31 Aug '08

--------------------------------------------------------------------------- Haskell Weekly News http://sequence.complete.org/hwn/20080830 Issue 83 - August 30, 2008 --------------------------------------------------------------------------- Welcome to issue 83 of HWN, a newsletter covering developments in the [1]Haskell community. This is the "better late than never" edition. As an excuse I could tell you that my home internet service has been horrible (now fixed) and I was away from home for a few days with my wife celebrating our third wedding anniversary. But instead, I give you a link to the [2]Uncyclopedia entry on Haskell. If you haven't already seen it, you should give it a read, being sure not to drink any milk at the same time, or at least pointing your nose away from the keyboard if you insist on drinking milk. Community News If Dell sends John Goerzen (CosmicRay) one more catalog, it will [3]actually be a federal crime. Announcements LogFloat 0.9. wren ng thornton [4]announced a new official release of the [5]logfloat package for manipulating log-domain floating numbers. This release is mainly for those who are playing with Transfinite rather than LogFloat, but the interface changes warrant a minor version change. validating xml lib - need some guidance. Marc Weber [6]asked for help developing an xml generating library validating the result against a given DTD. gsl-random 0.1 and monte-carlo-0.1. Patrick Perry [7]announced that he has started on [8]bindings for the random number generators and random distributions provided by the gsl. He has also written a [9]monad and transformer for doing monte carlo computations that uses gsl-random internally. For a quick tutorial in the latter package, see [10]his blog. Wired 0.1.1. Emil Axelsson [11]announced the first release of the hardware description library [12]Wired. Wired can be seen as an extension to Lava that targets (not exclusively) semi-custom VLSI design. A particular aim of Wired is to give the designer more control over the routing wires' effects on performance. darcs weekly news #1. Eric Kow [13]sent out the first edition of the new Darcs Weekly News! zip-archive 0.0. John MacFarlane [14]announced the release of the [15]zip-archive library for dealing with zip archives. The Monad.Reader - Issue 11. Wouter Swierstra [16]announced a new issue of [17]The Monad.Reader, with articles by David Place, Kenn Knowles, and Doug Auclair. First Monad Tutorial of the Season. Hans van Thiel [18]announced a new monad tutorial, [19]The Greenhorn's Guide to becoming a Monad Cowboy. "Real World Haskell" hits a milestone. Bryan O'Sullivan [20]proudly announced that the draft manuscript of Real World Haskell is [21]complete! It is now available online in its entirety. The authors expect the final book to be published around the beginning of November, and to weigh in at about 700 pages. Mueval 0.5.1, 0.6, 0.6.1, 0.6.2, 0.6.3, 0.6.4. Gwern Branwen [22]announced a number of new releases of [23]Mueval. Lambdabot now uses mueval for all its dynamic Haskell evaluation needs. Hoogle Database Generation. Neil Mitchell (ndm) [24]announced that a new release of the [25]Hoogle command line is out, including bug fixes and additional features. Upgrading is recommended.Two interesting features of Hoogle 4 are working with multiple function databases (from multiple packages), and running your own web server. Blog noise [26]Haskell news from the [27]blogosphere. * Real-World Haskell: [28]Source handed over to production. * Douglas M. Auclair (geophf): [29]Earning \bot-Trophies. * Douglas M. Auclair (geophf): [30]Scanner-parsers II: State Monad Transformers. * Douglas M. Auclair (geophf): [31]Scanner-parsers I: lifting functions. * >>> software engineering radio: [32]Episode 108: Simon Peyton Jones on Functional Programming and Haskell. A podcast interview with Simon Peyton Jones. * Neil Mitchell: [33]Running your own Hoogle on a Web Server. * Braden Shepherdson: [34]Announcing xmonad-light. Braden is rolling out a new configuration framework for xmonad, providing an easier learning curve for those not wanting to learn Haskell right away, and an easy transition to a more powerful Haskell configuration when they want it. * Gabor Grief: [35]Category. Gabor is excited that base-3.0 will include Control.Category. * Douglas M. Auclair (geophf): [36]Ten = 1+2+3+4. Solving an arithmetic puzzle with Haskell, Prolog-style. * Paul R Brown: [37]perpubplat now on github. * Douglas M. Auclair (geophf): [38]"Lucky you!"?. Doug shares some secrets of his success in getting Haskell/Dylan/Mercury/Prolog jobs. * Neil Mitchell: [39]Hoogle Database Generation. Neil releases a new command-line version of Hoogle, including bug fixes and additional features -- multiple function databases and the ability to run your own Hoogle server. * Patrick Perry: [40]A Monte Carlo Monad for Haskell. * Mads Lindstroem: [41]Proposal: Adding composability to WxHaskell. * Tom Moertel: [42]PXSL Tools now on Hackage and GitHub. * Gabor Greif: [43]Uploaded. Gabor uploads his first contribution to Hackage, [44]thrist-0.1. * Muad`Dib (vixey): [45]Tail Call Optimization doesn't exist in Haskell. * >>> Hans van Thiel: [46]The Greenhorn's Guide to becoming a Monad Cowboy. YAMT * Dan Piponi (sigfpe): [47]Untangling with Continued Fractions: Part 2. Dan continues his excellent series on rational tangles, this time showing the connection between tangles and rational numbers. * >>> Praki Prakash: [48]Learning Haskell Redux. * Gabor Greif: [49]Ca(ni)balized. Gabor creates his first cabal package! * Real-World Haskell: [50]Our writing is now complete!. * Tom Moertel: [51]Thinking algebraically: a functional-programming dividend that pays during your imperative-programming day job. * Jason Dagit: [52]Darcs 2 Real-World Push Performance Evaluation. * Well-Typed.Com: [53]What's wrong with make?. A lot, apparently: it's too static, and makes it easy to write incorrect rules. Duncan (dcoutts) provides an analysis. * Dougal Stanton: [54]Lord of the Flies as a window onto monadic IO. * Real-World Haskell: [55]A tighter page count estimate. Real World Haskell is going to be about 700 pages!? * Real-World Haskell: [56]Real World Haskell tutorial next month at DEFUN 2008. Quotes of the Week * shepheb: #haskell isn't so much on-topic when discussing Haskell, but off-off-topic. * chrisdone: it's neat how you learn haskell because you are drawn in by the purely functional paradigm, and then you find loads more things like algebraic data types, monad abstractions, arrows and applicative, lack of objects... so that when people say "well, it's not haskell, but at least X is functional", it's just not the same at all * lambdabot: [tristes_tigres] @vixen unsafe [lambdabot] you're turning me on :) * b\6: sometimes i make variables f and ck and find some reason to multiply them like f*ck if i'm having a bad day. * waynemokane: wow... thanks everyone - it looks like I have a full day of reading type signatures ahead of me. * mauke: call/cthulhu * seydar: monads are just like saran wrap. * b\6: keep this info private, but you can actually overclock your brain. the technique i use is to loop mplayer playing oggs but increase the speed like -speed 1.25 for 125% normal. your brain's speed increases accordingly, allowing you to solve problems much more easily. About the Haskell Weekly News New editions are posted to [57]the Haskell mailing list as well as to [58]the Haskell Sequence and [59]Planet Haskell. [60]RSS is also available, and headlines appear on [61]haskell.org. To help create new editions of this newsletter, please see the information on [62]how to contribute. Send stories to byorgey at cis dot upenn dot edu. The darcs repository is available at darcs get [63]http://code.haskell.org/~byorgey/code/hwn/ . References 1. http://haskell.org/ 2. http://uncyclopedia.org/wiki/Haskell 3. http://changelog.complete.org/posts/741-From-Dell,-a-Uniquely-Terrible-Expe… 4. http://article.gmane.org/gmane.comp.lang.haskell.cafe/43829 5. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/logfloat 6. http://article.gmane.org/gmane.comp.lang.haskell.cafe/43825 7. http://article.gmane.org/gmane.comp.lang.haskell.cafe/43764 8. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/gsl-random 9. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/monte-carlo 10. http://quantile95.com/2008/08/27/a-monte-carlo-monad-for-haskell/ 11. http://article.gmane.org/gmane.comp.lang.haskell.cafe/43658 12. http://www.cs.chalmers.se/~emax/wired/index.html 13. http://article.gmane.org/gmane.comp.lang.haskell.cafe/43626 14. http://article.gmane.org/gmane.comp.lang.haskell.cafe/43539 15. http://johnmacfarlane.net/zip-archive/ 16. http://article.gmane.org/gmane.comp.lang.haskell.cafe/43491 17. http://www.haskell.org/haskellwiki/The_Monad.Reader 18. http://article.gmane.org/gmane.comp.lang.haskell.cafe/43403 19. http://www.muitovar.com/monad/moncow.xhtml 20. http://article.gmane.org/gmane.comp.lang.haskell.cafe/43401 21. http://www.realworldhaskell.org/blog/2008/08/22/our-writing-is-now-complete/ 22. http://article.gmane.org/gmane.comp.lang.haskell.cafe/43372 23. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/mueval 24. http://neilmitchell.blogspot.com/2008/08/hoogle-database-generation.html 25. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/hoogle 26. http://planet.haskell.org/ 27. http://haskell.org/haskellwiki/Blog_articles 28. http://www.realworldhaskell.org/blog/2008/08/30/source-handed-over-to-produ… 29. http://logicaltypes.blogspot.com/2008/08/earning.html 30. http://logicaltypes.blogspot.com/2008/08/scanner-parsers-ii-state-monad.html 31. http://logicaltypes.blogspot.com/2008/08/scanner-parsers-i-lifting-function… 32. http://www.se-radio.net/podcast/2008-08/episode-108-simon-peyton-jones-func… 33. http://neilmitchell.blogspot.com/2008/08/running-your-own-hoogle-on-web-ser… 34. http://braincrater.wordpress.com/2008/08/28/announcing-xmonad-light/ 35. http://heisenbug.blogspot.com/2008/08/category.html 36. http://logicaltypes.blogspot.com/2008/08/ten-1234.html 37. http://mult.ifario.us/p/perpubplat-now-on-github 38. http://logicaltypes.blogspot.com/2008/08/lucky-you.html 39. http://neilmitchell.blogspot.com/2008/08/hoogle-database-generation.html 40. http://quantile95.com/2008/08/27/a-monte-carlo-monad-for-haskell/ 41. http://lindstroem.wordpress.com/2008/03/16/proposal-adding-composability-to… 42. http://blog.moertel.com/articles/2008/08/24/pxsl-tools-now-on-hackage-and-g… 43. http://heisenbug.blogspot.com/2008/08/uploaded.html 44. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/thrist 45. http://muaddibspace.blogspot.com/2008/08/tail-call-optimization-doesnt-exis… 46. http://www.muitovar.com/monad/moncow.xhtml 47. http://sigfpe.blogspot.com/2008/08/untangling-with-continued-fractions_23.h… 48. http://monadict.blogspot.com/2008/08/learning-haskell-redux-when-you-are.ht… 49. http://heisenbug.blogspot.com/2008/08/canibalized.html 50. http://www.realworldhaskell.org/blog/2008/08/22/our-writing-is-now-complete/ 51. http://blog.moertel.com/articles/2008/08/20/thinking-algebraically-a-functi… 52. http://blog.codersbase.com/2008/08/21/darcs-2-real-world-push-performance-e… 53. http://blog.well-typed.com/2008/08/whats-wrong-with-make/ 54. http://www.dougalstanton.net/blog/index.php/2008/08/21/lord-of-the-flies-as… 55. http://www.realworldhaskell.org/blog/2008/08/20/a-tighter-page-count-estima… 56. http://www.realworldhaskell.org/blog/2008/08/20/real-world-haskell-tutorial… 57. http://www.haskell.org/mailman/listinfo/haskell 58. http://sequence.complete.org/ 59. http://planet.haskell.org/ 60. http://sequence.complete.org/node/feed 61. http://haskell.org/ 62. http://haskell.org/haskellwiki/HWN 63. http://code.haskell.org/~byorgey/code/hwn/

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Named field syntax
by Maurício 30 Aug '08

30 Aug '08

Hi, I see that this works: data Test = Test Integer String This also works: data Test = Test {a::Integer,b::String} However, this doesn't: data Test = Test Integer {b::String} Is there some way to name only a single, or a few, of some data type fields? Thanks, Maurício

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ANN: LogFloat 0.9
by wren ng thornton 30 Aug '08

30 Aug '08

-------------------------------------------- -- Announcing: logfloat 0.9.1 -------------------------------------------- New official release of the logfloat package for manipulating log-domain floating numbers. This release is mainly for those who are playing with Transfinite rather than LogFloat, but the interface changes warrant a minor version change. If you're just using the LogFloat stuff you shouldn't notice any changes except that the constructors/destructors may be a bit pickier about being used when a polymorphic return type is expected. -------------------------------------------- -- Changes -------------------------------------------- * John Meacham pointed out that the Num and Ord superclasses on Transfinite may be onerous due to the growing disaffection with the Prelude. The Num requirement has been lifted, and the Ord requirement has been weakened to the new PartialOrd class. * Introduced a new class for partially ordered types: PartialOrd. * The transfinite-value respecting logarithm has been moved from Data.Number.LogFloat to Data.Number.Transfinite because it's more appropriate there. (It's still reexported by Data.Number.LogFloat.) * In the continuing battle between transfinite values and Rationals, I've added a new class RealToFrac for overloading the Prelude's realToFrac function. On GHC, primitive converters are used to improve both performance and correctness. For non-GHC compilers, only a few generic instances are given. For an older discussion on this topic, see: http://www.haskell.org/pipermail/haskell-prime/2006-February/000791.html * Due to all this development on trying to solve the issues of transfinite values, the "stability" of the package has been changed from stable to provisional. -------------------------------------------- -- Future Extensions -------------------------------------------- * A strict version. The current lazy version will move to Data.Number.LogFloat.Lazy, and the strict version will be the standard one in Data.Number.LogFloat * A signed version in Data.Number.LogFloat.Signed * A test suite -------------------------------------------- -- Links -------------------------------------------- Homepage: http://code.haskell.org/~wren/ Hackage: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/logfloat Darcs: http://code.haskell.org/~wren/logfloat Haddock (Darcs version): http://code.haskell.org/~wren/logfloat/dist/doc/html/logfloat/ -- Live well, ~wren

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Syntax of 'do'
by Maurício 30 Aug '08

30 Aug '08

Hi, http://haskell.org/haskellwiki/Keywords says that: ------------- [do is a] syntactic sugar for use with monadic expressions. For example: do { x ; result <- y ; foo result } is shorthand for: x >> y >>= \result -> foo result ------------- I did some tests hiding Prelude.>> and Prelude.>>= and applying >> and >>= to non-monadic types, and saw that 'do' would not apply to them. So, I would like to add the following to that text: ------------- as long as proper types apply: x :: Prelude.Monad a y :: Prelude.Monad b foo :: b -> Prelude.Monad c ------------- Is that correct (Haskell and English)? Thanks, Maurício

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