September 2008 - Haskell-Cafe

GADTs and instances
by Tony Hannan 06 Sep '08

06 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 06 Sep '08

06 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 06 Sep '08

06 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 05 Sep '08

05 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 05 Sep '08

05 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 05 Sep '08

05 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 05 Sep '08

05 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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