July 2011 - Haskell-Cafe

ANN: Monad.Reader Issue 18
by Brent Yorgey 05 Jul '11

05 Jul '11

I am pleased to announce that Issue 18 of The Monad.Reader is now available [1]. Issue 18 consists of the following three articles: * "MapReduce as a Monad" by Julian Porter * "Fun with Parallel Monad Comprehensions" by Tomas Petricek * "Attributed Variables: Their Uses and One Implementation" by Douglas M. Auclair Feel free to browse the source files. You can check out the entire repository using darcs (or simply visit the below URL in your browser): darcs get http://code.haskell.org/~byorgey/TMR/Issue18 Enjoy! [1] http://themonadreader.files.wordpress.com/2011/07/issue18.pdf

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Haskell and Databases
by Tobias Schoofs 05 Jul '11

05 Jul '11

I am studying Haskell database libraries. In particular I am looking at haskelldb and the Takusen Database.Enumerator library. In haskelldb, there are very good ideas aiming to represent database objects as (typeable) Haskell objects, instead of embedding SQL in plain strings. This is really an interesting approach that overcomes the hiatus between the database there and the processing logic here. Unfortunately, the library cannot deal with some obvious issues related to this concept; conflicts arise, for instance, when Haskell keywords are used as names of attributes or tables or when the same name is used for an attribute and for a table (which is perfectly legal in SQL). Also, haskelldb cannot cope with peculiarities of popular database tables, such as ISAM tables - which are case sensitive. Another, more conceptual issue, is the lack of database cursors. Database programs, usually, do not just issue isolated SQL statements, but implement a processing logic with nested queries and DML statements. A DB library should provide control structures to ease this kind of processing. A library that tackles this issue, is the Takusen Database.Enumerator, again, a library with very strong ideas. Unfortunately, in Takusen there is nothing like the haskelldb DSL. Concerning, cursors, Takusen uses iterators to stream query results. This makes a lot of sense. The iterators, however, cannot be used to implement the nesting of queries. The user has to provide an additional action which is responsible for all the cursor-related logic. The tools provided to deal with cursors resemble very much the instructions found in other database-oriented languages, such as PL/SQL. This leads to verbose code where cursor control is implemented in a quite "imperative" way. The user, however, should be concerned with getting her queries right, not with the "technical" details of fetching from cursors. In summary, what I miss is a database library that - integrates haskelldb approach with - a "functional-style" cursor concept Is anybody working on something like this? Thanks, Tobias

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haskell; json-rpc
by Комар Максим 05 Jul '11

05 Jul '11

Apologize in advance for the English language. I'm working with json-rpc interface (http://json-rpc.org/) via http. Requests and responses look like this: -> {"jsonrpc" : "2.0", "id" : "1", "method" : "user.authenticate", "params" : { "user" : "myUser", "password" : "myPassword" }} <- {"jsonrpc":"2.0","result":"97d4b59d2f2fbf703bb7aa257aae5254","id":"1"} -> {"jsonrpc" : "2.0", "id" : "2", "method" : "user.getObjects", "auth" : "97d4b59d2f2fbf703bb7aa257aae5254", "params" : { "alias" : "myUser" }} <- {"jsonrpc":"2.0","result":[{"userid":"3","alias":"myUser","name":"lala", "surname":"lala","passwd":"c5aba41f671a02a3c8a6f10a2a7d8a19", "url":"myUrl","autologin":"1","autologout":"0","lang":"ru_ru", "refresh":"30","type":"1","theme":"css_bb.css","attempt_failed":"0", "attempt_ip":"0.0.0.0","attempt_clock":"1282502966", "rows_per_page":"50"}],"id":"2"} How can i describe in Haskell my task? P.S. The first request is required for authentication (field "auth" in the following queries). Using the second query, I want to know "userid"). P.P.S. My first approach is to describe rpc query as following: data RpcQuery = RpcQuery { rpcVersion :: String, rpcId :: String, rpcMethod :: String, rpcAuth :: String, rpcParams :: AuthUser } deriving (Show) data AuthUser = AuthUser { authUser :: String, authPassword :: String } deriving (Show) instance JSON AuthUser where showJSON au = makeObj [ ("user", showJSON $ authUser au) , ("password", showJSON $ authPassword au) ] readJSON (JSObject obj) = let jsonObjAssoc = fromJSObject obj in do user <- mLookup "user" jsonObjAssoc >>= readJSON password <- mLookup "password" jsonObjAssoc >>= readJSON return $ AuthUser { authUser = user , authPassword = password } readJSON _ = fail "" But this scheme is not extended.

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Haskell Implementors Workshop 2011, Second CFT
by Ben Lippmeier 05 Jul '11

05 Jul '11

Call for Talks ACM SIGPLAN Haskell Implementors' Workshop http://haskell.org/haskellwiki/HaskellImplementorsWorkshop/2011 Tokyo, Japan, September 23rd, 2011 The workshop will be held in conjunction with ICFP 2011 http://www.icfpconference.org/icfp2011/ Important dates Proposal Deadline: 22nd July 2011 Notification: 8th August 2011 Workshop: 23rd September 2011 The Haskell Implementors' Workshop is to be held alongside ICFP 2011 this year in Tokyo, Japan. There will be no proceedings; it is an informal gathering of people involved in the design and development of Haskell implementations, tools, libraries, and supporting infrastructure. This relatively new workshop reflects the growth of the user community: there is a clear need for a well-supported tool chain for the development, distribution, deployment, and configuration of Haskell software. The aim is for this workshop to give the people involved with building the infrastructure behind this ecosystem an opportunity to bat around ideas, share experiences, and ask for feedback from fellow experts. We intend the workshop to have an informal and interactive feel, with a flexible timetable and plenty of room for ad-hoc discussion, demos, and impromptu short talks. Scope and target audience ------------------------- It is important to distinguish the Haskell Implementors' Workshop from the Haskell Symposium which is also co-located with ICFP 2011. The Haskell Symposium is for the publication of Haskell-related research. In contrast, the Haskell Implementors' Workshop will have no proceedings -- although we will aim to make talk videos, slides and presented data available with the consent of the speakers. In the Haskell Implementors' Workshop we hope to study the underlying technology. We want to bring together anyone interested in the nitty gritty details necessary to turn a text file into a deployed product. Having said that, members of the wider Haskell community are more than welcome to attend the workshop -- we need your feedback to keep the Haskell ecosystem thriving. The scope covers any of the following topics. There may be some topics that people feel we've missed, so by all means submit a proposal even if it doesn't fit exactly into one of these buckets: * Compilation techniques * Language features and extensions * Type system implementation * Concurrency and parallelism: language design and implementation * Performance, optimisation and benchmarking * Virtual machines and run-time systems * Libraries and Tools for development or deployment Talks ----- At this stage we would like to invite proposals from potential speakers for a relatively short talk. We are aiming for 20 min talks with 10 mins for questions and changeovers. We want to hear from people writing compilers, tools, or libraries, people with cool ideas for directions in which we should take the platform, proposals for new features to be implemented, and half-baked crazy ideas. Please submit a talk title and abstract of no more than 200 words to benl(a)cse.unsw.edu.au We will also have a lightning talks session which will be organised on the day. These talks will be 2-10 minutes, depending on available time. Suggested topics for lightning talks are to present a single idea, a work-in-progress project, a problem to intrigue and perplex Haskell implementors, or simply to ask for feedback and collaborators. Organisers ---------- * Rebekah Leslie (Portland State University) * Ben Lippmeier - co-chair (University of New South Wales) * Andres Loeh (Well-Typed LLP) * Oleg Lobachev (University of Marburg) * Neil Mitchell - co-chair (Standard Chartered) * Dimitrios Vytiniotis (Microsoft Research)

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ANNOUNCE: doctest-0.4.0
by Simon Hengel 04 Jul '11

04 Jul '11

I just uploaded a new version of doctest[1] to Hackage. WHAT IS doctest? ================ doctest is a port of Python's doctest[2] to Haskell. It can be used to verify, that examples in Haddock comments[3] do still work. This also provides you with a simple mechanism to write unit test, without the burden of maintaining a dedicated test suite. A basic example of usage is at [4]. WHAT'S NEW IN THIS VERSION? =========================== doctest's functionality is now exposed as a library, this allows for integration with existing test frameworks. Sakari Jokinen is working on integration with test-framework[5]. Thanks to Sakari Jokinen for both, his contributions to the doctest API and his work on test-framework-doctest! Cheers, Simon [1] http://hackage.haskell.org/package/doctest [2] http://docs.python.org/library/doctest.html [3] http://www.haskell.org/haddock/doc/html/ch03s08.html#id566093 [4] https://github.com/sol/doctest-haskell#readme [5] https://github.com/sakari/test-framework-doctest

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Haskell Platform on Ubuntu: Installation Problems
by Dmitri O.Kondratiev 04 Jul '11

04 Jul '11

Hi, I am trying to install Haskell Platform on latest Ubuntu desktop: ( uname -a: Linux frigate 2.6.38-8-generic #42-Ubuntu SMP Mon Apr 11 03:31:24 UTC 2011 x86_64 x86_64 x86_64 GNU/Linux ) I started with installing GHC. Ubuntu could only install version 6.12.3: without Cabal! Now I am trying to install haskell-platform which is a separate package. Ubuntu suggests to insatll version 2010.1.0.0.1 and fails (see bellow). What sould I do now to get GHC + Cabal? Should I: 1) De-insatll GHC 6.12.3 and all related libraries and try then installing Haskell Platform again? 2) Should I install Caball separately and forget about Haskell Platform? 3) Provide somehow for two different installations? Thanks! === Installation errors: This error could be caused by required additional software packages which are missing or not installable. Furthermore there could be a conflict between software packages which are not allowed to be installed at the same time. haskell-platform: Depends: ghc6 (< 6.12.1+) but 6.12.3-1ubuntu7 is to be installed Depends: libghc6-cgi-dev (>= 3001.1.7.2) but 3001.1.7.2-1build1 is to be installed Depends: libghc6-fgl-dev (< 5.4.2.2+) but 5.4.2.2-2build2 is to be installed Depends: libghc6-glut-dev (< 2.1.2.1+) but 2.1.2.1-1build1 is to be installed Depends: libghc6-haskell-src-dev (< 1.0.1.3+) but 1.0.1.3-2build1 is to be installed Depends: libghc6-html-dev (< 1.0.1.2+) but 1.0.1.2-3build2 is to be installed Depends: libghc6-hunit-dev (< 1.2.2.1+) but 1.2.2.1-2build2 is to be installed Depends: libghc6-mtl-dev (< 1.1.0.2+) but 1.1.0.2-10build2 is to be installed Depends: libghc6-network-dev (< 2.2.1.7+) but 2.2.1.7-1build2 is to be installed Depends: libghc6-opengl-dev (< 2.2.3.0+) but 2.2.3.0-2build2 is to be installed Depends: libghc6-parallel-dev (< 2.2.0.1+) but 2.2.0.1-1build1 is to be installed Depends: libghc6-parsec2-dev (< 2.1.0.1+) but 2.1.0.1-2build2 is to be installed Depends: libghc6-quickcheck2-dev (< 2.1.0.3+) but 2.1.0.3-1build2 is to be installed Depends: libghc6-regex-base-dev (< 0.93.1+) but 0.93.1-8build1 is to be installed Depends: libghc6-regex-compat-dev (< 0.92+) but 0.92-6build2 is to be installed Depends: libghc6-regex-posix-dev (< 0.94.1+) but 0.94.1-2build1 is to be installed Depends: libghc6-stm-dev (< 2.1.1.2+) but 2.1.1.2-5build2 is to be installed Depends: libghc6-xhtml-dev (>= 3000.2.0.1) but 3000.2.0.1-6build2 is to be installed Depends: libghc6-zlib-dev (< 0.5.2.0+) but 0.5.2.0-4build2 is to be installed Depends: libghc6-http-dev (>= 40000009) but 40000009-2build1 is to be installed Depends: libghc6-deepseq-dev (< 1.1.0.0+) but 1.1.0.0-2build1 is to be installed Depends: alex (< 2.3.3+) but 2.3.3-1 is to be installed Depends: happy (< 1.18.4-2+) but 1.18.4-2 is to be installed Depends: haddock but it is a virtual package

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IterIO: How to write use my inumReverse
by John Ky 04 Jul '11

04 Jul '11

Hi Haskell Cafe, I've defined the following reverse echo server that echos text back in reverse: module Programs.TcpEchoIterServer where import Control.Concurrent import Control.Exception import Control.Monad import Control.Monad.Trans import Data.IterIO import Data.IterIO.Inum import Network import System.IO import System.IO.Error (isEOFError) import qualified Data.ByteString.Lazy as L import qualified Data.ByteString.Lazy.Char8 as C iterHandle' :: (MonadIO m) => Handle -> IO (Iter L.ByteString m (), Onum L.ByteString m a) iterHandle' = iterHandle main = withSocketsDo $ do sListen <- listenOn (PortNumber 8000) putStrLn "Listening on Port 8000" forkIO $ forever $ do (sSession, hostname, port) <- accept sListen hSetBuffering sSession NoBuffering putStrLn ("Connected to " ++ hostname ++ ":" ++ show port) forkIO $ do (iter, enum) <- iterHandle' sSession enum |$ inumLines .| inumReverse .| inumUnlines .| iter putStrLn "Press <CTRL-D> to quit." exitOnCtrlD iterLines :: (Monad m) => Iter L.ByteString m [L.ByteString] iterLines = do line <- lineI return [line] iterUnlines :: (Monad m) => Iter [L.ByteString] m L.ByteString iterUnlines = (L.concat . (++ [C.pack "\n"])) `liftM` dataI iterReverse :: (Monad m) => Iter [L.ByteString] m [L.ByteString] iterReverse = do lines <- dataI return (map L.reverse lines) inumLines = mkInum iterLines inumUnlines = mkInum iterUnlines inumReverse = mkInum iterReverse exitOnCtrlD = try getLine >>= either (\e -> unless (isEOFError e) $ ioError e) (const exitOnCtrlD) It all works fine. My question is: Is it possible to rewrite inumReverse to be this: iterReverse :: (Monad m) => Iter L.ByteString m L.ByteString iterReverse = do line <- dataI return (L.reverse line) inumReverse = mkInum iterReverse And still be able to use it in the line: enum |$ inumLines .| {-- inumReverse goes in here somehow --} .| inumUnlines .| iter The reason I ask is that the Haskell function reverse has the type [a] -> [a], not [[a]] -> [[a]]. I thought perhaps the alternative inumReverse is cleaner than the original as it behaves more similarly to Haskell's own reverse function. Cheers, -John

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Data.Time
by Tony Morris 04 Jul '11

04 Jul '11

-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 I recently set out to write a library that required a decent time library. Having only had a flirt with Data.Time previously, I assumed it would be robust like many other haskell libraries. I don't know about consensus, but I have been massively let down. I won't go in to the details, since this is not the point -- I don't wish to complain - -- I wish to get on with it. So, assuming the consensus is in agreement, is there a reasonable alternative to Data.Time (I looked on hackage and nothing seemed to have come close)? Am I wrong in assuming Data.Time is pretty useless? If I am right, and there is no alternative, I see no option but to take an excursion into writing my own. Ultimately, I am just trying to avoid this. Tips welcome. - -- Tony Morris http://tmorris.net/ -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.10 (GNU/Linux) Comment: Using GnuPG with Mozilla - http://enigmail.mozdev.org/ iEYEARECAAYFAk4Ggx4ACgkQmnpgrYe6r61BRQCfbn+1jqNSjR+lxM+4h3gpvAMM VskAoKxqDCETyVAaOdoYDmFJGz1fOGd/ =IC7O -----END PGP SIGNATURE-----

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Re: [Haskell-cafe] How to ensure code executes in the context of a specific OS thread?
by Jason Dagit 04 Jul '11

04 Jul '11

On Sun, Jul 3, 2011 at 10:31 PM, David Barbour <dmbarbour(a)gmail.com> wrote: > > On Jul 3, 2011, Jason Dagit <dagitj(a)gmail.com> wrote: >> >> to correctly handle GUI events you need to use the original thread >> allocated to your process to check for events and to call the Cocoa >> framework functionality. >> >> I looked at the threading documentation in Control.Concurrent for GHC and >> it's not clear to me if this is even possible with GHC without restricting >> to the non-threaded RTS. > > The 'main' thread in GHC is the bound thread initially allocated to the > process. That doesn't seem to be the case for GHCI. > You can easily establish an event-loop in the main thread, and feed > events from other threads. Consider using Control.Concurrent.Chan. > If you need feedback, things get a bit trickier because you'll have special > cases based on whether you're starting in the main thread. Yes, that might work for compiled programs. What about GHCI? Jason

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Re: [Haskell-cafe] How to ensure code executes in the context of a specific OS thread?
by David Barbour 04 Jul '11

04 Jul '11

On Jul 3, 2011, Jason Dagit <dagitj(a)gmail.com> wrote: > to correctly handle GUI events you need to use the original thread > allocated to your process to check for events and to call the Cocoa > framework functionality. > > I looked at the threading documentation in Control.Concurrent for GHC and > it's not clear to me if this is even possible with GHC without restricting > to the non-threaded RTS. > The 'main' thread in GHC is the bound thread initially allocated to the process. You can easily establish an event-loop in the main thread, and feed events from other threads. Consider using Control.Concurrent.Chan. If you need feedback, things get a bit trickier because you'll have special cases based on whether you're starting in the main thread. Regards, Dave

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