- Haskell-Cafe - Haskell.org

Announce: GraphViz: Change of Maintainer
by Matthew Sackman 27 Apr '09

27 Apr '09

I no longer have the time to continue development of the (apparently, judging from the number of emails I'm getting, rather popular) graphviz package. Fortunately, Ivan Lazar Miljenovic who has extended it himself and sent me patches (some of which I did get round to folding in) has agreed to take on the package. Please direct all further queries to him! Many thanks Ivan! Matthew

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LAST CALL: Haskell Communities and Activities Report
by voigt＠tcs.inf.tu-dresden.de 27 Apr '09

27 Apr '09

Dear Haskellers, It is not yet too late to contribute to the May 2009 edition of the Haskell Communities and Activities Report. If you haven't already, please write an entry for your new project, or update your old entry. Please mail your entries to hcar(a)haskell.org in LaTeX format. More information can be found in the original Call for Contributions at http://www.haskell.org/pipermail/haskell/2009-April/021180.html I look forward to receiving your contributions. Thanks a lot, Janis. -- Dr. Janis Voigtlaender http://wwwtcs.inf.tu-dresden.de/~voigt/ mailto:voigt@tcs.inf.tu-dresden.de

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how can one load Prelude into ghci?
by Thomas Hartman 27 Apr '09

27 Apr '09

One reason it would be nice to be able to do this, is you could then get lambdabot :src style listings of prelude function definitions by using the ghci debugger and :list for prelude functions. is this possible? so far I have tried thartman@ubuntu:~/haskellInstalls/ghcInstalls/source/ghc-6.10.1/libraries/base>head Prelude.hs {-# OPTIONS_GHC -XNoImplicitPrelude #-} ----------------------------------------------------------------------------- thartman@ubuntu:~/haskellInstalls/ghcInstalls/source/ghc-6.10.1/libraries/base>ghci -i. Prelude.hs GHCi, version 6.10.1: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer ... linking ... done. Loading package base ... linking ... done. <command line>: module `Prelude' is not loaded thartman@ubuntu:~/haskellInstalls/ghcInstalls/source/ghc-6.10.1/libraries/base>ghci Prelude.hs GHCi, version 6.10.1: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer ... linking ... done. Loading package base ... linking ... done. <command line>: module `Prelude' is not loaded

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Memory usage of cabal install
by Krzysztof Kościuszkiewicz 27 Apr '09

27 Apr '09

Hello Haskell-Café, I have a problem with high memory usage of cabal-install. Whenever I try to install or upgrade a package, cabal manages to consume 1,3G of memory before I killed it (on a 32-bit machine with 1 GB of memory). Increasing verbosity does not help, memory consumption goes up after the message "Resolving dependencies..." shows up. I use ghc 6.8.2 and cabal-install version 0.5.1 using version 1.4.0.1 of the Cabal library. Is there a workaround? I would like to avoid fetching & building packages manually. -- Krzysztof Kościuszkiewicz "Simplicity is the ultimate sophistication" -- Leonardo da Vinci

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"subscribing" to the comments, online Real World Haskell
by Michael P Mossey 27 Apr '09

27 Apr '09

I love that Real World Haskell is online, here: <http://book.realworldhaskell.org/read/> I love that it has a comment section embedded with every paragraph. However, I would like some ability to subscribe to specific comments. I want to see if people have replied to me or what the latest discussion is. Anyone know if this is possible by a method other than manually copying down every URL? (In fact, I'm not sure that I can create a URL that goes directly to a part of a chapter.) Thanks, Mike

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old-time conflict when installing phooey
by Daryoush Mehrtash 27 Apr '09

27 Apr '09

When I try to install phooey I get conflict with old-time that I am not sure how to resolve. Any ideas? > cabal install phooey Resolving dependencies... cabal: dependencies conflict: ghc-6.10.1 requires old-time ==1.0.0.2 however old-time-1.0.0.2 was excluded because ghc-6.10.1 requires old-time ==1.0.0.1 Thanks Daryoush

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RE: [Haskell] ANN control-monad-exception-0.1: Explicitly typed exceptions
by Simon Peyton-Jones 27 Apr '09

27 Apr '09

Is there a Haskell Wiki page on extensible exceptions? The paper is a fine reference, it'd be cool to have a wiki page giving programmer-oriented guidance, saying what comes with GHC what other packages are available and some simple examples of how to use them. Simon | -----Original Message----- | From: haskell-bounces(a)haskell.org [mailto:haskell-bounces@haskell.org] On Behalf Of | Pepe Iborra | Sent: 25 April 2009 19:52 | To: haskell(a)haskell.org | Subject: [Haskell] ANN control-monad-exception-0.1: Explicitly typed exceptions | | The control-monad-exception package [1] provides explicitly typed | exceptions for Haskell. | In other words, this is a perfect example of bundling in a Haskell | library what for other | programming languages is a native feature. | | The type of a computation in the EM monad carries a list of the exceptions that | the computation may throw. A exception is raised with 'throw', which | in addition adds it | to the type, and captured with 'catch', which correspondingly removes | it from the type. | Only safe computations (all exceptions handled) can escape from the monad. | | The encoding used for the exception list is based on a phantom type | variable carrying a | @Throws@ constraint for every exception type. Catching an exception | @e@ satifies the constraint | @Throws e@ thus removing it from the type. It is possible to teach | Throws about exception subtyping | by manually inserting new instances declaring the subtyping relations | between exceptions. I don't | believe there is a better way to handle this, as the existential | wrapper encoding used | for Control.Exception.SomeException does not reveal the subtyping | relations, but ideas are | welcome. | | Example | -------- | GHCi infers the following types | | eval :: (Throws DivideByZero l, Throws SumOverflow l) => Expr -> EM l Double | eval `catch` \ (e::DivideByZero) -> return (-1) :: Throws | SumOverflow l => Expr -> EM l Double | runEM (eval `catch` \ (e::SomeException) -> return (-1)) :: Expr -> Double | | for the code below. | | | > import Control.Monad.Exception | > import Data.Typeable | | > data Expr = Add Expr Expr | Div Expr Expr | Val Double | > eval (Val x) = return x | > eval (Add a1 a2) = do | > v1 <- eval a1 | > v2 <- eval a2 | > let sum = v1 + v2 | > if sum < v1 || sum < v2 then throw SumOverflow else return sum | > eval (Div a1 a2) = do | > v1 <- eval a1 | > v2 <- eval a2 | > if v2 == 0 then throw DivideByZero else return (v1 / v2) | | > data DivideByZero = DivideByZero deriving (Show, Typeable) | > data SumOverflow = SumOverflow deriving (Show, Typeable) | | > instance Exception DivideByZero | > instance Exception SumOverflow | | | Comments and patches are welcome. | Cheers, | Pepe Iborra | | [1] - http://hackage.haskell.org/cgi-bin/hackage-scripts/package/control-monad- | exception | _______________________________________________ | Haskell mailing list | Haskell(a)haskell.org | http://www.haskell.org/mailman/listinfo/haskell

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Re: Is 78 characters still a good option? Was: [Haskell-cafe] breaking too long lines
by Jason Dusek 26 Apr '09

26 Apr '09

2009/04/26 Miguel <miguelimo38(a)yandex.ru>: > 2009/04/26 Jason Dusek <jason.dusek(a)gmail.com>: >> If we do not preserve the old ways, it'll be anarchy all the >> way down. > > How exactly are you going to preserve old ways? Whenever are we are presented with the a question of correct conduct, we must re-affirm our commitment to the practices handed down to us. When we revisit this question -- "Should we maintain the 80 column limit?" -- it as an opporunity to affirm this practice. Through this exercize, we perenially renew our faith in this convention, accepting the bonds of convention. This ritual -- the "line-length reminding ritual" -- is played out across all programming communities several times a year. Everyone says the same things. Then they all go away with the 80 column limit re-established in their minds. -- Jason Dusek

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installing Sqlite3 driver for windows
by Michael P Mossey 26 Apr '09

26 Apr '09

I'm working from Real World Haskell, trying to install HDBC and the Sqlite3 driver on Windows XP. It was easy enough to use Cabal to install HDBC. However, for the Sqlite3 driver things get fuzzy. I downloaded hdbc-sqlite3_2.1.0.0.zip, but don't know what I'm supposed to do with it. Also, it wasn't clear what to do with the sqlite3.dll file. On this page <http://software.complete.org/software/wiki/hdbc/FrequentlyAskedQuestions> it seems to imply you need to put it in 'gch --print-libdir'\include and %windir%\system32 but that page is really about building the driver from source. Maybe it only needs to go in the system32 directory. Thanks, Mike

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Haskell/JS -- better through typeclasses?
by Jason Dusek 26 Apr '09

26 Apr '09

I'd like to be able to translate Haskell to JavaScript. Many Haskell/JS bridges provide libraries for writing complete JavaScript programs in Haskell; some of them even include jQuery. However, my goals are more limited -- I'd like to be able to take a Haskell module and turn it into a JavaScript object. For example, I'd like to write a nice parser in Haskell and then reuse it on the client side. No need to handle all the DOM events or implement multi-threading. Of course, the place to start is by reading the commentary. A little bit of browsing suggests some questions of strategy: . Maybe a new backend is not the right thing? All the backends seem to be for real computers with real instruction sets. . Is it better to just work on transforming Core into JS directly? It seems that "External Core" is still in limbo. . Some translations strike me as baffling in principle. For example, a value like `ones`: ones = 1 : ones We'd want to avoid most native JavaScript containers, it seems; however, we are then unable to leverage the speed of native containers. It's entirely possible that translating Haskell to JavaScript may turn out not to be the best idea; maybe it is better to have a type class for types (for example, `Parser Char`) to provide their own translators? The it would be straightforward to prevent translation of programs that use concurrency libs, native ops or `IO`. -- Jason Dusek

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