- Haskell-Cafe - Haskell.org

Importing modules in GHCi
by Artyom Kazak 04 Jun '12

04 Jun '12

Hi Café, I would greatly appreciate your help. Entering the following in GHCi produces an error message… --> import Data.Time --> getCurrentTime >>= print <interactive>:1:20: No instance for (Show UTCTime) arising from a use of `print' Possible fix: add an instance declaration for (Show UTCTime) In the second argument of `(>>=)', namely `print' In the expression: getCurrentTime >>= print In an equation for `it': it = getCurrentTime >>= print …but when I create a file with `import Data.Time` in it, and load the file in GHCi, everything works as expected. Is it a bug in GHCi, or a feature? I’m using Haskell Platform 2012.2.0.1.

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Restricted Monad question
by Ismael Figueroa Palet 04 Jun '12

04 Jun '12

Hi, I was wondering about defining a restricted monad, and found out this blog post: http://blog.omega-prime.co.uk/?p=127 In GHC 7.4.1 still the recommended way is using the rmonad package, right? I guess the tricks with ConstraintKinds explained in the blog are used for internal implementation of RMonad? Thanks -- Ismael

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Edmonton Functional Programming Users Group meets June 5
by Kevin Charter 04 Jun '12

04 Jun '12

Hi Cafe, The Edmonton Functional Programming Users Group (EFPUG) is meeting tomorrow, June 5, at 6:00pm in room CS 2-49 of the Computing Science building on the University of Alberta north campus. We'll be having a hack night, exploring combinator parsing with a variety of combinator libraries, including Parsec, attoparsec, uu-parsinglib, and parsatron, a Clojure library inspired by Parsec. Everyone is welcome. https://groups.google.com/forum/?fromgroups&hl=en#!forum/edmonton-functiona… Kevin -- Kevin Charter kevin.charter(a)acm.org

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An attempt at an EDSL for platform independent 3D animation
by C K Kashyap 04 Jun '12

04 Jun '12

Hi All, I've written a Haskell program that allows you to describe "dance movements" and it spits out javascript that does animation on an HTML 5 canvas (that's the platform independent bit). https://github.com/ckkashyap/Dancer Please do check it out and let me know what you think - all you'd need is Haskell platform. It's really preliminary at this point - The only animation that I've built using it is "walk" - this makes a matchstick man walk (you have to stretch your imagination a bit). Regards, Kashyap

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FP freelancers group on linkedin
by Ivan Perez 04 Jun '12

04 Jun '12

Hi, A while ago somebody posed the question "What if you get hit by a bus", regarding the apparent lack of Haskell developers that could continue somebody's work if something happened to them. I just created a linkedin group for FP programmers available for hire on a per-project basis ([1]). I hope this enables new collaborations among us, and also that it helps potential clients understand that there are many Haskell professionals available and that they need not fear this language. I also think this may complement the great listing at haskellers.com created by snoyberg, as well as other linkedin groups related to FP. I'd be happy to add links to other "directories" on the group's description page. I'm open to suggestions. Regards, Ivan. [1] http://www.linkedin.com/groups/Functional-programming-freelancers-consultan…

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mapping a concept to a type
by Corentin Dupont 04 Jun '12

04 Jun '12

Hi everybody, I'm still working on implementing a nomic game in Haskell. Although the game is pretty advanced, I'm still confused by one fundamental question: A nomic game is composed of rules. A Rule is a sort of little program submitted by the player during the game. They come in two fashions: - a Normal rule, when executed, can change the state of the Game. - a Meta rule, when executed on another rule, can assess the legality of that rule and change the state of the Game. In those definitions, the Game can be seen as a data structure containing all the current state of the game. Sorry if it's a bit abstract, but as it's fairly complex, I'd prefer to keep it like this for the moment. Thus, I found that those definitions can be translated to two possible type definitions: type NormalRule = State Game () type MetaRule = Rule -> State Game Bool data Rule = MR MetaRule | NR NormalRule *** or *** newtype Rule = Rule {rule :: Maybe Rule -> (State Game Bool)} All rules submitted by the user must be of the type Rule. It is thus important that it is very clean. Which type do you prefer? I find both heavy and redundant. The first forces me to specify if I want an argument of not (with the constructors MR and NR), the second forces me to pass the Nothing argument in the case of a Normal rule, and to return a dummy value (for ex. return True). Do you know of a construction/abstraction that allows having or not an argument (a variable number of arguments, here zero or one)? Thanks! Corentin

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Re: [Haskell-cafe] Announce: Haskell Platform 2012.2.0.0
by Chris Wong 04 Jun '12

04 Jun '12

On Mon, Jun 4, 2012 at 1:16 PM, Jens Petersen <juhp(a)community.haskell.org> wrote: > Congratulations on the release! > Equally surprising to me is that the number of slashes > also seems to affect the CSS presentation of the website > in Chrome. > > // seems to give the Summer theme, > whereas / gives the Winter one! > > Kind of weird. Anyway I agree it would be better to avoid > the superfluous slashes if possible. Your browser might be caching the file. Try pressing Ctrl-F5 on the winter page to force a reload. > I am going build HP 2012.2 for Fedora 18 soon but > the first proper build may be after ghc-7.4.2 is released. > > Jens > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe

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Announce: Haskell Platform 2012.2.0.0
by Mark Lentczner 04 Jun '12

04 Jun '12

We're pleased to announce the next release of Haskell Platform: a single, standard Haskell distribution for everyone. Download Haskell Platform 2012.2.0.0: http://haskell.org/platform/ The specification, along with installers (including Windows, Macintosh, and Unix installers for a full Haskell environment) are available. Notable version changes with this release: ghc 7.4.1 cabal 1.14 & cabal-install 0.14 mtl 2.1.1 & transformers 0.3.0.0 alex 3.0.1 full package and version list: http://hackage.haskell.org/platform/changelog.html Thanks to: Joachim Breitner, George Colpitts, Duncan Coutts, Jason Dagit, Chris Dornan, Yitzchak Gale, Mikhail Glushenkov, Andres Löh, Gábor Páli, Jens Petersen, Don Stewart, David Terei, Johan Tibell, & Mark Wright for their efforts and help in putting together this release. -- The Platform Infrastructure Team Mark Lentczner, release manager --------------------------------------------------------------------- The Haskell Platform is a single, standard Haskell distribution for every system, in the form of a blessed library and tool suite for Haskell distilled from the thousands of libraries on Hackage, along with installers for a wide variety of systems. It saves developers work picking and choosing the best Haskell libraries and tools to use for a task. When you install the Haskell Platform, you get the latest stable compiler, an expanded set of core libraries, additional development tools, and cabal-install – so you can download anything else you need from Hackage.

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Announce: haskellifi-trayicon-0.0.1
by Ivan Perez 03 Jun '12

03 Jun '12

Hi everyone, I wrote a small app that suggests default passwords for visible wifi networks (currently only some Telefonica & Jazztel Wifis in Spain, but could be extended). Not a big deal (less than 400 lines of code with the GUI), but some people asked me to release it, so here it is. Disclaimer ======== THIS IS IMPORTANT. This program is not intended for hacker wannabes. This is meant ONLY for network administrators. Use *only* with permission of the wifi admin. Introduction ========= More often than not, network administrators have to deal with clients or other administrators how know very little about wifi security. It is not uncommon to find wifis with default passwords. This program suggests passwords for the wifis it detects, and presents the information in a visually attractive way (tray icon with popup menu) so that your customers/collaborators fully understand HOW EASY IT IS TO BREAK INTO THEIR NETWORKS. NOTE: at the present time, this only works with Telefonica Spain's WPA2 default passwords (and maybe Jazztel's). If the default password of your wifi can be automatically generated from "open" or publicly visible parameters, feel free to write the algorithm in Haskell, send it to me and I'll see if I can add it here. Instructions ========= Code, Installation, readme, and other instructions: https://github.com/ivanperez-keera/haskellifi-trayicon During execution, you should see an icon on your traybar or notification area. Left click will show a menu with the list of wifis it can suggest a password for. Clicking on one will copy the suggested password to the clipboard. Right click on the icon to clear the wifi list or quit the program. Feel free to send me bugs reports and suggestions. Hope it helps somebody. Cheers, Ivan.

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Re-exports of resourcet in conduit
by Myles C. Maxfield 03 Jun '12

03 Jun '12

To: Michael Snoyman CC: haskell-cafe Hello, I'm having a problem working with the conduit library, and was hoping you could help me out. Data.Conduit re-exports ResourceT, MonadResource, and MonadThrow (but not ExceptionT) from Control.Monad.Trans.Resource. I have a conduit which operates on a monad in the MonadThrow class. I am trying to figure out which MonadThrow class this should be (the Data.Conduit.MonadThrow class, or the Montrol.Monad.Trans.Resource.MonadThrow class, since apparently GHC doesn't recognize them as the same, even though one is just a re-export of the other). If a user of this conduit wants to chain this conduit up with something like sourceFile, the underlying monad has to be a member of Data.Conduit.MonadResource and whatever MonadThrow class I chose to use. I would like to be able to use an existing instance to lift the class of the inner monad to the class of the entire monad stack (so I don't have to tell the user of my conduit that they have to define their own instances), and the only rule that I can find that does that is the following from Data.Conduit: Data.Conduit.MonadThrow m => Data.Conduit.MonadThrow (Data.Conduit.ResourceT m) However, GHC doesn't seem to think that Control.Monad.Trans.Resource.ExceptionT is an instance of Data.Conduit.MonadThrow: No instance for (Data.Conduit.MonadThrow (ExceptionT IO)) arising from a use of `.....' Control.Monad.Trans.Resource has a similar instance: Control.Monad.Trans.Resource.MonadThrow m => Control.Monad.Trans.Resource.MonadThrow (Control.Monad.Trans.Resource.ResourceT m) but because sourceFile operates in the Data.Conduit.MonadResource class, and Control.Monad.Trans.Resource.ResourceT isn't a member of that class (it's only a member of Control.Monad.Trans.Resource.MonadResource), that doesn't help: No instance for (Data.Conduit.MonadResource (Control.Monad.Trans.Resource.ResourceT (ExceptionT IO))) arising from a use of `.....' It should be noted that neither module defines anything like the following: MonadResource m => MonadResource (ExceptionT m) It seems like the underlying problem here is that: 1) I am required to use the Control.Monad.Trans.Resource.ExceptionT class, because Data.Conduit doesn't re-export it 2) I am required to use the Data.Conduit.MonadResource class, because sourceFile and others require it 3) There doesn't seem to be an existing instance that bridges between the two. This seems like a fundamental flaw with re-exporting; it can only work if you re-export every single last thing from the original module. This doesn't seem tenable because the orignal module might not be under your control, so its author can add new symbols whenever he/she wants to. I see two solutions to this problem: 1) Re-export Control.Monad.Trans.Resource.ExceptionT in Data.Conduit. This will work until someone adds something to the resourcet package and someone wants to use the new addition and Data.Conduit.ResourceT in the same stack 2) Don't re-export anything in Data.Conduit; make sourceFile and others explicitly depend on types in another module, but this might break compatibility with existing programs if they use fully-qualified symbol names. 3) Make anyone who wants to use a monad stack in MonadThrow and MonadResource define their own instances. This is probably no good because it means that many different modules will implement the same instance in potentially many different ways. I feel like option 2) is probably the best solution here. I'm perfectly happy to issue a pull request for whichever option you think is best, but I don't know which solution you think is best for your project. What do you think? --Myles

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