June 2011 - Haskell-Cafe

haskellwiki slow/unresponsive
by Greg Weber 08 Jun '11

08 Jun '11

> > Those are definitely valid concerns. Has anyone made a wiki-like site with > Yesod? I hadn't heard of Yesod until I joined this mailing list, but I've > seen quite a bit of buzz around it since then. If a large enough chunk of > the community is backing a framework and focusing on making it secure and > reliable, then it should be possible to build applications with it (wikis, > blogs, etc.) that draw on the framework's strength and security. You may > still have security issues, but if they're continually addressed and > maintained at the framework level it benefits everyone building > applications > on top of that framework. I'm still relatively new to the Haskell > community > so I apologize if much of this has been addressed before! It is dead-simple to deploy a fast haskell web app, so haskell *might* have a chance at solving the problem, but likely it can be fixed much faster by doing a better job configuring the server or adding more resources. I think that security issues are a bit of a red herring. You can go through potential security issues one-by-one and basically make them impossible in a web/wiki framework (as we are doing in Yesod). Of course this is easier to do with a type system in Haskell. And of course some security is outside the scope of the software and needs to be established by procedure (admin server access should only be through ssh key, etc). Gitit uses darcs or git to store data, but through the command line interfaces. Unfortunately to my knowledge darcs does not expose a library interface. Gitit could be made faster and more secure by interfacing with libgit2. Even though it is may not be socially acceptable to state it, it is likely that gitit or any haskell library would be more secure in practice because it is more obscure. And the kind of security we really care about in the wiki- the wiki page content- can be easily replicated and restored. I definitely agree that the hard and boring part of switching to a haskell wiki is converting existing MediaWiki content and configuration to a new system- that would have to be investigated first. Unfortunately this is probably all bike-shedding and my original question of how can the community speed up the wiki hasn't been answered :) Greg Weber

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Does this library already exist? template haskell + generate/compile C code + dlopen
by Ryan Newton 08 Jun '11

08 Jun '11

The Nikola GPU programming system has a very neat, flexible approach to how you compile the EDSL-generated code. You can do it dynamically, calling nvcc at runtime, OR it can play a trick where it calls nvcc at compile time (via template haskell) and caches the result in a string literal within the TH-generated Haskell code. That is a pretty cool trick IMHO. Moreover, we can do that with any tool that generates C code or native code, as follows: At compile time, via Template Haskell: 1. call tool which creates C code 2. create temp files and call C compiler 3. load resulting object file as a bytestring and store it as a string constant Then at runtime: 1. put bytestring back into a file 2. call dlopen 3. call dlsym, get FunPtr, voila! Anyway, I was going to put together a simple library that encapsulates the above steps. Such a library could be used, for example, in making this stencil compiler project <http://people.csail.mit.edu/yuantang/> available to Haskell users as well as C++ users. (The compiler is written in Haskell already anyway!) But first I thought I'd ask if this already exists. Also, is there a better way to do it? In particular, is there any way to get static linking to work, rather than the silliness with strings, tempfiles, and dlopen. Thanks, -Ryan

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Proposal: remove "Stability" from haddock documentation on hackage
by Chris Smith 08 Jun '11

08 Jun '11

I got asked a question today about why Control.Applicative is labeled as "experimental" on Hackage. Perhaps that field is something of a failed experiment, and it remaining there is likely to confuse people. Just a thought... not sure of the best place to mention it. -- Chris Smith

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Local Haskell Meeting (Stammtisch) Leipzig, Germany, June 27
by Johannes Waldmann 08 Jun '11

08 Jun '11

Informal Haskell Stammtisch on Monday, June 27, 8 p.m., at Cafe Grundmann, Leipzig, Germany. http://www.cafe-grundmann.de/ Pre-registration (by email to me) is appreciated. Please bring ideas for our local Haskell Workshop (planned for end of September). See you - J.W.

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Twidge and identi.ca
by Mats Rauhala 08 Jun '11

08 Jun '11

Does twidge still support identi.ca? I was able to auth, but after that every single command returns 'twidge: user error (Bad response: 404)'. The commands work for twitter. -- Mats Rauhala MasseR

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Comment Syntax
by Guy 08 Jun '11

08 Jun '11

-- followed by a symbol does not start a comment, thus for example, haddock declarations must begin with -- |, and not --|. What might --| mean, if not a comment? It doesn't seem possible to define it as an operator.

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haskell platform question
by Tim Docker 08 Jun '11

08 Jun '11

I'd like to build the haskell platform against a recent GHC snapshot, for testing purposes. I see that I can download the source for the platform from: http://lambda.galois.com/hp-tmp/2011.2.0.1/haskell-platform-2011.2.0.1.tar.… with instructions: http://www.vex.net/~trebla/haskell/haskell-platform.xhtml However when I tried this some months ago, I found that some of the packages in the above source fail to build due to recent ghc changes (eg it's stricter handling of bang patterns). Is there a more recent "beta" snapshot of the platform? Or a darcs/git repo somewhere? I imagine wanting to build the platform against bleeding edge ghc would be a pretty common desire. Tim

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HUnit false-positive stumper
by KQ 08 Jun '11

08 Jun '11

While working on a project of mine recently I realized that a particular HUnit test should have been failing. After paring things down, I came up with this shocker: test_perhaps.hs: > module Main where >import Test.HUnit >main = runTestTT $ TestList [ True ~=? True > , False ~=? True > , TestCase $ assertEqual "both true" True True > , TestCase $ assertEqual "false true" False True > ] $ cabal-dev install ... $ ./cabal-dev/bin/test_perhaps.hs ### Failure in: 3 false true expected: False but got: True Cases: 4 Tried: 4 Errors: 0 Failures: 1 The shock here is that there was only one failure, whereas the "False ~=? True" should have failed. Environment: MacOS 10.5.8 (Leopard) GHC 6.12.3 cabal-dev 0.7.4.1 Cabal 1.10.1.0 HUnit 1.2.2.3 I even tried removing the dist and cabal-dev directories and reattempting but got the same results. I tried the same test in a different environment (Linux PC w/GHC 7.0.2, all other elements the same) and got correct results: $ ./cabal-dev/bin/test_perhaps ### Failure in: 1 expected: False but got: True ### Failure in: 3 false true expected: False but got: True Cases: 4 Tried: 4 Errors: 0 Failures: 2 $ I realize it's very difficult to debug someone's setup remotely, but I have to confess I'm really stumped at this point. The input program is pretty straightforward, and I *have* gotten test failures in the past on the Mac. I did do a "cabal update" the other day after being informed that my hackage file was getting old, but HUnit hasn't changed in months. I even tried unpacking HUnit and reducing the (non-)failing test case by using the definitions of the elements: > module Main where > > import Control.Monad (unless) > import Test.HUnit > > main = runTestTT $ TestList [ True ~=? True > , False ~=? True > , TestCase $ assertEqual "both true" True True > , TestCase $ assertEqual "false true" False True > , TestCase $ assertEqual "fa" False True > , TestCase $ assertEqual "f" False True > , TestCase $ (False @?= True) > , TestCase $ unless (False == True) (assertFailure "f") > ] The results are very strange: $ ./cabal-dev/bin/test_perhaps ### Failure in: 3 false true expected: False but got: True ### Failure in: 4 fa expected: False but got: True ### Failure in: 7 f Cases: 8 Tried: 8 Errors: 0 Failures: 3 $ The assertEqual form doesn't fail as it should if the label is a single character (test 5), but in its fully expanded form (test 7) it will fail. Huh?! At this point I'm thoroughly confused. I'm using 6.12.3 on the Mac because I don't have the newer MacOS release for which there's a Haskell Platform release and I haven't wanted to build GHC by hand. I wouldn't expect 6.12.3 to have issues like this, but I wouldn't expect issues like this anywhere. If anyone has any suggestions (other than upgrading to GHC 7.x) I'll be most appreciative. -- -KQ

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Non-advanced usage of Type classes
by Arnaud Bailly 07 Jun '11

07 Jun '11

Hello, In a recent thread, it has been asserted that defining type class is something you seldom need when programming in Haskell. There is one thing that as non-professional Haskell programmer I found type-classes useful for: Testing. This is probably very OO and is pretty much influenced by what I read in RWH but I find useful to define TC that abstract away from low-level interactions with other code, possibly IO related, in such a way that I can have one implementation for testing and one implementation for real work that is wired in caller context. This is what is called "mockist" TDD in some circles: Write code that expresses what it needs in its own terms, then implement "glue" to the code that provide the concrete behaviour. For example, while designing some program (a game...) I defined a type class thus: > class (Monad io) => CommandIO io where > readCommand :: io Command > writeResult :: CommandResult -> io () Then I defined in a module Commands.IO : > instance CommandIO IO where > readCommand = do input <- getLine > ... > writeResult r = putStrLn $ show r and for the purpose of testing I defined in some test module: > instance CommandIO (S.State ([Command],[CommandResult])) where > readCommand = do ((c:cs),rs) <- S.get > .... > writeResult r = do (cs,rs) <- S.get > ... Is this badly designed code that tries to mimic OO in a functional setting? If the answer is yes, how could I achieve same result (eg. testing the code that does command REPL) without defining type classes? Regards, Arnaud

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Distributing Haskell GUI apps for Windows/MacOS?
by Pedro Vasconcelos 07 Jun '11

07 Jun '11

Hello, I've implemented a Haskell program to play a modern board game. I've put it on Hackage (http://hackage.haskell.org/package/hstzaar) but would also like to distribute to people without the Haskell development tools. Are there any easy ways of making installers for Windows/MacOS (preferably using free or open source tools)? Best regards, Pedro

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