- Haskell-Cafe - Haskell.org

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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Cons of -XUndecidableInstances
by Scott Lawrence 07 Jun '11

07 Jun '11

According to the haskell-prime wiki[1], -XUndecidableInstances removes checks on the form of instance declaration, and just impose a depth limit to ensure termination (of compilation, I assume?). The listed Con is that this removes the clear boundary between legal and illegal programs, and behaviour may be implementation-dependent as the edge of that boundary is reached. How can I tell when I'm nearing that boundary? (And where are the sorts of things GHC does with types documented? I can't seem to find any good explanation of these things.) More specifically, I have class Model m a | m -> a where ... class Entropy d where ... instance (Model m a) => Entropy m where ... The first line requires MultiParamTypeClasses and FunctionalDependencies (the two seem to go together) - the third requires UndecidableInstances (since the type variable 'a' appears on the left but not the right). Is this likely to cause a problem? My guess is it shouldn't, since it's equivalent to class Model m a | m -> a where ... class Entropy d a where ... instance (Model m a) => Entropy m a where ... without bothering to actually use 'a' in Entropy - but one never knows... (Actually, a third type variable has to be introduced to Entropy to remove the UndecidableInstances dependency - "Constraint is no smaller than the instance head". This only increases the illogic in my humble eyes. These examples seem simple enough for GHC to handle nicely...) [1] http://hackage.haskell.org/trac/haskell-prime/wiki/UndecidableInstances -- Scott Lawrence

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