September 2010 - Haskell-Cafe

FFI and concurrency
by Johannes Waldmann 10 Sep '10

10 Sep '10

What's the story with FFI calls and concurrency? I have an expensive calculation performed by some C function, which I call from Haskell land. (This works like a charm.) I have several cores available. How could I run several of these calculations in parallel? Thanks, Johannes.

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Combining applicative with arrows?
by Nils Schweinsberg 10 Sep '10

10 Sep '10

Hey, I just wondered if you can define Applicative instances for arrows? Basicly what I thought of is: I have a type for my arrow which is "CollectA" (using HXT here): type CollectA a = SomeArrow XmlTree a And my datatype which I want to construct: data Test = Test { testString :: String , testInt :: Int } With the functions: collectString :: CollectA String collectInt :: CollectA Int Now, what I currently do is something like: collectTest :: CollectA Test collectTest = getChildren >>> someOtherArrow -- etc... >>> proc foo -> do s <- collectString -< foo i <- collectInt -< foo returnA -< Test s i With Applicatives that proc-do-part would become: collectTest' :: CollectA Test collectTest' = ... -- arrow stuff >>> Test <$> collectString <*> collectInt Is something like this possible? I don't understand that "proc" part good enough to see what it's actually doing there, maybe someone with a better understanding for this could help out? :) Thx, - Nils

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Will GHC 6.14 with LLVM use LLVM C compiler to compile external C Libraries
by Mathew de Detrich 10 Sep '10

10 Sep '10

Since GHC 6.14 will (hopefully) be use LLVM as a default backend, an idea has occured to me Should GHC also use the clang (C/C++->LLVM compiler) on external C library sources which are used with certain Haskell packages (such as gtk) when LLVM does become a default backend for GHC. The consensus is that since Clang will also produce LLVM 'assembler', it can be very easily linked with the LLVM 'assembler' produced by GHC's LLVM backend, making the process of using external C sources a lot easier. Parts of Clang required could even be integrated into GHC (although this may be tricky since its coded in C++). It should also hopefully make using Haskell packages on windows that use C sources less painful Clang could also make using FFI with C++ much easier (for reasons stated above) Thoughts?

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Handling platform- or configuration-specific code (or, my CPP complaints)
by Ben Millwood 09 Sep '10

09 Sep '10

Good evening, cafe, Having recently taken on maintenance of a package that depends on template-haskell, I've been in some discussion with users and dependencies of my package about how best to write a library that works with multiple incompatible versions of a dependency. The two main approaches that I'm aware of are: 1. Use CPP and the macros defined by Cabal to conditionally include or exclude code for each version. 2. Use Cabal file conditionals to select hs-source-dirs containing those parts of the code (or even TH to help generate those parts of the code) that are specific to each configuration. In my discussion with others and examination of existing libraries, it seems to me that 1. is the preferred option, but I personally can think of a number of reasons to prefer the second option: * CPP was not designed for Haskell, and even cpphs still shows symptoms of it, so we have to worry about things like its handling of string gaps or single quotes. * CPP is primarily a textual manipulation tool, rather than a symbolic one, which makes it even more easy to produce malformed code with it than with TH or similar. * On that note, it's difficult to statically analyse a file using CPP: parser tools like haskell-src-exts don't support it and it's not at all obvious how or if they ever could. With Cabal choosing source files based on configuration, all the source is valid, normal Haskell and is easy for computers and humans to understand. * It may require some significant digging into each source file to establish what configurations must be tested, or to add a new supported configuration or remove an old one, if they are chosen based on CPP conditional compilation. When Cabal chooses what is compiled it is fairly explicit what is chosen and what could be, and adding a new configuration is - at least in theory - as simple as adding a new file and conditional. * It's just not very pretty! Haskell code has a sort of aesthetic that I don't think CPP macros share - different function application syntax, for example. Of course the trouble is that when your conditional compilation is on the module level there are some things which just can't be done without code duplication, and there's a tendency for small and often quite incidental bits of a function definition to suddenly be in another module instead of where they're used. So I wonder what people think of the use of CPP in Haskell code, what alternatives people can propose, or what people hope to see in future to make conditional compilation of Haskell code more elegant and simple? I once pondered whether it would be a good idea to somehow make available to TH splices information from Cabal or GHC's configuration, so that one could do something like this: myFunctionDecl = if packageVersion "template-haskell" >= Version [2, 4] [] then [d| unTyVarBndr (PlainTV n) = n; unTyVarBndr (KindedTV n _) = n |] else [d| unTyVarBndr = id |] This exactly wouldn't make sense because KindedTV wouldn't exist in the earlier version so the quotation would object, but one could imagine something similar being useful.

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installing GHC/Haskell Platform in Linux without root permission
by Ali Razavi 09 Sep '10

09 Sep '10

Is it possible to install either of these (preferably the latter) somewhere in my home directory without having root permission? I tried the unknown linux package with configure --prefix set to a subdir in my home, to no avail. The problem seems to be due to some library registration which fails. thanks Ali

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Haskell, arrows and signal processing
by Anthony LODI 09 Sep '10

09 Sep '10

> The input and output are infinite streams. I have a few questions: > > 1) Is it possible to change it to use arrows? How would it look like? > 2) How would one implement an continuous time version? Yampa can do *exactly* what you're asking for. Unfortunately the code seems to be a little rough and there are many space leaks (some of which are alluded to in the source code comments). Definitely make sure to profile memory usage as you code.

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Hackage package download count
by Tony Hannan 09 Sep '10

09 Sep '10

Is there anywhere we can see the number of download for a particular package, especially ones you maintain yourself? Thanks, Tony

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Haskell, arrows and signal processing
by Rafael Gustavo da Cunha Pereira Pinto 09 Sep '10

09 Sep '10

Hi folks from the cafe!! Last weekend, I was wondering on how hard it would be to use Haskell for mixed-signal processing. Here is an example of an digital integrator: summation=zipWith (+) delay xs=(fromIntegral 0):xs integrator xs=let ws=summation (integrator xs) xs in delay ws The input and output are infinite streams. I have a few questions: 1) Is it possible to change it to use arrows? How would it look like? 2) How would one implement an continuous time version? For 2) I would like to implement something like that digital integrator, so I could create filters based on integrator loops like this one: x(t) ----(+)----> Integrator --|---> y(t) ^ | |--------------------| Note that x(t) would be a function, and I would expect to create a function y, based on this flow! Rafael Gustavo da Cunha Pereira Pinto -- Rafael Gustavo da Cunha Pereira Pinto

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BPMN and BPEL
by Hector Guilarte 09 Sep '10

09 Sep '10

Hello, Does anybody knows if there is some work related to BPMN and/or BPEL done in Haskell? Maybe some research or some papers? I'm about to begin a mapper from BPMN to BPEL and Haskell is my first option for doing it, but since my company is somehow married to .NET and nobody else knows anything about Functional Programming or Haskell, using it might be a problem for maintenance and scalability. If somebody can point out really good reasons on why I should use Haskell to do my work, please let me know them, they might help me convincing my bosses. On the other hand, if you believe Haskell is a bad language for this kind of task, and why C# or any other .NET language would be better, I'm welcome to hear your reasons, they might convince me. I know about F#, but the "problem" is not that Haskell is not .NET, the "problem" is that it is functional programming and as I said before, nobody here knows a bit about it. If I were to do it using FP, I would use Haskell for sure, not F#. Thank you very much, Héctor Guilarte

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Haskell Weekly News
by Malcolm Wallace 09 Sep '10

09 Sep '10

I miss the Haskell Weekly News. The most recent issue was published on 8th March 2010. The volunteer who produces it claimed on 27th April that he would be back in action "soon", implying that once a couple of weeks' worth of university classes were finished, HWN would return. So in the absence of any visible movement, and it now being Sept, can I appeal for a new volunteer to take over? There is more info here: http://www.haskell.org/haskellwiki/HWN Regards, Malcolm

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