January 2013 - Haskell-Cafe

Proposal: TypeDirectedNameResolution
by Johannes Waldmann 03 Mar '13

03 Mar '13

While browsing Haskell-Prime I found this: http://hackage.haskell.org/trac/haskell-prime/wiki/TypeDirectedNameResoluti… This is not some April Fool's day hoax? Because, it might actually turn Haskell into a somewhat usable (and marketable) language ... well, you know what I mean. Is there 'ghc -XTypeDirectedNameResolution' yet? - J.W.

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tplot (out of memory)
by Manish Trivedi 27 Feb '13

27 Feb '13

Hi, I am running into a weird out of memory issue. While running timeplot over an input file having ~800 rows. From below provided info, seems like machine has enough ram (1849MB). Please let me know if anyone has pointers. * # free -m * total used free shared buffers cached Mem: 3825 1975 1849 0 13 71 -/+ buffers/cache: 1891 1934 Swap: 4031 111 3920 *#time tplot -o out.png -or 1024x768 -k 'CurrentPerHour' 'lines' -k 'RequiredPerHour' 'lines' -if adgroup_delivery_chart.input -tf 'date %Y-%m-%d %H:%M:%OS' * tplot: user error (out of memory) real 0m0.026s user 0m0.018s sys 0m0.008s -Manish

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Re: [Haskell-cafe] FFI - Approaches to C/C++
by Casey Basichis 05 Feb '13

05 Feb '13

Hi Ertugrul, I'm not entirely sure what you mean. I'm intending on using Ogre for GUI - for which there is the Hogre bindings, but after emailing the DEV about it, I didn't get the impression from his advice that I should be using it for production code. Here is what he suggested: "It depends, really. Hogre is good for running Ogre from within Haskell, but it has its limitations. The number one thing people have been struggling with is handling input with hogre - there's Hois (Haskell wrapper for OIS) but it's not perfect (it misses input events), and the other option is having to write some C++ glue. Hogre is a solid proof of concept and you can do some demos with it, but if you're e.g. writing a game it might be a bit of a struggle. In the end it's about how much you value being able to write code in Haskell (or how allergic to C++ you are)." I'm on iOS so I imagine those difficulties are compounded. I am using several other C++ libraries for which there are no existing bindings and no Haskell alternative packages that are even remotely close. Are you suggesting it would be better to write all my own FFI bindings for all the needed libraries? Everything I read suggests that Haskells strengths are in transformation and that interaction is not its strong suit. I am interested in your thoughts and I am open to whatever, but you are the first to suggest that the mix is a bad idea. Thanks, Casey >>* I'm working on a project in Haskell and C++ where the former is the*>>* brains and the latter is for UI, interaction etc.* >That's a rather odd choice. Not exactly answering your question, but >questioning your project decisions, why would you do UI and interaction >in C++? You have the necessary Haskell bindings and libraries to write >everything cleanly in Haskell. > > >Greets, >Ertugrul

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Yet another Conduit question
by Simon Marechal 04 Feb '13

04 Feb '13

I am working with bulk sources and sinks, that is with a type like: Source m [a] Sink [a] m () The problem is that I would like to work on individual values in my conduit. I can have this: concat :: (Monad m) => Conduit [a] m a concat = awaitForever (mapM_ yield) But how can I do it the other way around ? I suppose it involves pattern matching on the different states my conduit might me in. But is that even possible to do it in a "non blocking" way, that is catenate data while there is something to read (up to a certain threshold), and send it as soon as there is nothing left to read ? Or doesn't that make any sense in the context of Conduits (in the sense that this conduit will be recheck for input before the upstream conduits will have a chance to operate) ? Another approach would be to have a map equivalent: conduitMap :: Conduit i m o -> Conduit [i] m [o] But I am not sure how to do this either ...

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Most used functions in hackage
by Casey Basichis 03 Feb '13

03 Feb '13

Hi, I'm guessing this is a long shot, but Is there any link that counts the use of all functions in all packages in Hackage and lists them by frequency or by other stats? I'm still new to haskell but I've been working my way through tons and tons of tutorials and books. It would be very helpful to target in on the current reality of the most critical functions. Anything like this out there? Thanks, Casey

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linking errors while compile hugs98 in macos
by Junior White 03 Feb '13

03 Feb '13

Hi Cafe, I downloaded the latest hugs98 source package, unzip and build, I get the following link errors. It seems many symbols are not defined, am I missing same depending libraries? This is my machine info: ➜ hugs98-plus-Sep2006 git:(master) ✗ uname -a Darwin lan-seimatoMacBook-Air.local 11.4.2 Darwin Kernel Version 11.4.2: Thu Aug 23 16:25:48 PDT 2012; root:xnu-1699.32.7~1/RELEASE_X86_64 x86_64 ================================ Preprocessing Network/Hackage/Version Undefined symbols for architecture x86_64: "_environ", referenced from: ___hscore_environ in ccuTP291.o (maybe you meant: ___hscore_environ) ld: symbol(s) not found for architecture x86_64 collect2: ld returned 1 exit status runhugs: Error occurred ERROR "libraries/bootlib/Foreign/C/Error.hs" - Error while running compilation command 'gcc -DNDEBUG=1 -g -no-cpp-precomp -flat_namespace -shared -fPIC -D__HUGS__ "-Ihugsdir/include" -o "libraries/bootlib/Foreign/C/Error.so" "libraries/bootlib/Foreign/C/Error.c" -Ipackages/base/include -Ighc/includes packages/base/cbits/PrelIOUtils.c' Undefined symbols for architecture x86_64: "_environ", referenced from: ___hscore_environ in ccYZo1zT.o (maybe you meant: ___hscore_environ) ld: symbol(s) not found for architecture x86_64 collect2: ld returned 1 exit status runhugs: Error occurred ERROR "libraries/bootlib/Foreign/Marshal/Alloc.hs" - Error while running compilation command 'gcc -DNDEBUG=1 -g -no-cpp-precomp -flat_namespace -shared -fPIC -D__HUGS__ "-Ihugsdir/include" -o "libraries/bootlib/Foreign/Marshal/Alloc.so" "libraries/bootlib/Foreign/Marshal/Alloc.c" -Ipackages/base/include -Ighc/includes packages/base/cbits/PrelIOUtils.c packages/base/cbits/dirUtils.c packages/base/cbits/consUtils.c'

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Monadic parser vs. combinator parser
by Jan Stolarek 03 Feb '13

03 Feb '13

I will be writing a parser in Haskell and I wonder how to approach the problem. My first thought was to use monadic parser, e.g. like the one described by Hutton and Meijer in "Monadic Parsing in Haskell" functional pearl. But then I stumbled upon this: https://github.com/alephnullplex/cradle/tree/master/code/src/Lbach/Parser Monadic parser seems extremely verbose and not very straightforward compared to this one. I started to wonder whether I should use monadic parser for the sake of it being monadic or should I just go with the combinator approach? Any thoughts will be appreciated before I shoot myself in the foot :) Janek

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Teaching haskell and fuzzy logic to high school students
by claudio 31 Jan '13

31 Jan '13

I'm an Italian IT teacher and I would like to introduce functional programming using haskell. As classroom activity, I would like to build-up a simple fuzzy inference engine. Does anyone know if someone has experience in teaching haskell to high school students or something has already been done with fuzzy logic and haskell? I found only this page: http://www.haskell.org/haskellwiki/AI/Logic/Fuzzy with some information and a link to a pubblication (with fee, I didn't download yet). Thanks Claudio

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Type classes, collections, sum types, closures, and a massive headache
by Bob Hutchison 31 Jan '13

31 Jan '13

Hi, I'm relatively new to Haskell, and consider myself to be towards the beginner side of the scale. Nevertheless, I've got this Haskell program I've been working on that's sitting around 11k lines right now. The pattern has been to let it grow to then knock it back by 'refactoring' or whatever you want to call it… doing it right the second time maybe… or the third time. All I want to get across is that though I consider myself a Haskell beginner I've still managed to produce something that is actually quite complex and of reasonable size in about three months. I'm still getting caught by stuff that I should not be caught by. So. Today I thought it was about time to simplify how new 'things' of a certain kind are added to the system. These things are some a cross between an event and an assertion of a fact in a rule based system. There are many different kinds of these things. I already have more than a dozen commonplace ones, and I expect there's a much larger number of more specialized ones that a user will want to add on their own. While they start out quite differently, they end up satisfying a common interface and follow the identical three or four state lifecycle. This sounded like a type class to me, and in fact, easily implemented as such. Now, this is how I got caught: it seems to be impossible to have collections of things with a common type class if they have different types. How is it that I've written that many lines of code in Haskell and I'm just noticing this now? (If I wasn't so annoyed, I'd look for something clever to reflect how loc count obviously doesn't mean much… but clever seems to be beyond me today). Is this true? Are there any GHC extensions that will let me around this? The immediate problem is mapping an input to the system, some json message containing a reference to the 'thing' (like a key of some kind). I have to take that reference and find the thing and operate on it. All operations are easily accommodated by a type class. However, since I can't have a collection with mixed types even if the types satisfy a type class, I can't quite see how to actually store the things so I can find them. So there are a couple of obvious ways to handle this. I could use an ADT and a sum type of all the known kinds of thing, but I already know that this has to be extended and that's going to be problematic with users doing this on their own. And the type signatures look ugly. So I think that's not the best. I could use an ADT that contains functions that correspond to the functions of the type class, and that close over the 'thing' in question. I think this could be made to work, but I'm concerned with walking into more nasty surprises… If anyone is able to make sense of what I wrote and has any suggestions I'd really appreciate hearing them. Thanks, Bob

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Call for Participation - ACCAT 2013
by Ulrike Golas 31 Jan '13

31 Jan '13

[We apologize if you receive multiple copies of this message.] CALL FOR PARTICIPATION ========== 8th International Workshop on Applied and Computational Category Theory ACCAT 2013 http://accat2013.zib.de/ Satellite Event of ETAPS 2013, Rome, March 17 2013 ========== Scope ===== Since the 1960s, the use of category theory in computer science has been a fruitful one, including applications to different areas such as automata theory, algebraic specification, and programming languages, among others. In recent years, techniques and methods from CT have been adopted as a standard research tool, and considered as such in different venues around the world. The ACCAT workshop on "Applied and Computational Category Theory" has been one of these venues. Since its inception in 2006, ACCAT provided a forum where invited contributors presented their own research on different facets of category theory applied to computer science. Following the tradition, the program of this year edition will include eight invited talks by top researchers in the area: Samson Abramsky Robin B. Cockett Barbara Koenig Ugo Montanari Till Mossakowski Dusko Pavlovic Andrzej Tarlecki Glynn Winskel Despite ACCAT's success, we believe that the current formula of the workshop should be deeply revised. Indeed, we believe that a fully fledged conference is missing where all kinds of applications of category theory to computer science can be presented (like the former CTCS conference, which somehow ended in 2006). Therefore, after the presentations, the workshop will end up with a general discussion among the invited speakers and the attendees. We hope that the outcome of the discussion could be a decision whether to push for such high-level workshop/conference on the application of category theory to computer science, or at least to verify the viability of a further meeting focusing on this issue. For more information, please contact Fabio Gadducci (gadducci(a)di.unipi.it) or Ulrike Golas (golas(a)zib.de) -- Dr. Ulrike Golas Konrad-Zuse-Zentrum für Informationstechnik Berlin Takustr. 7, 14195 Berlin, Germany Tel. +49 30 84185 - 318 golas(a)zib.de -- www.zib.de/golas

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