March 2010 - Haskell-Cafe

Re: [Haskell-cafe] Parsec to parse tree structures?
by John Lato 17 Mar '10

17 Mar '10

Hi Dave, > From: david fries <djf(a)gmx.ch> > > Hello Café > > Some time ago I wrote a parser for a project of one our customers. The > format was proprietary and binary. The data was structured as a tree > with tables pointing to sub tables farther in the file. (Well actually > there was one or two cases where branches joined together, so I guess it > was a directed graph.) Also it had no defined endianess, some tables > were bigendian others were little endian and some were in their very own > in-house-endianess. > All in all, the typical binary data structure that has been in use and > continuously extended for the last 15 years. You know the kind. > > My parser was written in C# and wasn't particularly elegant, but it > worked reliably. I was wondering how you would parse tree-like > structures with Parsec (or other functional parsers)? Up to know, all > examples I've seen were of sequential data. To parse trees, you'd > essentially need to be able to follow pointers, parse whatever is there > and then jump back. I guess I'd have to mess around with the internal > state of the parser to do that, which is something I'd rather avoid. Have you seen Edward Kmett's Parsing Trifecta talk/slides, available at http://comonad.com/reader/2009/iteratees-parsec-and-monoid/ ? I think a similar approach would allow you to parse this data structure. The trick is to create a data source that can read from arbitrary locations. Also see http://okmij.org/ftp/Haskell/Iteratee/Tiff.hs for an iteratee-based TIFF reader. TIFF files have a similar structure, so this code could apply pretty directly. The parsing is quite low-level, but you could use iteratee-parsec [1] or attoparsec-iteratee [2] to use parser combinators with this technique. [1] http://hackage.haskell.org/package/iteratee-parsec [2] http://hackage.haskell.org/package/attoparsec-iteratee

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Mini-announce: AC-Color 1.1.3
by Andrew Coppin 17 Mar '10

17 Mar '10

I just uploaded a new version of the ever-unpopular AC-Colour package. (Yes, there's a country where it's spelled that way.) What it already did: Provides a simple strict unboxed RGB colour type, with arithmetic. The three channels can be either Double or Word8. (And there's some low-level hackery to convert between the two. For some reason just using "floor" is awesomely slow, but talking to GHC.Prim directly is much, much faster... which unfortunately makes this package far less portable than it should be!) What it does now: Colour maps. Give it some control points, and it'll (linearly) interpolate between them to give you a function Double -> Colour. (I haven't seen anything else on Hackage that will do this out-of-the-box. I've seen linear blends between two colours, but not fully automatic piece-wise linear maps like this.) Now obviously none of this is terribly earth-shattering. But I find myself needing this often enough that it's annoying to have to spend 10 minutes coding and debugging it every single time I want it. Maybe somebody else will find a use for it too. I paid special attention to trying to make the colour maps as general as possible. (E.g., the parameter range isn't limited to [0..1], it's arbitrary. Going off the end of the range produces sensible results. You can make the map loop. And so on.) Maybe if I'm feeling ambitious I'll try 2D colour maps. (Of course, the problem there is that your control points need not be coplanar - or even form a regular grid!)

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ANNOUNCE: Mac and Windows installers for Leksah 0.8 prerelease available
by Hamish Mackenzie 17 Mar '10

17 Mar '10

We have uploaded first pass at Windows and OS X binaries for 0.8. They are both built using GHC 6.12.1. Installation: Make sure ghc 6.12.1 is installed. Take care that wget and grep are on your PATH. Please try them out if you can and let us know if you find any issues. http://leksah.org/news.html

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compiler management for Haskell a la RVM?
by Mark Wotton 17 Mar '10

17 Mar '10

RVM (at http://rvm.beginrescueend.com) is a rather nice tool for managing multiple ruby installations - it gives support for switching between ruby environments (similar to gcc_select and friends), and also exporting lists of packages so that you can easily bring up a given set of gems in any of the interpreters. Do we have any similar system for ghc/cabal? I quite frequently find myself switching between 6.10 and 6.12 for various things, and it's always a bit painful to get your environment up to speed. Is there a golden road for this stuff, or do the compiler hackers here just munge the PATH? cheers Mark

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th-kinds v0.0.0
by Louis Wasserman 17 Mar '10

17 Mar '10

I'm going to phrase this as a challenge, because I think that's likely to get the best response: I just released a package, th-kinds<http://hackage.haskell.org/package/th-kinds>, which attempts to automatically infer the kind of a specified type, type constructor, type family, type class, or pretty much anything else that has a kind. This package was developed in response to a sort-of challenge from Brent Yorgey on #haskell to create this functionality. So, uh, I stayed up last night until 5 am learning unification algorithms and implementing this...heh. It rolls its own kind inference, so I don't know if it's adequately powerful to handle all the wacky types you can construct. Things I have tested it on: - IO and ST - Many primitive types, including unboxed tuples and State# - mtl - newtype Fix f = Fix (f (Fix f)) - data Tree a = Leaf a | Bin (Tree a) (Tree a) - data Foo a b = a (tests proper defaulting behavior) - A type family from my old TrieMap package with kind -- I kid you not -- ((* -> *) -> ((* -> *) -> * -> *) -> (* -> *) -> * -> * -> *) Things that it will not work on, and that I don't think will change: - GADT types that cannot be reified by TH. Essentially, I think this is "the set of GADT data types that actually couldn't be implemented without GADTs." Not sure, though. In any event, at the moment, I don't think there's any hope of handling GADTs in TH at this point, so I don't really object to this problem. Anyway, y'all should attempt to break th-kinds. Tell me if you can construct a type for which my inference checker breaks, but not because you get an error message saying "Can't reify a GADT data constructor..." By the way, I'd like to bump these<http://hackage.haskell.org/trac/ghc/ticket/3916> tickets <http://hackage.haskell.org/trac/ghc/ticket/3920>. They're certainly related, but I'm not sure if they're genuinely equivalent. It's really irritating, though, that kind parsing is so broken in TH, and I think that a TH backend guru should be able to figure out where there's a foldr instead of a foldl... th-kinds includes a workaround to this bug, but if the bug is fixed, it'll break again. Yuck, bugs. Louis Wasserman wasserman.louis(a)gmail.com http://profiles.google.com/wasserman.louis

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ANNOUNCE: AusHac2010
by Alex Mason 17 Mar '10

17 Mar '10

G'day all, Over the last few days, Ivan Miljenovic and I have been gauging interest in having an Australian Haskell Hackathon. After some rather encouraging interest, we've decided to get the ball rolling by making a wiki page that we can use to organise the event: [http://www.haskell.org/haskellwiki/AusHac2010] If you're interested in coming, please put your name down on the list (or email me and I'll do it for you if you don't have an account). The more people we can get, the better it will be, and the more productive our work will be. So if you want to come along, play with some cool code, help out the community, and just meet an awesome bunch of like minded people, then we want to hear from you! Cheers, Alex Mason

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Proposal: Australian Hackathon
by Ivan Miljenovic 17 Mar '10

17 Mar '10

Would other Australians be interested in having our own Hackathon (why should all those northerners have all the fun)? I'm thinking about organising it to be in the July break between university semesters. There was a previous consideration a few years back to have an OzHaskell group (http://www.haskell.org/haskellwiki/OzHaskell) but nothing seems to have eventuated out of it. In terms of projects, here are some ideas: * A plotting library using Ben's newly released Gloss library (for people who can't or won't install Gtk2Hs to get Chart working; Alex Mason is interested in this) * Various graph-related project (graphviz, generic graph class, etc.; this assumes someone else apart from me cares about this stuff) * Hubris if Mark Wotton comes along * LLVM if David Terei comes So, at least as an initial listing, we'd need to have a listing of: 1) Who's interested 2) What dates are good 3) What projects people want to work on 4) Where we can host this Let's get the ball rolling! -- Ivan Lazar Miljenovic Ivan.Miljenovic(a)gmail.com IvanMiljenovic.wordpress.com

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SoC Proposals?
by Jeff Wheeler 17 Mar '10

17 Mar '10

Is there any way to propose a SoC idea right now? My account doesn't seem to have been created correctly, so I can't login to the Trac. I think it'd be interesting for a student to abstract the layout model in xmonad, separating it into an independent library. Other applications could then use this library, like Yi, for other types of tiling. I'd love to see other ideas related to Yi, too. It's a great project but on the verge of death (although the maintainer, JPB, is interested in mentoring). -- Jeff Wheeler Undergraduate, Electrical Engineering University of Illinois at Urbana-Champaign

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Extending Cabal's Preprocessors for GSoC?
by Diego Echeverri 16 Mar '10

16 Mar '10

Hi guys! Thomas Schilling suggested me to cross-post this here. Basically I'm thinking about the possibility of extending Cabal's Preprocessor mechanism for the GSoC. I've been working to provide uuagc with a Custom build mechanism for Cabal and in this process I have stumbled upon some difficulties (Dependency resolution and configurability). I believe is possible to extend the current preprocessing mechanism to provide the following: * Independence from physical representation (Currently the module is assumed to be built from a single file with the same name that the module) * Dependency checks (precompile when one of the files on which it depends changed. I had to trick cabal and duplicate functionality to do this for uuagc. There should be a better way!) * Composability (Add the ability to chain existing preprocessors) * Configurabily (Make easier to add configuration sections to the cabal file. I've been using "x-" sections but I guess is possible to make it look like the already-supported preprocessors). Any feedback about this? Suggestions are greatly appreciated. Diego Echeverri On Mon, Mar 15, 2010 at 10:33 AM, Thomas Schilling <nominolo(a)googlemail.com> wrote: > Yes, you sent it to the right list. In fact for discussing summer of > code it might not be a bad idea to cross-post to haskell-cafe to get > some feedback from Cabal users. > > In my experience, it is most important for Summer of Code to have a > clearly defined set of "deliverables", i.e., have a good idea of what > features you want to have working at the end of the 10 weeks. (It's > actually 12 weeks, but the last 2 weeks are expected to be used for > documenting stuff.) > > Your problem description sounds plausable, but for a GSoC project it's > good to also have an idea how to solve the problem. E.g., do you plan > to build a custom framework or just extend Cabal to support more kinds > of preprocessors? > > Duncan and others have been working on a true dependency tracking > system similar to Make, but I think there are too many open questions > to turn that into a GSoC project just yet. It does raise the > question, however, how much overlap there would be with your idea. > Maybe you can find a well-defined subset? > > In any case, try to ask haskell-cafe for feedback and apply for both > ideas. I think you can apply for up to 10 projects, so it's no > problem to apply for more than one. But keep in mind that the quality > of the proposals matters. > > On 14 March 2010 01:40, Diego Echeverri <diegoeche(a)gmail.com> wrote: >> Hi guys! >> >> I was wondering about the possibility to work on Cabal for the >> summer of code. I've been helping one of my teachers to >> provide (improve) a "plugin" for the uuagc (I haven't spent too >> much time on it, but here's the fork I'm working on >> http://github.com/diegoeche/uuagc) I have found how flexible the >> cabal libraries are but also some things haven't been >> straightforward to do (I've been using Cabal 1.6.0.3). >> >> Basically there's seems to be an assumption of a one-to-one >> relation of modules and the sources of the files which is not >> followed by the uuagc pre-processor. I can use List.ag and inside >> of it use an include declaration to List-Data.ag. If I modify >> List-Data.ag cabal won't precompile it again because it will only >> check dates against List.ag (cabal doesn't know that the >> file that generates List.hs depends not only in List.ag). I had >> to override the build hook, duplicate the check (this time >> getting all the dependencies) and delete the generated file to >> trick cabal to precompile again... and well... that's seems quite >> a nasty hack to me. >> >> This assumption also affect the possibility to make the output of >> the pre-processor not necessarily an .hs file (and not necessarily >> carrying the same name). Checking the code for the >> pre-processors (Distribution.Simple.PreProcess) I noticed it would >> be good to have the possibility to chain pre-processors. I believe >> both issues are pretty much connected. >> >> I also found the following proposal here: >> http://hackage.haskell.org/trac/summer-of-code/ticket/1581 which >> sound also interesting to me. (and there's already a mentor >> available!) >> >> Do you think this could be a good project for the summer of code? >> Maybe any other ideas I should explore? Did I write to the right >> list? (please excuse me if I didn't) >> >> Thanks! >> Diego Echeverri >> >> _______________________________________________ >> cabal-devel mailing list >> cabal-devel(a)haskell.org >> http://www.haskell.org/mailman/listinfo/cabal-devel >>

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Re: [Haskell-cafe] How to define instance for type synonym with parameter.
by Vasyl Pasternak 16 Mar '10

16 Mar '10

> > Aren't ErrorT and ReaderT instances of Applicative already? (At least in > 'transformers' package?) > > what difference between 'mtl' and 'transformers' packages ? 'mtl' goes with Haskell Platform as standard package, so what is the benefit of 'transformers' library ? Thanks, Vasyl

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