[Haskell-cafe] Extending Cabal's Preprocessors for GSoC?

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 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 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

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