Hi Günther,
But since I have the ambition to become a real haskeller I was gonna make myself acquainted with darcs. Should I skip that and head straight for git?
My extremely biased opinion is that you skip git and go straight for darcs ;-) Why I love darcs ================ Git is a fine revision control system; I like it and I intend to learn more about it. On the other hand, darcs does some unique, which makes it similar to Haskell (vs. other programming languages) in way, that one often has a hard time explaining to folks what it is that makes it so special (the good news is that the Haskell community is getting better and better at this! and hope that the same will happen for darcs). So let's give this a shot. Why darcs? In my opinion, the number one killer feature of darcs is that it knows how to merge changes without doing *any* guessing. It doesn't say "well, I have this patch that's supposed to tweak line number 4, but oops, I think I see that the line must have moved down to line number 7 because it looks sort of similar". Instead darcs knows with 100% certainty where the right place to apply your patch is. This is because it uses the full history of patches where everything goes. There is a lot of talk about a theory of patches, and this is largely what it's about in the darcs world, finding a precise way to talk about patches. See why some Haskellers love it? It's not just that it's written in Haskell; it's that the whole thing is driven by something which is very clean, powerful and which delivers a concrete impact in the real world. We love this kind of precision not just because we're theory-fans, but because it delivers the goods. Being able to apply and unapply patches precisely means that we can merge changes all we want -- we know that darcs will always preserve user intent -- plus we can adopt all sorts of new behaviours like cherry-picking incoming patches ("I'll take patch A, C and E; but not B and D please; oh, you tell me I can't have E without D? Very well, then") or cherry-picking changes that you want to undo ("Oops, I actually didn't mean to write that thing in line 58 and 98, but I still want to keep that thing in line 68, so I'll darcs revert them"... or "Oops, actually I've shouldn't have pulled in change C, but I still want to keep changes D E, so I'll just say 'darcs obliterate -p C' to get rid of it"). These kinds of operations happen seamlessly in darcs. There is no fuss, there is no worry; everything you can do in darcs, you can un-do. Anything you un-do in darcs, you can re-do. Darcs past, present and future ============================== So darcs has a clean theory which makes it possible to use a revision control system in a new seamless way. Why isn't everybody using it? Conflicts: It used to be the case that darcs had theoretical problems dealing with large conflicts (representing a conflict would take exponential space depending on the patches involved). It took the darcs community many years to address, but we eventually did, by releasing darcs 2. Darcs 2 repositories add enough information to the representation of conflicts which allows us to avoid the exponential blow-up most of the time. For the most part, the problem is solved. There are still cases where this blow-up can happen (we call them "conflict fights"), but they are well understood and can be avoided. In the meantime, we have some folks in the community who are doing some long-term work refining the darcs theory yet again. Performance: In addition to the conflicts problem, darcs has suffered from a lack of optimisation. (In fact, many users may have complained about patch theory problems, when they may really have been suffering more from performance issues or a mixture of the two). We have had and still have some practical day-to-day performance issues. For example, some of my larger repositories suffer when I use them on an NFS share. Rather than things being instantaneous, they would take a few seconds and I would get annoyed. Or perhaps, darcs would make too many connections over a network to fetch a lot of little files instead of sending them over in one pack. Now that darcs 2 is out, we have been focusing the bulk of our energy on tackling these basic performance problems. For other Haskellers in the room, I should point out that hacking darcs performance would be a great way for you to hone your skills and to give back to the community. No patch theory needed! Anyway, I wouldn't be too worried about performance issues. Darcs performs just fine for small to medium sized repositories. The darcs darcs repository has over 7000 patches over 6 years with over 160 contributors, no sweat. Windows: darcs support for Windows used to be a bit uneven. There were a few details here and there that none of us had time to get to (especially since none of us were using Windows). Things have gotten quite a lot better since, and we now have a dedicated Windows Czar who is helping us to bring our Windows support completely up to speed. GUIs: This too is changing, although it may take a while. Most of us are quite happy to use the command line interface to darcs, but we would love to see what kind of graphical interfaces folks will come out with in the future. In the meantime, we have recently exposed the darcs Haskell modules a sort of darcs library, which we hope will be useful to folks working on third party tools. Note that TortoiseDarcs has recently had a new release supporting darcs 2 Hosting: Aside from the haskell.org services, there was no hosting for darcs, commercial or otherwise. Now we have patch-tag.com which provides free hosting for open source projects and paid commercial hosting for private projects. So putting all this together: Darcs does precision merging, which allows you to do cool practical things like seamless merging and cherry picking (these make *my* life easier anyway). There used to be conflicts issues, but they are mostly solved. There are performance problems, but they only affect large repositories and they are getting better. There was poor Windows support, this has improved lots and has a dedicated czar. There is TortoiseDarcs if you're interested in a GUI. Hosting is available on haskell.org and patch-tag.com. Darcs and Haskell? ================== As a final note, darcs as a hacking project would be a good project to get interested in as a Haskeller. It's very much a real world program, so there are plenty of things to learn from it and plenty of things to teach it. We need lots of help. For example, we could use an army of Haskellers to scrub out our 2003 code and help us hunt down those pesky performance bugs. We now have a stable and more sustainable development team (I'm working on 20% darcs time at my job with the University of Brighton and have committed myself to spending no more than 8 hours a week on darcs to avoid burnout; a lot of our jobs have been assigned to dedicated managers -- hello!) and are adopting some new practices (hacking sprints, 6-month time based release schedule) to keep development running more smoothly. The future is bright. Come join us! Cheers, Eric P.S. Darcs will be hosting its second hacking sprint in Utrecht this 17-19 April as part of the Haskell Hackathon. It'll be a great chance to meet some darcs hackers and fix some easy bugs :-) P.P.S. All darcs patches require an inverse. Can a git user please write an inverse of this email so that Günther can make a more informed choice? -- Eric Kow http://www.nltg.brighton.ac.uk/home/Eric.Kow PGP Key ID: 08AC04F9