Interview: David Roundy of Darcs Revision Control

comforteagle writes "In the aftermath of our last interview with Tom Lord, regardless of personalities, it became apparent that the idea of decentralizing CVS is a big deal. Many mentioned darcs as an alternative to Arch. Mark Stosberg has interviewed project head-hancho David Roundy about darcs, his 'theory of patches,' what's next, and on using Haskell for the project."

Comments

Darcs is KISS

[Earlybird]

Among the plethora of emerging version control systems -- Subversion, Arch, Monotone and so on -- Darcs stands out for its simplicity and thoughtful design.

Like CVS, you can get productive within minutes; the same cannot be said for Arch or even Subversion. Let's see:

john@somewhere$ cd ~/myproject
john@somewhere$ darcs init

You now have a Darcs repository! Let's do something with it:

john@somewhere$ darcs add -r *
john@somewhere$ darcs record -am "Initial import."
Finished recording patch 'Initial import.'

Now your repository contains all your files. Let's look at the changelog:

john@somewhere$ darcs changes
Thu Nov 25 06:26:19 CET 2004 johndoe@example.com
* Initial import.

Now, where's the server? You need a server to share your repository, right? Nearly -- every repository is a potential server, as long as it's accessible either through the file system, through SSH/SFTP, HTTP or email. Let's go to another machine and check out the repository we just made:

jane@elsewhere$ darcs get john@somewhere:~/myproject
Copying patches...
.
Finished getting.

We now have a repository on Jane's box. Let's make a modification:

jane@elsewhere$ echo "#include <foo.h>" >>foo.c
jane@elsewhere$ darcs whatsnew --summary
M ./foo.c +1
jane@elsewhere$ darcs whatsnew
{
hunk ./foo.c 2
+#include <foo.h>
}

This last output, by the way, is Darcs' patch format. A "hunk" is a line-based diff. Other types of changes that may be contained in a changeset include renames, moves and binary changes. (Yes, you can also get a GNU-patch-compatible output similar to "cvs diff".)

Now let's commit and push the changes back to John's repository:

jane@elsewhere$ darcs record -am "Added a missing include."
jane@elsewhere$ darcs push -a
[...]
Finished applying...

Now we can go back to John's machine and look:

john@somewhere$ darcs changes
Thu Nov 25 06:26:10 CET 2004 janedoe@example.com
* Added missing include.

Thu Nov 25 06:26:19 CET 2004 johndoe@example.com
* Initial import.

(Note how Darcs generates a GNU-style changelog for you automatically.)

Where are the revision numbers, you ask? Well, they don't exist, because they're not needed. Darcs is changeset-oriented, not file-oriented. You can refer to a changeset by name, date, or a special hash identity.

Darcs changesets aren't just GNU patches; they have context, which means, for example, that someone can check out a repository, move a file "foo.c" into the directory "bar" and commit; meanwhile, another person, working on an older copy of the same repository, edits foo.c (which is still in its old location) and commits that. Darcs know that this edit should apply to foo.c in the new location -- and unlike CVS, you don't need to do anything similar to "cvs update" if you're committing files that have been changed on the server. In other words, people can freely commit changes, and the only kind of visible "conflict" will occur when you actually edit the exact same line.

Unlike CVS and Subversion, but like Arch and Monotone, Darcs is a distributed version control system. Repositories are islands which are constantly out of sync with each other, and Darcs' patch commutation system takes care of integration the changes that flow between them.

This system has several extremely useful effects:

• Offline mode. You can commit changes even if you're on the road with no access to the server. That's because your own working directory is a repository in its own righ

Re:Our experience CVS vs. DARCs

[David Roundy]

Darcs get (equivalent to CVS checkout) is the single least efficient command in darcs. People keep telling me I need to fix this, since it's the first thing users see, but it's really not an important command to optimize (apart from first impressions issues). When run locally (to create a new branch) it's fast.

And comparing darcs get with cvs checkout really isn't fair, since darcs gives you a copy of the full history of the repository, a separate branch on which to record changes before committing them to the centralized repository, and the ability to browse the history offline.

If you want a fast get, just run optimize --checkpoint on the parent repository (assuming you've tagged recently--if not, then tag the current state first), and then use the --partial flag when running darcs get. It'll still give you more flexibility than a cvs checkout, and will be much faster.

Re:Interesting app. non-troll questions

[David Roundy]

1. It's actually hard to use the patch commutation code to do any good outside the concept of a darcs repository.

1.5 I've thought about creating a C library for manipulating/querying darcs repositories, but haven't gotten around to it. The hard part would be of course designing the API. Ideally I'd like the interface to be such that programs using the library couldn't accidentally corrupt the repository.

2. Darcs requires ghc, since it uses some library code only available in ghc to do more efficient IO, string manipulation and to access zlib. It turns out to be a pain on many systems to link with the necesary libaries when using the interpereted version of ghc. So probably accessing darcs from perl will have to go through the executable until a C library is written (which could of course have perl bindings).

3. Rewriting darcs in perl (or parts of it) would be possible, but would be a pain. In particular, the commutation of patches which have conflicts is pretty complicated.