Source code management with darcs: a first look

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Tags: darcs, source, scm, monotone, haskell

I have been managing the LectroTest project with the monotone revision control system. For the last few months, monotone has been undergoing some growing pains that have made it less stable than I would like for everyday use. Thus I thought that I would give darcs a try.

I have been following the progress of darcs since it was first announced on the Haskell-Cafe mailing list on 9 April 2003. Darcs is written in Haskell, one of my favorite programming languages, and that was my initial draw. Still, until yesterday I had never used it for any of my projects because I felt it was immature and needed some more real-world testing before I committed work to it.

In the last three months, Darcs has gained mainstream attention (triggered by a favorable write-up in Linux Weekly News) and a growing user base. Under the gaze of these new eyeballs, darcs has matured much. I thought it was time for another look.

Darcs has a small, easy-to-understand command set and yet offers “modern” source-code management features such as distributed development (via HTTP, ssh, and email), change sets, and cherry picking. Want to start an experimental branch of your project? Just check out another copy and use it for the branch. Each working copy is a complete, independent repo. Want to publish a project repository to the world? Just copy it to a public web server. Want to start working on someone else’s project? A single “darcs get” gives you a complete, stand-alone copy. Your own personal branch. Start hacking.

To try darcs on something I was familiar with, I decided manage my LectroTest development with it. The first thing I did was change to the LT working directory and use “darcs init” to create a darcs repository there.

[tom@bifur Flippi]$ cd ~/work/research/perl/qc/  # LT root dir
[tom@bifur qc]$ darcs init
[tom@bifur qc]$ l
blib/                       mt.db
_build/                     mtdb.dump
Build*                      perl-Test-LectroTest-0.2007-1.src.rpm
Build.PL                    pod2htmd.tmp
Build.PL~                   pod2htmi.tmp
buildrpm*                   posts/
Changes~                    README
checkpods*                  t/
checkpods~                  Test-LectroTest-0.2001.tar.gz                    Test-LectroTest-0.2002.tar.gz                   Test-LectroTest-0.2003.tar.gz
CVS/                        Test-LectroTest-0.2004.tar
_darcs/                     Test-LectroTest-0.2004.tar.gz                Test-LectroTest-0.2005.tar.gz
lib/                        Test-LectroTest-0.2006.tar.gz
Makefile.PL                 Test-LectroTest-0.2007.tar.gz
MANIFEST                    Test-LectroTest-0.2008.tar.gz
MANIFEST~                   Test-LectroTest-0.2009.tar.gz
MANIFEST.bak                Test-LectroTest-0.201.tar.gz
MANIFEST.SKIP               tex/
META.yml                    THANKS~
monotone.db                 TODO
monotone.db.bak             TODO~
monotone.db.pre-changesets  toms-notes.txt
monotone.db-pre-sql3        toms-notes.txt~

You can see that there is a lot of accumulated cruft in my working directory, including CVS, monotone, and now darcs revision-control artifacts. To prevent Perl’s Module::Build from thinking the _darcs directory is meaningful, I added it to the manifest-skip file.

[tom@bifur qc]$ echo 'b_darcsb' >> MANIFEST.SKIP

Next I added my LectroTest sources, docs, and related files to the darcs repo.

[tom@bifur qc]$ darcs add Build.PL buildrpm Changes \
    toms-notes.txt tex/Makefile tex/titlepage.ltx

The “darcs whatsnew” command asks darcs to tell me what is changed in the working directory with respect to the repository state.

[tom@bifur qc]$ darcs whatsnew -s
A ./Build.PL
A ./Changes
A ./buildrpm
A ./checkpods
A ./tex/
A ./tex/Makefile
A ./tex/titlepage.ltx
A ./toms-notes.txt

The files that I added are new because I had not yet recorded them to the repository. Before I did that, I added the remaining LT assets.

[tom@bifur qc]$ darcs add t  # add the tests dir
[tom@bifur qc]$ darcs add t/*.t
[tom@bifur qc]$ darcs add lib
[tom@bifur qc]$ cd lib
[tom@bifur lib]$ l
[tom@bifur lib]$ darcs add Test
[tom@bifur lib]$ cd Test
[tom@bifur Test]$ l
LectroTest/     LectroTest::Tutorial.pod~
[tom@bifur Test]$ darcs add LectroTest
[tom@bifur Test]$ cd LectroTest
[tom@bifur LectroTest]$ l         Tutorial.pod           Tutorial.pod~      Tutorial.pod.bak
[tom@bifur LectroTest]$ darcs add *.pm *.pod

At this point, it looked like I had all of the files under darcs’s watchful eye.

[tom@bifur LectroTest]$ darcs w -s  # abbreviated: w -> whatsnew
A ./Build.PL
A ./Changes
A ./buildrpm
A ./checkpods
A ./lib/
A ./lib/Test/
A ./lib/Test/LectroTest/
A ./lib/Test/
A ./lib/Test/LectroTest/
A ./lib/Test/LectroTest/
A ./lib/Test/LectroTest/
A ./lib/Test/LectroTest/
A ./lib/Test/LectroTest/Tutorial.pod
A ./t/
A ./t/001.t
A ./t/002.t
A ./t/003.t
A ./t/004.t
A ./t/005.t
A ./t/compat.t
A ./tex/
A ./tex/Makefile
A ./tex/titlepage.ltx
A ./toms-notes.txt

[tom@bifur LectroTest]$ cd ../../..  # back up to project home

That looked right. It was time to record my changes. This was straightforward.

[tom@bifur qc]$ darcs record --all    # record all changes

Darcs needs to know what name (conventionally an email address) to use
as the patch author, e.g. 'Fred Bloggs <fred@bloggs.invalid>'.
If you provide one now it will be stored in the file
'_darcs/prefs/author' and used as a default in the future.  To change
your preferred author address, simply delete or edit this file.

What is your email address? Tom Moertel <>;
What is the patch name? Initial checkin of sources
Do you want to add a long comment? [yn] n
Finished recording patch 'Initial checkin of sources'

Now what did darcs think has changed?

[tom@bifur qc]$ darcs w -s
No changes!


One cool feature of darcs is that every working directory is also a complete, independent repository. To make a branch, then, is as simple as checking out a new repository.

Of course, because there is no central repository in the darcs model, “checking out” is a concept that does not really apply. Rather, what I must do is set up a new repository and then “push” my existing repository’s patches to it. I can push in many ways, including via ssh to a remotely hosted repository, but here I will just set up a new repo in /tmp and push to it on the local filesystem.

[tom@bifur qc]$ mkdir /tmp/lt && pushd /tmp/lt
/tmp/lt ~/work/research/perl/qc
[tom@bifur lt]$ darcs init   # set up new repo at /tmp/lt
[tom@bifur lt]$ popd
[tom@bifur qc]$ darcs push /tmp/lt   # push to repo at /tmp/lt
Sat Feb 12 01:26:15 EST 2005  Tom Moertel <>
  * Initial checkin of sources
Shall I push this patch? (1/1) [ynWvxqadjk], or ? for help: y
Finished applying...

Now, I can begin working on my new branch in the /tmp/lt working directory.

[tom@bifur qc]$ cd /tmp/lt
[tom@bifur lt]$ l
Build.PL  Changes    _darcs/  MANIFEST       posts/  tex/    TODO
buildrpm  checkpods  lib/     MANIFEST.SKIP  t/      THANKS  toms-notes.txt
[tom@bifur lt]$ emacs lib/Test/   # fix typo
[tom@bifur lt]$ darcs record
hunk ./lib/Test/ 38
-of your software.  LectroTest then checks your software see whether
+of your software.  LectroTest then checks your software to see whether
Shall I record this patch? (1/1) [ynWsfqadjk], or ? for help: y

What is the patch name? Fixed stupid typo in intro text of
Do you want to add a long comment? [yn] n
Finished recording patch 'Fixed stupid typo in intro text of'

Now my branch repository contains two patches:

[tom@bifur lt]$ darcs changes
Sat Feb 12 13:20:07 EST 2005  Tom Moertel <>
  * Fixed stupid typo in intro text of
Sat Feb 12 01:26:15 EST 2005  Tom Moertel <>
  * Initial checkin of sources

Because the typo that I fixed is not unique to my new branch, I ought to make sure that the original branch gets the fix, too. To do so, I just push it:

[tom@bifur lt]$ darcs push ~/work/research/perl/qc
Pushing to /home/thor/work/research/perl/qc...
Sat Feb 12 13:20:07 EST 2005  Tom Moertel <>
  * Fixed stupid typo in intro text of
Shall I push this patch? (1/1) [ynWvxqadjk], or ? for help: y
Finished applying...

And now my patch has been pushed back up to the mainstream branch! This is an attractive development model.

So far, I like darcs. Its source code management model is simple and powerful. Its command set is small enough to actually grok. Using darcs has me wondering why other SCM systems have made the problem seem so complicated. My life is complicated enough as it is.

I think I just switched to darcs.

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