How I stopped missing Darcs and started loving Git

Posted on December 10, 2007

About three years ago, I switched to Darcs as my primary source-code management system. It was simple, intuitive, and powerful, and it made managing my projects more fun and less frustrating than any centralized VCS ever had. That it was written in Haskell, one of my favorite programming languages, made it even better. I was hooked.

Since then, the distributed SCM landscape has changed. Darcs hasn’t improved much, but its competitors have made long strides, especially Git and Mercurial. Both are crazy fast, vigorously developed, and widely used on large, highly active real-world projects, such as the Linux kernel and Mozilla 2. In comparison, Darcs has stagnated.

When I started working for a new company recently, I had to consider whether to advocate Darcs or something else. In the end, I decided that Darcs would be a hard sell. Nobody else at the company uses Haskell, and having to explain how to avoid the occasional corner case seemed liked a losing proposition.

After researching and playing around with Git and Mercurial, I settled on Git. I like Git’s underlying hashed-blobs model better than Mercurial’s revlogs, and Git seems to have slightly more development momentum. Still, it was a close call. Either choice would have been completely reasonable.

Missing Darcs

When I started using Git on real projects, the one thing I really missed was the ability to easily amend earlier patches, something Darcs made trivial. Let me explain. The typical development workflow goes something like this:

Checkout copy of upstream code base.
Implement feature X.
Commit.
Implement independent feature Y.
Commit.
Implement independent feature Z.
Commit.
Push new features back upstream.

Now, what really happens is that when I’m implementing Y or Z, I’ll realize that I made a mistake in X. The trick is then fixing X so that my fix is part of the changeset/patch for X that ultimately gets pushed upstream in the last step. That way, the upstream folks will see only a single, clean patch for feature X – not a mishmash of patches that together represent X.

In Darcs, amending the original patch is easy because its patch theory lets me tweak the patch for X independently of the other patches. Darcs will simply ask me which patch I want to amend, and I’ll select the orignal patch for X:

$ emacs               # fix X
$ darcs amend-record  # amend original patch for X

Mon Dec 10 14:43:13 EST 2007  Tom Moertel <tom@moertel.com>
  * Implemented Z
Shall I amend this patch? [yNvpq], or ? for help: n

Mon Dec 10 14:42:12 EST 2007  Tom Moertel <tom@moertel.com>
  * Implemented Y
Shall I amend this patch? [yNvpq], or ? for help: n

Mon Dec 10 14:41:46 EST 2007  Tom Moertel <tom@moertel.com>
  * Implemented X
Shall I amend this patch? [yNvpq], or ? for help: y
hunk ./x 1
-X1
+X2
Shall I add this change? (1/?)  [ynWsfqadjkc], or ? for help: y
Finished amending patch:
Mon Dec 10 14:43:25 EST 2007  Tom Moertel <tom@moertel.com>
  * Implemented X

That’s it. The exact same process will work regardless of when I realize I need to fix X: before I start Y, while I’m implementing Y, after I’ve committed Y, while I’m working on Z, or after I’ve committed Z.

Learning to love Git

With Git, however, I can amend a commit only if I haven’t committed anything else before making my fix. In Git’s mind, Y depends on X, and Z depends on Y, even if they really are independent of one another.

So if I commit the original patch for X and then immediately realize I need to make a fix, before I start working on Y or Z, it’s easy:

$ emacs               # implement X
$ git commit -m 'Implemented X'

# discover problem in X

$ emacs               # fix X
$ git commit --amend  # amend original patch

More typically, it’s only while I’m working on Y that I’ll realize I need to fix X. Then it’s more complicated to amend the original commit:

$ emacs               # implement X
$ git commit -m 'Implemented X'
$ emacs               # start working on Y

# discover problem in X

$ git stash           # stash away half-completed work on Y
$ emacs               # fix X
$ git commit --amend  # amend original patch for X
$ git stash apply     # restore work on Y
$ emacs               # continue working on Y

While not as convenient as Darcs’s workflow, it’s perfectly workable.

Now let’s consider another fairly typical case: I commit X and Y and then start working on Z before I notice the problem in X. I used to think that Git couldn’t handle this case, but it can, thanks to git rebase —interactive:

$ emacs               # implement X
$ git commit -m 'Implemented X'
$ emacs               # implement Y
$ git commit -m 'Implemented Y'
$ emacs               # start working on Z

# discover problem in X

$ git stash           # stash away half-completed work on Z
$ emacs               # fix X
$ git commit -m 'Fixed X'
$ git rebase --interactive HEAD~3  # see comments below
$ git stash apply     # restore work on Z
$ emacs               # continue working on Z

The git rebase —interactive command is powerful. What the command does, as called in the snippet above, is invoke my editor of choice on a text file describing the last 3 commits (that’s the HEAD~3 part):

# Rebasing 3ad99a7..b9a8405 onto 3ad99a7
#
# Commands:
#  pick = use commit
#  edit = use commit, but stop for amending
#  squash = use commit, but meld into previous commit
#
# If you remove a line here THAT COMMIT WILL BE LOST.
#
pick 0885540 Implemented X
pick 320b115 Implemented Y
pick b9a8405 Fixed X

I can then edit the file to reorder, merge (squash), and/or remove the commits. In this example, I want to merge the fix for X into the original commit that implemented X. So I edit the file like so:

pick 0885540 Implemented X
squash b9a8405 Fixed X
pick 320b115 Implemented Y

Then I save the file, at which point Git takes over and makes the requested changes, merging the fix for X into the original commit for X. Now the log shows the original implementation and fix as one commit:

$ git log
commit f387d650976246c0854d028b040cca40e542be56
Author: Tom Moertel <tom@moertel.com>
Date:   Mon Dec 10 15:11:26 2007 -0500

    Implemented Y

commit 82a1c849ffd1bd688d5bc9d99be0e63548a89c4c
Author: Tom Moertel <tom@moertel.com>
Date:   Mon Dec 10 15:13:03 2007 -0500

    Implemented X

    Fixed X

commit 3ad99a7ef537b7ae99e435e0d2b4b0d03de92c65
Author: Tom Moertel <tom@moertel.com>
Date:   Mon Dec 10 15:11:14 2007 -0500

    Initial checkin

Once I figured out how to use git rebase —interactive, I stopped missing Darcs and started loving Git.