Pijul – A free and open source distributed version control system

Here is a very short video on a project called "Camp" that stalled out, nearly a decade ago. I think it very nicely explains how the user interface differs:

https://www.youtube.com/watch?v=iOGmwA5yBn0

The most important thing is that because there is no DAG, when you say "Darcs, pull this patch for me", like saying `git cherry-pick ABCDEF` -- the dependencies are automatically computed and pulled as well. You can sort of imagine it like if you had a git branch, and you ran 'cherry-pick' on one of the commits to your 'master' branch (because you wanted it). But, rather than pulling that one thing, 'cherry-pick' implicitly traversed the dependent patches and picks them all as well. But because there is no DAG, a dependency doesn't mean "parent commit". It means "the other patches that are mathematically required for this patch to work out". That means cherry-pick always works: you never have to calculate the dependencies yourself. To merge a patch is to implicitly merge all of its dependencies.

I've spent plenty of my time as an OSS maintainer dealing with merging multiple bug fixes from a development branch into stable branches. For example, a bug may already be fixed in HEAD when it's reported, but not STABLE, so you want to pull changes from HEAD into STABLE. Many times this requires multiple, carefully curated sequences of 'git cherry-pick' in order to correctly get the dependencies right. For example, the author may have made a small refactoring, then implemented the bugfix on top of that. Or it requires a complete reformulation or re-commit of a new change that matches the STABLE branch.

In a sense: this never happens with Darcs. If there's a bugfix, I say "Get me that bugfix patch". It always gets every dependent patch that is necessary, and never anything more. Every time. It always just works. Remember: no DAG. You aren't traversing parent commits. You are, in a sense, finding the transitive closure of "patches that cannot commute with this patch" (IIRC). That means: if a given patch does not commute with this patch, i.e. it is dependent, because we must apply them in a certain order, so there is a dependency -- then you also need that patch. And you need to apply that rule to that patch, and every patch it depends on, and so on and so forth (hence 'transitive closure')...

This allows a very powerful form of development, where features and bugfixes can coexist. But they do not necessarily need separate 'branches', so to speak. To merge a feature into a repository implicitly pulls its dependents, and the same with bugfixes. The net effect of this is that Darcs almost always gets merges correct, or it fails to do the merge at all. This kind of means that merges are sound ('kind of' because I don't know about an actual soundness proof, but the intuitive idea roughly is right): if Darcs pulls off the merge, then it's always correct, but it may not be able to always actually do that merge (perhaps not every merge is actually sensible, in the theoretical view of things, or perhaps the merge is sensible but the model doesn't allow it to handle that case).

The "fails to do so" is the tricky part, where Darcs 1 originally went exponential in some cases, though Darcs 2 mitigates this. It looks like Pijul will finally nail this problem dead, although admittedly I haven't looked over the theory.

Side note: Camp was originally envisioned to be the successor to Darcs, or at least the basis for "Darcs 3", using Coq to build formal proofs about the underlying patch theory to show it worked out correctly and avoided the harry bits that plagued Darcs 2. Unfortunately, it never panned out that way (due to time and lack of funding). The project was actually started by Ian Lynagh who worked at my current company before me and was one of the founders.