Pijul a FOSS distributed version control system
pijul.org
pijul.org
I think jj’s concept of being a front end for many backends and sharing a common UX over them is a good one, but without a pijul backend for existing tools I have a hard time seeing it catch on.
For example, diffs can't show context. They show lines removed and added, but can't show the previous and following line because of implementation details.
Here's a diff: https://nest.pijul.com/pijul/pijul/changes/OMGL7JBCLSHWTIYP7...
It's a little thing, but when looking at a diff, seeing three lines of context helps enormously.
What I'm waiting for before I switch is mainly a really good bridge to Git. At the first couple tech companies I worked at, I led efforts to switch from Subversion to Git, and I did it by experimenting and I eventually proving a viable path using Git's built-in, two-way SVN bridge, which is excellent.
Whenever I revisit Pijul, I play with importing a giant (pathological, tbh) Git repository. Unfortunately, it runs for days and has never successfully completed for me. But the moment that bridge is good enough, I'm in! (Performance is nice but reliability is obviously the essential thing, especially when it comes to one-time operations like the initial import.)
PS: pmeunier, if you're reading this: I love your work. When Pijul can reliably import Nixpkgs, write a killer blog post about it. I'm sure it'll blow up here on HN. :)
And thanks for your work to advance the state of the art in DVCS! :)
PPS: I wonder if Pijul's fundamentally better merging behavior could become especially salient for teams (or individuals) whose codebases are seeing a lot of churn with heavy LLM use, so that it can help make automatic resolution of merge queues cheaper or easier or more reliable.
Jujutsu steals it, though! You should probably give it a look.
In my work with high-performance engines, 'on-the-fly' graph resolution is usually the first thing to hit a performance wall compared to simple snapshot-based lookups. Pijul is a brilliant experiment in Category Theory applied to VCS, but until it can demonstrate that it doesn't degrade linearly with history size, Git's 'dumb' but fast snapshots will likely win the network effect battle.
> Why?
> Commutation
> In Pijul, independent changes can be applied in any order without changing the result or the version's identifier. This makes Pijul significantly simpler than workflows using git rebase or hg transplant. Pijul has a branch-like feature called "channels", but these are not as important as in other systems. For example, so-called feature branches are often just changes in Pijul. Keeping your history clean is the default.
This is a useless property because the graph is only encoded at the patch layer. In the real world you have far more semantic dependencies than patch dependencies. ie, I have a patch that adds a function that calls a function added in another patch. Pijul doesn't know about that.
> Merge correctness
> Pijul guarantees a number of strong properties on merges. The most important one is that the order between lines is always preserved. This is unlike 3-way merge, which may sometimes shuffle lines around. When the order is unknown (for example in the case of concurrent edits), this is a conflict, which contrasts with systems with "automatic" or "no conflicts" merges.
I can't remember being bitten by this, and you don't need Pijul to solve this. A merge algorithm that leverages `git blame` information would work just as well. It's just nobody cares enough to use such a thing.
> First-class conflicts
> In Pijul, conflicts are not modelled as a "failure to merge", but rather as the standard case. Specifically, conflicts happen between two changes, and are solved by one change. The resolution change solves the conflict between the same two changes, no matter if other changes have been made concurrently. Once solved, conflicts never come back.
Conflicts coming back is not an issue in git. For some reason people think they need to use rebase when they should almost always be using merge.
> Partial clones
> Commutation makes it possible to clone only a small subset of a repository: indeed, one can only apply the changes related to that subset. Working on a partial clone produces changes that can readily be sent to the large repository.
Git and other snapshot-based SCMs do this far far better. Git can checkout only a set of files or directories, and the tree-structure encoded in git objects in its db makes this very efficient. You could even build a fuse layer to lazily fetch content. With Pijul you would have to extremely carefully maintain your history to allow this. ie, when you have a patch that modifies 2 other patches, then those are merged forever if you need the changes in the merger. Imagine a PR that reformatted all files in the repo or changes a top level interface and fixed all users in the same PR. Whoops, everything is now interdependent, no more partial clones.
It's also pretty easy to find a set of patches that have to be applied in order. Then someone copies those patches onto another divergent tree, which has it's own set of custom patches without renaming. This is dangerous in git and probably sensible in pijul.
Haven't use pijul in practice, but it's not hard to imagine being better than git here.
Fossil's implementation is the best, since a cherry-picked commit always points back to its origin.
It still required intelligence (changes across files may not be tracked as dependent but actually are) but it was a different experience from what git provides.
But for a lot of work at scale (or people) mixing bits is important.
How do you do this? With submodules / subtrees?
"This is a useless property because the graph is only encoded at the patch layer. In the real world you have far more semantic dependencies than patch dependencies. ie, I have a patch that adds a function that calls a function added in another patch. Pijul doesn't know about that."
Darcs solved this in two different ways.
1. Dependencies. While Darcs patches would inherently "depend" on the last patch which affected the same lines, the committer could specify other patches as dependencies, handling exactly the case you described.
2. Practicality. In reality, there's no scenario where someone is pulling your "use function X" patches and also not pulling the "define function X" patch. They could if they really want to, but this would be a lot of effort for a deliberately bad result. It would be like, in Git, cherry-picking the "use" patches without the "define" patches. In neither system would this happen by accident.
"Conflicts coming back is not an issue in git. For some reason people think they need to use rebase when they should almost always be using merge."
There's a big difference between "conflicts shouldn't come back as long as everyone does what I want" and "conflicts don't come back". As long as you're using Git with other people, the rebase-lovers and their problems will be a perpetual issue. I've been on 3 teams in a row with this problem.
I deliberately moved away from Darcs after a few years - the benefit of snapshot VCS is that you don't just have the change, but you have the whole context in which the change happened. (Also branch discovery in Darcs basically doesn't exist; Pijul fixed this, at least!) I love Fossil and Mercurial for their adherence to accurate history.
I don't know who would want to put in the work of mapping out the semantic dependency graph because maybe some day someone might want to compose a slightly different set of patches. And even if everyone tried, it would surely still fail because that's so hard or impossible.
> There's a big difference between "conflicts shouldn't come back as long as everyone does what I want" and "conflicts don't come back". As long as you're using Git with other people, the rebase-lovers and their problems will be a perpetual issue. I've been on 3 teams in a row with this problem.
Just stop using rebase is much easier to socialize than let's all move to Pijul. It's also the correct thing to do.
> the benefit of snapshot VCS is that you don't just have the change, but you have the whole context in which the change happened.
I strongly agree with this and think it's the only tractable way to operate.