Comment by pmeunier | Hacker News Reader

Comment by pmeunier | Hacker News Reader

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Sure, but if you had read the manual[1], you would have learned that this is only the associativity part, which you could indeed use to provide a better merge algorithm in a snapshot-based VCS.

So:

1. You're 100% correct in saying that this can be done in a snapshot-based VCS like Git, Mercurial or SVN.

2. But you're also missing the main feature of Pijul, patch commutation, also explained in the manual. The idea is that you don't need to rebase anymore, two patches produced independently always commute, and hence patches don't need to change their identity when they are "rebased", or merged at any point in time. This has major usability benefits, such as being able to undo a "merge" (I prefer to call it a patch application) even after ten other "merges". Or choosing which part of a conflict to remove in order to solve it (if you don't choose to actually solve the conflict). With patch commutation, you don't need `git rerere` anymore, and you don't even need to branch very often, since branch are essentially an emulation of commutation (then of course Pijul still has branches, but they're quite different from Git's branches, in particular they are not as essential as in Git).

But then Pijul is still a young project, there are still a few bugs here and there. It is self-hosted, and we're quite happy using it for its own development (and the development of all companion libraries, Thrussh, Sanakirja…). But is it really ready for HackerNews-level flamewars?

[1] https://pijul.org/manual/why_pijul.html|the manual

> In Pijul, for any two patches A and B, either A and B commute, (in other words, A and B can be applied in any order), or A depends on B, or B depends on A.

This isn't exhaustive. What about the case where both patches A and B are worked on independently, and both depend on an original version of the codebase, let's call it P for parent.

A and B both diverge from P and therefore require a merge, or in git, a rebase. Over my years as a developer this scenario has caused approximately 90% of the issues with git. How does Pijul tackle it?

You can simply incorporate both patches A and B, you don't need a separate "merge" step.

If A and B have conflicts, then the resulting repo will have conflicts (but that's allowed in pijul) and you can resolve those conflicts with a third patch, C.

The nice thing is that this patch C can be used to resolve those same set of conflicts, even if someone else has a completely different history. In git, when you perform a rebase, the identity of commits is lost, and so you have to constantly re-resolve the same merge conflicts when merging between branches where one of the branches has been rebased.

That sounds great.

What about the situation where A and B don't have conflicts (as defined by the VCS), but still don't work. For example, based on a state of a piece of source code, e.g. A renames a method and all usages (works fine), and B introduces new code (but uses the original method name, also works fine). Merging them will not produce conflicts, but will not work.

In Git I always use "--exec 'mvn clean test'" when using "git rebase", to compile the source code (which would find such rename issues) and run unit tests (which would find other issues, e.g. A introduces a new mandatory column in a database table and B introduces an INSERT statement without that column).

What would be really cool is if one could say to Pijul "for any operations on this repository, execute this command to determine if the software is OK". And then forget about it. That way all operations it does it could check, and the user could never forget. I wish Git had something like that.

Or have I misunderstood something? I mean, simply relying on "no conflicts implies everything's fine" is not sufficient as far as I can see?

I would say that is somewhat outside the jurisdiction of the VCS. Sure you can have post-commit or CI hooks that ensure your code always compiles, but it doesn't need deep support from the VCS to do that.

However, the fact the pijul doesn't really treat the "no conflict" state as "special" is in some ways closer to what you want - the repo is just a collection of patches, and whether the result is "good" or "bad" is not determined by the VCS, but by whatever means you choose to employ.

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