4 ms·

Why not summarize it a little if you feel that info got lost. Technically if you need a patch you can generate it on the fly by comparing both objects (you can

by runyor 8y ago

Why not summarize it a little if you feel that info got lost. Technically if you need a patch you can generate it on the fly by comparing both objects (you can even do that in a bash script with `diff`, if you are willing to type in the logic to look up commit->tree->file->object-name first). So there shouldn't be anything lost. The only parameters I can see with storing diffs vs immutable objects is disk space vs processing time, which is also what git proofs by not storing old immutables and instead store diffs for old stuff (reducing space by increasing processing time).

vthriller 8y ago
> Technically if you need a patch you can generate it on the fly by comparing both objects Automatically generated patches might not work in the long run in certain situations (e.g. when standard 3-line context is repetitive, making the patch applicable at multiple places in the same file). Those patches also suck at conveying actual file changes (how many actual changes start with "- }" or contain lots of context braces?); that eventually prompted me to do lots of split commits in git whose sole purpose is to make automatic diffs more readable (e.g. separate commit for indentation fix). That brings the question: can Pijul store user-formatted patches?
pmeunier 8y ago
No, it's actually related to the ease of use and correctness. By design, Git lacks two fundamental properties that would make working with repositories much easier: 1. Git merge is not associative. This results in cases where Alice and Bob work together, Bob pulls Alice's work, and her lines get merged into places she's never seen (blocks of text newly introduced by Bob). 2. More importantly, Git is not commutative. This means that cherry-picking and rebasing are complex operations that change commits' ids. This forces Git users to branch (an extra step that needs to be done before starting to write), but more painfully, it sometimes forces them to do multiple steps of rebasing before being able to merge what they want, or to solve the same conflict again and again after an unlucky cherry-picking. Of course, all this could maybe be prevented by stricter planning and a more vertical organisation. But this is not how many people write code in 2019. Continuous delivery, for instance, means that teams no longer know long in advance what they are going to over the course of the project. Also, the best developers don't work in vertical teams where they get continuously told what to work on.
feanaro 8y ago
Sorry, I was in a hurry and I think I mixed up articles since I think another one recently appeared on HN which pointed to a page with a much better explanation. The main idea behind Pijul (as I understand it) is that it makes merging divergent branches correct and predictable by making patch application an associative operation.[1] What this means is that that, given patches A, B and C, it should be irrelevant whether you are: 1. starting with A, applying B on top, followed by applying C 2. starting with A, applying the result of C applied on top of B all at once In other words, (AB)C = A(BC) This isn't always what happens in git because it doesn't work with patches on an abstract level. Instead, it always works with states of the branch heads (and sometimes the state of their branching point, i.e. BASE, in case of a 3-way merge). As to why this matters, [2] and [3] has practical test cases where the difference of these two approaches is observable. [3] is an example of plausible C code where the non-patch approach may produce an incorrect result. It also demonstrates the way the patch algebraic approach is able to take individual changes into account and apply edits from the second branch in the correct place in the first branch, even though the affected code has moved in the first branch in the meantime. The focus on patches has other important implications, such as the fact that branches then simply become sets of patches and cherry-picking retains the identity of patches instead of creating new, unique commits. The other important aspect of Pijul is use of efficient data structures that are naturally suited to the problem in order to avoid suboptimal algorithmic complexity. This is explained in more detail here[4]. [1]: https://pijul.org/manual/why_pijul.html https://pijul.org/manual/why_pijul.html [2]: https://tahoe-lafs.org/%7Ezooko/badmerge/simple.html https://tahoe-lafs.org/%7Ezooko/badmerge/simple.html [3]: https://tahoe-lafs.org/%7Ezooko/badmerge/concrete-good-semantics.html https://tahoe-lafs.org/%7Ezooko/badmerge/concrete-good-seman... [4]: https://pijul.org/model/ https://pijul.org/model/
- runyor 8y ago
  I disagree with some of that. But first thanks for explaining more in-depth what is behind pijul. The main subthread I started because I didn't understand what it provides. If it can combine easier UX with more efficient diffing, then I think it is a very valuable contribution, actually. Now some more in-depth bla bla if interesting: > (AB)C = A(BC) great feature request, I agree. > This isn't always what happens in git correct. > because it doesn't work with patches on an abstract level. Instead, it always works with states Incorrect though. States, patches, these are just trade-offs. You can represent either in the other completely. Like you can build a list using a tree structure if you just allow one branch. Or you can also build a tree on top of a list structure, if your traversal algorithm knows which item-index to pick for a certain subtree's children. All trade-offs. That doesn't mean "Pijul does better diffs" would be wrong, though. It can still be the case. But it doesn't mean that git would need huge refactoring to also implement that better-diff-algorithm. In the end implementing this better algorithm in git might be trivial for a git core developer if you can explain to him how it works. If you think about it a diff between two states has an unlimited way of being represented. And considering minimal steps to generate the diffs with adding lines to the diff and removing lines from the diff, the whole thing is an abstract tree. Basically to achieve associative patches one needs to make sure to always traverse this tree in the same order. Git traverses greedily though, using the very first diff that is good enough as a final result. Probably the idea behind this was also smart. Do it quickly for now, and optimize it if needed later.

by runyor 8y ago

vthriller 8y ago

> Technically if you need a patch you can generate it on the fly by comparing both objects Automatically generated patches might not work in the long run in certain situations (e.g. when standard 3-line context is repetitive, making the patch applicable at multiple places in the same file). Those patches also suck at conveying actual file changes (how many actual changes start with "- }" or contain lots of context braces?); that eventually prompted me to do lots of split commits in git whose sole purpose is to make automatic diffs more readable (e.g. separate commit for indentation fix). That brings the question: can Pijul store user-formatted patches?

pmeunier 8y ago

No, it's actually related to the ease of use and correctness. By design, Git lacks two fundamental properties that would make working with repositories much easier: 1. Git merge is not associative. This results in cases where Alice and Bob work together, Bob pulls Alice's work, and her lines get merged into places she's never seen (blocks of text newly introduced by Bob). 2. More importantly, Git is not commutative. This means that cherry-picking and rebasing are complex operations that change commits' ids. This forces Git users to branch (an extra step that needs to be done before starting to write), but more painfully, it sometimes forces them to do multiple steps of rebasing before being able to merge what they want, or to solve the same conflict again and again after an unlucky cherry-picking. Of course, all this could maybe be prevented by stricter planning and a more vertical organisation. But this is not how many people write code in 2019. Continuous delivery, for instance, means that teams no longer know long in advance what they are going to over the course of the project. Also, the best developers don't work in vertical teams where they get continuously told what to work on.

feanaro 8y ago

Sorry, I was in a hurry and I think I mixed up articles since I think another one recently appeared on HN which pointed to a page with a much better explanation. The main idea behind Pijul (as I understand it) is that it makes merging divergent branches correct and predictable by making patch application an associative operation.[1] What this means is that that, given patches A, B and C, it should be irrelevant whether you are: 1. starting with A, applying B on top, followed by applying C 2. starting with A, applying the result of C applied on top of B all at once In other words, (AB)C = A(BC) This isn't always what happens in git because it doesn't work with patches on an abstract level. Instead, it always works with states of the branch heads (and sometimes the state of their branching point, i.e. BASE, in case of a 3-way merge). As to why this matters, [2] and [3] has practical test cases where the difference of these two approaches is observable. [3] is an example of plausible C code where the non-patch approach may produce an incorrect result. It also demonstrates the way the patch algebraic approach is able to take individual changes into account and apply edits from the second branch in the correct place in the first branch, even though the affected code has moved in the first branch in the meantime. The focus on patches has other important implications, such as the fact that branches then simply become sets of patches and cherry-picking retains the identity of patches instead of creating new, unique commits. The other important aspect of Pijul is use of efficient data structures that are naturally suited to the problem in order to avoid suboptimal algorithmic complexity. This is explained in more detail here[4]. [1]: https://pijul.org/manual/why_pijul.html https://pijul.org/manual/why_pijul.html [2]: https://tahoe-lafs.org/%7Ezooko/badmerge/simple.html https://tahoe-lafs.org/%7Ezooko/badmerge/simple.html [3]: https://tahoe-lafs.org/%7Ezooko/badmerge/concrete-good-semantics.html https://tahoe-lafs.org/%7Ezooko/badmerge/concrete-good-seman... [4]: https://pijul.org/model/ https://pijul.org/model/

runyor 8y ago
I disagree with some of that. But first thanks for explaining more in-depth what is behind pijul. The main subthread I started because I didn't understand what it provides. If it can combine easier UX with more efficient diffing, then I think it is a very valuable contribution, actually. Now some more in-depth bla bla if interesting: > (AB)C = A(BC) great feature request, I agree. > This isn't always what happens in git correct. > because it doesn't work with patches on an abstract level. Instead, it always works with states Incorrect though. States, patches, these are just trade-offs. You can represent either in the other completely. Like you can build a list using a tree structure if you just allow one branch. Or you can also build a tree on top of a list structure, if your traversal algorithm knows which item-index to pick for a certain subtree's children. All trade-offs. That doesn't mean "Pijul does better diffs" would be wrong, though. It can still be the case. But it doesn't mean that git would need huge refactoring to also implement that better-diff-algorithm. In the end implementing this better algorithm in git might be trivial for a git core developer if you can explain to him how it works. If you think about it a diff between two states has an unlimited way of being represented. And considering minimal steps to generate the diffs with adding lines to the diff and removing lines from the diff, the whole thing is an abstract tree. Basically to achieve associative patches one needs to make sure to always traverse this tree in the same order. Git traverses greedily though, using the very first diff that is good enough as a final result. Probably the idea behind this was also smart. Do it quickly for now, and optimize it if needed later.