Hacker News Reader

Comment by leeoniya | Hacker News Reader

> Semantic diff – Can we figure out how to use version control to have more context-aware merges? Can you believe that we still rely on a text diffing algorithm from 1976 (and its shortcomings)? Git still has trouble with file renaming. GitHub Copilot, but for merge conflicts? Semantic diff has been tried before, but language-specific implementations will likely never work.

in case anyone missed it: https://github.com/Wilfred/difftastic

View on HN

> Difftastic output is intended for human consumption, and it does not generate patches that you can apply later.

To me this looks like integrating it with tools of actual conflict resolution during merges will be a bit harder than one would like. I'd be glad to be wrong.

git mv?

That's just a convenience command. Git doesn't actually record moves as anything different from a delete and an add. Many of the querying commands (eg `git log`) use heuristics to show moved files, but they end up wrong fairly often, especially if you don't mess with the parameter(s) of the heuristics.

In my experience they are correct all of the time for simple renames. It's when you move a file and make substantial edits that it gets confused.

I think it's reasonable to argue that git shouldn't get confused in this scenario, but you could also do your renames in one commit and your changes in another.

Would it make sense to make one commit for the move and one for the changes?

I wish I didn't have to think about it and git supported file renaming properly so a unit of change that hasn't much sense when it is split can stay a unit, but without it yes, I think it makes sense to commit renames separately so history can be tracked more easily.

I think git can be configured on how hard it tries to find renames from similarity between a deleted file and an added file.

Yes, in some cases, but it depends on your workflow. Some projects require each commit on the main branch to be fully working and pass all the tests.

No, because you often have to make changes to the file to get it to compile after loving to a new location. It's much more annoying to review history with changes that can't possibly compile.

This isn't necessarily an issue, depending on what you mean by "review". The one case I really run into is bisects, but even for that you can tell it to just skip commits that won't build.

No, it makes no difference. Git doesn't look at the full history when performing a merge, it just looks for similarity between the paths and files in the commits being merged, and the common base commit (which you can discover by running `git merge-base`). It doesn't keep any metadata about whether something was a move: as far as the actual data structures are concerned, you deleted file a/b/c and added file d/e/f, that have the same file content hash.

You can find more information on how Git now does it at https://blog.palantir.com/optimizing-gits-merge-machinery-1-... . I think this first started to be released in v2.31.0, and completed in v2.33.0. v2.34.0 switched the default merge strategy to the new 'ort' strategy mentioned in that blog.

The "git log" "follow renames" logic isn't the same as conceptually related merge logic.

I think there are unfortunately still cases where what the GP is suggesting improves the UX, i.e. I think some shortcuts are taken when following a file if the content doesn't change.

IIRC this matters particularly for very large renames, at some point during revision walking we'll give up trying to match your A.txt to B.txt, but if they're the same...

> It's when you move a file and make substantial edits that it gets confused.

It's your POV that it's the same file. One could argue that it's a new file and the content of the old one embed in the file. That's the huge problem with git users, people cling desperately to the idea of changes, when git is just about snapshots.

People "cling desperately" to the way they are actually working. That Git uses an alien model that happens to somewhat match what I'm doing doesn't mean I'm "desperately clinging" to the actual reality of what I'm doing (editing a file).

So if someone creates a new file and integrate the content of the old file to reach the same folder state than yours, you think s/he does something different?

Yes? Editing a file is different from creating a new file with some code copied from somewhere else, obviously. They may reach the same end state, but that doesn't make them the same operation.

It also happens that one is much, much more common in software development than the other, so it's a much better mental model of what SEs do.

> They may reach the same end state, but that doesn't make them the same operation.

So you're clinging to your mental model. Thanks for proving my point.

This is how work is actually done, and my mental model matches how I and my colleagues and yours as well actually work.

I could contort my mental model to make it match how Git works, but it is a contortion that some tool imposes on me.

We work in computing, I would expect everyone's mental models to be computational, not equational - different algorithms with the same result are still different. QuickSort is not the same as Merge sort just because they have the same inputs and outputs.

The issue is that the path you used to go from A to B is yours, one could use a totally different path. But what really matters is that your product is now in state B. That's what you ship, not the path.

Git has both concepts, snapshots and changes. You'd have to contort yourself more than a bit to describe common uses of "git rebase" without viewing it as changes, for instance.

You may be aluding to git's underlying storage being snapshot based, but it actually also has diffs in pack files.

> but it actually also has diffs in pack files

Storage optimization, absolutely unrelated to the diff between two snapshots.

The semantic layer uses both, and the implementation layer uses both. Saying that it's one or the other just seems misguided.

I don't understand your point. Mine was just that git could decide to store only snapshots, it would change nothing except for low level commands that deal directly with objects storage.

Internally just rms and adds. The "Rename" feature is just a UI feature displayed when the files have identical or near-identical content.

Reply on news.ycombinator.com