2. Let’s start with some questions:
What is a repository?
What is a commit?

5. To the terminal!

6. What is a repository? Repository . ├── .git │ ├── branches
│ ├── COMMIT_EDITMSG
│ ├── config
│ ├── description
│ ├── FETCH_HEAD
│ ├── HEAD
│ ├── hooks
│ ├── index
│ ├── info
│ ├── logs
│ ├── objects
│ ├── ORIG_HEAD
│ ├── packed-refs
│ └── refs
├── some files
├── more files
└── even more files
Repository

7. What is a commit? $ git cat-file -p 2dfc9fe
tree fcfde60a5d645769d536f9b7c ed225a14
parent e4fdd0e4e49842d50f75e4d80f555fa783c0709f
author Alan Du
committer Alan Du
Store fractional seconds as a u32 instead of a f64
Drops precision down to the nanoseconds

9. What are branches? Refs are “nicknames” for commits
Branches are “auto-updating” refs (via HEAD)

10. What does this mean practically?
git checkout COMMIT
git checkout BRANCH
git checkout REF

11. What does this mean practically?
Is it:
git checkout origin/master
git pull origin/master
Or is it:
git checkout origin master
git pull origin master

12. What does this mean practically?
Is it:
git checkout origin/master
git pull origin/master
Or is it:
git checkout origin master
git pull origin master

13. What does this mean practically?
git bisect start
git bisect bad COMMIT
git bisect good COMMIT
Binary search to find which commit introduced the bug

14. What does this mean practically?
Merge, baby, merge? (git merge)
Or rebase all the things? (git rebase)

15. So clearly we’re done right?

16. Git is a “snapshot”-based system
Commits store the current tree
There’s no first-class notion of diff or patch!!!
Git’s ability to deal with diffs is ad-hoc and fundamentally flawed

17. What’s wrong with snapshots?
git cherry-pick
Let’s say you’re the cpython project and have:
master
3.6
3.5
3.4
2.7

18. What’s wrong with snapshots?
git cherry-pick
Cherry-picking changes the identity of the commit!
So if you cherry-pick two related commits in the wrong order..

20. It gets worse A
/ \
B B New feature!

21. It gets worse A
/ \
B B New feature!
| |
A A Revert the change!

22. It gets worse A / \ B B New feature! |\ /| A X A Revert the change!
|/ \|
B B Merge each other’s work

23. It gets worse A / \ B B New feature! |\ /| A X A Revert the change!
|/ \|
B B Merge each other’s work
\ /
B WTF?

24. ≠ It gets worse Git’s merge algorithm is fundamentally flawed
In general:
B1 – B2
/ \ \
A \ \
\ \ \
A1 – A2 – A3
B1 – B2
/ \
A \
\ \
A1 ––––– A3 ≠

26. The Fundamental Problem
is that diffs in git are second-class citizens
So… why don’t we just make diffs first-class citizens?
Can only add or delete lines (no editing!)
Each patch records the “ancestry” of each line (or deletion)

27. Patches Compose

28. Commutative Diagram

29. The Merge Commutative Diagram

30. Not a merge

31. Not a merge

32. But that isn’t unique! We want the “smallest” merge
(In the language of category theory, we want the pushout).

33. One Technical Detail
The pushout isn’t always a file!

34. One Technical Detail The pushout isn’t always a file!
In category theory terms, need the free co-completion

35. How does this help us?
How to revert shoes?

36. Quick Summary What are: Git repositories? Git commits? Git branches?
What’s wrong with snapshot-based systems?
Baby patch theory (inspired by Darcs and Pijul)

37. References https://git-scm.com/book/en/v2
good-semantics.html
the-inside-out

38. Questions?