Darcs what, why and how. Ganesh Sittampalam London HUG, 24 th April 2013

Transcription

1 Darcs what, why and how Ganesh Sittampalam London HUG, 24 th April 2013

2 So what is it? A distributed version control system like git, mercurial, etc and unlike svn, cvs, etc Based on patches as first-class objects unlike git, mercurial, etc

3 Hasn t git won? If Darcs didn t exist, it d be worth writing now A fundamentally different model Hybrid future - bridges to git etc

4 First-class patches? A philosophical difference like Haskell versus C# A practical difference Cherry-picking is cheap and reversible Merges are deterministic

5 Darcs philosophy A simple, intuitive user model (we like to think) A simple command set Commands are interactive by default

6 What doesn t darcs have? Multi-head repos Good conflict handling Short revision identifiers Github Lots of tools Lots of users Extreme speed

7 Demo

8 Rebase Coming in darcs 2.10 Much less necessary than in tree-based systems No need to tidy up history to remove merges Conflict-free cherry-picking isn t a rebase Basic operations: suspend, unsuspend

9 Demo

10 Commands Local init record unrecord amend-record revert unrevert rollback tag Specific patch types add move remove replace Querying changes whatsnew diff annotate Remote get put pull push obliterate (unpull) send apply

11 What is a darcs patch? A description of how to change a tree e.g. remove text X starting at lines 3 and add text Y instead Add file A Rename file B to C Replace token X with token Y in file Z

12 Darcs patches are invertible e.g. remove X and add Y at line 3 remove Y and add X at line 3 An important part of reversible cherry-picking Darcs underapproximates dependencies, most systems overapproximate them

13 Deterministic merging Merge algorithm part of patch type No merge commits Merges are reproducible Merging is associative

14 Internally, the inverse of merging is commuting: Tree A B? intermediate B final A A A A base B Tree B initial B? We write AB BA To cherry pick, commute unwanted patches to the end

15 A darcs repository is set of patches Stored in an order - this shows up in the UI Two repositories with the same patches but in different orders will be equivalent Patches retain their identity in different contexts But can change their representation

16 A bit more theory No clear formalism But there are some laws we believe are true e.g. AB B A B A AB B -1 A A B -1

17 Permutivity A C B C A B A B C B C A

18 Permutivity A ABC C B C A B A B C B C A

19 Permutivity A ABC C B C B A C A B A B C B C A

20 Permutivity A ABC C A B C B B A C B C A A B C B C A

21 Permutivity A ABC C A B C B B A C B C A C B A A B C B C A

22 Permutivity A ABC C A B C B B A C B C A C B A B C B A C C A B A

23 Permutivity A ABC C A B C B B A C B C A C B A B C B A C C A B A C B A

24 Permutivity A ABC C A B C B B A C B C A C B A B C B A C C A B A C B A B C A

25 Patch witnesses Patches need to be moved between contexts Representation sometimes changes Easy to get it wrong

26 (AB)C? data Patch = Null Seq Patch Patch commute :: (Patch, Patch) Maybe (Patch, Patch) commute (Seq a b, c) = do (c, a ) commute (a, c ) (c,b ) commute (b, c ) return (c, Seq a b )

27 data Patch a b where Null :: Patch a a Seq :: Patch a b Patch b c Patch a c Bring in GADTs! data PatchPair a b where Pair :: Patch a b Patch b c PatchPair a c commute :: PatchPair a c Maybe (PatchPair a c) commute (Pair (Seq a b) c) = do Pair c a commute (Pair a c) Pair c b commute (Pair b c ) return (Pair c (Seq a b ))

28 witnessed.hs:14:37: Couldn't match type `b' with `b1' `b' is a rigid type variable bound by a pattern with constructor Pair :: forall a c b. Patch a b -> Patch b c -> PatchPair a c, in an equation for `commute' at witnessed.hs:13:10 `b1' is a rigid type variable bound by a pattern with constructor Seq :: forall a c b. Patch a b -> Patch b c -> Patch a c, in an equation for `commute' at witnessed.hs:13:16 Expected type: Patch b1 c Actual type: Patch b c In the second argument of `Pair', namely `c' In the first argument of `commute', namely `(Pair a c)' In a stmt of a 'do' block: Pair c' a' <- commute (Pair a c)

29 What s next? Release 2.10: rebase, fast annotate Finish and develop git bridge Improve hosting Graphical tools Multi-head repos Better conflict handling Develop more patch types

30 More info Hosting: Participating in GSoC (via haskell.org) - please apply!