Darcsden improvements and Darcs sprint

This post is intended to be a short summary of my SoC project, as well as my recent trip to Darcs sprint.

Introduction

I am finishing up this post on the train back from the Autumn 2015 Darcs sprint. Today (Sept 20, Sun) was a very fun day full of darcs chatting and coding. By the end of the day we’ve heard a number of presentations

• Ganesh described his work on "stash" command for darcs (naming subject to change!). It involves some refactoring of the rebase code. I hope we would hear more from him on that, because the internal workings are actually quite interesting — I believe it’s the first time singleton types and DataKinds are used in the darcs codebase;
• Florent Becker gave a presentation about Pijul and the theory behind it — A Categorical Theory of Patches by Samuel Mimram and Cinzia Di Giusto, see arXiv:1311.3903;
• Vinh Dang talked about his improvements on the darcs wiki (it’s about time to organize the website), his goal was to make it more accessible to the newcomers;
• Yours truly gave a small presentation, outline of which you will find below:

Looking back

I have spent this summer hacking on DarcsDen as part of the Google Summer of Code program.

My basic goal was to create a "local" version of darcsden. It was not a trivial task to install darcsden (and probably installation is still not very easy!). It uses a third-party software like Redis and CouchDB. During my coding process I modifed darcsden such that it now can be a good choice for local (or lightweight single user) darcs UI. The local darcsden version can be used without any databases, tracking the repositories in the local file system. This way darcsden can be used by a developer on her local computer, like darcsum, (for working with/comparing repositories) as well as a replacement for darcsweb/cgit — a single user web front for darcs repositories.

Besides that a user of a local version can use darcsden’s interactive UI for recording new patches, as well as a command-line tool den for a quick way of browsing the repositories.

Installing darcsden-local is currently not as easy as I want to it be, but I hope that soon you will be able to install it just by running cabal install darcsden or brew install darcsden. As for now, one could do the following:

1. darcs get --lazy http://hub.darcs.net/co-dan/darcsden-local
2. cabal install . or stack install

This should install the darcsden binary and all the related css/js files. You can start darcsden by running darcsden --local. If you open your web browser you should see a list of repositories in the current directory.

However, you might have repositories scattered all over the place, and scanning your whole system for darcs repositories is just inefficient. For this purposes darcs keeps a list of repositories in a file inside your ~/.darcs directory. You can manage that list either by hand, or using the command-line den tool:

• den $PATH — add $PATH to the list of repositories in ~/.darcs/darcsden_repos (if it’s not already present there), start darcsden server in the background and launch the browser pointing to $PATH;
• den — the same as den .;
• den --add $PATH — add $PATH to the list of repositories in ~/.darcs/darcsden_repos;
• den --remove $PATH — remove $PATH from the list of repositories in ~/.darcs/darcsden_repos.

In order to further customize darcsden, one can tweak the configuration file located at ~/.darcs/darcsden.conf. Apart from the usual darcsden settings one may pay attention to the following variables:

• homeDir (default .), points to the "root" directory with repositories. If the list file ~/.darcs/darcsden_repos is not present darcsden will recursively search repositories in that directory
• unLocal, pwLocal: the username and the password of the "local" user

The user/password credentials are required for editing the repositories and recording new patches. However, the den binary should automatically pick them up from the config file and log you in.

Once you are logged in, and you have unrecorded changes in the repository, you can use darcsden UI to record a new patch.

DarcsDen record

Below you can see an example of recording and merging patches from a branch.

DarcsDen merge

Darsden allows you to create forks/branches of your repositories, and it keeps track of the patch dependencies in your branches.

More "internal" changes:

• Instead of having to specify some parts of the configuration in DarcsDen.Settings, darcsden now uses runtime flags: –hub for using hub-specific modifications, –local for using the local backend and no flag for default behaviour
• The flag actually choose what are called instances — something that a bit less fine-grained than settings. Instances allow you to pick backend, overwrite settings, modify the looks of the front page.
• HTTP-testing using wreq. The previous test suite used selenium and it got bit-rotten. The wreq-based is easier to run and perhaps slightly easier to maintain.
• HTTP auth, which is used as part of the local instance; the den tool utilizes it to log the user in automatically.
• Support for repositories inside directories and nested repositories.
• All the backend code that is used for handling repositories and meta-data on the file system.
• Functionality for downloading zipped dist archives of darcs repositories.
• Assorted mini-fixes

What now?

During the sprint I hacked together some code for viewing suspended patches along the regular ones. The next step would be to have a similar interface for managing the suspended patches.

We have also discussed the possibility of adding rewrite rules implementing short-cut fusion for the directed types in Darcs. In order to see if it’s really worth it we would have to bring back to life the benchmarking suite (or at least check on it!).

It was a really exciting weekend for me and I was delighted to meet some of my IRC friends. As it turns out, it is a small world and despite being from different parts of it we have a bunch of common IRL friends, professors. As the French would (probably not) say, très bien. The next darcs sprint will probably be in January, and probably in Europe, again.