Darcsden improvements (GSoC 2015 project)

Current status


First meeting


Progress so far

  1. Basic local (i.e. under a given root directory) viewing functionality; ability to browse local repositories, view files, list repositories (under user or global)
  2. Nested repositories (rough edges!)
  3. Metadata fetching from _darcs/darcsden


  • Writing tests is got to be the most boring part of programming…
  • But they are useful.
  • How to do UI test? Set up the Selenium testing environment.
  • Darcsden needs more fine-grained automatic tests.
  • What to implement:
    1. HTTP tests: do not use the HTTP library, use conduit-http or something else. In HTTP tests it is hard to test the output for matches; HTTP tests will be used for testing handlers (whether they fail or not).
    2. Doing HTTP tests is the easiest method of testing handlers, routes, and viewes.
    3. Particular functions can be tested directly.

Coding/recording style

  • Submitting patches upstream ASAP
  • Pulling from upstream frequently

Second meeting


Progress so far

  1. Parent repositories, branches, compare files
  2. (Almost) automatic testing for some of the functionality
    • Tests check for status codes (i.e. check for exceptions/404s)
    • Tests for descriptions
    • More to do: tests for parents, patches, comparisons
  3. Some patches for ‘vanilla’ darcsden.


I had to modify darcs and hsp to get everything working. Why?

Relative vs absolute paths

Most of the operations in darcs rely on the current directory, so darcsden sometimes uses commands like withCurrentDirectory. This is bad if you have relative paths. There are two ways of taclking this

  1. Modifying darcs with additional functionality.
  2. Making absolute paths required in parts of darcsden and making sure that this is preserved under functions.

Previously I used to modify darcs, but meddling with folder handling in darcs can be proven dangerous.

HSP and strange HTML

As for the hsp problems, it used to generate very weird HTML, that is parsable in theory, but, as it turns out, not in practice. Possible solutions (other than patching hsp): removing whitespaces inside tags & filing a report for taggy.

To get an impression of the HTML, it looked like this:

    ><link rel="shortcut icon" href="http://hub.darcs.net/public/images/favicon.ico"
    ><link rel="stylesheet" href="http://hub.darcs.net/public/bootstrap/css/bootstrap.min.css" type="text/css"
    ><link rel="stylesheet" href="http://hub.darcs.net/public/css/main.css" type="text/css"
    ><link rel="stylesheet" href="http://hub.darcs.net/public/css/combined.css" type="text/css"
    ><script src="http://hub.darcs.net/public/js/combined.js" type="text/javascript"


Majority of the issues with nested repositories are actually issues with nested directories. The biggest question right now is how do we handle the routing process? We don’t want to break the current routing too much, because that would require some severe changes to the codebase.

Possible hacks/solutions: moving repoURL function in the typeclass, and overloading it in such a way that repoURL “/my/repo/” = “/localrepo?path=/my/repo&dispatch=”, then then (repoURL “/my/repo” ++ “/patches”) would be “/localrepo?path=/my/repo&dispatch=/patches”. The downside is that we would have to implement dispatching ourselves, duplicating code and data. Another idea is to end the repo path part with double slash: //. For example, http://localhost/local/darcs/screened//patches. Probably we can modify the routing mechanism somehow to account for that.

Future ideas

Command line tooling

Command line tools: running darcsden-open in a directory should start a darcsden instance (if it’s not already running), and open a web browser pointing to the web page with that repository. Running something like darcsden-init should initialize darcs repo (if not already initialized), prompt user for metadata, and symlink the repository to a darcsden directory (unless the repository is already indexed by darcsden).

Local-only functionality

One important functionality that should be present for the local darcsden is the ability to see unrecorded changes (most preferably they should be marked and easily distinguished).

If darcsden is running fully locally (as opposed to single-user public serving darcsden), then the user should basically be always logged in as an admin. (modulo making sure someone else can’t get access by finding the right port!)

Other stuff

Listing the directories and searching for darcs repositories: could it be possible to use pipes/dirstream for that? Might give better results if you have a sophisticated directory structure and a lot of repositories. We would have to run everything in SafeT/Pipes, but we can utilize BackendPermanentM for that.

<notdan> Heffalump: can you grab this? http://hub.darcs.net/co-dan/darcsde​n-local/patch/20150610201308-21ae5
<Heffalump>	(random feature request: be able to darcs pull single patches from URLs like the above)

N-th meeting

(I sort of forgot to summarize a couple of last meetings and now I lost track of the meeting count)


Command line tool den is in development. Multiple bug/tweaks resolves in darcsden-local. Routing issues have been completely fixed (or, rather, avoided altogether).

Near-future plans

  1. Merge the changes from upstream (oh boy, I can already imagine sorting out the conflicts; that’s what you get for not doing your homework and merging early!)
  2. Extract darcsden-local into a runtime parameter (what about -DHUB? let’s hear from sm on that)
  3. Customize the frontpage business (add a frontpage of the type [User bp] -> [Repository bp] -> DDXML to the settings)
  4. (Related to 2) Add “darcden-in-local-mode” flag to the settings or somewhere like that. This will make sure that darcsden-local is sort of integrated in the “core” of darcsden, but that’s not a problem. Based on this flag we will be displaying and turning on/off certain features like unrecorded changes, repository renaming, etc
  5. The den program should start the darcsden-local server if it’s not already running. Use a lock file in /tmp for that, or something.
  6. The HTML file with redirect hack for den.

Meeting on the 12th of July

Status of darcsden-local

  • The local branch compiles with freshly released darcs-2.10.1.
  • The local username/password is stored in the configuration file.
  • Merged with some upstream changes.
  • New frontpage for the local instance.
  • The list of repositories is computed at the startup and cached (need to figure a way to force the re-computation – another handler?).
  • --local and --hub are run-time flags.
  • New typeclass for maximized customization and code reuse: DenInstance. This typeclass allows us to override the front page. In the future we shall add more features to it.
  • Live instance at http://den.updog.xyz/local.


  1. Rename EInstance to DenConfig
  2. Merge with upstream once again.
  3. Redo the indexing of repositories.
  4. Merge darcsden-cli into the main repo.
  5. Make sure that the patches button doesn’t cause the darcsden to list all the repositories/repository forks.
  6. Write a blog post about using existential types for picking/packaging up backends

The goal is to merge darcsden-local in one or two weeks.

The indexing mechanism

Instead of searching for the repositories in a directory we should maintain an explicit list of repositories (in ~/.darcs/ or ~/.darcsden/). Existing repositories can be added to that list from the command line.

Alternative idea: use search patterns to specific only a small subset of directories to search. foo/* looks for repositories in foo and foo/** looks for repositories in foo and subdirectories of foo.

Mini-meeting on the 15th

  • A huge rebase of the patches
  • The repository list is grabbed from ~/.darcs/darcsden_repos (or windows equivalent). if the file is not present we do a recursive search of the repositories in homeDir
  • den --add repo1 repo2 for adding repositories to the list


How to make sure that den and darcsden are in sync?

I used to have code in darcsden that would touch a file in the /tmp folder, and then den would check for that file before starting a server. If the file is present it would not start the webserver, it would just open the URL. There are a number of issues with this approach

  1. Portability
  2. Hard to remove that temp file on exit (where exactly do we hook in signal handlers?)
  3. That file would have to be either hardcoded or read from the configuration file. Either way any other program can touch that file.

A separate issue: den tries to start a web-server, but the port is already in use. This resuts in an exception that we can catch and try to start a web server on a different port. Problem: startHTTP never returns, so we never know if we have actually been successful at starting the server. But if we don’t know the port that we are using, we cannot launch a web browser!

An alternative would be to ping the hostname/port see if its running. Repeat until we find a fresh port. Potential issues:

  1. Something other than darcsden can be running on that port. Or, darcsden can be running at that port. We have no idea.
  2. Port is busy, but the server is not accepting connections.
  3. Connection times out/takes a lot of time to establish – this introduces a lag in the program.

The copout: just try to bind on the port mentioned in the settings; if it’s already allocated – too bad; exit the program.


  • one idea for the cli tool - it should automatically add a repo if you launch the tool on that repo.
  • “Last modified”: the root directory modification date is a pretty bad measurement; check the timestamp of _darcs/patches

Meeting on the 23rd of July

What’s on the agenda: rebase, try to merge, repeat. The prime goal at this point is to do what it takes to help Simon merge the -local branch.

Some progress have been made towards the “record” feature. The UI is very clunky for now, but the basic idea is that you select the changes from the list (hold down Ctrl to select multiple changes). After you press the record button you are taken to the next screen where you have a “full” list of patches (the ones that you’ve selected + deps) where you can enter your message and confirm the record. The full list of changes can be obtained in two ways. One way (which is implemented now) is to interpret a selected change as a “definite yes”. That means get that change and everything it depends on.

The UI is implemented as Selectable widget from JQuery UI. It would be much nicer to have a drag&drop UI where you would be able to drag the changes and it would drop both the change and it’s dependencies. For that I need to

  1. Precompute the dependency graph for the unrecorded changes on the server
  2. Serialize that to JSON
  3. Write the JavaScript part (yuck!) for reading that JSON and handling the UI

NB: to compute the dependency graph we should try to commute changes until we get a conflict


Race condition in darcsden-cli? The browser can error out because it may try to connect to the server before the server is started. Use MVars for synchronization.

How to rewrite history?

Rewriting history is art. We need to make the history cleaner so it is easier to merge the branch, but we also don’t want to get rid of, well, the history of the repository. Heffalump and sm offer some advice and discuss the matter.

Project idea

I (Daniil Frumin) want to work on the improvements for the darcsden project. Darcsden is a web platform, akin to github or bitbucket, for hosting darcs repositories. There is a couple of improvements I had in mind in particular.

Idea 1: Develop Darcsden as a local darcs UI

Right now it is not a trivial task to install darcsden. It uses a third-party software like Redis and CouchDB. However, I believe that Darcsden can be a good choice for local (or lightweight single user) darcs hosting/UI. I plan on abstracting Darcsden from the concrete DB interface, making it more flexible, so it could work with CouchDB and with plain text files as well. Making issue tracking use plain text files is also a first step to a distributive issue tracking.

As Ganesh noted, reusing darcsden for the purpose of local GUI can be beneficial. So, after implementing this idea, a user could use darcsden locally, or on her server, as a replacement for cgit/git-web.

Idea 2: Enhance the easiness of obtaining code from Darcsden

A user should have an option to download the current source code in .zip or .tar.gz format. It should also be possible for user to download a .zip archive based on the specific set of patches she picks.

Idea 3: Improved patch submission

Ideally, the users should be able to contribute to projects on Darcsden without configuring any third-party software (like sendmail), and without forking the repository (like it is done on GitHub).

The idea is that a user X works on a repository R that he cloned from DarcsDen’s user Y. When X wants to submit his patches he should be able to “send” a bundle of patches via HTTP to darcsden. The patches thus will be registered in Y’s repository and Y can apply them.

Partial bits of HTTP push support are already implemented, at least the HTTP support on the darcs part, but the code might have bitrotted by now, due to the lack of integration on the darcsden side.

Ideas (1) and (2) are “plan minimum”, the bare minimum that I would like to achieve. If everything goes smoothly, I will be able to finish idea (3) as well.

Frequently Raised Questions

  • In what ways will this project benefit the wider Haskell community?

Darcs is a very interesting DVCS, and Darcsden is a de-facto web interface used for darcs hosting. The popular website http://hub.darcs.net, which hosts a variety of Haskell-related projects, is powered by Darcsden.

  • Can you give some more detailed design of what precisely you intend to achieve?
  1. During the community bonding period, and the beginning of the working period, I would like to spend time working on abstracting the database interface further, which is necessary for the idea (1). I would also like to start fostering discussion about the possible design choices necessary for part (3), i.e. how the submitted bundles should be presented?

  2. During the next stage, up to mid-term evaluations, I plan on making Darcsden available for local use, without any other third-party software. I plan on having at least a beta-version available for the mid-term deadlines. This would involve implementing additional backends (based on SQLite or plain-text files). I would also like to start working on making darcsden available as a single-user instance (similar to gitweb).

  3. During the third stage, until the beginning of September, I plan on finishing the work on idea (1) and idea (2). If I have further time, I would like to start implementing the HTTP patch push support for Darcsden. This would require making some design decision beforehand (see point (1)).

  4. From the beginning of September and until the “pencils down” deadline, I plan on finishing things up, and writing the documentation, where necessary.

  • What relevant experience do you have? e.g. Have you coded anything in Haskell? Have you contributed to any other open source software? Been studying advanced courses in a related topic?

I have been coding in Haskell for a bunch of years now. In 2013 I successfully completed a Haskell GSoC: http://www.google-melange.com/gsoc/project/details/google/gsoc2013/difrumin/5662278724616192. I wrote an interactive pastebin that compiled Haskell code to JavaScript, using GHCJS, and rendered Haskell diagram as images, or as interactive widgets.

I have contributed to a bunch of open-source Haskell projects, including Diagrams, GHCJS, Hastache, and Darcsden itself. Granted, it has been over a year since I wrote my last patch to Darcsden/darcs, but I still remember my way around the code, and I will be able to familiarize myself with the updated codebase fairly quickly.

  • Why do you think you would be the best person to tackle this project?

Firstly, I love darcs, and I use it for my personal projects and writings. Secondly, I love programming in Haskell. In fact, Haskell spoiled me to the point that I curse myself when I have to code something not in Haskell. Finally, I have previously contributed to Darcsden and darcs.

  • In what ways do you envisage interacting with the wider Haskell community during your project? e.g. How would you seek help on something your mentor wasn’t able to deal with? How will you get others interested in what you are doing?

I would blog regularly about my progress, and post updates to the darcs-devel mailing list, mentioning problems and pitfalls I’ve encountered on the way. I would also be up for having weekly/bi-weekly public “meetings” in the IRC channel, if that would be deemed necessary. If I get stuck and my mentor is not being able to deal with some problems I will not hesitate to seek help on haskell-cafe mailing list or on reddit/stackoverflow. I also plan on rolling out a work-in-progress version of the project online for people to play with. The code for the project will be available on http://hub.darcs.net.