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Notes for Julia Contributors

Hi! Thanks for checking out Julia. If you have questions or problems, the Julia dev list is a good place to post them, but you should check out the online Julia docs first. If you have changes you'd like to contribute, you'll need a free GitHub account. (If you're reading this on GitHub, you probably already have one.) You'll also want a working copy of Julia, built from source (you can still contribute with a binary install, but it's harder and not really recommended). In list form, here's what to do to become a contributor:

  • Join the dev list.

  • Create a GitHub account.

  • Fork Julia.

  • Build the software and libraries (the first time takes a while, but it's fast after that). Detailed build instructions are in the README. Julia depends on several external packages; most are automatically downloaded and installed, but are less frequently updated than Julia itself.

  • Keep Julia current. Julia is a fast-moving target, and many details of the language are still settling out. Keep your repository up-to-date and rebase your work in progress frequently.

  • Learn to use git, the version control system used by GitHub and the Julia project. Try a tutorial such as the one provided by GitHub.

  • Relax and have fun.

How to file a bug report

A useful bug report filed as a GitHub issue provides information about how to reproduce the error.

  1. Before opening a new GitHub issue:

    • Try searching the existing issues or the julia-users mailing list to see if someone else has already noticed the same problem.
    • Try some simple debugging techniques to help isolate the problem.
      • Try running the code with the debug build of Julia with make debug, which produces the usr/bin/julia-debug.
      • Consider running julia-debug with a debugger such as gdb or lldb. Obtaining even a simple backtrace is very useful.
      • If Julia segfaults, try following these debugging tips to help track down the specific origin of the bug.
  2. If the problem is caused by a Julia package rather than core Julia, file a bug report with the relevant package author rather than here.

  3. When filing a bug report, provide where possible:

    • The full error message, including the backtrace.
    • A minimal working example, i.e. the smallest chunk of code that triggers the error. Ideally, this should be code that can be pasted into a REPL or run from a source file. If the code is larger than (say) 50 lines, consider putting it in a gist.
    • The version of Julia you are using as provided by the versioninfo() command. Occasionally, the longer output produced by versioninfo(true) may be useful also, especially if the issue is related to a specific package.
  4. When pasting code blocks or output, put triple backquotes (```) around the text so GitHub will format it nicely. You can format code statements by surrounding it in single backquotes (`). Be aware that the @ sign tags users on GitHub, so references to macros should always be in single backquotes. See GitHub's guide on Markdown for more formatting tricks.

Submitting your contributions

Contributing a Julia package

Julia has a built-in package manager based on git. A number of packages across many domains are already available for Julia. Developers are encouraged to provide their libraries as a Julia package. The Julia manual provides instructions on creating Julia packages.

For developers who need to wrap C libraries so that they can be called from Julia, the Clang.jl package can help generate the wrappers automatically from the C header files.

Writing tests

There are never enough tests. Track code coverage at Coveralls, and help improve it.

  1. Go visit

  2. Browse through the source files and find some untested functionality (highlighted in red) that you think you might be able to write a test for.

  3. Write a test that exercises this functionality---you can add your test to one of the existing files, or start a new one, whichever seems most appropriate to you. If you're adding a new test file, make sure you include it in the list of tests in test/choosetests.jl. may be helpful in explaining how the testing infrastructure works. Submit the test as a pull request (see

Issue 9493 has a more detailed discussion on code coverage.

Improving documentation

By contributing documentation to Julia, you are agreeing to release it under the MIT License.

Julia's documentation is stored in the doc directory, and like everything else can be modified using git. However, for small changes one can also use GitHub's web interface:

  • Navigate to
  • Click doc
  • If you want to modify an entry in the help for Julia's standard library, click stdlib
  • Pick the file you want to edit (for example, base.rst)
  • Select the master branch (if not browsing it already)
  • Click "Edit"
  • Click on the icon that looks like a fullscreen symbol ("Zen" mode)
  • Search for the function you want to change
  • Make your changes
  • Exit Zen mode
  • Provide a title, and optionally a longer description of your change
  • Submit your change

Julia's documentation is built with Sphinx, which supports (and Julia's docs rely heavily on) ReST directives. To build the documentation locally, run

make -C doc html


make -C doc latex

from Julia's root directory. Sometimes errors only show up in one of them, so if you're preparing a pull request it is nice if you've checked both formats before you submit.

Contributing to core functionality or base libraries

By contributing code to Julia, you are agreeing to release it under the MIT License.

The Julia community uses GitHub issues to track and discuss problems, feature requests, and pull requests. You can make pull requests for incomplete features to get code review. The convention is to prefix the pull request title with "WIP:" for Work In Progress, or "RFC:" for Request for Comments when work is completed and ready for merging. This will prevent accidental merging of work that is in progress.

Note: These instructions are for adding to or improving functionality in the base library. Before getting started, it can be helpful to discuss the proposed changes or additions on the mailing list or in a GitHub issue---it's possible your proposed change belongs in a package rather than the core language. Also, keep in mind that changing stuff in the base can potentially break a lot of things. Finally, because of the time required to build Julia, note that it's usually faster to develop your code in stand-alone files, get it working, and then migrate it into the base libraries.

Add new code to Julia's base libraries as follows:

  1. Edit the appropriate file in the base/ directory, or add new files if necessary. Create tests for your functionality and add them to files in the test/ directory. If you're editing C or Scheme code, most likely it lives in src/ or one of its subdirectories, although some aspects of Julia's REPL initialization live in ui/.

  2. Add any new files to sysimg.jl in order to build them into the Julia system image.

  3. Add any necessary export symbols in exports.jl.

  4. Include your tests in test/Makefile and test/choosetests.jl.

Build as usual, and do make clean testall to test your contribution. If your contribution includes changes to Makefiles or external dependencies, make sure you can build Julia from a clean tree using git clean -fdx or equivalent (be careful – this command will delete any files lying around that aren't checked into git).

Note: You can run specific test files with make:

make test-bitarray

or with the runtests.jl script, e.g. to run test/bitarray.jl and test/math.jl:

./usr/bin/julia test/runtests.jl bitarray math

Make sure that Travis greenlights the pull request with a Good to merge message.

Code Formatting Guidelines

General Formatting Guidelines for Julia code contributions

  • 4 spaces per indentation level, no tabs
  • use whitespace to make the code more readable
  • no whitespace at the end of a line (trailing whitespace)
  • comments are good, especially when they explain the algorithm
  • try to adhere to a 92 character line length limit
  • use upper camel case convention for modules, type names
  • use lower case with underscores for method names

General Formatting Guidelines For C code contributions

  • 4 spaces per indentation level, no tabs
  • space between if and ( (if (x) ...)
  • newline before opening { in function definitions
  • f(void) for 0-argument function declarations
  • newline between } and else instead of } else {
  • if one part of an if..else chain uses { } then all should
  • no whitespace at the end of a line

Git Recommendations For Pull Requests

  • Avoid working from the master branch of your fork, creating a new branch will make it easier if Julia's master changes and you need to update your pull request.
  • Try to squash together small commits that make repeated changes to the same section of code so your pull request is easier to review, and Julia's history won't have any broken intermediate commits. A reasonable number of separate well-factored commits is fine, especially for larger changes.
  • If any conflicts arise due to changes in Julia's master, prefer updating your pull request branch with git rebase versus git merge or git pull, since the latter will introduce merge commits that clutter the git history with noise that makes your changes more difficult to review.
  • If you see any unrelated changes to submodules like deps/libuv, deps/openlibm, etc., try running git submodule update first.
  • Avoid committing changes to auto-generated files such as doc/helpdb.jl, which is a frequent source of conflicts and can be re-generated later.
  • Descriptive commit messages are good.
  • Using git add -p or git add -i can be useful to avoid accidentally committing unrelated changes.
  • GitHub does not send notifications when you push a new commit to a pull request, so please add a comment to the pull request thread to let reviewers know when you've made changes.
  • When linking to specific lines of code in discussion of an issue or pull request, hit the y key while viewing code on GitHub to reload the page with a URL that includes the specific version that you're viewing. That way any lines of code that you refer to will still make sense in the future, even if the content of the file changes.
  • Whitespace can be automatically removed from existing commits with git rebase.
    • To remove whitespace for the previous commit, run git rebase --whitespace=fix HEAD~1.
    • To remove whitespace relative to the master branch, run git rebase --whitespace=fix master.

Getting help

While getting familiar with Julia, remember to check out the docs, keeping in mind that they are searchable. (If you use a script blocker then you'll have to unblock that page.) The source code is an excellent source of examples (and it's mostly pretty approachable). If you're still stumped, post something on the dev list, but you may want to search the archives first to see if there's already been a discussion about what you're stuck on.


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