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Notes for Julia Contributors

Hi! If you are new to the Julia community: welcome, and thanks for trying Julia. Please be sure to respect our community standards in all interactions.

Learning Julia

The learning page has a great list of resources for new and experienced users alike. This tutorial video is one recommended starting point, as is the "Invitation to Julia" workshop video from JuliaCon 2015 (slide materials here). The Julia documentation covers the language and core library features, and is searchable. (note: Javascript required).

Before filing an issue

  • Reporting a potential bug? Please read the "How to file a bug report" section to make sure that all necessary information is included.

  • Contributing code? Be sure to review the contributor checklist for helpful tips on the tools we use to build Julia.

  • Library feature requests are generally not accepted on this issue tracker. New libraries should be developed as packages. Discuss ideas for libraries at the Julia Discourse forum. Doing so will often lead to pointers to existing projects and bring together collaborators with common interests.

Contributor Checklist

  • 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 the repository up-to-date and rebase work-in-progress frequently to make merges simpler.

  • 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.

  • Review discussions on the Julia Discourse forum.

  • For more detailed tips, read the submission guide below.

  • 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 Discourse forum 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.

  1. 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.

  2. 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 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.
  1. When pasting code blocks or output, put triple backquotes (```) around the text so GitHub will format it nicely. Code statements should be surrounded by 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 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.

Package Compatibility Across Releases

Sometimes, you might find that while your package works on the current release, it might not work on the upcoming release or nightly. This is due to the fact that some Julia functions (after some discussion) could be deprecated or removed altogether. This may cause your package to break or throw a number of deprecation warnings on usage. Therefore it is highly recommended to port your package to latest Julia release.

However, porting a package to the latest release may cause the package to break on earlier Julia releases. To maintain compatibility across releases, use Compat.jl. Find the fix for your package from the README, and specify the minimum version of Compat that provides the fix in your REQUIRE file. To find the correct minimum version, refer to this guide.

Writing tests

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

  1. Go visit https://coveralls.io/r/JuliaLang/julia.

  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. http://julia.readthedocs.org/en/latest/stdlib/test/ may be helpful in explaining how the testing infrastructure works.

  4. Run make test-all to rebuild Julia and run your new test(s). If you had to fix a bug or add functionality in base, this will ensure that your test passes and that you have not introduced extraneous whitespace.

  5. Submit the test as a pull request (PR).

Coveralls shows functionality that still needs "proof of concept" tests. These are important, as are tests for tricky edge cases, such as converting between integer types when the number to convert is near the maximum of the range of one of the integer types. Even if a function already has some coverage on Coveralls, it may still benefit from tests for edge cases.

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.

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

or

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.

Existing docstrings now live primarily in base/docs/helpdb.jl. It is a goal over time to move the docstrings inline to their respective method definitions. If you want to edit the body of a method docstring, run the doc/genstdlib.jl script to regenerate the restructured text files after you have already rebuilt Julia. (From the top-level source directory, you can do this with make julia-genstdlib.) If you want to edit an existing docstring signature, you first have to change the signature in the doc/stdlib ..function or ..data definition (not the auto-generated content) and then edit the helpdb.jl or inline method docstrings. The existing signatures in the doc/stdlib/*.rst files are pattern matched to base docstrings and the new content overwrites the content in doc/stdlib/. The signature definitions must be in sync or else the pattern match will fail and documentation will be lost in the result. To add entirely new methods to the stdlib documentation, first add the signature in the appropriate doc/stdlib/*.rst file before writing the docstring, rebuilding Julia, and re-running doc/genstdlib.jl. Pattern matching requires that multiline method signatures' inter-line character alignment in doc/stdlib/*.rst match that in the corresponding docstring. In the following example,

"""
    foo(bar, baz,
        qux, quux)

Foo `bar`, `baz`, `qux`, and `quux`.
"""

will only match entries in doc/stdlib/*.rst beginning with

.. function:: foo(bar, baz,
                  qux, quux)

Note that the second line of the signature is indented by four spaces relative to foo(bar, baz, in the first line of the signature. This leading indent matches the indent used in the docstring exactly. If it did not match, such as in the following example,

.. function:: foo(bar, baz,
                 qux, quux)

where three spaces instead of four are used then running genstdlib.jl will print a warning and not update the docstring.

It is encouraged to write all new docstrings in Markdown markup. If you need to write a more complicated docstring that contains cross-references or citations it can be written in a restructured text codeblock. Many of the existing docstrings are currently restructured text codeblocks and these will be transitioned to Markdown over time. RST codeblocks are delineated with the triple-quote (```rst ```) Makdown codeblock syntax. The content of the codeblock is spliced directly into the final restructured text document unmodified.

Examples written within docstrings can be used as testcases known as "doctests" by annotating code blocks with jldoctest.

```jldoctest
julia> uppercase("Docstring test")
"DOCSTRING TEST"
```

A doctest needs to match an interactive REPL including the julia> prompt. To run doctests you first run make julia-genstdlib then make -C doc doctest.

For new functionality and other substantial changes, add a brief summary to NEWS.md. The news item should cross reference the pull request (PR) parenthetically, in the form ([#pr]); after adding this, run ./julia doc/NEWS-update.jl from the julia directory to update the cross-reference links. To add the PR reference number, first create the PR, then push an additional commit updating NEWS.md with the PR reference number.

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 (PR). 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 Julia Discourse forum 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
  • it is generally preferred to use ASCII operators and identifiers over Unicode equivalents whenever possible
  • in docstring refer to the language as "Julia" and the executable as "julia"

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.
  • 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.

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