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Contributor Info


The git repository for the project is hosted on Github at chapel-lang/chapel. Anyone can read the repository. It is open source!

This document contains a mixture of tips for git beginners and specific Chapel workflow recommendations.

Below are instructions for setting up a github account, developing a feature,and submitting pull requests.

Note: A contributor license agreement must be signed before any contributing pull requests can be merged.

Developer Workflow


  1. Discuss design changes or big development efforts
  2. Get set up
  3. Create new branch
  4. Develop and test contributions locally
    1. Add new tests
  5. Request feedback on your changes
    1. Push your work to your feature branch
    2. Ask for feedback on your branch early (optional)
    3. Submit pull request
    4. Find a reviewer
    5. Work with your reviewers
  6. Get ready to merge
  7. Watch automatic testing and address issues

HOWTO and Git details

Policy details

Discuss design changes or big development efforts

Before embarking on a big project, please open an issue on the GitHub page and/or send an email to chapel-developers with a prefix of [Design] in the subject header for discussion with the community and to ensure that you are aware of any parallel efforts in that area. It may also be a good idea to ask for input from the user community via the chapel-users mailing list.

Get set up

This should only need to happen once per developer.

Note: these are expected to evolve over time as the governance of Chapel is migrated from Cray to an external/community body (the major elements are likely to be similar, though the specific people involved are likely to change and grow).

  1. Set up a github account. The "Free" plan is sufficient for contributing to Chapel.
  2. Make sure you have configured your environment to work with git. See initial git setup instructions.
  3. Use the GitHub web interface to create a fork of the Chapel repo by visiting and clicking the 'Fork' button (see also Fork the repo). Then configure your local git and check out your fork
  4. If you're working on a long-term effort, announce it on the chapel-developers mailing list to make sure toes are not being stepped on, work is not being pursued redundantly, etc. Similarly, fundamental changes to the language or architecture should be circulated to the chapel-developers and/or chapel-users lists to make sure effort is not wasted.
  5. Sign a Chapel contributor license agreement and mail it, with your GitHub ID.

Create new branch

This should happen once for every new effort.

Develop your feature, bug fix, etc on your fork. To create a new branch, use the New branch command. Using a concisely named branch is encouraged.

Develop and test contributions locally

Your contribution will take the form of a series of commits. While including sensible commit messages is a good idea, it is more important to have a good merge message once the pull request is going in. Likewise, it is OK to have many small commits that reflect the history of development rather than commits for the feature. See Development commands for how to perform some common operations during development.

As you work, you will want to periodically bring in changes from the main Chapel project to your feature branch (described in Development commands), to avoid code drift.

Add new tests

You will probably need to create new tests for your feature. See Creating a Simple Test in Test System for more information on this process.

Any addition/change to the Chapel test system should pass testing when that test/directory is run with start_test (and performance tests should also pass testing for start_test -performance).

Request feedback on your changes

Push your work to your feature branch

Push your changes to your feature branch on GitHub to enable others to see your work (see How to push for command details). Note that if you have already created a pull request from a feature branch, pushing your work to that feature branch will update the pull request.

Ask for feedback on your branch early (optional)

Not ready to merge your changes, but still want to see if your work is going in the right direction? Feel free to ask for early feedback! Exposing the code is generally done by:

Discussion can take place in:

Submit pull request

See How to open a PR for the sequence of steps necessary.

Contributors should be reasonably confident in the testing done on their code before asking for a final review. Should additional testing resources be needed, you can request help from a member of the core Chapel team when creating your pull request.

In working with your reviewers, you will no doubt change your pull request. Just do your local development and then update your feature branch as in Push your work to your feature branch

Please follow the Pull request guidance and keep PRs reasonably sized.

Find a reviewer

  • Once your PR is ready, you'll need to request a review. If you know who you'd like to review it, @ mention them in a comment on the PR and ask them to have a look. If you don't know their Github id, you can find them in the chat room or send them an email. If you don't know who should review the change, send an email to the chapel-developers list requesting a review and linking to the PR. Such an email should have a subject line starting with [PR].

    Note: Ideally, someone should volunteer to review your pull request within a day or two. If this doesn't happen, feel free to make some noise. Ideally the review should take place within a few days, though timing may vary depending on other deadlines.

  • See Reviewer responsibilities for details on what performing a review on another contributor's code entails.

Work with your reviewers

  • Iterate with the reviewer until you're both satisfied. One should generally try to do whatever their reviewer asks. Sometimes, a reviewer will ask for something really hard. Try to make sure they understand the magnitude of the request, and try to discuss if it's really necessary to do before merging. If you can't come to agreement, one of you should bring other developers (individually or via chapel-developers) into the conversation to get a broader opinion. One of the jobs of the reviewer is to serve as a proxy for other developers, or to bring those developers into the conversation if they feel unqualified to do so.

Get ready to merge

Before the change can be merged, go through this checklist to ensure:

If you did not have the resources to perform at least a full correctness test run, this is the point at which a reviewer would do so for you. Contributors are expected to have verified any new tests work before asking a reviewer to do this.

Details on how to run tests may be found at Test System, and details on the appropriate amount of testing before merging the final PR may be found at Testing your patch.

  • Once the pull request is approved, it can be merged. This can be done by either the reviewer or developer (given sufficient permissions), as decided between the two of them. See How to merge a PR for steps to perform this.
  • If you are reviewing code from an external contributor without push privileges, go through the checklist once more before merging the change.

After the final version of the change has been agreed upon, the person making the merge should follow the steps for How to merge a PR.

Watch automatic testing and address issues

  • In short order, a smoke-test will be run against the commit to make sure that nothing basic has been broken by it. Monitor the chapel-test-results-regressions mailing list to make sure that nothing breaks.
  • For the day or two after the commit has gone in, check the chapel-test-results-regressions mailing list to ensure that there are no new failures caused by your commit. Use the chapel-developers mailing list if you are unsure (a member of the core Chapel team will be tasked with diagnosing any testing failures on any given night, but it's nice when developers notice the issue first themselves to save wasted effort).

HOWTO and Git details

Initial Git Setup

Follow the GitHub directions to setup a new account.

If you plan to use ssh to push/pull, setup SSH keys.

Configure your local git

git config --global 'Thomas Van Doren'
git config --global ''

# linux/mac
git config --global core.autocrlf input

# windows
git config --global core.autocrlf true

# If using ssh keys, verify access to github. It should respond with a
# message including your github username.

# Clone your fork of the chapel repo!
git clone<github_username>/chapel.git

# Or, if using HTTPS instead of SSH.
git clone<github_username>/chapel.git

# Set up remotes for github
cd chapel
git remote add upstream
# Make sure it works, get up-to-date without modifying your files
git fetch upstream
# Change remote for upstream push to "no_push"
git remote set-url --push upstream no_push
# Optionally add remotes for commonly viewed branches
git remote add <branch_owner_username><branch_owner_username>/chapel.git

New branch command

git checkout -b <branch_name>

Development commands

Stage a file/dir for commit:

git add path/to/file

# (sort of) similar to: svn add path/to/file

Delete a file/dir and stage the change for commit:

git rm [-r] path/to/dir/or/file

# similar to: svn delete path/to/dir/or/file

Move a file/dir:

git mv orig/path/a.txt new/path/to/b.txt

# similar to: svn move orig/path/a.txt new/path/to/b.txt

Copy a file/dir and stage target for commit:

cp <src> <target>
git add <target>

# similar to: svn copy <src> <target>

Get the status of files/dirs (staged and unstaged):

git status

# similar to: svn status

Get the diff of unstaged changes:

git diff

# similar to: svn diff

Get the diff of staged changes (those that were staged with git add):

git diff --cached

Backing out unstaged changes:

git checkout path/to/file/a.txt

# similar to: svn revert path/to/file/a.txt

Committing staged changes:

git commit [-m <message>]

# similar to: svn commit [-m <message>]

Bring in changes from the main Chapel project:

git fetch upstream
git merge upstream/master

# or:
git pull upstream <branch_name>

# with feature branch checked out:
git merge [--no-ff] upstream/master

If there are conflicts, you will be asked to resolve them. Once the affected files have been fixed, stage them with git add, and then call git commit to finish the merge process.

If you want to understand the changes that occurred upstream, see Read commit messages for changes from the main Chapel project below.

How to modify git history

The following commands may cause problems if the changes they overwrite have been pulled by other repositories.

Fixing a commit message:

git commit --amend

Un-do the last commit (leaving changed files in your working directory):

git reset --soft HEAD~1

Reapplying changes from the current branch onto an updated version of master:

git rebase master

Reapplying changes from the current branch onto an updated version of upstream/master, without updating your local master (note: you will need to perform a pull next time you checkout your local master):

git fetch upstream
git rebase upstream/master

Pushing such changes to your repository (again, this may cause problems if other repositories have pulled the changes):

git push -f origin <branch_name>

Read commit messages for changes from the main Chapel project

To view only the commits that happened on master (in other words, the old svn commits and the merge commits for pull requests):

git log --first-parent

# or with line wrapping
git log --first-parent | less

# or including files changed
git log --first-parent -m --stat

# or similar to svn log
git log --first-parent -m --name-status

More logging commands are described in Other logging commands below.

How to push

git push origin <branch_name>

# or if you don't like typing your complicated branch name,
# you can use this command to push the current branch:
git push origin HEAD

# if you forgot your branch name, you can get it by running
git branch

# it is the starred one...

Note that -f is necessary if you've modified changes on your feature branch (see How to modify git history).

How to open a PR:

  • Submit a pull request with your changes (make sure you have synced with the main repo).

    To do this, after pushing your changes to your feature branch on GitHub, you can use the GitHub web interface to create a pull request. Visit<username>/chapel

    and look for a "Compare & pull request" button for your feature branch. Alternatively, navigate to your feature branch, and click the green icon next to the branch dropdown to "Compare, review, create a pull request".

    Next, put in a message to your reviewer about the purpose of your pull request and give the pull request a useful title. Your PR message will introduce the changes to reviewers and form the basis for the merge message. See Final merge message for recommendations on what that commit message should look like.

    You will have to have signed a contributor license agreement (CLA).

    Your pull request will be available at a URL like:<number>

    and you can discuss the patch with your reviewers there.

How to merge a PR:

If you have commit privileges (see Who has or needs commit access to the main repository?), navigate to the pull request:

go to


and click the friendly green button "Merge pull request" (it is possible to merge the pull request from the command line also and the pull request page has details). When you click "Merge pull request", you will need to enter a commit message. See Final merge message for a reminder on what that commit message should entail (generally, this will closely resemble the PR message).

More information on using git

Additional docs available online at:

Git help pages can be viewed with:

git help <command>

Other git commands

Update to HEAD:

(If you use this command on a feature branch, you'll just be updating to the latest work stored on github. See Development commands for how to update a feature branch with new changes from the main Chapel project)

git pull

# or:
git fetch origin
git merge origin/master # replace master with whatever branch you're on

# similar to: svn update

Update to specific revision number:

git checkout <commit sha1>

# similar to: svn update -r<revision number>

To view "dirty" files, or all those files that are not tracked (includes ignored files):

git ls-files --others

If you've gotten your master branch mucked up but haven't pushed the branch with errors to your remote fork, you can fix it with the following series of commands:

# This will save your old master state to a different branch name, removing
# the name "master" from the list of branches you can access on your fork
git branch -m <name for old, messed up master>

# You will get a message indicating you are in a "detached HEAD state".  This
# is expected (and desired).  Now the repository you are in is in line with
# your fork's master branch.
git checkout origin/master

# This will save the state of the repository right now to a new branch, named
# master.
git checkout -b master

At this point, a git push origin master should work as expected. Remember, do not try this with a master branch that has been corrupted on your remote fork.

An alternate method, if you know or can easily find out the last commit that should be kept:

# on any branch that contains commits you do not want.
git branch <new branch name>

# do not use --hard if you wish to leave untracked files in your tree
git reset --hard <last commit you want to keep>

Other logging commands

To view commits grouped by author (for example, show me commits by author from tag to now):

git shortlog --numbered --no-merges

# With commit sha1 and relative date:
git shortlog --numbered --no-merges \
  --format='* %Cred[%h]%Creset %s %Cgreen(%cr)%Creset'

# Set alias
git config --global \
  'shortlog --numbered --no-merges \
   --format=\'* %Cred[%h]%Creset %s %Cgreen(%cr)%Creset\''

# Show commits by author between 1.8.0 and releases:
git sl 1.8.0..

Finding a Pull Request by Commit

Suppose you have figured out that a particular commit is causing a problem and you'd like to view the pull request discussion on GitHub. You can go to<commit-hash>

and GitHub shows the pull request number at the bottom of the commit message complete with a link to the pull request page.

Policy details

Who has or needs commit access to the main repository?

Core team members have commit access to the main repository. Reviewers on the core team can pull, review, and merge your pull requests. Even the developers that have write access to the Chapel repository need to have all non-trivial changes reviewed. Developers who have been given write access can merge trivial changes (e.g. small bug fixes, documentation changes) without review.

If you will need commit/push access to the main repository, chapel-lang/chapel, send a request including your github username to chapel_admin _at_

Third-party code

If your work will require committing any third-party code that you are not developing yourself (or code that you've developed as a standalone package), alert the chapel-developers mailing list of this as, presently, such code packages must be approved by Cray leadership before being committed.

Here are some guiding questions to determine whether a third-party package you rely on should be committed to the chapel repository:

  • How large is the third-party code you wish to include?
    • If the code is very large, perhaps it would be better to add directions on how to install this dependency.
  • Under what license does this code operate?
    • We try not to add dependencies on code that is under GPL or LGPL, as those licenses have copyleft properties and force derivative works to be distributed under the same license.
      • Is there an alternate package with a more permissive license that can accomplish the same purpose?
        • If so, we recommend relying on that package instead.
        • If not, it would be better to add directions on how to install this dependency.
  • How easy is this code to obtain?
    • Will it be installed by default on an ordinary machine?
      • If so, we do not need to redistribute it ourselves.
  • How much of the Chapel implementation will rely on this code?
    • The compiler for ordinary Chapel? A commonly used runtime configuration?
      • In these cases, we will probably want to include the code in our distribution.
    • A standard or package module that is not included by default?
      • Depending on the circumstances, it might be better to just include directions on how to install this code.
  • Do we require Chapel-specific modifications to the code in order to use it?
    • If so, we will probably want to distribute this package, or at least include the modifications and an easy way to install them.

Please include the answers to these questions when you contact the chapel-developers mailing list, if you believe the code should be included or you remain uncertain.

Testing your patch

  • Changes to the Chapel implementation should not cause regressions. Developers are responsible for doing a degree of testing that's appropriate for their change (described in the following bullets) and then can rely on nightly regression testing to worry about the full cross-product of configurations.
    • At a minimum, patches should pass correctness testing for the full test/ directory hierarchy for:
      • CHPL_*_PLATFORM=linux64
      • CHPL_*_COMPILER=gnu
      • CHPL_COMM=none
      • CHPL_TASKS=<default>
    • Most developers will start by focusing on a subdirectory of tests that exercise the features they changed, or test/release/ as a suite of tests that exercises a rich and important slice of the language.
    • Changes that are likely to affect multi-locale executions should also be tested against tests that exercise multi-locale capabilities with CHPL_COMM=gasnet. A common subset is: test/release/, test/multilocale/, and test/distributions/.
    • Changes that are likely to cause portability issues should be tested against different platforms and compilers to avoid fallout in the nightly testing to the extent possible.
  • Note that the quickest way to do testing is to use the parallel testing system across a large number of workstations. If you have limited testing resources available to you, you can request that a member of the core Chapel team help.

Work-in-progress pull request

This is a special kind of pull request that is not yet intended to be merged. Such PRs are created to take advantage of what the Github PR interface provides, such as public comment history and quick links between the WIP PR and other related issues and pull requests. They allow the developer to get early feedback on a change.

The status of the WIP PR should be clearly stated, including what steps need to be taken before the PR is ready for final review.

It is generally advisable to "close" such PRs until they are ready for final review and testing, as their development can span a large period of time and would thus add clutter to the list of open PRs.

It is perfectly acceptable to abandon such PRs (especially in favor of a cleaned up version of the code) when the git history becomes too large, so long as a link to the original PR is provided when the change is eventually merged, to preserve the discussion.

Pull request guidance

  • It is considered good practice to keep PRs (pull requests) to a reasonable size. This ensures that the PR will be reviewed in a timely manner and will receive a higher level of attention per line of code during review.
    • When submitting a PR, the contributor should ask themselves if their contribution can be separated into smaller logical chunks or independent parts. Reviewers will also be pondering the same question and may request a break up of the contribution into smaller PRs.
    • Breaking up a PR can sometimes require a great deal of effort and creativity, and may not be feasible at all, due to the intertwined nature of the code.
    • Ideally, the size of the PR should be proportional to the expected value to the developer and user community. For example, a new module introduced as a 1000-line PR is acceptable, while a set of new tests introduced as a 1000-line PR is not.

Final merge message

  • start with a single topic line with at most 75 characters
  • then have a blank line
  • then have a more detailed explanation including motivation for the change and how it changes the previous behavior
  • use present tense (e.g. "Fix file iterator bug")
  • manually wrap long lines in the explanation to 75 or 80 characters

It is good practice for the pull request title to match the topic line, and the detailed explanation to match the pull request description.

Git history is clear

It's not generally possible to completely remove a commit from git by the time it makes it in to the master branch. So be very careful not to commit anything that you might regret later (e.g., sensitive code, code owned by anyone other than yourself). Ideally, the review will catch such issues, but the ultimate responsibility is on the developer.

Reviewer responsibilities

  • If you're reviewing a commit from a developer outside the Chapel core team, be sure they have signed the contributor license agreement (see the Developer Workflow instructions for this). If the developer cannot or will not sign the agreement, bring the situation to the attention of the Chapel project leadership.

    Care may need to be taken when committing third-party code that originates from a different git[hub] repository. As an example, in one case in the past we brought in a copy of an outside commit that had originally been made in the git repository belonging to one of our third-party packages. We did that by using git-am to commit a copy of their raw commit (in git-send-mail format) to the appropriate third-party directory in the Chapel repository. For the commit in our repo, their developer was listed as the author, but the Chapel core team member who did the Chapel commit was listed as the contributor. Had we instead split the original commit apart into its constituent meta-information and patch parts and committed just the patch using git-apply, the Chapel core team member would have been listed as both author and contributor. In the end it didn't matter because although the outside developer couldn't sign our contributor's agreement, their IP attorneys decided that given their license (which was BSD), their commit constituted publishing the work rather than contributing it, and what Chapel did with it afterward was not their concern. Also, we would have picked up the same commit the next time we updated our third-party release of that package. Nevertheless, this gives an example of how tricky this kind of situation can be, and shows why decisions may need to be made (or at least understood) at a high level.

What Copyright Should I Use?

By signing a Contributor Agreement, you have agreed that code you contribute will be governed by the license and copyright of the project as a whole. A standard block of license text is required at the top of every compiler, runtime, and module code file. Browse other files of the same type to see the required license block.

Additional copyrights may also be applied, as appropriate.