Contribution Guide

The CUE project welcomes all contributors.

This document is a guide to help you through the process of contributing to the CUE project, which is a little different from that used by other open source projects. We assume you have a basic understanding of Git and Go.

Becoming a contributor


The first step is registering as a CUE contributor and configuring your environment. Here is a checklist of the required steps to follow:

  • Step 0: Decide on a single Google Account you will be using to contribute to CUE. Use that account for all the following steps and make sure that git is configured to create commits with that account's e-mail address.
  • Step 1: Sign and submit a CLA (Contributor License Agreement).
  • Step 2: Configure authentication credentials for the CUE Git repository. Visit, click on “Generate Password” (top right), and follow the instructions.
  • Step 3: Register for Gerrit, the code review tool used by the CUE team, by visiting this page. The CLA and the registration need to be done only once for your account.
  • Step 4: Install git-codereview by running: GO111MODULE=on go get (or just go install if you are using Go 1.16)

The rest of this chapter elaborates on these instructions. If you have completed the steps above (either manually or through the tool), jump to Before contributing code.

Step 0: Select a Google Account

A contribution to CUE is made through a Google account with a specific e-mail address. Make sure to use the same account throughout the process and for all your subsequent contributions. You may need to decide whether to use a personal address or a corporate address. The choice will depend on who will own the copyright for the code that you will be writing and submitting. You might want to discuss this topic with your employer before deciding which account to use.

Google accounts can either be Gmail e-mail accounts, G Suite organization accounts, or accounts associated with an external e-mail address. For instance, if you need to use an existing corporate e-mail that is not managed through G Suite, you can create an account associated with your existing e-mail address.

You also need to make sure that your Git tool is configured to create commits using your chosen e-mail address. You can either configure Git globally (as a default for all projects), or locally (for a single specific project). You can check the current configuration with this command:

$ git config --global  # check current global config
$ git config           # check current local config

To change the configured address:

$ git config --global   # change global config
$ git config            # change local config

Step 1: Contributor License Agreement

Before sending your first change to the CUE project you must have completed one of the following two CLAs. Which CLA you should sign depends on who owns the copyright to your work.

You can check your currently signed agreements and sign new ones at the Google Developers Contributor License Agreements website. If the copyright holder for your contribution has already completed the agreement in connection with another Google open source project, it does not need to be completed again.

If the copyright holder for the code you are submitting changes—for example, if you start contributing code on behalf of a new company—please send mail to the cue-dev mailing list. This will let us know the situation so we can make sure an appropriate agreement is completed and update the AUTHORS file.

Step 2: Configure git authentication

The remaining two steps only apply if you wish to contribute through Gerrit, which is the source of truth for the CUE project. You can also send Pull Requests to the mirror at

The main CUE repository is located at, a Git server hosted by Google. Authentication on the web server is made through your Google account, but you also need to configure git on your computer to access it. Follow this steps:

  • Visit and click on “Generate Password” in the page's top right menu bar. You will be redirected to to sign in.
  • After signing in, you will be taken to a page with the title “Configure Git”. This page contains a personalized script that when run locally will configure Git to hold your unique authentication key. This key is paired with one that is generated and stored on the server, analogous to how SSH keys work.
  • Copy and run this script locally in your terminal to store your secret authentication token in a .gitcookies file. If you are using a Windows computer and running cmd, you should instead follow the instructions in the yellow box to run the command; otherwise run the regular script.

Step 3: Create a Gerrit account

Gerrit is an open-source tool used by CUE maintainers to discuss and review code submissions.

To register your account, visit and sign in once using the same Google Account you used above.

Step 4: Install the git-codereview command

Changes to CUE must be reviewed before they are accepted, no matter who makes the change. A custom git command called git-codereview simplifies sending changes to Gerrit.

Install the git-codereview command by running,

$ GO111MODULE=on go get

or with Go 1.16:

$ go install

Make sure git-codereview is installed in your shell path, so that the git command can find it. Check that

$ git codereview help

prints help text, not an error.

On Windows, when using git-bash you must make sure that git-codereview.exe is in your git exec-path. Run git --exec-path to discover the right location then create a symbolic link or just copy the executable from $GOPATH/bin to this directory.

Before contributing code

Check the issue tracker

Whether you already know what contribution to make, or you are searching for an idea, the issue tracker is always the first place to go. Issues are triaged to categorize them and manage the workflow.

Most issues will be marked with one of the following workflow labels:

  • NeedsInvestigation: The issue is not fully understood and requires analysis to understand the root cause.
  • NeedsDecision: the issue is relatively well understood, but the CUE team hasn‘t yet decided the best way to address it. It would be better to wait for a decision before writing code. If you are interested on working on an issue in this state, feel free to “ping” maintainers in the issue’s comments if some time has passed without a decision.
  • NeedsFix: the issue is fully understood and code can be written to fix it.

You can use GitHub's search functionality to find issues to help out with. Examples:

Open an issue for any new problem

Excluding very trivial changes, all contributions should be connected to an existing issue. Feel free to open one and discuss your plans. This process gives everyone a chance to validate the design, helps prevent duplication of effort, and ensures that the idea fits inside the goals for the language and tools. It also checks that the design is sound before code is written; the code review tool is not the place for high-level discussions.

Sending a change via Gerrit

It is not possible to fully sync Gerrit and GitHub, although things are improving, so we recommend learning Gerrit. It's different but powerful and familiarity with help you understand the flow.


This is an overview of the overall process:

  • Step 1: Clone the CUE source code from and make sure it's stable by compiling and testing it once:
$ git clone
$ cd cue
$ go test ./...
$ go install ./cmd/cue
  • Step 2: Prepare changes in a new branch, created from the master branch. To commit the changes, use git codereview change; that will create or amend a single commit in the branch.
$ git checkout -b mybranch
$ [edit files...]
$ git add [files...]
$ git codereview change   # create commit in the branch
$ [edit again...]
$ git add [files...]
$ git codereview change   # amend the existing commit with new changes
$ [etc.]
  • Step 3: Test your changes, re-running go test.
$ go test ./...    # recompile and test
  • Step 4: Send the changes for review to Gerrit using git codereview mail (which doesn't use e-mail, despite the name).
$ git codereview mail     # send changes to Gerrit
  • Step 5: After a review, apply changes to the same single commit and mail them to Gerrit again:
$ [edit files...]
$ git add [files...]
$ git codereview change   # update same commit
$ git codereview mail     # send to Gerrit again

The rest of this section describes these steps in more detail.

Step 1: Clone the CUE source code

In addition to a recent CUE installation, you need to have a local copy of the source checked out from the correct repository. You can check out the CUE source repo onto your local file system anywhere you want as long as it's outside your GOPATH. Either clone from or from GitHub:

$ git clone   # or
$ cd cue
$ go test ./...
# go install ./cmd/cue

Step 2: Prepare changes in a new branch

Each CUE change must be made in a separate branch, created from the master branch. You can use the normal git commands to create a branch and add changes to the staging area:

$ git checkout -b mybranch
$ [edit files...]
$ git add [files...]

To commit changes, instead of git commit, use git codereview change.

$ git codereview change
(open $EDITOR)

You can edit the commit description in your favorite editor as usual. The git codereview change command will automatically add a unique Change-Id line near the bottom. That line is used by Gerrit to match successive uploads of the same change. Do not edit or delete it. A Change-Id looks like this:

Change-Id: I2fbdbffb3aab626c4b6f56348861b7909e3e8990

The tool also checks that you've run go fmt over the source code, and that the commit message follows the suggested format.

If you need to edit the files again, you can stage the new changes and re-run git codereview change: each subsequent run will amend the existing commit while preserving the Change-Id.

Make sure that you always keep a single commit in each branch. If you add more commits by mistake, you can use git rebase to squash them together into a single one.

Step 3: Test your changes

You‘ve written and tested your code, but before sending code out for review, run all the tests for the whole tree to make sure the changes don’t break other packages or programs:

$ go test ./...

Step 4: Send changes for review

Once the change is ready and tested over the whole tree, send it for review. This is done with the mail sub-command which, despite its name, doesn't directly mail anything; it just sends the change to Gerrit:

$ git codereview mail

Gerrit assigns your change a number and URL, which git codereview mail will print, something like:

remote: New Changes:
remote: math: improved Sin, Cos and Tan precision for very large arguments

If you get an error instead, check the Troubleshooting mail errors section.

If your change relates to an open GitHub issue and you have followed the suggested commit message format, the issue will be updated in a few minutes by a bot, linking your Gerrit change to it in the comments.

Step 5: Revise changes after a review

CUE maintainers will review your code on Gerrit, and you will get notifications via e-mail. You can see the review on Gerrit and comment on them there. You can also reply using e-mail if you prefer.

If you need to revise your change after the review, edit the files in the same branch you previously created, add them to the Git staging area, and then amend the commit with git codereview change:

$ git codereview change     # amend current commit
(open $EDITOR)
$ git codereview mail       # send new changes to Gerrit

If you don't need to change the commit description, just save and exit from the editor. Remember not to touch the special Change-Id line.

Again, make sure that you always keep a single commit in each branch. If you add more commits by mistake, you can use git rebase to squash them together into a single one.

Good commit messages

Commit messages in CUE follow a specific set of conventions, which we discuss in this section.

Here is an example of a good one:

math: improve Sin, Cos and Tan precision for very large arguments

The existing implementation has poor numerical properties for
large arguments, so use the McGillicutty algorithm to improve
accuracy above 1e10.

The algorithm is described at

Fixes #159

First line

The first line of the change description is conventionally a short one-line summary of the change, prefixed by the primary affected package.

A rule of thumb is that it should be written so to complete the sentence “This change modifies CUE to _____.” That means it does not start with a capital letter, is not a complete sentence, and actually summarizes the result of the change.

Follow the first line by a blank line.

Main content

The rest of the description elaborates and should provide context for the change and explain what it does. Write in complete sentences with correct punctuation, just like for your comments in CUE. Don't use HTML, Markdown, or any other markup language.

Referencing issues

The special notation “Fixes #12345” associates the change with issue 12345 in the CUE issue tracker When this change is eventually applied, the issue tracker will automatically mark the issue as fixed.

If the change is a partial step towards the resolution of the issue, uses the notation “Updates #12345”. This will leave a comment in the issue linking back to the change in Gerrit, but it will not close the issue when the change is applied.

If you are sending a change against a subrepository, you must use the fully-qualified syntax supported by GitHub to make sure the change is linked to the issue in the main repository, not the subrepository. All issues are tracked in the main repository's issue tracker. The correct form is “Fixes cuelang/cue#159”.

The review process

This section explains the review process in detail and how to approach reviews after a change has been mailed.

Common beginner mistakes

When a change is sent to Gerrit, it is usually triaged within a few days. A maintainer will have a look and provide some initial review that for first-time contributors usually focuses on basic cosmetics and common mistakes. These include things like:

  • Commit message not following the suggested format.
  • The lack of a linked GitHub issue. The vast majority of changes require a linked issue that describes the bug or the feature that the change fixes or implements, and consensus should have been reached on the tracker before proceeding with it. Gerrit reviews do not discuss the merit of the change, just its implementation. Only trivial or cosmetic changes will be accepted without an associated issue.


The CUE community values very thorough reviews. Think of each review comment like a ticket: you are expected to somehow “close” it by acting on it, either by implementing the suggestion or convincing the reviewer otherwise.

After you update the change, go through the review comments and make sure to reply to every one. You can click the “Done” button to reply indicating that you‘ve implemented the reviewer’s suggestion; otherwise, click on “Reply” and explain why you have not, or what you have done instead.

It is perfectly normal for changes to go through several round of reviews, with one or more reviewers making new comments every time and then waiting for an updated change before reviewing again. This cycle happens even for experienced contributors, so don't be discouraged by it.

Voting conventions

As they near a decision, reviewers will make a “vote” on your change. The Gerrit voting system involves an integer in the range -2 to +2:

  • +2 The change is approved for being merged. Only CUE maintainers can cast a +2 vote.
  • +1 The change looks good, but either the reviewer is requesting minor changes before approving it, or they are not a maintainer and cannot approve it, but would like to encourage an approval.
  • -1 The change is not good the way it is but might be fixable. A -1 vote will always have a comment explaining why the change is unacceptable.
  • -2 The change is blocked by a maintainer and cannot be approved. Again, there will be a comment explaining the decision.

Submitting an approved change

After the code has been +2'ed, an approver will apply it to the master branch using the Gerrit user interface. This is called “submitting the change”.

The two steps (approving and submitting) are separate because in some cases maintainers may want to approve it but not to submit it right away (for instance, the tree could be temporarily frozen).

Submitting a change checks it into the repository. The change description will include a link to the code review, which will be updated with a link to the change in the repository. Since the method used to integrate the changes is Git's “Cherry Pick”, the commit hashes in the repository will be changed by the submit operation.

If your change has been approved for a few days without being submitted, feel free to write a comment in Gerrit requesting submission.

Miscellaneous topics

This section collects a number of other comments that are outside the issue/edit/code review/submit process itself.

Copyright headers

Files in the CUE repository don't list author names, both to avoid clutter and to avoid having to keep the lists up to date. Instead, your name will appear in the change log and in the CONTRIBUTORS file and perhaps the AUTHORS file. These files are automatically generated from the commit logs periodically. The AUTHORS file defines who “The CUE Authors”—the copyright holders—are.

New files that you contribute should use the standard copyright header:

// Copyright 2018 The CUE Authors
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// See the License for the specific language governing permissions and
// limitations under the License.

(Use the current year if you're reading this in 2019 or beyond.) Files in the repository are copyrighted the year they are added. Do not update the copyright year on files that you change.

Troubleshooting mail errors

The most common way that the git codereview mail command fails is because the e-mail address in the commit does not match the one that you used during the registration process.

If you see something like...

remote: Processing changes: refs: 1, done
remote: ERROR:  In commit ab13517fa29487dcf8b0d48916c51639426c5ee9
remote: ERROR:  author email address XXXXXXXXXXXXXXXXXXX
remote: ERROR:  does not match your user account.

you need to configure Git for this repository to use the e-mail address that you registered with. To change the e-mail address to ensure this doesn't happen again, run:

$ git config

Then change the commit to use this alternative e-mail address with this command:

$ git commit --amend --author="Author Name <>"

Then retry by running:

$ git codereview mail

Quickly testing your changes

Running go test ./... for every single change to the code tree is burdensome. Even though it is strongly suggested to run it before sending a change, during the normal development cycle you may want to compile and test only the package you are developing.

In this section, we'll call the directory into which you cloned the CUE repository $CUEDIR. As CUE uses Go modules, The cue tool built by go install will be installed in the bin/go in your home directory by default.

If you're changing the CUE APIs or code, you can test the results in just this package directory.

$ cd $CUEDIR/cue
$ [make changes...]
$ go test

You don't need to build a new cue tool to test it. Instead you can run the tests from the root.

$ cd $CUEDIR
$ go test ./...

To use the new tool you would still need to build and install it.

Specifying a reviewer / CCing others

You can specify a reviewer or CC interested parties using the -r or -cc options. Both accept a comma-separated list of e-mail addresses:

$ git codereview mail -r -cc,

Synchronize your client

While you were working, others might have submitted changes to the repository. To update your local branch, run

$ git codereview sync

(Under the covers this runs git pull -r.)

Reviewing code by others

As part of the review process reviewers can propose changes directly (in the GitHub workflow this would be someone else attaching commits to a pull request).

You can import these changes proposed by someone else into your local Git repository. On the Gerrit review page, click the “Download ▼” link in the upper right corner, copy the “Checkout” command and run it from your local Git repo. It will look something like this:

$ git fetch refs/changes/21/13245/1 && git checkout FETCH_HEAD

To revert, change back to the branch you were working in.

Set up git aliases

The git-codereview command can be run directly from the shell by typing, for instance,

$ git codereview sync

but it is more convenient to set up aliases for git-codereview's own subcommands, so that the above becomes,

$ git sync

The git-codereview subcommands have been chosen to be distinct from Git‘s own, so it’s safe to define these aliases. To install them, copy this text into your Git configuration file (usually .gitconfig in your home directory):

	change = codereview change
	gofmt = codereview gofmt
	mail = codereview mail
	pending = codereview pending
	submit = codereview submit
	sync = codereview sync

Sending multiple dependent changes

Advanced users may want to stack up related commits in a single branch. Gerrit allows for changes to be dependent on each other, forming such a dependency chain. Each change will need to be approved and submitted separately but the dependency will be visible to reviewers.

To send out a group of dependent changes, keep each change as a different commit under the same branch, and then run:

$ git codereview mail HEAD

Make sure to explicitly specify HEAD, which is usually not required when sending single changes.