Version 19 (modified by dterei, 6 years ago) (diff)


Guidelines for using git with GHC

GHC uses git for revision control. This page describes various GHC-specific conventions for using git, together with some suggestions and tips for using git effectively.

Existing darcs users see: Git For Darcs Users. If you have an existing source tree in darcs and need to convert patches, see Darcs To Git. Simon PJ's git notes are GIT SPJ.

General Guidelines

  • Try to make small patches (i.e. work in consistent increments).
  • Separate changes that affect functionality from those that just affect code layout, indendation, whitespace, filenames etc. This means that when looking at patches later, we don't have to wade through loads of non-functional changes to get to the important parts of the patch.
  • If possible, commit often. This helps to avoid conflicts.
  • Discuss anything you think might be controversial before pushing it.


Please make sure you have setup git to use the correct name and email for your commits. Use the same name and email on all machines you may push from.

$ git config --global "Firstname Lastname"Sets the name of the user for all git instances on the system
$ git config --global ""

This will set your name and email globally. To set it for just the GHC repo, remove the --global flag. Also, the environment variables GIT_COMMITTER_NAME, GIT_COMMITTER_EMAIL, GIT_AUTHOR_NAME and GIT_AUTHOR_EMAIL will override git-config settings if they are defined.

Commit messages

We have a simple convention for commit log messages:

  • If your patch fixes breakage in the build, then begin the patch name with "FIX BUILD". e.g.
      FIX BUILD Use the right find on Windows systems; fixes bindist creation
  • If your patch fixes a bug, then include the ticket number in the form #NNNN in the patch name, e.g.
      withMVar family have a bug (fixes #767)
    Trac will then create a link from the commit to the ticket, making navigation easier.

Line endings

Files in GHC repos should use Unix conventions for line endings. If you are on Windows, ensure that git handles line-endings sanely by running:

git config --global core.autocrlf false

To find out what files in your tree have windows (CRLF) line endings, use

find . -name '*hs' | xargs file | grep CRLF

Do this before you commit them!

Workflow with validate

All changes to GHC and the libraries need to be validated before they can be pushed to the main repositories. Validation can take a while - 30 minutes on a 4-core machine is typical - so ideally you want to be validating changes while you are working in a separate tree. In fact, there are other compelling reasons to have two trees in your development workflow, one for working in and one for validation:

  • validation uses build settings that are different to the ones you would normally use while developing: it adds more libraries (DPH), builds extra ways (dynamic libraries), and builds all the documentation, so you don't want to use the same build for validation and ordinary development. In the development tree we use build settings optimised for development: -O0 -DDEBUG for the compiler, minimal libraries and ways so that rebuilding is fast.
  • Having two trees elimiantes a common source of breakage in the main repository: with one tree it is easy to add new files but forget to commit them. Your tests will work, but the build will be broken for others. If you have to pull your changes into a separate tree for testing, you'll notice the missing files before you push.

The typical workflow is to work in the development tree, pull into the validate tree, validate, and then push from the validate tree. But what if validate fails? There are two options:

  1. discard the patch in the validate tree (using some instance of git reset) and go back to the working tree to fix it
  2. or, add a new patch in the validate tree to fix the problem and re-validate

(1) is more for "back to the drawing board" kinds of failure, whereas (2) is for cases where you just need to fix a warning or some other minor error exposed by validate.

Setting up the trees

Let's call the two trees ghc-working and ghc-validate.

Set up your repos like this:

$ git clone ghc-working
$ cd ghc-working
$ ./sync-all --testsuite --no-dph get
$ cd ..
$ git clone ghc-working ghc-validate
$ cd ghc-validate
$ ./sync-all --testsuite get
$ ./sync-all -r remote set-url origin
$ ./sync-all -r `pwd`/../ghc-working remote add working
$ ./sync-all -r <account> remote set-url --push origin

(where <account> is your account on; omit this step if you don't have one, you can still submit patches via the mailing list (using git format-patch will help you with this) or send a pull request to get your changes in GHC).

Now you have ghc-working and ghc-validate repos, and additionally the ghc-validate repo tree is set up with a remote working pointing to the ghc-working tree, and pushing from ghc-validate will push changes via SSH to

The rebase workflow

How do we move patches from ghc-working and ghc-validate? There are several options here. One is to just use sync-all pull working and do merging as usual. This works fine, but results in extra "merge commits" that aren't particularly helpful and clutter the commit logs and the mailing list. A better approach is to rebase patches before committing. This is done as follows:

  1. Pull from ghc-working into ghc-validate: ./sync-all pull working
  2. Rebase onto origin/master: ./sync-all pull --rebase. You may encounter conflicts, in which case git will tell you what to do (usually fix the conflict and then git rebase --continue in the appropriate repository), then you can resume with ./sync-all --resume pull --rebase at the top.
  3. Check what you have relative to origin: ./sync-all new
  4. ./validate
  5. if validate went through, ./sync-all push (you might like to check once more what will be pushed: ./sync-all new).

If push fails because patches have been pushed by someone else while you were validating, it is acceptable to git pull --rebase in that repository and push if there are no conflicts (no need to validate again).

Now, the patches pushed this way are different (have different hashes) from the patches that you originally committed in ghc-working, and if you try to pull these patches in ghc-working again, confusion and conflicts will ensue. Fortunately there's an easy solution: just rebase again in ghc-working, and git will notice that your patches are already upstream and will discard the old versions. It's as simple as

 $ cd ghc-working
 $ ./sync-all pull --rebase

If rebase encounters a conflict at any point, it will tell you what to do. After fixing the conflict and completing the rebase manually, you can then resume the pull with ./sync-all --resume pull --rebase.

There is a slight tweak to this workflow that you might find more convenient: do a ./sync-all pull --rebase in the ghc-working tree prior to pulling into ghc-validate. This lets you fix conflicts in ghc-working rather than in ghc-validate, and test the resolution before validating. The downside is that you might now have to do a lot of rebuilding in your ghc-working tree if there are a lot of changes to pull.

Contributing patches

Please write your patch and then rebase to the latest version of GHC HEAD before sending to us. You can use the following command to send patches via email:

git send-email <hash-id> -1

where <hash-id> is the hash of the commit to send. If you'd prefer to create patch files and send them via email another way (or attach them to trac tickets) then you can use this command:

git format-patch [-o <outputdir>] <revision range>

Where <revision range> specifies the commit that git should stop at when going from HEAD backwards, creating a patch for each commit in the range <revision range>..HEAD.

Applying patches from email

git am -3 <email>

The stable branch