How to contribute to Project Mentat

This project is very new, so we’ll probably revise these guidelines. Please comment on a bug before putting significant effort in, if you’d like to contribute.

You probably want to quickly read the front page of the wiki to get up to speed.


  • Follow the Style Guide (see below).
  • Keep work branches in your own GitHub fork; rebase your own branches at will.
  • Squash or rebase branches before merging to master so the commits make sense on their own.
  • Get a SGTM from someone relevant before merging.
  • Keep commits to master bisect-safe (i.e., each commit should pass all tests).
  • Sign-off commits before merging (see below).
  • Make sure your commit message: references the issue or bug number, if there is one; identifies the reviewers; and follows a readable style, with the long description including any additional information that might help future spelunkers (see below).
Frobnicate the URL bazzer before flattening pilchard. (#123) r=mossop,rnewman.

The frobnication method used is as described in Podder's Miscellany, page 15.
Note that this pull request doesn't include tests, because we're bad people.

Signed-off-by: Random J Developer <>


  • Fork this repo at
  • Clone your fork locally. Make sure you use the correct clone URL.
    git clone

    Check your remotes:

    git remote --verbose

    Make sure you have an upstream remote defined:

    git remote add upstream
  • Create a new branch to start working on a bug or feature:
    git checkout -b some-new-branch
  • Do some work, making sure you signoff every commit:
    git commit --signoff --message "Some commit message"
  • Rebase your work during development and before submitting a pull request, avoiding merge commits, such that your commits are a logical sequence to read rather than a record of your fevered typing.
  • Make sure you’re on the correct branch and are pulling from the correct upstream (currently rust):
    git checkout some-new-branch
    git pull upstream rust --rebase

    Or using git reset --soft (as described in a tale of three trees)

  • Update your fork with the local changes on your branch:
    git push origin some-new-branch
  • Submit a pull request. It would be helpful if you also flagged somebody for review, by typing their @username in the comments section.

Addressing review comments

Adding more commits

After submitting a pull request, certain review comments might need to be addressed. All you have to do is commit your new work, and simply update your fork with the local changes on your branch again. The pull request will automatically update with your new changes.

Signoff earlier commits

If you forgot to signoff some earlier commits, do an incremental rebase on the branch you’re working on. Find the earliest commit hash you want to change, e.g., “1234567” (via git log), then use it in the rebase command to start an interactive rebase. Type edit instead of pick for the commits you want to edit.

git rebase --interactive '1234567^'
git commit --amend --signoff --no-edit
git rebase --continue

Squashing earlier commits

While you’re working, committing often is a good idea. However, it might not make sense to have commits that are too granular or don’t make sense on their own before closing a pull request and merging back to upstream master. Find the earliest commit hash you want to change, e.g., “1234567” (via git log), then use it in the rebase command to start an interactive rebase. Type squash instead of pick for the commits you want to squash into their parents.

git rebase --interactive '1234567^'

Properly set name and email

Update your .gitconfig with the proper information. You might need to update the earlier commits and sign them off as well, see above.

git config --global "Foo Bar"
git config --global
git commit --amend --reset-author --no-edit

Style Guide

Our Rust code approximately follows the Rust style guide. We use four-space indents, with categorized and alphabetized imports; see the examples in the tree. We try to follow these guidelines, too.

We do not automatically use rustfmt because it tends to make code incrementally worse, but you should be prepared to consider its suggestions.

An example of ‘good’ Rust code, omitting the license block:


extern crate foo;

use std::borrow::Borrow;
use std::error::Error;
use std::iter::{once, repeat};

use rusqlite;

use mentat_core::{

type MyError = Box<Error + Send + Sync>;

pub type Thing = Borrow<String>;

pub fn foo_thing(x: Thing) -> Result<(), MyError> {
    // Do things here.

Our JavaScript code follows the airbnb style with a few exceptions. The precise rules are likely to change a little as we get started so for now let eslint be your guide.

Our ClojureScript code (no longer live) doesn’t follow a specific style guide.

How to sign-off your commits

To help tracking who did what, we have a “sign-off” procedure on patches. This avoids the need for physically signed “[Committers|Contributors] License Agreements”.

The sign-off is a simple line at the end of the commit message, which certifies that you wrote it or otherwise have the right to pass it on as an open-source patch. The rules are pretty simple: if you can certify the below:

Developer's Certificate of Origin 1.1

By making a contribution to this project, I certify that:

(a) The contribution was created in whole or in part by me and I
    have the right to submit it under the open source license
    indicated in the file; or

(b) The contribution is based upon previous work that, to the best
    of my knowledge, is covered under an appropriate open source
    license and I have the right under that license to submit that
    work with modifications, whether created in whole or in part
    by me, under the same open source license (unless I am
    permitted to submit under a different license), as indicated
    in the file; or

(c) The contribution was provided directly to me by some other
    person who certified (a), (b) or (c) and I have not modified

(d) I understand and agree that this project and the contribution
    are public and that a record of the contribution (including all
    personal information I submit with it, including my sign-off) is
    maintained indefinitely and may be redistributed consistent with
    this project or the open source license(s) involved.

then you just add a line saying

Signed-off-by: Random J Developer <>

using your real name (sorry, no pseudonyms or anonymous contributions).

If you’re using the command line, you can get this done automatically with

$ git commit --signoff

Some GUIs (e.g., SourceTree) have an option to automatically sign commits.

If you need to slightly modify patches you receive in order to merge them, because the code is not exactly the same in your tree and the submitters’. If you stick strictly to rule (c), you should ask the submitter to submit, but this is a totally counter-productive waste of time and energy. Rule (b) allows you to adjust the code, but then it is very impolite to change one submitter’s code and make them endorse your bugs. To solve this problem, it is recommended that you add a line between the last Signed-off-by header and yours, indicating the nature of your changes. While there is nothing mandatory about this, it seems like prepending the description with your mail and/or name, all enclosed in square brackets, is noticeable enough to make it obvious that you are responsible for last-minute changes. Example:

Signed-off-by: Random J Developer <>
[ struct foo moved from foo.c to foo.h]
Signed-off-by: Lucky K Maintainer <>

This practice is particularly helpful if you maintain a stable branch and want at the same time to credit the author, track changes, merge the fix, and protect the submitter from complaints. Note that under no circumstances can you change the author’s identity (the From header), as it is the one which appears in the change-log.