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Contributing

It's pretty tricky to keep all the language tracks in good working order, and it's not uncommon that people discover typos in the README or the test suites, inconsistencies between the README and the tests, missing edge cases, factual errors, logical errors, and confusing ambiguities.

We welcome contributions of all sorts and sizes, from reporting issues to submitting patches, to hanging out in the support chat to help people get unstuck.

We are grateful for any help in making Exercism better!


This guide covers several common scenarios pertaining to improving the language tracks themselves. There are other guides about contributing to other parts of the Exercism ecosystem.

Table of Contents

We Will Gladly Help You Help Us

It can be confusing and intimidating to figure out how to fix even a tiny thing in a small project, much less a sprawling 50 repository beast like Exercism.

We'll do everything we can to help you get started.

The two best ways to get help are to

We are happy to help out with all sorts of things, including figuring out the whole git and pull request thing.

Don't be shy, we're a friendly bunch!

If you have questions that you're not comfortable asking out in the open, email Katrina at kytrinyx@exercism.io.

Code of Conduct

Help us keep Exercism welcoming. Please read and abide by the Code of Conduct.

Overview

Each language track is implemented in its own repository. This has several benefits:

  • It's easier to get started contributing, since you don't need to wade through setup instructions for 20 different languages.
  • There's less noise for people who are maintaining a language track, since they won't be seeing pull requests and issues about languages they're not maintaining.
  • Build tools can be tailored to each language.
  • Continuous integration runs more quickly, since it only needs to install a single language environment, and run the tests for one single track.

We use the following terminology:

  • Language - A programming language.
  • Track - A collection of exercises in a programming language.
  • Problem - A generic problem description.
  • Exercise - A language-specific implementation of a problem description.

Language Track Repositories

We've given each language track an ID, which is a url-friendly version of the language name. For example, C++ has the ID cpp. This ID is used throughout the Exercism ecosystem.

Each language-specific repository can be found under the Exercism GitHub organization, named with the track ID, prefixed with x.

https://github.com/exercism/x{TRACK_ID}

For example, the C++ repository is exercism/xcpp.

Problem Metadata

Many languages implement an exercise based on the same generic problem description. So you might have a "leap year" exercise in Haskell, JavaScript, Go, Ruby, and Python. The basic premise will be the same, but each language will tailor the exercise to fit the idioms and idiosyncrasies of that language.

We try to keep the generic descriptions generic--we avoid implementation-specific examples, and try not to be too prescriptive about suggesting how a problem might be solved.

The README of each exercise is pieced together from various bits and pieces of this shared metadata, along with some information that is custom to the language track in question.

Some of the problems also have a JSON file containing canonical test cases. These are used to hand-craft a test suite generator, allowing us to quickly regenerate test suites when edge cases or errors are discovered.

The generic problem descriptions live in the exercises folder in this repository (exercism/x-common).

Updating an Exercise Test Suite

Once you find the correct repository, you can fork it and then clone it.

The README in each individual language track repository covers how to set up the development environment for that language.

Often all you need is a running language environment along with the relevant testing library.

If the test suite was generated, then editing the solution will require a couple of extra steps. This is covered in detail in a separate section of this guide.

The test suite is usually named with test or Test in the filename, though some language tracks have other conventions (e.g. spec is fairly common, and sometimes it's just a matter of a different file extension).

If you're unsure where to make the change, ask us, and we'll help you figure it out.

Once you've updated the test suite, there are a few things you'll want to check.

  • Make sure the reference solution is still passing.
  • If the exercise is versioned, and the change will mean that existing solutions on the site will not pass the new tests, then increment the version number, both in the test and in the reference solution.
  • Run the full, track-level test suite, if available. If the track has a way to automatically run all the tests against their reference solutions, it will be documented in the README.
  • Run configlet, the track-level linter.

You can also take a look at the .travis.yml file to see what the continuous integration system will do to verify the track.

Take a look at our pull request guidelines. You don't need to get it perfect the first time around; we'll work with you to get the patch merged.

Updating a Generated Test Suite

Some language tracks are experimenting with generating test suites from shared test data. This is because various interesting edge cases are discovered as people discuss solutions, but these edge cases are usually then only added to a single language track. By standardising the inputs and outputs, it becomes easier to and improve the exercises across all the languages.

There are two possible scenarios, described below.

  1. You want to change or add inputs or outputs.
  2. You want to change something about the test suite itself.

Once you've made the change, then follow the instructions about verifying your change and submitting a patch as described above, in the section about updating an exercise test suite.

Changing Inputs and Outputs

If you want to add a new test or change some inputs or outputs, then the change needs to be made in the exercism/x-common repository, not directly to the test suite itself.

Find the JSON file for the problem in question. For example, if you want to change the Clock problem, then look for exercises/clock/canonical-data.json.

Submit a pull request with the change.

When that pull request has been merged, then the various languages that implement that problem will need to have their test suites regenerated. Track maintainers can do this, though we're always happy if you want to submit a patch with the regenerated test suite.

The instructions for regenerating a test suite should be described in the README of the language-specific repository.

Follow the guidelines for setting up a development environment, verifying the change, and submitting a pull request, as described in the main section about updating an exercise test suite.

Changing the Structure

If you are not changing inputs/outputs, but rather the structure of the test suite, then that change will need to be made within the generator itself. It lives in the language-specific repository along with the exercises, and the process for regenerating the exercise should be described in the README of the repository.

Follow the guidelines for setting up a development environment, verifying the change, and submitting a pull request, as described in the main section about updating an exercise test suite.

Tweaking a README

The Exercism exercise README treads a very fine line between useful ambiguity and confusing vagueness. Because the README is the same whether you're solving the problem in C++ or in Lua, the problem description needs to be high-level enough to allow for the syntactic, semantic, and philosophical differences in the various languages.

In other words: no specific references to syntax or data structures of a specific language can be used to further clarify a problem.

However, within this purposeful ambiguity might lie some opportunities for making an exercise description more clear. Typical issues to be attentive to:

  • poorly worded sentences
  • outdated information
  • incorrect directives
  • typos

Each language's test suite provides the precise specification for the exercise, which allows the user to view the problem from a perspective that is interesting and idiomatic for that specific language.

In addition, there's some language-specific content that gets woven into the README, usually a quick reminder about how to run the tests, and where to find more documentation.

Updating a Generic Problem Description

Fork this repository.

Each generic problem is identified by a slug. For example, the problem Crypto Square is crypto-square. There are two metadata files for each problem:

  • exercises/<slug>/description.md which contains the generic problem description which makes up the bulk of the README, and
  • exercises/<slug>/metadata.yml which contains a short one-line description of the problem as well as other metadata, such as the source that inspired the problem in the first place.

There aren't any rules about what a good exercism problem README looks like. If in doubt, open up a GitHub issue describing your suggestion.

Once you've made your change submit a pull request.

Updating Language-Specific Additions

Each language track may optionally contain a SETUP.md file in the root of the repository. This file should contain helpful, generic information about solving an exercism problem in the target language.

The contents of the SETUP.md file gets included in the README.md that gets delivered along with the test suite and any supporting files when a user runs the exercism fetch command from their terminal.

It would also be useful to explain in a generic way how to run the tests. Remember that this file will be included with all the problems, so it gets confusing if we refer to specific problems or files.

If a language track has specific expectations, these should also be documented here.

Porting an Exercise to Another Language Track

To get a list of all the exercises that can be ported to a track, go to the url http://exercism.io/languages/:track_id/contribute.

For example here is the list of exercises that have not yet been implemented for the Ruby track: http://exercism.io/languages/ruby/contribute

Each unimplemented exercise links to existing implementations of the exercise in other language tracks, so that people can use those example solutions and test suites as inspiration.

We are also extracting canonical inputs and outputs for a given exercise and storing them in JSON format in the x-common repository. We've accomplished this on a few exercises, but there are many more to do.

Although this page is now implemented, you can still get this information from the raw data served by the API endpoint http://x.exercism.io/v3/tracks/:track_id/todo.

For example, here's the list of exercises that have not yet been implemented in the Elm track: http://x.exercism.io/v3/tracks/elm/todo

It can be pretty unwieldy to read the JSON directly. To make it easier, install a browser extension that formats the JSON nicely, or copy/paste the response body into jsonlint.com and click "validate JSON", which not only validates it, but pretty-prints it.

The description of the problem can be found in the x-common repository, in a folder named after the problem slug: exercises/<slug>/description.md.

When you decide to implement an exercise

  • check that there are no open pull requests for the same exercise
  • open a "work in progress" (WIP) pull request

The way to open a WIP pull request even if you haven't done any work yet is:

  • Fork and clone the repository
  • Check out a branch for your the exercise
  • Add an empty commit git commit --allow-empty -m "dibs: I will implement exercise <slug>" (replace with the actual name of the exercise).
  • Push the new branch to your repository, and open a pull request against that branch.

Once you have added the actual exercise, then you can rebase your branch onto the upstream master, which will make the WIP commit go away.

The exercise should consist of, at minimum:

You will need to add the exercise to "exercises" section of the config.json file in the track. The order in which the exercises are listed there is the order in which they are fetched by default by exercism fetch. Typically, exercises are ordered by difficulty, unless there is a particular reason to do otherwise.

Each language track might have additional guidance on how to order their exercises or additional requirements on new exercise files; check the README in the repository for the track.

Providing Feedback on the Site for an Exercise You Implemented

Once you've created an exercise, you'll probably want to provide feedback to people who submit solutions to it. By default you only get access to exercises you've submitted a solution for.

You can fetch the problem directly using the CLI:

$ exercism fetch <track_id> <slug>

Go ahead and submit the reference solution that you wrote when creating the problem. Remember to archive it if you don't want other people to comment on it.

Implementing a Completely New Exercise

A problem must have a unique slug. This slug is used as

  • the directory name within each language-specific repository
  • the directory name for the folder that contains the metadata files (in this repository)
  • to identify the exercise in config.json

In exercism/x-common

  • Create exercises/<slug>/description.md and exercises/<slug>/metadata.yml.
  • Bonus: exercises/<slug>/canonical-data.json with inputs/outputs for the test suite.
  • Submit a pull request.

In exercism/x

  • Do the same as when porting an exercise. Reference the PR in x-common if it hasn't been merged yet, this must not be merged until the exercism/x-common PR is merged.

Improving Consistency By Extracting Shared Test Data

The goal is to have a file exercises/<slug>/canonical-data.json for each exercise in this repository. This JSON file should contain the generic (i.e. language independent) test data for that exercise. With this the quality of the tracks can be improved, e.g. fixes or new tests can be propagated to all the tracks. They can then be implemented manually or by running the test generator. (See this issue for more background info.)

The following steps help you extracting the canonical test data.

  1. Select the exercise for which you want to create the shared test data from the list of open todos
  2. Now you need to find the existing tests. You can get a list with links to the all of those one of two ways
    • Search for the exercise in this list http://x.exercism.io/v3/problems/data-todos (your browser might need a JSON viewer plugin/add-on to show this)
    • Run this command in your terminal curl http://x.exercism.io/v3/problems/data-todos | jq '.[] | map(select(.["slug"] == "accumulate"))'. Replace accumulate with the name of the exercise you have chosen.
  3. Open the links in different browser tabs, navigate to the test files and read through the tests. Look for test cases that all/many of the languages have in common and identify outliers that only appear in one or two languages. It might be helpful to make some notes on your findings.
  4. Now it's time to write the canonical-data.json file. Look at examples like the test data for hamming and check the readme to find out how to best structure the JSON file. From what you learned in step 3 decide for a sensible set of test cases. The following considerations might help with that.

    • Try to stick to test cases that already occur in many languages so it is feasible for the track maintainers to adapt to the canonical set.
    • All tests should be essential to the problem. Ensure people can get the first test passing easily. Then each following test should ideally add a little functionality. People should be able to solve the problem a little at a time. Avoid test cases that don't introduce any new aspect and would already pass anyway.
    • The test cases should not require people to follow one very specific way to solve the problem, e.g. the tests should avoid checking for helper functions that might not be present in other implementations.
    • Performance tests should not be included in the canonical test data.

    If you are unsure about all these considerations just make a pull request with an initial proposal and ask the community for help.

Writing a New Test Suite Generator

TODO: elaborate.

Track Anatomy

Each track should have the following structure:

├── .gitignore
├── .travis.yml
├── LICENSE
├── README.md
├── SETUP.md
├── bin
│   └── fetch-configlet
├── config.json
├── docs
│   ├── ABOUT.md
│   ├── INSTALLATION.md
│   ├── LEARNING.md
│   ├── RESOURCES.md
│   └── TESTS.md
└── exercises
    └── hello-world
        ├── hello-world_example.file
        ├── hello-world.file
        └── hello-world_test.file

The example template for a track can be found at x-template.

  • LICENSE - The MIT License (MIT)
  • README.md - a thorough explanation of how to contribute to the track.
  • SETUP.md - this should contain any track specific, problem agnostic information that will be included in the README.md of every exercise when fetched. Include information on how to run tests, how to get help, etc.
  • bin - scripts and other files related to running the track's tests, etc.
  • config.json - the track-level configuration. It contains configuration for which exercises (and in which order) are a part of the track, which exercises are deprecated, the track id, name of the language, and the location of the repository. Optionally, it may include a regex to recognize what files are part of the test suite (usually /test/i, but sometimes /spec/i or other things). If the test pattern is not included, then /test/i is assumed.
  • docs - the documentation for the track.These files are served to the exercism.io help site via the x-api. It should contain at minimum:

    • INSTALLATION.md - about how to get the track's language set up locally.
    • TESTS.md - how to run the tests for the track's individual exercises.

    Some nice to haves:

    • ABOUT.md - a short, friendly blurb about the track's language. What types of problems does it solve really well? What is it typically used for?
    • LEARNING.md - a few notes about where people might want to go to learn the track's language from scratch. These are the the resources you need only when first getting up to speed with a language (tutorials, blog posts, etc.).
    • RESOURCES.md - references and other useful resources. These resources are those that would commonly be used by a developer on an ongoing basis (core language docs, api docs, etc.).
  • exercises - all exercises for the track should live in subdirectories of this directory. Each exercise should have a test file, an example file that should pass all tests, and a template file that is a stub to help the user get started with the exercise. The example file should be used for the CI build.

Starting a New Track

If you're interested in adding problems for a language that we don't yet have, email Katrina and she'll set up a new repo for that language.

Then you can fork and clone per usual.

In order to launch the track needs:

  • At least 10 problems implemented.
  • A handful of people who can check in regularly and provide feedback on solutions.
  • Documentation in docs/ for how to get started / run the tests

Description of what is required for docs/ can be found in the Track Anatomy.

Once that is in place, the repository needs to be added as a submodule to exercism/x-api, and the "active" key in config.json must be flipped to true.

We don't deploy x-api automatically, so it will go live the next time the submodules are updated (daily, for the most part).

Beta-Testing a Language Track


For a track that is set as "active": false in the config.json, exercism fetch will not automatically pull down problems. You can still test the language by fetching problems directly, e.g.:

exercism fetch cpp bob

This will allow you to do some dry-run tests of fetching exercises, double checking the instructions for each problem and submitting the problem solution for peer review.

It is recommended that you configure a Travis continuous integration build with your language track to verify that your example problem solutions satisfy the tests provided for each problem.

You can include advice and helpful links for your language track in the SETUP.md file.

Maintaining a Track

Maintaining a language track generally consists of:

  • Reviewing/merging pull requests.
  • Discussing improvements in the exercises.
  • Implementing or porting new exercises.
  • Improving the development tooling (e.g. implementing continuous integration).
  • Language-Specific support.
  • Adding/improving language-specific documentation.

Ideally a track will have several maintainers, for two reasons:

  • more lively
  • spread the workload

More Lively

We've noticed that as soon as there are at least two people maintaining the same track we get rich discussions about quality and idioms. There's a lot more activity, and it's a lot more fun.

Spread the Workload

We don't want to burn people out, and it's really nice to be able to go on vacation or get busy at work without worrying too much about a growing backlog of unanswered issues and unreviewed and unmerged pull requests.

Caveat

There's a small chance that when more people are involved there's a bit of diffusion of responsibility (worth googling and reading about if you haven't heard the term before).

Reviewing/Merging Pull Requests

In general:

  • Avoid merging your own pull requests (but it's fine if it's really simple).
  • If the change is significant, get a second opinion.
  • If it's insignificant or simple or uncontroversial, go ahead and merge.
  • If nobody else responds within a certain amount of time, go ahead and merge it anyway, if you feel like it's good enough (we can always fix things later).

Many maintainers have mentioned that they like to get a second pair of eyes even for simple fixes, because it's so easy to for typos and really silly things to slip in.

Use Branches (Probably, Mostly)

Even for simple fixes (documentation, typos) branches let others see what's going on in the repository. If it's insignificant, go ahead and merge it yourself.

Sometimes it's just silly to create a branch. In that case, go ahead and put it in master, unless there's a track-level policy about not doing that.

Claiming Issues

When you start working on an issue, claim it (either assign it to yourself or just add a comment that you're taking it).

If you have a big list of similar, related things, it's fine to create a single issue with a todo list, and people can claim individual things in the comment thread.

Implementing / Reviewing New Exercises

The tracks should implement the exercise idiomatically in the language at hand, without veering too far from the README as described (does expanding the exercise introduce new ideas or just add more work? Is this better off treated as a new, separate exercise?).

If there are interesting corner cases, then these should be added to the README, they help make the discussions better.

Exercises should not enforce a single way to solve the problem, if possible. The more interesting exercises allow several approaches, and create rich opportunities for discussing trade-offs when people submit their solutions.

Skipping Uninteresting Exercises

Don't be afraid to 'forego' exercises that don't make sense in the language, or that are not particularly interesting.

Useful Tidbits

Here are a few bits and pieces that are referenced from some of the scenarios in this guide.

Pull Request Guidelines

  1. Put the name of the exercise in the subject line of the commit. E.g. hamming: Add test case for strands of unequal length
  2. Don't submit unrelated changes in the same pull request.
  3. If you had a bit of churn in the process of getting the change right, squash your commits.
  4. If you had to refactor in order to add your change, then we'd love to see two commits: First the refactoring, then the added behavior. It's fine to put this in the same pull request, unless the refactoring is huge and would make it hard to review both at the same time.

Once you've submitted a pull request, one or more of the track maintainers will review it. Some tracks are less active and might not have someone checking in every day. If you don't get a response within a couple of days, feel free to ping us in the support chat.

Exercise Versioning

It's only when we get a bunch of people having conversations about the solutions that we really discover what makes a problem interesting, and in what way it can be improved.

Some changes to the test suites will invalidate existing solutions that people have submitted.

We think this is totally fine, however sometimes people start leaving feedback saying this doesn't pass the tests. This is technically true, but since the solution passed the tests at the time it was written, it's generally more useful to just discuss the code as it is, rather than enforce strict adherence to the most recent version of the tests.

Some language tracks have implemented a simple, manual versioning system to help avoid unnecessary discussions about failing the current test suites.

If the exercise is versioned, then the test suite will probably have a book-keeping type test at the very bottom that asserts against a value in the reference solution. If the change you're making is backwards-incompatible, then please increment the version in both the test suite and the reference solution.

Anatomy of an Exercise

TODO: expand on notes below.

README

TODO

Test Suite

TODO

Supporting Files

TODO (boilerplate, header files, etc)

Reference Solution

The reference solution is named something with example or Example in the path.

The solution does not need to be particularly great code, it is only used to verify that the exercise is coherent.

If you change the test suite, then make sure the reference solution is fixed to pass the updated tests.

config.json

Each language track has a config.json file. Important keys are:

  • problems - actively served via exercism fetch
  • deprecated - implemented, but aren't served anymore
  • foregone - will not be implemented in the track
  • ignored - these directories do not contain problems

The configlet tool uses those categories to ensure that

  1. all the problems are implemented,
  2. deprecated problems are not actively served as problems, and
  3. foregone problems are not implemented.

In addition, it will complain about problems that are implemented but are not listed in the config under the problems key. This is where the ignored key is useful. Ignored directories don't get flagged as unimplemented problems.

A problem might be foregone for a number of reasons, typically because it's a bad exercise for the language.

The config.json also has an optional test_pattern key. This is a regex that test filenames will match. If test files contain /test/, then this key can be deleted.

Track-Level Linting With Configlet

If the config.json file is incomplete or broken, a lot of other things break. To make things easier we made a small tool to help verify the config: https://github.com/exercism/configlet#configlet

You can download the latest release from the releases page in the configlet repo, or you can use the bin/fetch-configlet command from the root of the language track repository, which will make a guess at what operating system and architecture you have and attempt to download the right one.

Verify the config by calling bin/configlet . (notice the dot). This says _check the config of the language track that is stored right here).

Git Basics

If you're concerned that you haven't done it right, don't worry. Submit your pull request, and we'll help you get the details sorted out.

Getting the Code

We recommend forking the project first, and then cloning the fork.

git clone git@github.com:<YOUR_USERNAME>/<REPO_NAME>.git

This will give you a remote repository named origin that points to your own copy of the project.

In addition to this, we recommend that you add the original repository as a secondary remote.

git remote add upstream https://github.com/exercism/<REPO_NAME>.git

The names origin and upstream are pretty standard, so it's worth getting used to the names in your own work.

Branches

When working on your fork it tends to make things easier if you never touch the master branch.

The basic workflow becomes:

  • check out the master branch
  • pull from upstream to make sure everything is up to date
  • push to origin so you have the latest code in your fork
  • check out a branch
  • make the changes, commit them
  • rebase onto the upstream master (and resolve any conflicts)
  • push your branch up to origin
  • submit a pull request

If you're asked to tweak your work, you can keep pushing it to the branch, and it automatically updates the pull request with the new commits.

Commit Messages

Commit messages are communication and documentation. They're a log of more than just what happened, they're about why it was done.

The longer a project is active, the more people involved, the larger the codebase, the more important it is to have good commit messages.

There's an excellent lightning talk by Ryan Geary called Do Your Commit Messages Suck?. It's funny and enlightening, and you should watch it.

Tim Pope wrote an article that has very clear guidelines about how to write excellent commit messages. Please read it.

Examples

Imagine that you're submitting a new problem called "spinning-wheel" to the Ruby track.

Here's a fairly typical set of commits that you might end up making:

433a0e1 added spinning wheel tests
1f7d4aa pass spinning wheel
cf21737 oops
be4c410 rename example file
bb89a77 update config

All of these commits are about a single thing: adding a new problem. They should be a single commit. They don't have to start out that way (life is messy), but once you're done, you should squash everything into one commit, and rename it cohesively:

f4314e5 add spinning wheel problem

Resetting master

If you've already made changes on your master so that it has diverged from the upstream you can reset it.

First create a backup of your branch so that you can find any changes. Just in case.

git checkout master
git checkout -b backup
git checkout master

Next, fetch the most recent changes from the upstream repository and reset master to it.

git fetch upstream
git reset --hard upstream/master

If you do a git log at this point you'll see that you have exactly the commits that are in the upstream repository. Your commits aren't gone, they're just not in master anymore.

To put this on your GitHub fork, you'll probably need to use the --force flag:

git push --force origin master

Squashing

Squashing commits into a single commit is particularly useful when the change happened in lots of little (sometimes confusing) steps, but it really is one change.

There are a number of ways to accomplish this, and many people like to use an interactive rebase, but it can be tricky if you haven't set git up to open your favorite editor.

An easier way to do this is to un-commit everything, putting it back into the staging area, and then committing it again.

Using the example from above, we have 5 commits that should be squashed into one.

433a0e1 added spinning wheel tests
1f7d4aa pass spinning wheel
cf21737 oops
be4c410 rename example file
bb89a77 update config

To un-commit them, use the following incantation:

$ git reset --soft HEAD^^^^^

Notice that there are 5 carets, one for each commit that you want to un-commit. You could also say:

$ git reset --soft HEAD~5

If you do a git status now, you'll see all the changed files, and they're staged and ready to commit. If you do a git log, you'll see that the most recent commit is from someone else.

Next, commit, as usual:

$ git commit -m "Add spinning wheel problem"

Now if you do a git status you may get a warning saying that your origin and your branch have diverged. This is completely normal, since the origin has 5 commits and you have 1 (different) one.

The next step is to force push this up to your branch, which will automatically update the pull request, replacing the old commits with the new one.

$ git push --force origin spinning-wheel

Resources

If you're completely new to git, there are a number of resources that can help get you feeling more comfortable with it.

If you've been using git for a while, but it feels like repeating magical incantations (while praying that nothing goes wrong), then you may find these helpful:

Rebasing

You'll often be asked to rebase your branch before we merge a pull request as Exercism likes to keep a linear project commit history. This is accomplished with git rebase. It takes the current branch and places all the commits at the front of the branch that you're rebasing with.

For example, rebasing the current branch on top of upstream/master:

 git rebase upstream/master

Project commit history:

                       -- current branch --
                      /
--- master branch ----
Interactive Rebase

The rebase command has an option called -i, --interactive which will open an editor with a list of the commits which are about to be changed. This list accepts commands, allowing the user to edit the list before initiating the rebase action.

Using the example from above, we have 5 commits that should be squashed into one.

433a0e1 added spinning wheel tests
1f7d4aa pass spinning wheel
cf21737 oops
be4c410 rename example file
bb89a77 update config

To interactively rebase, use the following incantation:

$ git rebase -i HEAD~5

This will bring up an editor with the following information:

pick 433a0e1 added spinning wheel tests
pick 1f7d4aa pass spinning wheel
pick cf21737 oops
pick be4c410 rename example file
pick bb89a77 update config

#
# Commands:
#  p, pick = use commit
#  r, reword = use commit, but edit the commit message
#  e, edit = use commit, but stop for amending
#  s, squash = use commit, but meld into previous commit
#  f, fixup = like "squash", but discard this commit's log message
#  x, exec = run command (the rest of the line) using shell
#
# These lines can be re-ordered; they are executed from top to bottom.
#
# If you remove a line here THAT COMMIT WILL BE LOST.
#
# However, if you remove everything, the rebase will be aborted.
#
# Note that empty commits are commented out

By choosing the reword command for the top commit and choosing the fixup command for the remaining commits, you will be able to squash the commits into one commit and provide a descriptive summary of the entire change

reword 433a0e1 added spinning wheel tests
fixup 1f7d4aa pass spinning wheel
fixup cf21737 oops
fixup be4c410 rename example file
fixup bb89a77 update config

Further Reading

et cetera

TODO: add more sections:

  • how to merge something locally (for example when there are conflicts, or if you want to fix a small thing without nagging the contributor about it)