# evolve extension for Mercurial

## Changeset Evolution with Mercurial

evolve is a Mercurial extension for faster and safer mutable history. It implements the changeset evolution concept for Mercurial.

- It offers a safe and simple way to refine changesets locally and propagate those changes to other repositories.

- It can automatically detect and handle the complex issues that can arise from exchanging draft changesets.

- It even makes it possible for multiple developers to safely rewrite the same parts of history in a distributed way.

- It fully respects the Phases concept so users will only be able to rewrite parts of the history that are safe to change. Phases have been part of Mercurial since early 2012.

## Installation and setup

We recommend you subscribe to the evolve-testers mailing list to stay up to date with the latest news and announcement.

Using pip:

    pip install --user hg-evolve

Then add in your hgrc config:

    [extensions]
    evolve=

You can easily edit the hgrc of a repository using hg config –local. Alternatively, you can edit your user configuration with hg config –edit.

## Table of Contents

- Evolve: User Guide
  - Life without `evolve`
  - Life with `evolve` (basic usage)
  - Changeset obsolescence under the hood
  - Life with `evolve` (advanced usage)
- Evolve: Shared Mutable History
  - Sharing with a single developer
  - Sharing with multiple developers: code review
  - Getting into trouble with shared mutable history
  - Conclusion
- Evolve: Concepts
  - Phases
  - Obsolete changesets
  - Unstable changesets (orphan, phase-divergent, content-divergent)
  - Hidden (and visible) changesets
  - Why is this changeset visible?
- From MQ To Evolve, The Refugee Book
  - Cheat sheet
  - Replacement details
- Evolve Commands Reference
  - amend
  - evolve
  - fold
  - metaedit
  - next
  - obslog
  - pdiff
  - previous
  - prune
  - pstatus
  - split
  - touch
  - uncommit
- Known limitations of the current documentation
  - Features not covered by tutorials
  - Unreferenced Documents

## What is Changeset Evolution?

With core Mercurial, changesets are permanent and immutable. You can commit new changesets to modify your source code, but you cannot modify or remove old changesets.

For years, Mercurial has included various commands that allow history modification: `rebase`, `histedit`, `commit --amend` and so forth. However, there’s a catch: until now, Mercurial’s various mechanisms for modifying history have been unsafe, in that changesets were destroyed (“stripped”) rather than simply hidden and still easy to recover.

`evolve` makes things better by changing the behaviour of most existing history modification commands so they use a safer mechanism (changeset obsolescence, covered below) rather than the older, less safe strip operation.

`evolve` is built on infrastructure in core Mercurial:

> - Phases (starting in Mercurial 2.1) allow you to distinguish mutable and immutable changesets.

> - Changeset obsolescence (starting in Mercurial 2.3) is how Mercurial knows how history has been modified, specifically when one changeset replaces another. In the obsolescence model, a changeset is neither removed nor modified, but is instead marked obsolete and typically replaced by a successor. Obsolete changesets usually become hidden as well. Obsolescence is a disabled feature in Mercurial until you start using `evolve`.

Some of the things you can do with `evolve` are:

> - Fix a mistake immediately: “Oops! I just committed a changeset with a syntax error—I’ll fix that and amend the changeset so no one sees my mistake.” (While this is possible using default features of core Mercurial, changeset evolution makes it safer.)

> - Fix a mistake a little bit later: “Oops! I broke the tests three commits back, but only noticed it now—I’ll just update back to the bad changeset, fix my mistake, amend the changeset, and evolve history to update the affected changesets.”

> - Remove unwanted changes: “I hacked in some debug output two commits back; everything is working now, so I’ll just prune that unwanted changeset and evolve history before pushing.”

> - Share mutable history with yourself: say you do most of your programming work locally, but need to test on a big remote server somewhere before you know everything is good. You can use `evolve` to share mutable history between two computers, pushing finely polished changesets to a public repository only after testing on the test server.

> - Share mutable history for code review: you don’t want to publish unreviewed changesets, but you can’t block every commit waiting for code review. The solution is to share mutable history with your reviewer, amending each changeset until it passes review.

> - Explore and audit the rewrite history of a changeset. Since Mercurial is tracking the edits you make to a changeset, you can look at the history of these edits. This is similar to Mercurial tracking the history of file edits, but at the changeset level.

## Why the evolve extension?

Mercurial core already has some support for changeset evolution so why have a dedicated extension?

The long-term plan for `evolve` is to add it to core Mercurial. However, having the extension helps us experiment with various user experience approaches and technical prototypes. Having a dedicated extension helps current users deploy the latest changes quickly and provides developers with low latency feedback.

Whenever we are happy with a experimental direction in the extension, the relevant code can go upstream into Core Mercurial.

## Development status

While well underway, the full implementation of the changeset evolution concept is still a work in progress. Core Mercurial already supports many of the associated features, but for now they are still disabled by default. The current implementation has been usable for multiple years already, and some parts of it are used in production by multiple projects and companies (including the Mercurial project itself, Facebook, Google, etc…).

However, there are still some areas were the current implementation has gaps. This means some use cases or performance issues are not handled as well as they currently are without evolution. Mercurial has been around for a long time and is strongly committed to backward compatibility. Therefore turning evolution on by default today could regress the experience of some of our current users. The features will only be enabled by default at the point where users who do not use or care about the new features added by evolution won’t be negatively impacted by the new default.

You can find the evolution roadmap in the wiki.

## Resources

> - For a practical guide to using `evolve` in a single repository, see the user guide.

> - For more advanced tricks, see sharing mutable history.

> - To learn about the concepts underlying `evolve`, see concepts (incomplete).

> - If you’re coming from MQ, see the MQ migration guide (incomplete).