.. Copyright © 2014 Greg Ward <greg@gerg.ca>

==================================
Changeset Evolution with Mercurial
==================================

`evolve`_ is a Mercurial extension for faster and safer mutable history.

.. _`evolve`: https://www.mercurial-scm.org/wiki/EvolveExtension

Installation and setup
----------------------

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

.. _`evolve-testers`: https://www.mercurial-scm.org/mailman/listinfo/evolve-testers

Using pip::

    pip install --user hg-evolve

Then add in your `hgrc` config::

   [extensions]
   evolve=

You can easily edit the `hgrc` of repository using `hg config --local` or your
user configuration `hg config --edit`.

.. toctree::
   :maxdepth: 2

   user-guide
   sharing
   concepts
   from-mq


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.
But 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 made hidden.

``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 an
    invisible 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 existing
    features of core Mercurial, ``evolve`` 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.

``evolve`` is experimental!
---------------------------

The long-term plan for ``evolve`` is to add it to core Mercurial.
However, it is not yet stable enough for that. In particular:

  * The UI is unstable: ``evolve``'s command names and command options
    are not completely nailed down yet. They are subject to occasional
    backwards-incompatible changes. If you write scripts that use
    evolve commands, a future release could break your scripts.

  * There are still some corner cases that aren't handled yet. If you
    think you have found such a case, please check if it's already
    described in the Mercurial bug tracker (https://bz.mercurial-scm.org/).
    Bugs in ``evolve`` are files under component "evolution": use
    `this query`_ to view open bugs in ``evolve``.

.. _`this query`: https://bz.mercurial-scm.org/buglist.cgi?component=evolution&bug_status=UNCONFIRMED&bug_status=CONFIRMED&bug_status=NEED_EXAMPLE

Next steps:
-----------

  * 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).

.. _`user guide`: user-guide.html
.. _`sharing mutable history`: sharing.html
.. _`concepts`: concepts.html
.. _`MQ migration guide`: from-mq.html