Re: master^ is not a local branch -- huh?!?

From: Ron Garret <ron1@flownet.com>
Date: Feb 1, 2010, 23:56 UTC
Message-ID: <ron1-ABA66E.15563101022010@news.gmane.org>
In-Reply-To: <7vwrywplxz.fsf@alter.siamese.dyndns.org>

In article <7vwrywplxz.fsf@alter.siamese.dyndns.org>,
 Junio C Hamano <gitster@pobox.com> wrote:

Show 14 quoted lines

> Ron Garret <ron1@flownet.com> writes:
> 
> >> For my rather fresh eye it looks more like unnamed (anonymous?) branch
> >> than a temporary one. Doesn't detached HEAD behave exactly like a
> >> regular HEAD but pointing to the tip of an unnamed branch?
> >
> > I strongly concur with this.
> >
> > And as long as I'm weighing in, it would also help to prevent confusion 
> > if it were made clear that this unnamed branch doesn't actually come 
> > into existence unless and until you do a commit.
> 
> This shows that you are still thinking a branch is a line (or multiple
> lines).  It is not.

The git user's guide says it is:

"When we need to be precise, we will use the word "branch" to mean a line of development..."

But I understand that a branch is not necessarily a line. In general it's a DAG. I get that.

The manual goes on to say:

"...and "branch head" (or just "head") to mean a reference to the most recent commit on a branch."

There are two ways this can be interpreted:

1.  A commit pointed to by a branch head cannot have any descendants.  
Because if it did it would not be the most recent commit on that branch, 
and by definition a branch head must point at the most recent commit.

2.  Any commit can be considered a branch head for a branch consisting 
of (for example) the transitive closure of its ancestors.

(Note that if I were being really strict with my terminology I would have to say: any commit can be considered the most recent commit for a branch consisting of... Because a branch head by definition is a *reference* to a commit, so strictly speaking a commit cannot be a branch head. The problem is that there are three concepts in play, but only two terms by which to refer to them.)

I personally think interpretation #1 makes more sense, and is a better match to most people's intuitions about what it means to be a branch head. But in neither case does simply moving HEAD "create" a branch. Even under interpretation 2, all those anonymous branches were there all along. You don't "create" anything, temporary or otherwise, simply by moving HEAD.

rg