Re: [RFC] Applying a graft to a tree and "rippling" the changes through

From: Linus Torvalds <torvalds@osdl.org>
Date: Nov 18, 2005, 17:25 UTC
Message-ID: <Pine.LNX.4.64.0511180915560.13959@g5.osdl.org>
In-Reply-To: <437DDDB1.60103@b-i-t.de>

On Fri, 18 Nov 2005, sf wrote:

> 
> I would go even further: The resulting tree does not depend on anything. A
> tree is a tree is a tree.

Agreed.

> A commit is really just the statement: "I changed the tree from state A to
> state B". After all, the commit message does not describe the new state
> (neither the old state) but does describe the changes.

No.

A commit is _not_ just the statement of change between two trees.

It's a statement of _history_ of the resulting tree. This is important. It's why a commit does not just point to the previous tree, but to the previous commit(s).

And that's really important. If you don't have a history of what has happened, then all the trees are just random collections of files. They're not a project any more.

So a commit doesn't just say "tree A changed into tree B". It very much says something much much stronger, which includes how you got to A in the first place.

The history is also what allow a commit to be meaningful. Without that history, you could never say "I fixed the bug that was introduced by xyz", which is often a very integral part of _why_ the change happened.

There are lots of changelog entries that don't necessarily make sense without knowing what went before them, so history is actually important even in a local sense - it's often what allows the explanation to make sense.

			Linus