Darcs understanding (Re: questions from a new bzr user)

[Jan Hudec]

On Thu, May 04, 2006 at 21:23:07 -0500, John Arbash Meinel wrote:
> One thing I would like to mention about Darcs, which was one of the
> show-stoppers for me. Was that it doesn't maintain an accurate history.
> Because of the "theory of patches", when you merge someone else, it can
> actually change the history of your branch.
> 
> As an example:
>  I committed patch "A, B, C" in my branch, and then I merge you, where
> you committed patch "D". In bzr, this would show up as:
> 
>   A
>   | \
>   B  D
>   |  |
>   C  |
>   | /
>   E
> 
> My understanding of Darcs, is that it might decide to commute patch C &
> D, so that you end up with
> 
>   A
>   |
>   B
>   |
>   D
>   |
>   C

Actually no. What it does is generating a minimal partial ordering of
the patches (under which B does not necessarily depend on A, C does not
necessarily depend on either A or B and D does not necessarily depend on
A). Patches don't depend on each other iff applying them in different
order yields the same result.

After the merge, Darcs does NOT produce new revision. The revision of
your tree is simply {A, B, C, D}. Any set of patches can always be
merged, with the result possibly containing conflict markers.

> This also has deeper implications when it comes to revision integrity.
> Because C might be changed by applying D, one cannot create a
> cryptographic signature for C. Inherent in the model is that revisions
> change based on what other revisions you have applied.
> I believe the benefit of this, is that you have more conflict free
> merges. The downside is a very large lack of stability.

You can't create cryptographic signature for C. That's not what you want
to create cryptographic signature for, since C is a patch, but NOT a
revision. A revision is {A, B, C} and {A, B, C, D} and you COULD create
cryptographic signature for that (except I don't think darcs makes any
provisions for it).

> I do believe darcs has a lot of very nice things going for it. And I
> think we should keep and eye on it, and see what things we can
> incorporate. But I thought you should be aware of something I considered
> important, that I didn't see mentioned.

I didn't actually try darcs, but I think I understand the 'patch
algebra' thing quite well. For one thing it does not actually give you
full cherry-picking. Only independent patches can be picked. Though
there would be a useful way to record proper reverse patch -- but I
think that can be done with revision model bzr has as well.

-- 
						 Jan 'Bulb' Hudec <bulb at ucw.cz>
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