[Haskell-cafe] A small Darcs anomoly

[Jason Dagit]

On Sun, Apr 24, 2011 at 2:05 AM, Andrew Coppin
<andrewcoppin at btinternet.com>wrote:

> I've discovered something interesting.
>
> Darcs stores history as a partially-ordered set of changes. This is a
> beautiful and elegant idea. In theory, this lets me apply any combination of
> changes, possibly generating file "versions" which have never actually
> existed before. (E.g., the new type checker from GHC 7.0 embedded in the GHC
> 6.6 codebase - not that I imagine it would compile, but in principle I can
> do it.)
>
> So I was a little surprised to discover that... Darcs doesn't actually
> support doing this. Darcs is only really interested in the result of
> applying *all* changes in a repo. If you want to apply some subset of
> changes, you need to make a seperate repo containing only the changes you
> want applied.
>
> It seems daft to me that you would design a sophisticated system for
> splitting history into independent chunks, and then not let me manipulate
> them independently.
>

This is because of a deliberate choice that was made by David Roundy.  In
darcs, you never have multiple branches within a single darcs repository
directory tree.  To get the effect you want, you simply create two
repositories.  One having only the patches for ghc 6.6 and one having the
patches of ghc 7.0 and then you pull just the patches you want from 7.0 into
6.6.  There are options to 'darcs get' that help you select the right set of
patches to help you create the two repositories.

If you're interested in the details of how to do it, I would suggest asking
on the darcs-users mailing list.

Jason
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