[darcs-users] The theory of patches

[Antonio Regidor García]

 --- "Stephen J. Turnbull" <stephen at xemacs.org> escribió: 
> >>>>> "Antonio" == Antonio Regidor García <a_regidor at yahoo.es> writes:
> 
>     Antonio> A general hunk patch can't replace a token replace patch
>     Antonio> (and vice versa), but a concrete hunk patch can replace a
>     Antonio> concrete token replace patch (and vice versa).
> 
> I'm still confused by this.  Is there a difference between a "concrete
> hunk patch" and a "concrete token replace patch"?  A tree pair is a
> tree pair.

Yes, they are the same. This sentence was not much clear, sorry. A general hunk patch is different
from a general token replace patch (they are different functions that transform trees). On the
other hand, let's suppose that you have a concrete patch P Q and want to interchange P and Q.
Let's say P = (R,S) and Q = (S,T). In David's notation, you obtain Q' P', where Q' "does the same
change" than Q, and P' the same than P. In my description, P and Q commute if there are two
general patches p and q so that P = (R,p(R)), Q=(S,q(S)), and P Q = (R,q(R)) (q(R),p(q(R))). You
have to _choose_ p and q, because many different general patches can generate P and Q (and from
your example, general patches that make P and Q commute). So P can be for example generated by a
general token replace patch or a general hunk patch. So, in this sense, you can say that P is a
concrete hunk patch or a concrete token replace patch. But a general token replace patch is never
the same than a general hunk patch. For commutativity, you can (in some cases) choose a general
hunk patch or a general token replace patch to generate P, and thus interchange P and Q in two
different ways. It's not a very good wording, anyway.

David says in the manual that he needs invertibility for theorem 2. And that he uses this theorem
in the implementation, but don't know if it is necessary. So I will study the implementation
before writing other theory.

Best regards,

Antonio Regidor García


		
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