Am 08.09.20 um 21:47 schrieb James Cook: >>>> In your case, for two adjacent /sequences/ A;B, and equivalences A~A', >>>> B~B', we need to have that A;B commutes iff A';B' commutes. Now, suppose >>>> you have patches a;b;b^;c, where none of the adjacent pairs commute. >>>> You'd have to show that this implies that a;c commutes neither (taking >>>> e.g. A=[a] and B=[b;b^;c]). But you can't, since it is not true. A >>>> counter example consists of 3 hunks a, b, c, where a and b overlap, b >>>> and c overlap, but a and c do not overlap. More concretely, take the >>>> initial state as file f=[1,2,3] and >>>> >>>> a=hunk f 1 [1] [1a] >>>> b=hunk f 1 [1,2,3] [1b,2b,3b] >>>> c=hunk f 3 [3] [3c] >>> >>> I'm still not sure I understand the problem. I agree that in your >>> example, it's possible to commute [a] with [c] but not [a] with >>> [b;b^;c]. But it is possible to "rearrange" a;b;b^;c into b;b^;c;a if >>> "rearrange" is defined broadly enough to allow the following three >>> steps: a;b;b^;c -> a;c -> c;a -> b;b^;c;a. >> >> Commutation means that we may have to re-arrange the /content/ of the >> patches we commute, such that they make sense in their new context. You >> abstracted that part away here. If we re-add it, the sequence becomes: >> >> a;b;b^;c -> a;c -> c';a' -> b';b'^;c';a' >> >> But what is the new b'? It should be clear that, in general, it cannot >> be the same as b: if b depends on a (which is what we assumed), then >> that means that b makes no sense without having a before it. >> >> Take the above three hunk patches and tell me how b' should be defined >> such that the resulting sequence b';b'^;c';a' makes sense. > > Oops, I didn't pay enough attention to your concrete example. > > Did you mean b = hunk f 1 [1a,2,3] [1b,2b,3b]? (I'm guessing hunk f n > [u,v,w] [x,y,z] means lines n..n+2 of file f start out as [u,v,w] and > end up as [x,y,z]; is that right?)
Yes to both, sorry I messed up the example. For reference, here is the fixed example in full: a=hunk f 1 [1] [1a] b=hunk f 1 [1a,2,3] [1b,2b,3b] c=hunk f 3 [3] [3c] > Then yes, I guess the order b;b^;c;a is not possible. > > I really want this to work, though, May I ask why? I mean, really, this is what commutation of patches is all about: to determine if we /can/ re-arrange the internals of patches in such a way that they make sense in a different context and in such a way that the patch laws hold. > so now I wonder what happens if we > just accept the situation: even though c can be rearranged to b;b^;c, The point is that you can /not/ do that. Yes, the /effect/ of b;b^;c is the same as that of c, that is, you can apply both to the same tree and get the same result. But with respect to commutation they are not equivalent. > and a;c can be rearranged to c;a Remember that in the general commutation may result in a different pair c';a'. (In my example above a=a' and c=c', but this is a special case.) > it turns out we can't combine the > two to rearrange a;c to b;b^;c;a. This means "rearrangement" is not permutative. Permutivity is essential for proving that merging is order-independent. This is *the* key feature that distinguishes Darcs from e.g. git. > I guess it's kind of ugly that changing the context can change what > rearrangements are possible: So far, patch theory has been designed precisely to avoid that, which is why in Darcs we allow re-arrangement of a sequence only via commutation. > in this case, the rearrangement c -> > b;b^;c may be possible or not depending on whether a patch named "a" > has been applied. This is cause to be suspicious, but is there an > obvious reason it's unworkable? I can't answer this question in any exact or precise way. My gut feeling is that it's going to be an ugly mess, but I haven't tried it so I may be wrong. I should perhaps also point out that there are some operations in Darcs for which the context is important; for instance, conflict markup in general requires access to the complete context. Cheers Ben _______________________________________________ darcs-users mailing list darcs-users@osuosl.org https://lists.osuosl.org/mailman/listinfo/darcs-users