Re: [darcs-devel] Darcs and git: plan of action

Re: [darcs-devel] Darcs and git: plan of action

[Ray Lee]

On Mon, 2005-04-18 at 08:20 -0400, David Roundy wrote:
> Putting darcs patches *into* git is more complicated, since we'll want to
> get them back again without modification.  Normal "hunk" patches would be
> no problem, provided we never change our diff algorithm (which has been
> discussed recently, in the context of making hunks better align with blocks
> of code).  We could perhaps tell users not to use "replace" patches.  But
> avoiding "mv" patches would be downright silly.

Okay, I still haven't used git yet (and have only toyed around with
darcs for a bit), so take what I'm saying with a grain of salt.
Regardless, I think you may be asking the wrong question. The tracking
of renames was bandied about pretty thoroughly on-list from Wednesday
through Friday (for far better commentary and insight, see Linus'
messages with subject: Merge with git-pasky II.)

git does track changesets that describe the parent tree(s) and the
result. The trees track filenames and hashes. So, doing a fairly
straightforward compare on two trees will let you immediately discover
renames that have occurred, as the filename in the tree changed while
the hash didn't.

So, the question then becomes, can an outside tool cheaply derive all
the information that darcs would need to perform it's work? The renames
should be easy, as long as no content changed during the rename. As for
token replacement (and whitespace changes, etc.), that could be
discovered via domain-specific parsers (something specific per language,
for example). Linus tossed a link to one such tool (hmm, where was it.
Sheesh. You sure right a lot, dude :-).)

	http://minnie.tuhs.org/Programs   (see Ctcompare)

...which should be viewed more as a proof-of-concept than a mergeable
code-set. It does show that diff's vocabulary is sadly lacking in
expressiveness, and improving that, I think, would be a useful area to
expend effort. 

Again, I may be off here, especially considering I've a backlog of a
couple hundred messages to read since the weekend. (You guys need to go
outside more often.)

Ray

Re: [darcs-devel] Darcs and git: plan of action

[linux]

> Hell no.
> 
> The commit _does_ specify the patch uniquely and exactly, so I really 
> don't see the point. You can always get the patch by just doing a
>
> 	git diff $parent_tree $thistree
>
> so putting the patch in the comment is not an option.

Er... no.

One of darcs' big points is that it has at least two fundamentally
different *kinds* of patches.  One is the classic diff(1) style.

The other is "replace very instace of identifier `foo` with identifier`bar`".

Note that merging such a patch with another that adds a new instance
of "foo" has a quite different effect from a similar diff-style patch.
Even though both have identical effects on the tree to which they were
initially merged.

And darcs is specifically intended to support additional kinds of patches.
Again, all in order that the patch can work better when applied to
trees *other* that the one it was originally developed against.


Anyway, the point is that, in the darcs world, it is NOT possible to
reconstruct a patch from the before and after trees.

Re: [darcs-devel] Darcs and git: plan of action

[Ray Lee]

On Mon, 2005-04-18 at 21:04 +0000, linux@horizon.com wrote:
> The other is "replace very instace of identifier `foo` with identifier`bar`".

That could be derived, however, by a particularly smart parser [1].
Alternately, that itself could be embedded in the comment for patches
sourced from darcs. Of course, that means patches from others are less
commutable than from other darcs users, but that's the price you'd pay
for relying on the user to explicitly note a token rename.

  [1] An example: http://minnie.tuhs.org/Programs/Ctcompare/index.html

As for "darcs mv", that can be derived from the before/after pictures of
the trees.

> And darcs is specifically intended to support additional kinds of patches.

Anything missing out of what I listed above? (darcs has adddir and
addfile, IIRC, but those are trivially discovered via inspection of the
trees as well, I think.)

> Anyway, the point is that, in the darcs world, it is NOT possible to
> reconstruct a patch from the before and after trees.

Not yet, and maybe not ever, but I think we can certainly get closer to
discovering what the coder was thinking during a changeset.

Ray

Re: [darcs-devel] Darcs and git: plan of action

[Kevin Smith]

Ray Lee wrote:
> On Mon, 2005-04-18 at 21:04 +0000, linux@horizon.com wrote:
> 
>>The other is "replace very instace of identifier `foo` with identifier`bar`".
> 
> 
> That could be derived, however, by a particularly smart parser [1].

No, it can't. Seriously. A darcs replace patch is encoded as rules, not
effects, and it is impossible to derive the rules just by looking at the
results. Not difficult. Impossible. You could guess, but that's not good
enough for darcs to be able to reliably commute the patches later.

I am curious whether Linus's suggestion about including the
corresponding darcs patch id in the git commit comments would be good
enough.

> As for "darcs mv", that can be derived from the before/after pictures of
> the trees.

Perhaps. If a file is moved and edited within the same commit, I'm not
sure that you can be certain whether it was done with d 'darcs mv' or
not. Requiring separate checkins for the rename and the subsequent
modify would make things easier on SCM's, but is impractical in real
life. Automated refactoring tools, for example, perform the
rename+modify as an atomic operation.

Now, git might not need to deal with any of this, because it only needs
to work with the kernel project. But darcs does have to deal with this
wide range of uses, as does just about any other SCM.

I'm *not* advocating cluttering up git with features that are not
directly needed for kernel development. I'm just trying to clarify the
facts so everyone can understand what darcs is trying to do.

Kevin

Re: [darcs-devel] Darcs and git: plan of action

[Ray Lee]

On Mon, 2005-04-18 at 21:05 -0400, Kevin Smith wrote:
> >>The other is "replace very instace of identifier `foo` with identifier`bar`".
> > That could be derived, however, by a particularly smart parser [1].
> 
> No, it can't. Seriously. A darcs replace patch is encoded as rules, not
> effects, and it is impossible to derive the rules just by looking at the
> results. Not difficult. Impossible.

Okay, either I'm a sight stupider than I thought, or I'm not
communicating well. Same net effect either way, I 'spose.

If I do a token replace in an editor (say one of those fancy new-fangled
refactoring thangs, or good ol' vi), a token-level comparator can
discover what I did. That link I sent is an example of one such beast.

> You could guess, but that's not good
> enough for darcs to be able to reliably commute the patches later.

Who said anything about guessing? If a user replaces all instances of
foo with bar, that's as close to proof as you can ever get, without
recording intent of the user at the time it's done. Now, I realize that
darcs *does* record intent, but I claim that's immaterial.

Perhaps I'm clueless; it's happened before, I'm resigned to it happening
again. So, tell it to me with full jargon, if you will. When it comes
down to brass tacks, why does my suggestion place weaker guarantees
about the quality of the resulting patch operator?

> > As for "darcs mv", that can be derived from the before/after pictures of
> > the trees.
> 
> Perhaps. If a file is moved and edited within the same commit, I'm not
> sure that you can be certain whether it was done with d 'darcs mv' or
> not.

Agreed. But then you go lart the committer of that patch.

> Requiring separate checkins for the rename and the subsequent
> modify would make things easier on SCM's, but is impractical in real
> life.

Eh? Why? "darcs mv" *is* a commit. Just because it doesn't seem to look
like one doesn't change the fact that you just invoked the SCM.

> Automated refactoring tools, for example, perform the
> rename+modify as an atomic operation.

And that's harder, I agree. But unless I'm missing some nifty
refactoring editor out there that integrates with darcs during the edit
session, the user *still* has to tell the SCM about the rename manually.

> Now, git might not need to deal with any of this, because it only needs
> to work with the kernel project.

It'd be unfortunate if git were limited to such a small developer base.

> I'm *not* advocating cluttering up git with features that are not
> directly needed for kernel development.

I'm not claiming you are. We want the same thing -- a nuanced SCM that
can take some of the drudge-work away from this stuff.

Ray

Re: [darcs-devel] Darcs and git: plan of action

[Ray Lee]

On Tue, 2005-04-19 at 02:55 +0200, Juliusz Chroboczek wrote:
> > But avoiding "mv" patches would be downright silly.
> 
> Aye, that will require some metadata on the git side (the hack,
> suggested by Linus, of using git hashes to notice moves won't work).

Okay, I'm coming to believe I missed something. So, why won't it work?

Ray

Re: [darcs-devel] Darcs and git: plan of action

[Kevin Smith]

Ray Lee wrote:
> On Mon, 2005-04-18 at 21:05 -0400, Kevin Smith wrote:
> 
>>>>The other is "replace very instace of identifier `foo` with identifier`bar`".
>>>
>>>That could be derived, however, by a particularly smart parser [1].
>>
>>No, it can't. Seriously. A darcs replace patch is encoded as rules, not
>>effects, and it is impossible to derive the rules just by looking at the
>>results. Not difficult. Impossible.
> 
> 
> Okay, either I'm a sight stupider than I thought, or I'm not
> communicating well. Same net effect either way, I 'spose.
> 
> If I do a token replace in an editor (say one of those fancy new-fangled
> refactoring thangs, or good ol' vi), a token-level comparator can
> discover what I did. That link I sent is an example of one such beast.

The big feature of a darcs replace patch is that it works forward and
backward in time. Let me try to come up with an example that can help
explain it. Hopefully I'll get it right. Let's start with a file like
this that exists in a project for which both you and I have darcs repos:

cat
dog
fish

Now, you change it to:

cat dog
dog
fish

while I simultaneously do a replace of "dog" with "plant", resulting in:

cat
plant
fish

We merge. The final result in both of our trees is:

cat plant
plant
fish

Notice that just by looking at my diffs, you can't tell that I used a
replace operation. I didn't just replace the instances of "dog" that
were in my file at that moment. I conceptually replaced all instances,
including ones that aren't there yet.

Now, I should mention here that I personally dislike the replace
operation, and I think it is more dangerous than helpful. However, other
darcs users are quite happy with it, and it certainly is a creative and
powerful feature.

Other creative patch types have also been dreamed of. For example, a
powerful language-specific refactoring operation has been discussed as a
far-future possibility. That would be safe, and cool.

>>Automated refactoring tools, for example, perform the
>>rename+modify as an atomic operation.
> 
> And that's harder, I agree. But unless I'm missing some nifty
> refactoring editor out there that integrates with darcs during the edit
> session, the user *still* has to tell the SCM about the rename manually.

Although there are no such nifty refactoring tools available today, they
will exist at some point. If they existed today, the world would be a
better place.

Even without tools, many shops have policies against checking in code
that won't compile. If you rename a java class, you must simultaneously
perform the rename and modify the class name inside. If you commit
between those steps, it's broken. [I do realize that the kernel doesn't
have java code, by the way.]

I should also mention that I currently believe that Linus is correct
that explicit rename tracking is not required for git. I have every hope
that his plan for handling the more general case of "moved text" will
take care of renames as a side effect. I don't know if that will be
sufficient to allow a two-way lossless gateway between git and darcs or
other systems that do track renames explicitly.

Kevin

Re: [darcs-devel] Darcs and git: plan of action

[Juliusz Chroboczek]

> > Aye, that will require some metadata on the git side (the hack,
> > suggested by Linus, of using git hashes to notice moves won't work).

> So, why won't it work?

Because two files can legitimately have identical contents without
being ``the same'' file from the VC system's point of view.

In other words, two files may happen to have the same contents but
have distinct histories.

                                        Juliusz

Re: [darcs-devel] Darcs and git: plan of action

[David Roundy]

On Mon, Apr 18, 2005 at 08:38:25AM -0700, Linus Torvalds wrote:
> On Mon, 18 Apr 2005, David Roundy wrote:
> > .... In particular, it would make life (that is, life interacting back
> > and forth with git) easier if we were to embed darcs patches in their
> > entirety in the git comment block.
> 
> Hell no.

I was afraid that would be the response...

> The commit _does_ specify the patch uniquely and exactly, so I really 
> don't see the point. You can always get the patch by just doing a
> 
> 	git diff $parent_tree $thistree
> 
> so putting the patch in the comment is not an option.

The issue is that in darcs the parent and child trees *don't* uniquely or
exactly specify the patch.  In fact, even the output of git diff will
depend on what version of diff you're using (e.g. if someone were to use
BSD diff rather than GNU diff).

> > As I say, it's a bit ugly, and before we explore the idea further, it would
> > be nice to know if this would cause Linus to vomit in disgust and/or refuse
> > patches from darcs users.
> 
> That's definitely the case. I will _not_ be taking random files etc just 
> to keep other peoples stuff straightened up.

Okay.

> > Another slightly less noxious possibility would be to store the darcs
> > patch as a "hidden" file, if git were given the concept of
> > commit-specific files.
> 
> No, git will not track commit-specific files. There's the comment
> section, and that _is_ the commit-specific file. But I will refuse to
> take any comments that aren't just human-readable explanations, together
> with maybe one extra line of
> 
> 	# Darcs ID: 780c057447d4feef015a905aaf6c87db894ff58c
> 
> (others will want to track _their_ PR numbers etc) and that's it. The 
> actual darcs data that that ID refers to can obviously be maintained in 
> _another_ git archive, but it's not one I'm going to carry about.

The trouble is that the philosophy of darcs and git are about as orthogonal
as one can come.  Git treats the content as fundamental, where in darcs the
changes are fundamental.  Since in darcs there can be different changes
that lead from the same parent to the same child--and these differences are
meaningful when merges happen---when interacting with git, we either need
to restrict darcs to only describe changes in a way that can be uniquely
determined by a parent and child, or we need to have extra metadata
somewhere.

For bidirectional functionality, we either need to avoid the use of
advanced darcs features, or we need to include that information in git
somehow, or we need to keep a parallel darcs archive holding that
information.

Would a small amount of human-readable change information be acceptable in
the free-form comment area? In the rename thread I got the impression this
would be okay for renames.  For example,

rename foo bar

or (this is less important, but you might consider it to be a useful
human-readable comment)

replace [_a-zA-Z0-9] old_variable new_variable file/path

Currently these two patch types account for almost the sum total of the
cases where different patches lead to the same resulting trees.
-- 
David Roundy

Re: [darcs-devel] Darcs and git: plan of action

[David Roundy]

On Tue, Apr 19, 2005 at 02:55:05AM +0200, Juliusz Chroboczek wrote:
> [Using git as a backend for Darcs.]
...
> >>  1. remove the assumption that patch IDs have a fixed format.  Patch
> >>  IDs should be opaque blobs of binary data that Darcs only compares
> >>  for equality.
> 
> > I'm not really comfortable with this,
> 
> Why?

I'm not clear why it would be necesary, and it takes the only immutable
piece of information regarding a patch, and makes it variable.  Just seems
dangerous and complicated, and I'm not sure why we'd need to do it.

> Suppose I record a patch in Darcs; it gets a Darcs id.  I push it into
> git, at which point it gets a git id, whether we want it to or not.
> What do we do when we pull that patch back into darcs?
> 
> Either we arbitrarily discard one of the ids (which one?), or we keep
> both.  If there's more pulling/pushing going on on the git side, we
> definitely need to keep both.

Or alternatively, we could have a one-to-one mapping between git IDs and
darcs IDs, which is what I'd do.

> > I think when dealing with git (and probably also with *any* other SCM
> > (arch being a possible exception), we need to consider the exchange
> > medium to be not a patch, but a tag.
> 
> We're thinking in opposite directions -- you're thinking of the alien
> versions as integrals of Darcs patches, I'm thinking of Darcs patches
> as derivatives of alien versions.
> 
>   You:  alien version = Darcs tag
> 
>   Me:   Darcs patch = pair of successive alien versions
> 
> My gut instinct is that the second model can be made to work almost
> seamlessly, unlike the first one.  But that's just a guess.

The problem is that there is no sequence of alien versions that one can
differentiate.  Git has a branched history, with each version that follows
a merge having multiple parents.  How do you define that change?  It's easy
enough to do if we tag each git version in darcs, since we know what the
two parents are, and we know what the final state is, but there *is* no
translation from a single git ID either to a single patch(1) patch, or to a
single darcs patch--unless you treat its parents as tags.

The key is that we can't make git work like darcs, so we'll have to make
darcs work like git.  If we do it right (automatically tagging like crazy
people), darcs users between themselves can cherry-pick all they like,
without introducing inconsistencies or losing interoperability with git.

To summarize how I'd see the mapping between git information and darcs, a
git commit would be composed of one darcs patch and one darcs tag.  With
this mapping, I don't believe we lose any information, and I believe we'll
be able to (except that patches would have to be uniquely determined by a
pair of trees) simply translate the darcs system right back again, since
it's a one-to-one correspondence of information.

My proposed mapping:

tree 6ff0e9f3d131bd110d32829f0b14f07da8313c45
# This is a darcs tag ID
parent abd62b9caee377595a9bf75f363328c82a38f86e
# This is the context of both a patch and tag.
author James Bottomley <James.Bottomley@SteelEye.com> 1113879319 -0700
# This is the author and date of the patch
committer Linus Torvalds <torvalds@ppc970.osdl.org.(none)> 1113879319 -0700
# This is the author and date of the tag
# Everything below would be the name and long comment of the patch

[PATCH] SCSI trees, merges and git status

Doing the latest SCSI merge exposed two bugs in your merge script:

1) It doesn't like a completely new directory (the misc tree contains a
   new drivers/scsi/lpfc)
2) the merge testing logic is wrong.  You only want to exit 1 if the
   merge fails. 


-- 
David Roundy
http://www.darcs.net

Re: [darcs-devel] Darcs and git: plan of action

[David Roundy]

On Mon, Apr 18, 2005 at 06:42:11PM -0700, Ray Lee wrote:
> On Mon, 2005-04-18 at 21:05 -0400, Kevin Smith wrote:
> > You could guess, but that's not good enough for darcs to be able to
> > reliably commute the patches later.
>
> Who said anything about guessing? If a user replaces all instances of
> foo with bar, that's as close to proof as you can ever get, without
> recording intent of the user at the time it's done. Now, I realize that
> darcs *does* record intent, but I claim that's immaterial.

The problem is, how do you know how to define a token? That's also included
in a darcs patch.  And a darcs user may choose not to use a replace patch,
if (for example) he's renaming a local variable, since he might not want to
mess with other functions in the same file.

Guessing the author's intent cannot reliably reproduce the author's stated
intent.  Either we need to include that information in one form or another
(and in one location or another), or we've got to simply disallow replaces
(and moves?) when interacting with git.
-- 
David Roundy
http://www.darcs.net

Re: [darcs-devel] Darcs and git: plan of action

[Petr Baudis]

Dear diary, on Tue, Apr 19, 2005 at 02:20:55PM CEST, I got a letter
where Juliusz Chroboczek <Juliusz.Chroboczek@pps.jussieu.fr> told me that...
> > The problem is that there is no sequence of alien versions that one can
> > differentiate.  Git has a branched history, with each version that follows
> > a merge having multiple parents.
> 
> Yep.  I've just realised that this morning.  Is there some notion of
> ``primary parent'' as in Arch?  Can a changeset have 0 parents?

Yes, the root commit. Usually, there is only one, but there may be
multiple of them theoretically.

-- 
				Petr "Pasky" Baudis
Stuff: http://pasky.or.cz/
C++: an octopus made by nailing extra legs onto a dog. -- Steve Taylor

Re: [darcs-devel] Darcs and git: plan of action

[Tupshin Harper]

Linus Torvalds wrote:

>(In other words: if it looks like something a careful human _could_ have
>written, it's certainly ok. But if it looks like something a careful human
>would have used a script to generate 40 entries of, it's bad).
>
>		Linus
>  
>
This is the way that darcs would currently represent a "darcs replace 
foo bar" on 15 files, which is obviously exactly what you are objecting to:
[global foo to bar
tupshin@tupshin.com**20050419155539] {
replace ./dir1/file1 [A-Za-z_0-9] foo bar
replace ./dir1/file2 [A-Za-z_0-9] foo bar
replace ./dir1/file3 [A-Za-z_0-9] foo bar
replace ./dir1/file4 [A-Za-z_0-9] foo bar
replace ./dir1/file5 [A-Za-z_0-9] foo bar
replace ./dir2/file1 [A-Za-z_0-9] foo bar
replace ./dir2/file2 [A-Za-z_0-9] foo bar
replace ./dir2/file3 [A-Za-z_0-9] foo bar
replace ./dir2/file4 [A-Za-z_0-9] foo bar
replace ./dir2/file5 [A-Za-z_0-9] foo bar
replace ./dir3/file1 [A-Za-z_0-9] foo bar
replace ./dir3/file2 [A-Za-z_0-9] foo bar
replace ./dir3/file3 [A-Za-z_0-9] foo bar
replace ./dir3/file4 [A-Za-z_0-9] foo bar
replace ./dir3/file5 [A-Za-z_0-9] foo bar
}

I see two possible complementary ways to address this:
1) allow something akin to the above form in git free-form comments as a 
*technical* solution, while leaving it up to the individual repository 
owner whether to accept such patches on aesthetic grounds.
2) explore adding a different format to darcs that would allow a files 
affected to be represented more compactly.


I suspect that any use of wildcards in a new format would be impossible 
for darcs since it wouldn't allow darcs to construct dependencies, 
though I'll leave it to david to respond to that.

At a minimum, something like:
replace ./dir1/[file1|file2|file3|file4|file5] [A-Za-z_0-9] foo bar
replace ./dir2/[file1|file2|file3|file4|file5] [A-Za-z_0-9] foo bar
replace ./dir3/[file1|file2|file3|file4|file5] [A-Za-z_0-9] foo bar
should be pretty feasible.

I don't believe, however, that it would ever be 100% reliable to try to 
look at a one line replace description and combine it with the actual 
changes and end up with a correct darcs replace patch.

-Tupshin

Re: [darcs-devel] Darcs and git: plan of action

[Linus Torvalds]

On Tue, 19 Apr 2005, Tupshin Harper wrote:
> 
> I suspect that any use of wildcards in a new format would be impossible 
> for darcs since it wouldn't allow darcs to construct dependencies, 
> though I'll leave it to david to respond to that.

Note that git _does_ very efficiently (and I mean _very_) expose the 
changed files.

So if this kind of darcs patch is always the same pattern just repeated
over <n> files, then you really don't need to even list the files at all.  
Git gives you a very efficient file listing by just doing a "diff-tree"  
(which does not diff the _contents_ - it really just gives you a pretty
much zero-cost "which files changed" listing).

So that combination would be 100% reliable _if_ you always split up darcs 
patches to "common elements". 

And note that there does not have to be a 1:1 relationship between a git
commit and a darcs patch. For example, say that you have a darcs patch
that does a combination of "change token x to token y in 100 files" and
"rename file a into b". I don't know if you do those kind of "combination 
patches" at all, but if you do, why not just split them up into two? That 
way the list of files changed _does_ 100% determine the list of files for 
the token exchange.

		Linus

Re: [darcs-devel] Darcs and git: plan of action

[Patrick McFarland]

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On Monday 18 April 2005 10:05 pm, Kevin Smith wrote:
> The big feature of a darcs replace patch is that it works forward and
> backward in time. Let me try to come up with an example that can help
> explain it. Hopefully I'll get it right. Let's start with a file like
> this that exists in a project for which both you and I have darcs repos:
>
> cat
> dog
> fish
>
> Now, you change it to:
>
> cat dog
> dog
> fish
>
> while I simultaneously do a replace of "dog" with "plant", resulting in:
>
> cat
> plant
> fish
>
> We merge. The final result in both of our trees is:
>
> cat plant
> plant
> fish
>
> Notice that just by looking at my diffs, you can't tell that I used a
> replace operation. I didn't just replace the instances of "dog" that
> were in my file at that moment. I conceptually replaced all instances,
> including ones that aren't there yet.

I think that's the best explanation of how it works. And that is partially why 
darcs is so powerful.

-- 
Patrick "Diablo-D3" McFarland || pmcfarland@downeast.net
"Computer games don't affect kids; I mean if Pac-Man affected us as kids, we'd 
all be running around in darkened rooms, munching magic pills and listening to
repetitive electronic music." -- Kristian Wilson, Nintendo, Inc, 1989

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Re: Darcs and git: plan of action

[Ray Lee]

(Sorry for the delayed reply -- I'm living on tape delay for a bit.)

On Mon, 2005-04-18 at 22:05 -0400, Kevin Smith wrote:
> >>>>The other is "replace very instace of identifier `foo` with identifier`bar`".
> >>>
> >>>That could be derived, however, by a particularly smart parser [1].
> >>
> >>No, it can't. Seriously. A darcs replace patch is encoded as rules, not
> >>effects, and it is impossible to derive the rules just by looking at the
> >>results. Not difficult. Impossible.
> >  
> > If I do a token replace in an editor (say one of those fancy new-fangled
> > refactoring thangs, or good ol' vi), a token-level comparator can
> > discover what I did. That link I sent is an example of one such beast.
> 
> The big feature of a darcs replace patch is that it works forward and
> backward in time.

That's *not* a feature of the token replace patch, however. That's a
feature of the darcs commutation machinery, correct? (With the obvious
caveat that darcs can only *do* the commutation if it has correctly
nuanced darcs-style token replace patches, rather than mere ASCII
textual diffs.)

> Let me try to come up with an example that can help
> explain it. Hopefully I'll get it right. Let's start with a file like
> this that exists in a project for which both you and I have darcs repos:
> 
> cat
> dog
> fish
> 
> Now, you change it to:
> 
> cat dog
> dog
> fish
> 
> while I simultaneously do a replace of "dog" with "plant", resulting in:
> 
> cat
> plant
> fish
> 
> We merge. The final result in both of our trees is:
> 
> cat plant
> plant
> fish

Okay, that all makes sense.

> Notice that just by looking at my diffs, you can't tell that I used a
> replace operation.

Here's where we disagree. If you checkpoint your tree before the
replace, and immediately after, the only differences in the
source-controlled files would be due to the replace. And since the
language of the file is known (and thereby the tokenization -- it *is*
well-defined), then a tokenizer that compares the before and after trees
(for just the files that changed, obviously), can discover what you did,
and promote the mere ASCII diff into a token-replace diff. (The same
sort of idea could be done for reindention, I'd hope.)

> I didn't just replace the instances of "dog" that
> were in my file at that moment. I conceptually replaced all instances,
> including ones that aren't there yet.

Well yes, that's exactly what we want. And the key point of all of this
is that there's no magic here. The darcs machinery does all the
commutations such that the patches can wiggle together without
conflicts. To do it's job, of course, it needs nuanced patches, rather
than the quite literal ones generated by diff.

We agree on everything except that it's provable that one can discover a
replace operation, given a before and after tree.

> Now, I should mention here that I personally dislike the replace
> operation, and I think it is more dangerous than helpful. However, other
> darcs users are quite happy with it, and it certainly is a creative and
> powerful feature.

It's creative alright, though I had the same misgivings. In my common
code workflow, I almost never have global tokens -- all my replaces
would be per function, so I never saw an opportunity to use it when I
was screwing around with darcs.

> Other creative patch types have also been dreamed of. For example, a
> powerful language-specific refactoring operation has been discussed as a
> far-future possibility. That would be safe, and cool.

<subliminal> indention patch type, indention patch type... </subliminal>

> > > Automated refactoring tools, for example, perform the
> > > rename+modify as an atomic operation.
> > [...]
> Although there are no such nifty refactoring tools available today, they
> will exist at some point.

Yeah, I spent some time drooling over the refactoring editors before
slapping myself and deciding I'd wait for others to live on that
bleeding edge for a while. I've had to clean up too much code from other
people.

> Even without tools, many shops have policies against checking in code
> that won't compile. If you rename a java class, you must simultaneously
> perform the rename and modify the class name inside. If you commit
> between those steps, it's broken.

I'm trying hard to find a nice way to say that's silly. I'm failing. My
suggestion in that case would be that the local coder commit many
patches to a local repository, one of which is the rename. Then upon
completion of the refactoring, the set of patches is committed to the
group repository. Tags before and after preserve the repository's
precondition that it always compiles.

> [I do realize that the kernel doesn't have java code, by the way.]

Don't worry, I didn't think that you did :-).

Ray

Re: Darcs and git: plan of action

[Tupshin Harper]

Ray Lee wrote:

>Here's where we disagree. If you checkpoint your tree before the
>replace, and immediately after, the only differences in the
>source-controlled files would be due to the replace.
>
This is assuming that you only have one replace and no other operations 
recorded in the patch. If you have multiple replaces or a replace and a 
traditional diff  recorded in the same patch, then this is not true.

> And since the
>language of the file is known (and thereby the tokenization -- it *is*
>well-defined), then a tokenizer that compares the before and after trees
>(for just the files that changed, obviously), can discover what you did,
>and promote the mere ASCII diff into a token-replace diff. (The same
>sort of idea could be done for reindention, I'd hope.)
>  
>
See above for one set of limitations on this. A more fundamental problem 
comes back to intent. If I have a file "foo" before:
a1
a2
and after:
b1
b2
is that a "replace [_a-zA-Z0-9] a b foo" patch, or is that a
-a1
-a2
+b1
+b2
patch? Note that this comes down to heuristics, and no matter what you 
use, you will be wrong sometimes,  *and* the choice that is made can 
substantively affect the contents of the repository after additional 
patches are applied.

>We agree on everything except that it's provable that one can discover a
>replace operation, given a before and after tree.
>  
>
It's provable that you can not.

-Tupshin

Re: [darcs-devel] Darcs and git: plan of action

[Kevin Smith]

Ray Lee wrote:
> On Mon, 2005-04-18 at 22:05 -0400, Kevin Smith wrote:
> 
>>Notice that just by looking at my diffs, you can't tell that I used a
>>replace operation.
> 
> 
> Here's where we disagree. If you checkpoint your tree before the
> replace, and immediately after, the only differences in the
> source-controlled files would be due to the replace. 

But I might have manually changed those tokens, or I might have done it
with a replace operation. Just looking at the diffs, those two cases
would look identical and be indistinguishable. The only way to know
whether or not a darcs replace was done was to look at the patch metadata.

Pop quiz:

Here is revision 1 of my file:

    abcde

Here is revision 2:

    wow

Now, did I do that with a darcs replace, or just by typing?

Kevin

Re: [darcs-devel] Darcs and git: plan of action

[Ray Lee]

On Tue, 2005-04-19 at 19:03 -0400, Kevin Smith wrote:
> Pop quiz:
> Here is revision 1 of my file:
>     abcde
> 
> Here is revision 2:
>     wow

> Now, did I do that with a darcs replace, or just by typing?

I'm still not communicating well.

Give me a case where assuming it's a replace will do the wrong thing,
for C code, where it's a variable or function name.

Ray

Re: Darcs and git: plan of action

[Ray Lee]

On Tue, 2005-04-19 at 16:00 -0700, Tupshin Harper wrote:
> Ray Lee wrote:
> 
> >Here's where we disagree. If you checkpoint your tree before the
> >replace, and immediately after, the only differences in the
> >source-controlled files would be due to the replace.
> >
> This is assuming that you only have one replace and no other operations 
> recorded in the patch. If you have multiple replaces or a replace and a 
> traditional diff  recorded in the same patch, then this is not true.

I had a precondition on my argument (not quoted), that the code was
checkpointed before and after. Obviously, a large set of changes in one
patch is a problem. However, a darcs replace is (effectively) a commit
on its own, so I was limiting myself to the same situation under a
different system.

> A more fundamental problem comes back to intent. If I have a file
> "foo" before:
> a1
> a2
> and after:
> b1
> b2
> is that a "replace [_a-zA-Z0-9] a b foo" patch, or is that a
> -a1
> -a2
> +b1
> +b2
> patch?

Okay, so in reading the online darcs manual (yet) again, I now see that
it allows regular expressions for the match and replace, which means
multiple unique tokens could change atomically. (Does anyone actually
*use* regexes? Sounds like a cannon that'd be hard to aim.)

Regardless, I only care about code, not free text. If it's in a language
that doesn't do some use-'em-as-you-need-'em duck typing spiel
(<cough>python</cough), then the context of your patch (namely, the
file) already has those tokens somewhere in them. And I bet that if
*you* looked at that file, you could tell if it was a replace or a mere
textual diff. Am I wrong?

> Note that this comes down to heuristics, and no matter what you 
> use, you will be wrong sometimes,  *and* the choice that is made can 
> substantively affect the contents of the repository after additional 
> patches are applied.

Unless I'm missing something, the darcs replace patch can already do the
wrong thing. If I do a replace patch on a variable introduced in a local
tree, then do a darcs replace on it before committing it to a shared
repository, and coder B introduces a variable of the same original name
in my copy, then there's a chance that the replace patch will
incorrectly apply upon his newly introduced variable. No?

> It's provable that you can not.

I'm still not seeing the problem, at least when it comes to ANSI C.

Ray

Re: Darcs and git: plan of action

[Tupshin Harper]

Ray Lee wrote:

> I'm still not communicating well.
>
>Give me a case where assuming it's a replace will do the wrong thing,
>for C code, where it's a variable or function name.
>
>Ray
>
>-
>
I think you are communicating fine, but not fully understanding darcs.

try this:
initial patch creates hello.c
#include <stdio.h>

int main(int argc, char *argv[])
{
  printf("Hello world!\n");
  return 0;
}

second patch:
replace ./hello.c [A-Za-z_0-9] world universe

third patch, for conceptual clarity, created in another repository that 
had seen the first patch, but not the second (adds function wide_world):
hunk ./hello.c 3
+void wide_world()
+{
+  printf("Hello wide world\n");
+}
+
hunk ./hello.c 11
+  wide_world();
}

If patch2 was a replace patch, then the result of running the combined 3 
patch version would be:
Hello universe!
Hello wide universe

but if patch2 was a non-replace patch, then the result would be:
Hello universe!
Hello wide world

-Tupshin

Re: Darcs and git: plan of action

[Tupshin Harper]

Ray Lee wrote:

>it allows regular expressions for the match and replace, which means
>multiple unique tokens could change atomically. (Does anyone actually
>*use* regexes? Sounds like a cannon that'd be hard to aim.)
>  
>
Yes, and replace patches need to be used very carefully.

>Regardless, I only care about code, not free text. If it's in a language
>that doesn't do some use-'em-as-you-need-'em duck typing spiel
>(<cough>python</cough), then the context of your patch (namely, the
>file) already has those tokens somewhere in them. And I bet that if
>*you* looked at that file, you could tell if it was a replace or a mere
>textual diff. Am I wrong?
>  
>
Yes. See my hello world example from my last email.

>
>Unless I'm missing something, the darcs replace patch can already do the
>wrong thing. 
>
Yes, depending on how you define wrong. Darcs replace is fully 
predictable, and poorly chosen replaces can lead to incorrect results 
after future patches are applied.

>If I do a replace patch on a variable introduced in a local
>tree, then do a darcs replace on it before committing it to a shared
>repository, and coder B introduces a variable of the same original name
>in my copy, then there's a chance that the replace patch will
>incorrectly apply upon his newly introduced variable. No?
>  
>
Absolutely correct, and the exact reason why replace patches need to be 
used *very* selectively.

>  
>
>>It's provable that you can not.
>>    
>>
>
>I'm still not seeing the problem, at least when it comes to ANSI C.
>
>Ray
>  
>
See hello world example in my other email. You can argue that it is an 
existing problem in darcs, but really, it just points out the fact that 
a computer is *incapable* of knowing whether it is safe to use a replace 
patch based on a diff because replace patches are dangerous if not used 
intelligently.

-Tupshin

Re: [darcs-devel] Darcs and git: plan of action

[Ray Lee]

Thanks for your patience.

On Tue, 2005-04-19 at 16:32 -0700, Tupshin Harper wrote:
> >Give me a case where assuming it's a replace will do the wrong thing,
> >for C code, where it's a variable or function name.

> try this:
> initial patch creates hello.c
> #include <stdio.h>
> 
> int main(int argc, char *argv[])
> {
>   printf("Hello world!\n");
>   return 0;
> }
> 
> second patch:
> replace ./hello.c [A-Za-z_0-9] world universe

Aha! Okay, I now see at least part of issue: we're using different
definitions of 'token.' Yours is quite sensible, in that it matches the
darcs syntax. However, I'm claiming a token is defined by the file's
language, and that a replace patch on anything but a token as per those
language standards is a silly thing.

In your example, I'd claim you did an inter-token edit, as the natural
token there was "Hello world!\n".

With that, let me restate what I think is possible.

One should be able to discover renames (replaces) of user identifiers in
C code programmatically. Is that everything darcs replace does?
Obviously not. Is that what users would usually *want*? If I were using
it, that's what I'd want (especially including the limited scope of
replacement -- user identifiers such as variable or function names,
etc.). But then I'm not a lurker on the darcs user list, so I don't know
how usage of darcs replace plays out in actual practice.

So, it's a subset. Is it a useful subset? Yes, as it addresses what
happens during refactoring, which is when I'd usually see this getting
used. (Syntactically ignorant search and replace is so, y'know,
*1970s*.)

Any clearer?

Ray

Re: [darcs-devel] Darcs and git: plan of action

[Ray Lee]

On Tue, 2005-04-19 at 10:22 +0200, Juliusz Chroboczek wrote:
> > > Aye, that will require some metadata on the git side (the hack,
> > > suggested by Linus, of using git hashes to notice moves won't work).
> 
> > So, why won't it work?
> 
> Because two files can legitimately have identical contents without
> being ``the same'' file from the VC system's point of view.
> 
> In other words, two files may happen to have the same contents but
> have distinct histories.

Eh, let's not talk using integral/summation view across all the patches
that ever could have come in against the file. We're hamstringing
ourselves if we do that, and it's not what darcs does. darcs looks at a
differential view of the changes, and for a mv, it looks at it when it
happens.

darcs does a "darcs mv" to commit a "file move patch" to whatever
logging or patch repository it keeps below the surface.

The equivalent in git would be to have a given tree, move a file via
bash's mv, and then checkpoint a new tree. (I'm sure there's details in
there, but that's plumbing, and what we have Petr for.)

A differential comparison of the two trees shows no content changed, but
a file label was modified. Ergo, a rename occurred.

QED.

~r.

Re: [darcs-devel] Darcs and git: plan of action

[Juliusz Chroboczek]

> However, I'm claiming a token is defined by the file's language, and
> that a replace patch on anything but a token as per those language
> standards is a silly thing.

Please recall the context of this discussion: getting Darcs to grok
git repositories.

You are arguing that it should be possible to design a set of
heuristics that Do The Right Thing often enough.  And you are probably
right.

But the point is immaterial as nobody has stepped up to implement in
Darcs the sort of heuristics you have in mind.  Partly because nobody
has time, but mostly because we don't like heuristics, we prefer Darcs
to remain deterministic.

So while yes, it might be possible to get about using heuristics, it
seems rather unlikely that that's what we'll do.

                                        Juliusz

Re: [darcs-devel] Darcs and git: plan of action

[Juliusz Chroboczek]

> We're talking about interoperating with a Git repository here,
> right?  Even if we got the metadata in there, doesn't Git have to
> understand a replace patch for things to work out?

> 0. All three are in sync to begin with.

> 1. CC creates a token-replace patch, sends the changes in normal hunk
> format to AA.

> 2. BB makes changes, sends a normal hunk patch to AA and CC.  AA will
> apply the hunk normally.  For CC the token replace might apply here
> and so the result could be different.

3. when AA and CC try to sync, they will get spurious merge conflicts.

> Isn't this a potential problem?

It is.  In a heterogeneous environment they will get spurious merge
conflicts.

                                        Juliusz

Re: [darcs-devel] Darcs and git: plan of action

[David Roundy]

On Tue, Apr 19, 2005 at 02:25:18PM +0200, Petr Baudis wrote:
> Dear diary, on Tue, Apr 19, 2005 at 02:20:55PM CEST, I got a letter
> where Juliusz Chroboczek <Juliusz.Chroboczek@pps.jussieu.fr> told me that...
> > > The problem is that there is no sequence of alien versions that one
> > > can differentiate.  Git has a branched history, with each version
> > > that follows a merge having multiple parents.
> > 
> > Yep.  I've just realised that this morning.  Is there some notion of
> > ``primary parent'' as in Arch?  Can a changeset have 0 parents?
> 
> Yes, the root commit. Usually, there is only one, but there may be
> multiple of them theoretically.

Incidentally (and completely off-topic for this thread), wouldn't there be
a sha1 tree hash corresponding to a completely empty directory, and
couldn't one use that as the parent for the root? Would there be any reason
to do so? Just a silly thought...
-- 
David Roundy
http://www.darcs.net

Re: [darcs-devel] Darcs and git: plan of action

[David Roundy]

On Tue, Apr 19, 2005 at 02:20:55PM +0200, Juliusz Chroboczek wrote:
> [Removing Linus from CC, keeping the Git list -- or should we remove it?]

I think leaving much of this on git would be appropriate, since there are
issues of how to relate to git that should be relevant.

> > If we do it right (automatically tagging like crazy people), darcs
> > users between themselves can cherry-pick all they like, without
> > introducing inconsistencies or losing interoperability with git.
> 
> You've lost me here.  How can you cherry-pick if every tag depends on
> the preceding patches?  Or are you thinking of pulling just the patch
> and not the tag -- in that case, what happens when you push to git a
> Darcs patch that depends on a patch that originated with git?

Yes, I'm thinking of pulling patches from one darcs repo to another.  If we
cherry-pick in this way, we need to create a "git-tag" for each patch that
we pull without its associated tag.  To git, this would look like two
separate changes that have the same commit log, except that they have
different parents and different commiters and commit dates.

I don't think this will be a problem for git, and since darcs will
recognize the two patches as the identical darcs patch (we'll need to put
somewhere in the git commit log a magic word indicating that this patch
originated in darcs), there won't be a problem for darcs either.

In case I haven't been clear (which seems likely), the scenario is that
darcs user 1 makes the following changes to his darcs version of a
git-based repository:

changes in 1: A -> B
tags in 1:    A1   B1

Darcs user 2 wants B, but not A, and didn't do any development:

changes in 2: B
tags in 2:    B2

User 2 pushes to git, and now git has (where P is the parent of both of the
above):

git:
P -> B/B2  (where B/B2 is the commit log with B2 as "committer info" and B
            as the "author info and long comment)

User 1 pushes (everything) to git and merges the two (patch M, which has
two parents, B1 and B2:

git:

   ->B/B2---------
  /               \
P--> A/A1 -> B/B1---> M

It's a little lame, and if user 2 doesn't do any real work, the git-using
person might be annoyed, but I think it's doable.
-- 
David Roundy
http://www.darcs.net

Re: [darcs-devel] Darcs and git: plan of action

[David Roundy]

On Tue, Apr 19, 2005 at 06:11:43PM -0700, Ray Lee wrote:
> > second patch:
> > replace ./hello.c [A-Za-z_0-9] world universe
> 
> Aha! Okay, I now see at least part of issue: we're using different
> definitions of 'token.' Yours is quite sensible, in that it matches the
> darcs syntax. However, I'm claiming a token is defined by the file's
> language, and that a replace patch on anything but a token as per those
> language standards is a silly thing.

The trouble is that a token based on language standards is also wrong,
unless your file at all times is syntactically correct.  It also means (for
C in particular) that the result of the token replace isn't uniquely
determined by the combination of the token replace patch and the file it
applies to, since you need parse any header files in order to tokenize the
C file.  In the case of header files, it may not be possible to tokenize
them uniquely, since they may tokenize differently depending on what other
header files are included before them.  And of course, none of this may be
possible if you haven't run autoconf and configure, since you may not
actually *have* the header files in the first place...

In a (reasonably) general-purpose tool like darcs, I think it's better to
stick with a simpler definition of token that doesn't require a complete
integrated development environment.

It's also true that often you want to modify headers and string contents
simultaneously with the change of the code itself.  When I replace
get_pseudowavefunction with get_atomic_orbital, I also want to modify

// We call get_pseudowavefunction to get the atomic orbital...

and

printf("Error in get_pseudowavefunction!\n");
-- 
David Roundy
http://www.darcs.net

Re: [darcs-devel] Darcs and git: plan of action

[Ralph Corderoy]

Hi Ray,

> Give me a case where assuming it's a replace will do the wrong thing,
> for C code, where it's a variable or function name.

How about two patches.

    1.  s/foo/bar/ throughout file because foo() has been decided upon
    as the name of a new globally visible forthcoming function but was
    already in use as a static function.

    2.  Add definition of new foo().

Patch 1 mustn't be a `darcs replace' despite it changing every occurence
of the C token foo into bar.

Cheers,


Ralph.