# Re: [darcs-devel] Darcs and git: plan of action
18 messages from 2005-04-18 to 2005-04-20. Participants: linux@horizon.com, Ray Lee, Kevin Smith, David Roundy, Patrick McFarland, Tupshin Harper, Juliusz Chroboczek, Ralph Corderoy.
Thread: https://gitlist.dev/t/119
## linux@horizon.com, 2005-04-18 23:33
Subject: Re: [darcs-devel] Darcs and git: plan of action
Message-ID: <20050418210436.23935.qmail@science.horizon.com>
URL: https://gitlist.dev/e/20050418210436.23935.qmail%40science.horizon.com
```
> 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.
```
## Ray Lee, 2005-04-19 00:07
Subject: Re: [darcs-devel] Darcs and git: plan of action
Message-ID: <1113869248.23938.94.camel@orca.madrabbit.org>
URL: https://gitlist.dev/e/1113869248.23938.94.camel%40orca.madrabbit.org
In-Reply-To: <20050418210436.23935.qmail@science.horizon.com>
```
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
```
## Kevin Smith, 2005-04-19 01:05
Subject: Re: [darcs-devel] Darcs and git: plan of action
Message-ID: <42645969.2090609@qualitycode.com>
URL: https://gitlist.dev/e/42645969.2090609%40qualitycode.com
In-Reply-To: <1113869248.23938.94.camel@orca.madrabbit.org>
```
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
```
## Ray Lee, 2005-04-19 01:42
Subject: Re: [darcs-devel] Darcs and git: plan of action
Message-ID: <1113874931.23938.111.camel@orca.madrabbit.org>
URL: https://gitlist.dev/e/1113874931.23938.111.camel%40orca.madrabbit.org
In-Reply-To: <42645969.2090609@qualitycode.com>
```
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
```
## Kevin Smith, 2005-04-19 02:05
Subject: Re: [darcs-devel] Darcs and git: plan of action
Message-ID: <4264677A.9090003@qualitycode.com>
URL: https://gitlist.dev/e/4264677A.9090003%40qualitycode.com
In-Reply-To: <1113874931.23938.111.camel@orca.madrabbit.org>
```
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
```
## David Roundy, 2005-04-19 11:05
Subject: Re: [darcs-devel] Darcs and git: plan of action
Message-ID: <20050419110521.GC28269@abridgegame.org>
URL: https://gitlist.dev/e/20050419110521.GC28269%40abridgegame.org
In-Reply-To: <1113874931.23938.111.camel@orca.madrabbit.org>
```
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
```
## Patrick McFarland, 2005-04-19 22:08
Subject: Re: [darcs-devel] Darcs and git: plan of action
Message-ID: <200504191808.26559.pmcfarland@downeast.net>
URL: https://gitlist.dev/e/200504191808.26559.pmcfarland%40downeast.net
In-Reply-To: <4264677A.9090003@qualitycode.com>
```
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
```
## Ray Lee, 2005-04-19 22:40
Subject: Re: Darcs and git: plan of action
Message-ID: <1113950442.29444.31.camel@orca.madrabbit.org>
URL: https://gitlist.dev/e/1113950442.29444.31.camel%40orca.madrabbit.org
In-Reply-To: <4264677A.9090003@qualitycode.com>
```
(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.
indention patch type, indention patch type...
> > > 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
```
## Tupshin Harper, 2005-04-19 23:00
Subject: Re: Darcs and git: plan of action
Message-ID: <42658D95.7020404@tupshin.com>
URL: https://gitlist.dev/e/42658D95.7020404%40tupshin.com
In-Reply-To: <1113950442.29444.31.camel@orca.madrabbit.org>
```
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
```
## Kevin Smith, 2005-04-19 23:03
Subject: Re: [darcs-devel] Darcs and git: plan of action
Message-ID: <42658E38.1020406@qualitycode.com>
URL: https://gitlist.dev/e/42658E38.1020406%40qualitycode.com
In-Reply-To: <1113950442.29444.31.camel@orca.madrabbit.org>
```
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
```
## Ray Lee, 2005-04-19 23:06
Subject: Re: [darcs-devel] Darcs and git: plan of action
Message-ID: <1113951972.29444.42.camel@orca.madrabbit.org>
URL: https://gitlist.dev/e/1113951972.29444.42.camel%40orca.madrabbit.org
In-Reply-To: <42658E38.1020406@qualitycode.com>
```
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
```
## Ray Lee, 2005-04-19 23:21
Subject: Re: Darcs and git: plan of action
Message-ID: <1113952916.29444.60.camel@orca.madrabbit.org>
URL: https://gitlist.dev/e/1113952916.29444.60.camel%40orca.madrabbit.org
In-Reply-To: <42658D95.7020404@tupshin.com>
```
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
(python 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
```
## Tupshin Harper, 2005-04-19 23:32
Subject: Re: Darcs and git: plan of action
Message-ID: <426594F9.4090002@tupshin.com>
URL: https://gitlist.dev/e/426594F9.4090002%40tupshin.com
In-Reply-To: <1113951972.29444.42.camel@orca.madrabbit.org>
```
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
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
```
## Tupshin Harper, 2005-04-19 23:38
Subject: Re: Darcs and git: plan of action
Message-ID: <42659678.9090605@tupshin.com>
URL: https://gitlist.dev/e/42659678.9090605%40tupshin.com
In-Reply-To: <1113952916.29444.60.camel@orca.madrabbit.org>
```
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
>(pythonfile) 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
```
## Ray Lee, 2005-04-20 01:11
Subject: Re: [darcs-devel] Darcs and git: plan of action
Message-ID: <1113959503.29444.91.camel@orca.madrabbit.org>
URL: https://gitlist.dev/e/1113959503.29444.91.camel%40orca.madrabbit.org
In-Reply-To: <426594F9.4090002@tupshin.com>
```
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
>
> 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
```
## Juliusz Chroboczek, 2005-04-20 07:52
Subject: Re: [darcs-devel] Darcs and git: plan of action
Message-ID: <7i3btlubpm.fsf@lanthane.pps.jussieu.fr>
URL: https://gitlist.dev/e/7i3btlubpm.fsf%40lanthane.pps.jussieu.fr
In-Reply-To: <1113959503.29444.91.camel@orca.madrabbit.org>
```
> 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
```
## David Roundy, 2005-04-20 11:55
Subject: Re: [darcs-devel] Darcs and git: plan of action
Message-ID: <20050420115547.GI29945@abridgegame.org>
URL: https://gitlist.dev/e/20050420115547.GI29945%40abridgegame.org
In-Reply-To: <1113959503.29444.91.camel@orca.madrabbit.org>
```
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
```
## Ralph Corderoy, 2005-04-20 17:11
Subject: Re: [darcs-devel] Darcs and git: plan of action
Message-ID: <200504201711.j3KHBtn22691@blake.inputplus.co.uk>
URL: https://gitlist.dev/e/200504201711.j3KHBtn22691%40blake.inputplus.co.uk
In-Reply-To: <1113951972.29444.42.camel@orca.madrabbit.org>
```
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.
```