Re: my git problem

From: Jakub Narebski <jnareb@gmail.com>
Date: Apr 30, 2008, 08:17 UTC
Message-ID: <m3prs7oixp.fsf@localhost.localdomain>
In-Reply-To: <20080429171519.GA21310@fieldses.org>

"J. Bruce Fields" <bfields@fieldses.org> writes:

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> On Mon, Apr 28, 2008 at 12:28:38PM -0700, Linus Torvalds wrote:
>> 
>> 
>> On Mon, 28 Apr 2008, Johannes Schindelin wrote:
>>> 
>>> On Mon, 28 Apr 2008, Andrew Morton wrote:
>>>> 
>>>> That's missing the "logical" bit :)
>>> 
>>> Heh, you're right.  I am too used to Git to think how other people would 
>>> feel about these things... :-)
>> 
>> No, you are both wrong.
>> 
>> You're wrong because apparently you never did abstract algebra and set 
>> theory in school.

[...]

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>> If you know math, git actually does the rigth and very much the *logical* 
>> thing.
>> 
>> So ".." is a simple difference, while "..." is a more complex difference. 
>> 
>> They mean different things for different operation types, but that is 
>> again something a math person takes for granted (ie in algebra, a "+" or 
>> "-" is just a random operation that follows certain rules: "a-b" means one 
>> thing for the set of real numbers, and something *totally* different if 
>> you are talking about set algebra).

[...]

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> I can sorta buy the argument that "A...B" means most generally "some
> kind of difference between the three sets A, A^B, and B", and that in
> the context of "git diff" it's most sensible to take ordering into
> account and produce some approximation of a diff between A^B and B.  I'd
> personally have found an entirely separate operator simpler to
> understand.  But perhaps there's only so many keys on the keyboard.

IMHO adding support for a..b and a...b to git-diff is a bit of trick, as a..b and a...b were created to represent a set of revisions (a revision range).

If we have linear history:

   *---*---*---a---*---*---b

then a..b notation for a revision range is very natural, and having git-diff interprete "a..b" as "a b" (for git-diff only endpoints matter) to allow copy'n'pasting between git-log and git-diff, and between git-fetch messages and git-diff was a good extension.

Now if the history is not linear, as in example below:

   *---*---*---x---*---*---b
                \
                 \-*---a

then "a..b", which is shortcut for "b ^a" (b --not a), returns x..b range (set) of revisions. If you read "a..b" as "what's in 'b' since 'a'" it makes perfect sense. But "git diff a..b" is still "git diff a b", not "git diff x b".

It would be perhaps as good notation to have "git diff a..b" mean "git diff x b", i.e. be diff between endpoints of "git log a..b", and have "git diff a...b" be "git diff a b", i.e. to be diff between endpoints^W points of "git log a...b"... but if there is no clean winner, simplicity of implementation wins.

-- 
Jakub Narebski
Poland
ShadeHawk on #git