Re: [RFC/PATCH] tag: make list exclude !<pattern>

From: Junio C Hamano <gitster@pobox.com>
Date: Feb 13, 2012, 06:37 UTC
Message-ID: <7vsjifgrwl.fsf@alter.siamese.dyndns.org>
In-Reply-To: <4F389FB1.2070706@alum.mit.edu>

Michael Haggerty <mhagger@alum.mit.edu> writes:

> Of *course* they operate on different namespaces.  But part of the way
> that revisions are selected using rev-list is by *selecting or excluding
> refnames* from which it should crawl.

I am appalled if that is truly the understanding of yours, after having taken more than a few patches from you to fairly core parts of Git.

"rev-list A ^B" does not say "include A and exclude B from which rev-list should crawl" AT ALL. We _actively_ crawl from both A and B. It is that what are reachable from B is painted in a color different from the color in which we paint what are reachable from A.

A better pair you could have mentioned would be for-each-ref vs rev-parse (not rev-list). What Tom wanted with "do not show the refs that match the pattern" he originally wanted to give to "tag --list" would be

	for-each-ref A ^B

that is "show ref that matches A but do not show if it also matches B", while what you want to say is "I want to paint A in positive color and paint B in negative color, and I want to get a canonical notation to do so", it is spelled with rev-parse, not for-each-ref, like this:

	rev-parse A ^B

In other words,

	git rev-list $(git rev-parse A ^B)

would be the equivalent to "git rev-list A ^B".

Maybe you are troubled that there are multiple concepts of negation, which ultimately comes from the undeniable fact that for-each-ref and rev-parse operate on entities in different concept domain (refnames and objects)? And if we decide to use "^", then these two different concepts of negation are both expressed with the same operator "prefix ^", leading to confusion?

I am kind of sympathetic to that argument, and it might be a better idea to avoid using "^" as the negation for matching operator, in order to make it more apparent to the users that for-each-ref and rev-parse operate on different concepts. But at the same time, if you know these are distinct concepts, using the same "^" operator as a consistent way to express the "negation" that is applicable in each concept domain does make it easier for users as they gain experience. I tend to avoid making things too hard for experienced users for the sake of flattening the very early learning curve when possible, because nobody will stay novice forever.