Re: [PATCH v5 02/14] trailer: process trailers from file and arguments

From: Christian Couder <chriscool@tuxfamily.org>
Date: Feb 14, 2014, 23:29 UTC
Message-ID: <20140215.002950.97819205213642217.chriscool@tuxfamily.org>
In-Reply-To: <xmqqfvnlgyel.fsf@gitster.dls.corp.google.com>

From: Junio C Hamano <gitster@pobox.com>

Show 19 quoted lines

>
> Christian Couder <chriscool@tuxfamily.org> writes:
> 
>> For example some people might want:
>>
>>     if_exists = overwrite
>>     if_missing = add
>>
>> while others might want:
>>
>>     if_exists = overwrite
>>     if_missing = do_nothing
>>
>> and I don't see how we can say that with just:
>>
>>     action = do_Y_if_X_and_Z
> 
> Yes, but then we go back to my original question: why exists and
> missing are so special,

Because they are completely disjoint, easy to understand, and they can avoid a lot of combinatorial explosion we would have if we used only one "action" variable, while still providing lot of expressiveness.

They are just a good tradeoff for the special problem we have.

> and why there aren't two kinds of exists
> (i.e. "there exists an entry with the same <key, value>" vs "there
> exists an entry with the same <key>").

Because it doesn't improve expressiveness much, doesn't remove much combinatorial explosion and make it significantly more difficult to understand, compared to only "if_exists" and "if_missing".

Show 5 quoted lines

> I would have understood your
> "this is not too hard to understand for users" if you had three
> (i.e. "missing", in addition to these two flavours of "exists"), but
> with only two, I do not see how it is useful in a hypothetical
> situation like above.

You mean that you do not see why:

     if_exists = overwrite
     if_missing = do_nothing

is simple and expressive?

Show 6 quoted lines

> For example, how would you express something like this only with
> "if-exists" vs "if-missing"?
> 
> 	if_exists_exactly = ignore
>         if_exists_with_different_value = append
>         if_missng = prepend_to_the_beginning

First, previously in the discussion you said that you didn't want us to talk about the "where = (after | before)" part, because you could see that it was orthogonal to the other stuff, but now it looks like you want again to put that on the table.

Then yes, it is not possible to express the above with what I implemented. But it could be possible with only "if-exists" vs "if-missing" like this:

	if_exists = append_if_different
	if_missing = prepend

And yes I think it is much better than:

	if_exists_exactly = ignore
	if_exists_with_different_value = append
	if_missng = prepend_to_the_beginning

because we can still easily express things like:

	if_exists = append_if_different_neighbor
	if_missing = prepend

while it would be more difficult to understand with "if_exists_exactly", "if_exists_with_different_value" and "if_missing".

Thanks, Christian.