Re: How is the ^{sha256} peel syntax supposed to work?

From: Jonathan Nieder <jrnieder@gmail.com>
Date: Aug 29, 2018, 20:46 UTC
Message-ID: <20180829204623.GD7547@aiede.svl.corp.google.com>
In-Reply-To: <87y3cpc6bt.fsf@evledraar.gmail.com>

Hi,

Ævar Arnfjörð Bjarmason wrote:

> On Wed, Aug 29 2018, Jonathan Nieder wrote:

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>> In other words, I want the input format and output format completely
>> decoupled.  If I pass ^{sha1}, I am indicating the input format.  To
>> specify the output format, I'd use --output-format instead.
>
> This is also a reasonable thing to want, but I don't see how it can be
> sensibly squared with the existing peel syntax.

All the weight here is on the word "sensibly". Currently, ^{thing} means "act on the object" and @{thing} means "act on the ref". This ^{sha1} syntax is really a new kind of modifier, ~{thing}, meaning "act on the string".

That said, we can make it do anything we want. There is nothing forcing us to make it more similar to ^{commit} than to ^{/searchstring}, say.

In that context:

[...]

>> Ævar Arnfjörð Bjarmason wrote:

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>>> Similarly, I think it would be very useful if we just make this work:
>>>
>>>     git rev-parse $some_hash^{sha256}^{commit}
>>>
>>> And not care whether $some_hash is SHA-1 or SHA-256, if it's the former
>>> we'd consult the SHA-1 <-> SHA-256 lookup table and go from there, and
>>> always return a useful value.
>>
>> The opposite of this. :)
>
> Can you elaborate on that?

What I'm saying is, regardless of the syntax used, as a user I *need* a way to look up $some_hash as a sha256-name, with zero risk of Git trying to outsmart me and treating $some_hash as a sha1-name instead.

Any design without that capability is a non-starter.

[...]

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> I.e. if I'm using this in a script I'd need:
>
>     if x = git rev-parse $some_hash^{sha256}^{commit}
>         hash = x
>     elsif x = git rev-parse $some_hash^{sha1}^{commit}
>         hash = x
>     endif

Why wouldn't you use "git rev-parse $some_hash^{commit}" instead?

Thanks, Jonathan