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From:   Junio C Hamano <gitster@pobox.com>
To:     ZheNing Hu <adlternative@gmail.com>
Cc:     ZheNing Hu via GitGitGadget <gitgitgadget@gmail.com>,
        Git List <git@vger.kernel.org>,
        Christian Couder <christian.couder@gmail.com>,
        Hariom Verma <hariom18599@gmail.com>,
        Karthik Nayak <karthik.188@gmail.com>
Subject: Re: [PATCH 2/3] [GSOC] ref-filter: support %(contents) for blob, tree
References: <pull.959.git.1621763612.gitgitgadget@gmail.com>
        <19413cfdb1ea50de401ab958b954a550b6514688.1621763612.git.gitgitgadget@gmail.com>
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Date:   Wed, 26 May 2021 02:11:54 +0900
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ZheNing Hu <adlternative@gmail.com> writes:

> So I don't know if adding %(header) will cause duplication of functions.

I do not think you can duplicate the feature of %(header) with
other existing placeholders.  You may want to dump the headers of
series of commits so that you can find if there is any commit with
unusal header elements, but %(tree), %(parents), etc. would by
definition be the known ones, so any combination of them will not be
a substitute.

But nobody is asking for it right now, and your cat-file --batch
does not need it right away.

What I wanted to say in the message you are responding to was *not*
that you would want to add %(header) for completeness right now.
But thinking beyond your immediate need (i.e. the "whole thing") and
imagining how various pieces, including the ones that do not exist
yet, would fit together, would help you to avoid designing the parts
you immediately need in a way you would regret in the future.