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Re: [PATCH] pretty: Add %(trailer:X) to display single trailer

On Sun, Oct 28, 2018 at 01:50:25PM +0100, Anders Waldenborg wrote:

> This new format placeholder allows displaying only a single
> trailer. The formatting done is similar to what is done for
> --decorate/%d using parentheses and comma separation.

Displaying a single trailer makes sense as a goal. It was one of the
things I considered when working on %(trailers), actually, but I ended
up needing something a bit more flexible (hence the ability to dump the
trailers in a parse-able format, where I feed them to another script).
But your ticket example makes sense for just ordinary log displays.

Junio's review already covered my biggest question, which is why not
something like "%(trailers:key=ticket)". And likewise making things like
comma-separation options.

But my second question is whether we want to provide something more
flexible than the always-parentheses that "%d" provides. That has been a
problem in the past when people want to format the decoration in some
other way.

We have formatting magic for "if this thing is non-empty, then show this
prefix" in the for-each-ref formatter, but I'm not sure that we do in
the commit pretty-printer beyond "% ". I wonder if we could/should add a
a placeholder for "if this thing is non-empty, put in a space and
enclose it in parentheses".

> diff --git a/Documentation/pretty-formats.txt b/Documentation/pretty-formats.txt
> index 6109ef09aa..a46d0c0717 100644
> --- a/Documentation/pretty-formats.txt
> +++ b/Documentation/pretty-formats.txt
> @@ -211,6 +211,10 @@ endif::git-rev-list[]
>    If the `unfold` option is given, behave as if interpret-trailer's
>    `--unfold` option was given.  E.g., `%(trailers:only,unfold)` to do
>    both.
> +- %(trailer:<t>): display the specified trailer in parentheses (like
> +  %d does for refnames). If there are multiple entries of that trailer
> +  they are shown comma separated. If there are no matching trailers
> +  nothing is displayed.

It might be worth specifying how this match is done. I'm thinking
specifically of whether it's case-sensitive, but I wonder if there
should be any allowance for other normalization (e.g., allowing a regex
to match "coauthored-by" and "co-authored-by" or something).