Re: Fwd: [PATCH 2/2] pretty.c: allow date formats in user format strings

From: Jeff King <peff@peff.net>
Date: Mar 7, 2011, 16:17 UTC
Message-ID: <20110307161758.GB11934@sigill.intra.peff.net>
In-Reply-To: <AANLkTikaN=wsg6RLFaFxh=L3RCYjKkVGFR4VTrQ=KRZk@mail.gmail.com>

On Sun, Mar 06, 2011 at 09:54:01PM +0000, Will Palmer wrote:

Show 10 quoted lines

> On Sat, Mar 5, 2011 at 8:00 PM, Jeff King <peff@peff.net> wrote:
> > You can now do "%ad(short)" or similar (using any format
> > that works for --date). This makes some formats like %aD
> > redundant (since you can do "%ad(rfc)"), but of course we
> > keep them for compatibility.
> >
> 
> The more I see long formats like this, the more I think it would make
> sense to make formats %(likeThis), the way for-each-ref does.
> Ideally, these formats could even be unified, at some point.

Yeah, I totally agree. One problem is that everytime an extended format comes up it gets bikeshedded to death as everybody mentions their favorite format and/or feature, and then nobody codes it.

Show 6 quoted lines

> I tried this a long while ago, as part of my attempt to make all
> pre-defined formats work in terms of format strings, but that turned
> into too much of a bloated mess to bother submitting. I don't know
> if there's enough interest in such a thing to justify trying again (or to
> justify rebasing the bloated version, cleaning it up and submitting it
> as-is, for that matter)

I think there is interest. I'd be curious to see what you have. A few days ago, when working on this series, I tried to make a minimally-invasive change to allow "%(ad)" to work alongside "%ad", with a generic arguments format like %(ad:flag:key=value). Which would allow existing shorthand, for-each-ref-style %(refname:short), and leave room for arbitrary extension of each placeholder (alongside more human-readable placeholder names).

The problem I ran into was the internal code interface. We parse the format string each time we expand it. This works OK for simple printf-like stuff. But ideally we can handle something like:

  %(ad:key=embedded\:colon:key2=embedded\)paren)

It's hard to make a nice interface to that which doesn't involve copying the quoted string out into a non-quoted version. But we don't want to be doing a bunch of parsing and allocation per-expansion. It's slow, and this expansion happens inside a fairly tight loop in many cases (e.g., during rev-list).

So I think the whole thing needs to be factored into two phases: a parsing phase where we build some internal parse tree, and then an expansion phase where we walk the parse tree for each commit (or ref, or whatever is being expanded).

> Point is: we're going to keep having more and more format options,
> I think that's a given. At some point, these short mnemonics will just
> stop making sense, and it makes sense to have an escape plan when
> that happens.

Agreed. And I think it is possible to do it in a backwards-compatible way; support %(longname:options) for everything, and keep short-hands like %h and %ad for existing elements without options.

-Peff