Having tried out `field`, some comments (from [[smcv]]):

The general concept looks great.

The `pagetemplate` hook seems quite namespace-polluting: on a site containing
a list of books, I'd like to have an `author` field, but that would collide
with IkiWiki's use of `<TMPL_VAR AUTHOR>` for the author of the *page*
(i.e. me). Perhaps it'd be better if the pagetemplate hook was only active for
`<TMPL_VAR FIELD_AUTHOR>` or something? (For those who want the current
behaviour, an auxiliary plugin would be easy.)

> No, please.  The idea is to be *able* to override field names if one wishes to, and choose, for yourself, non-colliding field names if one wishes not to.  I don't wish to lose the power of being able to, say, define a page title with YAML format if I want to, or to write a site-specific plugin which calculates a page title, or other nifty things.
>It's not like one is going to lose the fields defined by the meta plugin; if "author" is defined by \[[!meta author=...]] then that's what will be found by "field" (provided the "meta" plugin is registered; that's what the "field_register" option is for).
>--[[KathrynAndersen]]

From a coding style point of view, the `$CamelCase` variable names aren't
IkiWiki style, and the `match_foo` functions look as though they could benefit
from being thin wrappers around a common `&IkiWiki::Plugin::field::match`
function (see `meta` for a similar approach).

I think the documentation would probably be clearer in a less manpage-like
and more ikiwiki-like style?

> I don't think ikiwiki *has* a "style" for docs, does it?  So I followed the Perl Module style. And I'm rather baffled as to why having the docs laid out in clear sections... make them less clear. --[[KathrynAndersen]]

If one of my branches from [[todo/allow_plugins_to_add_sorting_methods]] is
accepted, a `field()` cmp type would mean that [[plugins/contrib/report]] can
stop reimplementing sorting. Here's the implementation I'm using, with
your "sortspec" concept (a sort-hook would be very similar): if merged,
I think it should just be part of `field` rather than a separate plugin.

	# Copyright © 2010 Simon McVittie, released under GNU LGPL >= 2.1
	package IkiWiki::Plugin::fieldsort;
	use warnings;
	use strict;
	use IkiWiki 3.00;
	use IkiWiki::Plugin::field;

	sub import {
		hook(type => "getsetup", id => "fieldsort",  call => \&getsetup);
	}

	sub getsetup () {
		return
			plugin => {
				safe => 1,
				rebuild => undef,
			},
	}

	package IkiWiki::PageSpec;

	sub check_cmp_field {
		if (!length $_[0]) {
			error("sort=field requires a parameter");
		}
	}

	sub cmp_field {
		my $left = IkiWiki::Plugin::field::field_get_value($_[2], $_[0]);
		my $right = IkiWiki::Plugin::field::field_get_value($_[2], $_[1]);

		$left = "" unless defined $left;
		$right = "" unless defined $right;
		return $left cmp $right;
	}

	1;