brief review of field; fieldsort plugin

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+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.)
+
+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?
+
+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;