### compiler
-As a compiler, ikiwiki starts by calling the `refresh` hook. Then it checks
-the wiki's source to find new or changed pages. The `needsbuild` hook is
-then called to allow manipulation of the list of pages that need to be
-built.
-
-Now that it knows what pages it needs to build, ikiwiki runs two
-compile passes. First, it runs `scan` hooks, which collect metadata about
-the pages. Then it runs a page rendering pipeline, by calling in turn these
-hooks: `filter`, `preprocess`, `linkify`, `htmlize`, `postscan`,
-`pagetemplate`, `sanitize`, `format`.
-
-After all necessary pages are built, it calls the `change` hook. Finally,
-if a page is was deleted, the `delete` hook is called, and the files that
-page had previously produced are removed.
+As a compiler, ikiwiki starts by calling the
+[[`refresh`|plugins/write#refresh]] hook. Then it checks the wiki's source to
+find new or changed pages. The [[`needsbuild`|plugins/write#needsbuild]] hook
+is then called to allow manipulation of the list of pages that need to be
+built.
+
+Now that it knows what pages it needs to build, ikiwiki runs two compile
+passes. First, it runs [[`scan`|plugins/write#scan]] hooks, which collect
+metadata about the pages. Then it runs a page rendering pipeline, by calling
+in turn these hooks: [[`filter`|plugins/write#filter]],
+[[`preprocess`|plugins/write#preprocess]],
+[[`linkify`|plugins/write#linkify]], [[`htmlize`|plugins/write#htmlize]],
+[[`indexhtml`|plugins/write#indexhtml]],
+[[`pagetemplate`|plugins/write#pagetemplate]],
+[[`sanitize`|plugins/write#sanitize]], [[`format`|plugins/write#format]].
+
+After all necessary pages are built, it calls the
+[[`changes`|plugins/write#changes]] hook. Finally, if a page was deleted, the
+[[`delete`|plugins/write#delete]] hook is called, and the files that page had
+previously produced are removed.
### cgi
Alice browses to a page and clicks Edit.
-* Ikiwiki is run as a cgi. It assigns Alice a session cookie, and,
- by calling the `auth` hooks, sees that she is not yet logged in.
-* The `sessioncgi` hooks are then called, and one of them,
- from the [[editpage]] plugin, notices that the cgi has been told "do=edit".
-* The [[editpage]] plugin calls the `canedit` hook to check if this
- page edit is allowed. The [[signinedit]] plugin has a hook that says not:
- Alice is not signed in.
-* The [[signinedit]] plugin then launches the signin process. A signin
- page is built by calling the `formbuilder_setup` hook.
+* Ikiwiki is run as a cgi. It assigns Alice a session cookie, and, by calling
+ the [[`auth`|plugins/write#auth]] hooks, sees that she is not yet logged in.
+* The [[`sessioncgi`|plugins/write#sessioncgi]] hooks are then called, and one
+ of them, from the [[editpage]] plugin, notices that the cgi has been told
+ "do=edit".
+* The [[editpage]] plugin calls the [[`canedit`|plugins/write#canedit]] hook
+ to check if this page edit is allowed. The [[signinedit]] plugin has a hook
+ that says not: Alice is not signed in.
+* The [[signinedit]] plugin then launches the signin process. A signin page is
+ built by calling the [[`formbuilder_setup`|plugins/write#formbuilder]]
+ hook.
Alice signs in with her openid.
-* The [[openid]] plugin's `formbuilder` hook sees that an openid was
- entered in the signin form, and redirects to Alice's openid provider.
-* Alice's openid provider calls back to ikiwiki. The [[openid]] plugin
- has an `auth` hook that finishes the openid signin process.
+* The [[openid]] plugin's [[`formbuilder`|plugins/write#formbuilder]] hook
+ sees that an openid was entered in the signin form, and redirects to Alice's
+ openid provider.
+* Alice's openid provider calls back to ikiwiki. The [[openid]] plugin has an
+ [[`auth`|plugins/write#auth]] hook that finishes the openid signin process.
* Signin complete, ikiwiki returns to what Alice was doing before; editing
a page.
-* Now all the `canedit` hooks are happy. The [[editpage]] plugin calls
- `formbuilder_setup` to display the page editing form.
+* Now all the [[`canedit`|plugins/write#canedit]] hooks are happy. The
+ [[editpage]] plugin calls
+ [[`formbuilder_setup`|plugins/write#formbuilder]] to display the page
+ editing form.
Alice saves her change to the page.
-* The [[editpage]] plugin's `formbuilder` hook sees that the Save button
- was pressed, and calls the `checkcontent` and `editcontent` hooks.
- Then it saves the page to disk, and branches into the compiler part
- of ikiwiki to refresh the wiki.
+* The [[editpage]] plugin's [[`formbuilder`|plugins/write#formbuilder]] hook
+ sees that the Save button was pressed, and calls the
+ [[`checkcontent`|plugins/write#checkcontent]] and
+ [[`editcontent`|plugins/write#editcontent]] hooks. Then it saves the page
+ to disk, and branches into the compiler part of ikiwiki to refresh the wiki.
## Types of plugins
The function is passed no values. It's ok for the function to call
`error()` if something isn't configured right.
-### refresh
+### <a name="refresh">refresh</a>
hook(type => "refresh", id => "foo", call => \&refresh);
It's useful for plugins that need to create or modify a source page. The
function is passed no values.
-### needsbuild
+### <a name="needsbuild">needsbuild</a>
hook(type => "needsbuild", id => "foo", call => \&needsbuild);
-This allows a plugin to manipulate the list of files that need to be
-built when the wiki is refreshed. The function is passed a reference to an
-array of files that will be rebuilt, and can modify the array, either
-adding or removing files from it.
+This allows a plugin to observe or even manipulate the list of files that
+need to be built when the wiki is refreshed.
+
+As its first parameter, the function is passed a reference to an array of
+files that will be built. It should return an array reference that is a
+modified version of its input. It can add or remove files from it.
-### scan
+The second parameter passed to the function is a reference to an array of
+files that have been deleted.
+
+### <a name="scan">scan</a>
hook(type => "scan", id => "foo", call => \&scan);
The function is passed named parameters "page" and "content". Its return
value is ignored.
-### filter
+### <a name="readtemplate">readtemplate</a>
+
+ hook(type => "readtemplate", id => "foo", call => \&readtemplate);
+
+Runs on the raw source of a page or `*.tmpl` file that is being
+used as a template, before it is parsed by [[!cpan HTML::Template]].
+For instance, the [[plugins/templatebody]] plugin uses this to return
+the content of the [[ikiwiki/directive/templatebody]] directive (if there
+is one) instead of the page's full content.
+
+The function is passed named parameters:
+
+* `id`: the name under which the template was looked up,
+ such as `page.tmpl` or `note`
+* `page`: the name of the template as a page or attachment in the wiki,
+ such as `templates/note`, or `undef` if it's outside the wiki (e.g. in
+ `/usr/share/ikiwiki/templates`)
+* `content`: the content of the corresponding file
+* `untrusted`: true if the template was loaded from the wiki or an underlay,
+ false if it was loaded from a trusted location
+
+It should return the replacement content.
+
+### <a name="filter">filter</a>
hook(type => "filter", id => "foo", call => \&filter);
-Runs on the raw source of a page, before anything else touches it, and can
-make arbitrary changes. The function is passed named parameters "page",
+Runs on the full raw source of a page, before anything else touches it, and
+can make arbitrary changes. The function is passed named parameters "page",
"destpage", and "content". It should return the filtered content.
-### preprocess
+### <a name="preprocess">preprocess</a>
Adding a preprocessor [[ikiwiki/directive]] is probably the most common use
of a plugin.
be linkified and passed through markdown (or whatever engine is used to
htmlize the page) along with the rest of the page.
-### linkify
+### <a name="linkify">linkify</a>
hook(type => "linkify", id => "foo", call => \&linkify);
for the links on the page and adds them to `%links` (typically by calling
`add_link`).
-### htmlize
+### <a name="htmlize">htmlize</a>
hook(type => "htmlize", id => "ext", call => \&htmlize);
If `hook` is passed an optional "longname" parameter, this value is used
when prompting a user to choose a page type on the edit page form.
-### postscan
+### <a name="indexhtml">indexhtml</a>
- hook(type => "postscan", id => "foo", call => \&postscan);
+ hook(type => "indexhtml", id => "foo", call => \&indexhtml);
This hook is called once the page has been converted to html (but before
the generated html is put in a template). The most common use is to
update search indexes. Added in ikiwiki 2.54.
-The function is passed named parameters "page" and "content". Its return
-value is ignored.
+The function is passed named parameters "page", "destpage", and "content".
+Its return value is ignored.
-### pagetemplate
+### <a name="pagetemplate">pagetemplate</a>
hook(type => "pagetemplate", id => "foo", call => \&pagetemplate);
-[[Templates|wikitemplates]] are filled out for many different things in
+[[Templates]] are filled out for many different things in
ikiwiki, like generating a page, or part of a blog page, or an rss feed, or
a cgi. This hook allows modifying the variables available on those
templates. The function is passed named parameters. The "page" and
hook(type => "templatefile", id => "foo", call => \&templatefile);
-This hook allows plugins to change the [[template|wikitemplates]] that is
+This hook allows plugins to change the [[template|templates]] that is
used for a page in the wiki. The hook is passed a "page" parameter, and
-should return the name of the template file to use, or undef if it doesn't
-want to change the default ("page.tmpl"). Template files are looked for in
-/usr/share/ikiwiki/templates by default.
+should return the name of the template file to use (relative to the
+template directory), or undef if it doesn't want to change the default
+("page.tmpl").
+
+### pageactions
+
+ hook(type => "pageactions", id => "foo", call => \&pageactions);
+
+This hook allows plugins to add arbitrary actions to the action bar on a
+page (next to Edit, RecentChanges, etc). The hook is passed a "page"
+parameter, and can return a list of html fragments to add to the action
+bar.
-### sanitize
+### <a name="sanitize">sanitize</a>
hook(type => "sanitize", id => "foo", call => \&sanitize);
The function is passed named parameters: "page", "destpage", and "content",
and should return the sanitized content.
-### format
+### <a name="format">format</a>
hook(type => "format", id => "foo", call => \&format);
The function is passed named parameters: "page" and "content", and
should return the formatted content.
-### delete
+### build_affected
+
+ hook(type => "build_affected", id => "foo", call => \&build_affected);
+
+This hook is called after the directly changed pages have been built,
+and can cause extra pages to be built. If links and backlinks were provided
+by a plugin, this would be where that plugin would rebuild pages whose
+backlinks have changed, for instance. The [[trail]] plugin uses this hook
+to rebuild pages whose next or previous page has changed.
+
+The function should currently ignore its parameters. It returns a list with
+an even number of items (a hash in list context), where the first item of
+each pair is a page name to be rebuilt (if it was not already rebuilt), and
+the second is a log message resembling
+`building plugins/write because the phase of the moon has changed`.
+
+### <a name="delete">delete</a>
hook(type => "delete", id => "foo", call => \&delete);
-Each time a page or pages is removed from the wiki, the referenced function
+After a page or pages is removed from the wiki, the referenced function
is called, and passed the names of the source files that were removed.
-### change
+### rendered
- hook(type => "change", id => "foo", call => \&render);
+ hook(type => "rendered", id => "foo", call => \&rendered);
-Each time ikiwiki renders a change or addition (but not deletion) to the
+After ikiwiki renders a change or addition (but not deletion) to the
wiki, the referenced function is called, and passed the names of the
source files that were rendered.
+(This hook used to be called "change", but that was not accurate.
+For now, plugins using the old hook name will still work.)
+
+### <a name="changes">changes</a>
+
+ hook(type => "changes", id => "foo", call => \&changes);
+
+After ikiwiki renders changes to the wiki, the referenced function is
+called, and passed the names of the source files that were added, modified,
+or deleted.
+
### cgi
hook(type => "cgi", id => "foo", call => \&cgi);
Note that cgi hooks are called as early as possible, before any ikiwiki
state is loaded, and with no session information.
-### auth
+### <a name="auth">auth</a>
hook(type => "auth", id => "foo", call => \&auth);
if the name is set to the name of a user who is not registered,
a basic registration of the user will be automatically performed.
-### sessioncgi
+Auth plugins can use the loginselector helper plugin to let the user
+select which authentication method to use.
+
+### <a name="sessioncgi">sessioncgi</a>
hook(type => "sessioncgi", id => "foo", call => \&sessioncgi);
object and a session object. To check if the user is in fact signed in, you
can check if the session object has a "name" parameter set.
-### canedit
+### <a name="canedit">canedit</a>
hook(type => "canedit", id => "foo", call => \&canedit);
but is passed the named parameters `cgi` (a CGI object), `session` (a
session object), `src`, `srcfile`, `dest` and `destfile`.
-### checkcontent
+### <a name="checkcontent">checkcontent</a>
hook(type => "checkcontent", id => "foo", call => \&checkcontent);
that can be run to perform whatever action is necessary to allow the user
to post the content.
-### editcontent
+### <a name="editcontent">editcontent</a>
hook(type => "editcontent", id => "foo", call => \&editcontent);
It can modify the content as desired, and should return the content.
-### formbuilder
+### <a name="formbuilder">formbuilder</a>
hook(type => "formbuilder_setup", id => "foo", call => \&formbuilder_setup);
hook(type => "formbuilder", id => "foo", call => \&formbuilder);
* `description` is a short description of the option.
* `link` is a link to further information about the option. This can either
be a [[ikiwiki/WikiLink]], or an url.
+* `htmldescription` is displayed instead of the description by websetup.
* `advanced` can be set to true if the option is more suitable for advanced
users.
* `safe` should be false if the option should not be displayed in unsafe
strictly required.
* `section` can optionally specify which section in the config file
the plugin fits in. The convention is to name the sections the
- same as the tags used for [[plugins|plugin]] on this wiki.
+ same as the tags used for [[plugins]] on this wiki.
### genwrapper
hook(type => "genwrapper", id => "foo", call => \&genwrapper);
This hook is used to inject C code (which it returns) into the `main`
-function of the ikiwiki wrapper when it is being generated.
+function of the ikiwiki wrapper when it is being generated.
+
+The code runs before anything else -- in particular it runs before
+the suid wrapper has sanitized its environment.
+
+### disable
+
+ hook(type => "disable", id => "foo", call => \&disable);
+
+This hook is only run when a previously enabled plugin gets disabled
+during ikiwiki setup. Plugins can use this to perform cleanups.
## Exported variables
The `%wikistate` hash can be used by a plugin to store persistant state
that is not bound to any one page. To set a value, use
-`$wikistate{$id}{$key}=$value, where `$value` is anything Storable can
+`$wikistate{$id}{$key}=$value`, where `$value` is anything Storable can
serialize, `$key` is any string you like, and `$id` must be the same as the
"id" parameter passed to `hook()` when registering the plugin, so that the
state can be dropped if the plugin is no longer used.
$links{"foo"} = ["bar", "baz"];
+### `%typedlinks`
+
+The `%typedlinks` hash records links of specific types. Do not modify this
+hash directly; call `add_link()`. The keys are page names, and the values
+are hash references. In each page's hash reference, the keys are link types
+defined by plugins, and the values are hash references with link targets
+as keys, and 1 as a dummy value, something like this:
+
+ $typedlinks{"foo"} = {
+ tag => { short_word => 1, metasyntactic_variable => 1 },
+ next_page => { bar => 1 },
+ };
+
+Ordinary [[WikiLinks|ikiwiki/WikiLink]] appear in `%links`, but not in
+`%typedlinks`.
+
### `%pagesources`
The `%pagesources` has can be used to look up the source filename
of a page. So the key is the page name, and the value is the source
filename. Do not modify this hash.
+Attachments also appear in this hash, with the same key and value.
+
$pagesources{"foo"} = "foo.mdwn";
+ $pagesources{"logo/ikiwiki.png"} = "logo/ikiwiki.png";
+
### `%destsources`
The `%destsources` hash records the name of the source file used to
create each destination file. The key is the output filename (ie,
-"foo/index.html"), and the value is the source filename that it was built
-from (eg, "foo.mdwn"). Note that a single source file may create multiple
+"foo/index.html"), and the value is the name of the page that it was built
+from (eg, "foo"). Note that a single source file may create multiple
destination files. Do not modify this hash directly; call `will_render()`.
-
- $destsources{"foo/index.html"} = "foo.mdwn";
+
+Attachments also appear in this hash, with the same key and value.
+
+ $destsources{"foo/index.html"} = "foo";
+ $destsources{"logo/ikiwiki.png"} = "logo/ikiwiki.png";
## Library functions
### `template($;@)`
-Creates and returns a [[!cpan HTML::Template]] object. The first parameter
-is the name of the file in the template directory. The optional remaining
+Creates and returns a [[!cpan HTML::Template]] object. (In a list context,
+returns the parameters needed to construct the obhect.)
+
+The first parameter is the name of the template file. The optional remaining
parameters are passed to `HTML::Template->new`.
+Normally, the template file is first looked for in the templates/ subdirectory
+of the srcdir. Failing that, it is looked for in the templatedir.
+
+Wiki pages can be used as templates. This should be done only for templates
+which it is safe to let wiki users edit. Enable it by passing a filename
+with no ".tmpl" extension. Template pages are normally looked for in
+the templates/ directory. If the page name starts with "/", a page
+elsewhere in the wiki can be used.
+
+If the template is not found, or contains a syntax error, an error is thrown.
+
+### `template_depends($$;@)`
+
+Use this instead of `template()` if the content of a template is being
+included into a page. This causes the page to depend on the template,
+so it will be updated if the template is modified.
+
+Like `template()`, except the second parameter is the page.
+
### `htmlpage($)`
Passed a page name, returns the base name that will be used for a the html
* `filter` is a reference to a function, that is called and passed a page,
and returns true if the page should be filtered out of the list.
* `sort` specifies a sort order for the list. See
- [[ikiwiki/PageSpec/sorting]] for the avilable sort methods.
+ [[ikiwiki/PageSpec/sorting]] for the avilable sort methods. Note that
+ if a sort method is specified that depends on the
+ page content (such as 'meta(foo)'), the deptype needs to be set to
+ a content dependency.
* `reverse` if true, sorts in reverse.
* `num` if nonzero, specifies the maximum number of matching pages that
will be returned.
If the directory name is not absolute, ikiwiki will assume it is in
the parent directory of the configured underlaydir.
-### `displaytime($;$)`
+### `displaytime($;$$)`
Given a time, formats it for display.
The optional second parameter is a strftime format to use to format the
time.
+If the third parameter is true, this is the publication time of a page.
+(Ie, set the html5 pubdate attribute.)
+
### `gettext`
This is the standard gettext function, although slightly optimised.
This is the standard ngettext function, although slightly optimised.
-### `urlto($$;$)`
+### `urlto($;$$)`
Construct a relative url to the first parameter from the page named by the
second. The first parameter can be either a page name, or some other
destination file, as registered by `will_render`.
-If the third parameter is passed and is true, an absolute url will be
-constructed instead of the default relative url.
+Provide a second parameter whenever possible, since this leads to better
+behaviour for the [[plugins/po]] plugin and `file:///` URLs.
+
+If the second parameter is not specified (or `undef`), the URL will be
+valid from any page on the wiki, or from the CGI; if possible it'll
+be a path starting with `/`, but an absolute URL will be used if
+the wiki and the CGI are on different domains.
+
+If the third parameter is passed and is true, the url will be a fully
+absolute url. This is useful when generating an url to publish elsewhere.
### `newpagefile($$)`
filename of the page. For example, `targetpage("foo", "rss", "feed")`
will yield something like `foo/feed.rss`.
-### `add_link($$)`
+### `add_link($$;$)`
This adds a link to `%links`, ensuring that duplicate links are not
added. Pass it the page that contains the link, and the link text.
+An optional third parameter sets the link type. If not specified,
+it is an ordinary [[ikiwiki/WikiLink]].
+
+### `add_autofile($$$)`
+
+Sometimes you may want to add a file to the `srcdir` as a result of content
+of other pages. For example, [[plugins/tag]] pages can be automatically
+created as needed. This function can be used to do that.
+
+The three parameters are the filename to create (relative to the `srcdir`),
+the name of the plugin, and a callback function. The callback will be
+called if it is appropriate to automatically add the file, and should then
+take care of creating it, and doing anything else it needs to (such as
+checking it into revision control). Note that the callback may not always
+be called. For example, if an automatically added file is deleted by the
+user, ikiwiki will avoid re-adding it again.
+
+This function needs to be called during the scan hook, or earlier in the
+build process, in order to add the file early enough for it to be built.
+
## Miscellaneous
### Internal use pages
it might return the current revision ID of the file, and use that
information later when merging changes.
-#### `rcs_commit($$$;$$)`
+#### `rcs_commit(@)`
+
+Passed named parameters: `file`, `message`, `token` (from `rcs_prepedit`),
+and `session` (optional).
-Passed a file, message, token (from `rcs_prepedit`), user, and ip address.
Should try to commit the file. Returns `undef` on *success* and a version
of the page with the rcs's conflict markers on failure.
-#### `rcs_commit_staged($$$)`
+#### `rcs_commit_staged(@)`
-Passed a message, user, and ip address. Should commit all staged changes.
-Returns undef on success, and an error message on failure.
+Passed named parameters: `message`, and `session` (optional).
+
+Should commit all staged changes. Returns undef on success, and an
+error message on failure.
Changes can be staged by calls to `rcs_add`, `rcs_remove`, and
`rcs_rename`.
Remove a file. The filename is relative to the root of the srcdir.
Note that this should not commit the removal, it should only prepare for it
-to be committed when `rcs_commit` (or `rcs_commit_staged`) is called. Note
-that the new file may be in a new subdir that is not yet in version
-control; the subdir can be added if so.
+to be committed when `rcs_commit` (or `rcs_commit_staged`) is called.
#### `rcs_rename($$)`
{
rev => # the RCSs id for this commit
- user => # name of user who made the change,
+ user => # user who made the change (may be an openid),
+ nickname => # short name for user (optional; not an openid),
+
committype => # either "web" or the name of the rcs,
when => # time when the change was made,
message => [
],
}
-#### `rcs_diff($)`
+#### `rcs_diff($;$)`
+
+The first parameter is the rev from `rcs_recentchanges`.
+The optional second parameter is how many lines to return (default: all).
-The parameter is the rev from `rcs_recentchanges`.
Should return a list of lines of the diff (including \n) in list
-context, and the whole diff in scalar context.
+context, and a string containing the whole diff in scalar context.
#### `rcs_getctime($)`
This is used to get the page creation time for a file from the RCS, by looking
it up in the history.
+If the RCS cannot determine a ctime for the file, return 0.
+
+#### `rcs_getmtime($)`
+
+This is used to get the page modification time for a file from the RCS, by
+looking it up in the history.
+
It's ok if this is not implemented, and throws an error.
+If the RCS cannot determine a mtime for the file, return 0.
+
#### `rcs_receive()`
This is called when ikiwiki is running as a pre-receive hook (or
It should examine the incoming changes, and do any sanity
checks that are appropriate for the RCS to limit changes to safe file adds,
-removes, and changes. If something bad is found, it should exit
-nonzero, to abort the push. Otherwise, it should return a list of
-files that were changed, in the form:
+removes, and changes. If something bad is found, it should die, to abort
+the push. Otherwise, it should return a list of files that were changed,
+in the form:
{
file => # name of file that was changed
The list will then be checked to make sure that each change is one that
is allowed to be made via the web interface.
+#### `rcs_preprevert($)`
+
+This is called by the revert web interface. It is passed a RCS-specific
+change ID, and should determine what the effects would be of reverting
+that change, and return the same data structure as `rcs_receive`.
+
+Like `rcs_receive`, it should do whatever sanity checks are appropriate
+for the RCS to limit changes to safe changes, and die if a change would
+be unsafe to revert.
+
+#### `rcs_revert($)`
+
+This is called by the revert web interface. It is passed a named
+parameter rev that is the RCS-specific change ID to revert.
+
+It should try to revert the specified rev, and leave the reversion staged
+so `rcs_commit_staged` will complete it. It should return undef on _success_
+and an error message on failure.
+
+This hook and `rcs_preprevert` are optional, if not implemented, no revert
+web interface will be available.
+
+### `rcs_find_changes($)`
+
+Finds changes committed since the passed RCS-specific rev. Returns
+a hash of the files changed, a hash of the files deleted, and the
+current rev.
+
+This hook is optional.
+
+### `rcs_get_current_rev()`
+
+Gets a RCS-specific rev, which can later be passed to `rcs_find_changes`.
+
+This hook is optional.
+
### PageSpec plugins
It's also possible to write plugins that add new functions to
they match; "created_before(foo)" is influenced by the metadata of foo;
while "glob(*)" is not influenced by the contents of any page.
+### Sorting plugins
+
+Similarly, it's possible to write plugins that add new functions as
+[[ikiwiki/pagespec/sorting]] methods. To achieve this, add a function to
+the IkiWiki::SortSpec package named `cmp_foo`, which will be used when sorting
+by `foo` or `foo(...)` is requested.
+
+The names of pages to be compared are in the global variables `$a` and `$b`
+in the IkiWiki::SortSpec package. The function should return the same thing
+as Perl's `cmp` and `<=>` operators: negative if `$a` is less than `$b`,
+positive if `$a` is greater, or zero if they are considered equal. It may
+also raise an error using `error`, for instance if it needs a parameter but
+one isn't provided.
+
+The function will also be passed one or more parameters. The first is
+`undef` if invoked as `foo`, or the parameter `"bar"` if invoked as `foo(bar)`;
+it may also be passed additional, named parameters.
+
### Setup plugins
The ikiwiki setup file is loaded using a pluggable mechanism. If you look