it's not really hard. This page is a complete reference to everything a
plugin might want to do. There is also a quick [[tutorial]].
+[[!template id="note" text="""
+Ikiwiki is a compiler
+
+One thing to keep in mind when writing a plugin is that ikiwiki is a wiki
+*compiler*. So plugins influence pages when they are built, not when they
+are loaded. A plugin that inserts the current time into a page, for
+example, will insert the build time.
+
+Also, as a compiler, ikiwiki avoids rebuilding pages unless they have
+changed, so a plugin that prints some random or changing thing on a page
+will generate a static page that won't change until ikiwiki rebuilds the
+page for some other reason, like the page being edited.
+
+The [[tutorial]] has some other examples of ways that ikiwiki being a
+compiler may trip up the unwary.
+"""]]
+
[[!toc levels=2]]
+## Highlevel view of ikiwiki
+
+Ikiwiki mostly has two modes of operation. It can either be running
+as a compiler, building or updating a wiki; or as a cgi program, providing
+user interface for editing pages, etc. Almost everything ikiwiki does
+is accomplished by calling various hooks provided by plugins.
+
+### 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`, `indexhtml`,
+`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.
+
+### cgi
+
+The flow between hooks when ikiwiki is run as a cgi is best illustrated by
+an example.
+
+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.
+
+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.
+* 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.
+
+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.
+
## Types of plugins
Most ikiwiki [[plugins]] are written in perl, like ikiwiki. This gives the
explain how to write both sorts of plugins, albeit with an emphasis on perl
plugins.
-## Considerations
+## Plugin interface
-One thing to keep in mind when writing a plugin is that ikiwiki is a wiki
-*compiler*. So plugins influence pages when they are built, not when they
-are loaded. A plugin that inserts the current time into a page, for
-example, will insert the build time. Also, as a compiler, ikiwiki avoids
-rebuilding pages unless they have changed, so a plugin that prints some
-random or changing thing on a page will generate a static page that won't
-change until ikiwiki rebuilds the page for some other reason, like the page
-being edited.
+To import the ikiwiki plugin interface:
+
+ use IkiWiki '3.00';
+
+This will import several variables and functions into your plugin's
+namespace. These variables and functions are the ones most plugins need,
+and a special effort will be made to avoid changing them in incompatible
+ways, and to document any changes that have to be made in the future.
+
+Note that IkiWiki also provides other variables and functions that are not
+exported by default. No guarantee is made about these in the future, so if
+it's not exported, the wise choice is to not use it.
## Registering plugins
This allows for plugins to perform their own processing of command-line
options and so add options to the ikiwiki command line. It's called during
-command line processing, with @ARGV full of any options that ikiwiki was
+command line processing, with `@ARGV` full of any options that ikiwiki was
not able to process on its own. The function should process any options it
-can, removing them from @ARGV, and probably recording the configuration
-settings in %config. It should take care not to abort if it sees
+can, removing them from `@ARGV`, and probably recording the configuration
+settings in `%config`. It should take care not to abort if it sees
an option it cannot process, and should just skip over those options and
-leave them in @ARGV.
+leave them in `@ARGV`.
### checkconfig
hook(type => "checkconfig", id => "foo", call => \&checkconfig);
This is useful if the plugin needs to check for or modify ikiwiki's
-configuration. It's called early in the startup process. The
-function is passed no values. It's ok for the function to call
+configuration. It's called early in the startup process. `%config`
+is populated at this point, but other state has not yet been loaded.
+The function is passed no values. It's ok for the function to call
`error()` if something isn't configured right.
### refresh
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
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.
+### indexhtml
+
+ 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", "destpage", and "content".
+Its return value is ignored.
+
### pagetemplate
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
The function is passed named parameters: "page", "destpage", and "content",
and should return the sanitized content.
-### postscan
-
- hook(type => "postscan", id => "foo", call => \&postscan);
-
-This hook is called once the full page body is available (but before the
-format hook). 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.
-
### format
hook(type => "format", id => "foo", call => \&format);
describing the option. There can also be an item named "plugin", which
describes the plugin as a whole. For example:
- return
+ return
+ plugin => {
+ description => "description of this plugin",
+ safe => 1,
+ rebuild => 1,
+ section => "misc",
+ },
option_foo => {
type => "boolean",
description => "enable foo?",
safe => 1,
rebuild => 0,
},
- plugin => {
- description => "description of this plugin",
- safe => 1,
- rebuild => 1,
- },
* `type` can be "boolean", "string", "integer", "pagespec",
or "internal" (used for values that are not user-visible). The type is
the plugin) will require a wiki rebuild, false if no rebuild is needed,
and undef if a rebuild could be needed in some circumstances, but is not
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.
### 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.
-## Plugin interface
+The code runs before anything else -- in particular it runs before
+the suid wrapper has sanitized its environment.
-To import the ikiwiki plugin interface:
+### disable
- use IkiWiki '3.00';
+ hook(type => "disable", id => "foo", call => \&disable);
-This will import several variables and functions into your plugin's
-namespace. These variables and functions are the ones most plugins need,
-and a special effort will be made to avoid changing them in incompatible
-ways, and to document any changes that have to be made in the future.
+This hook is only run when a previously enabled plugin gets disabled
+during ikiwiki setup. Plugins can use this to perform cleanups.
-Note that IkiWiki also provides other variables and functions that are not
-exported by default. No guarantee is made about these in the future, so if
-it's not exported, the wise choice is to not use it.
+## Exported variables
-### %config
+Several variables are exported to your plugin when you `use IkiWiki;`
+
+### `%config`
A plugin can access the wiki's configuration via the `%config`
hash. The best way to understand the contents of the hash is to look at
your ikiwiki setup file, which sets the hash content to configure the wiki.
-### %pagestate
+### `%pagestate`
The `%pagestate` hash can be used by plugins to save state that they will need
next time ikiwiki is run. The hash holds per-page state, so to set a value,
Note that page state does not persist across wiki rebuilds, only across
wiki updates.
-### %wikistate
+### `%wikistate`
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
"id" parameter passed to `hook()` when registering the plugin, so that the
state can be dropped if the plugin is no longer used.
-### Other variables
+### `%links`
+
+The `%links` hash can be used to look up the names of each page that
+a page links to. The name of the page is the key; the value is an array
+reference. Do not modify this hash directly; call `add_link()`.
+
+ $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`
-If your plugin needs to access data about other pages in the wiki. It can
-use the following hashes, using a page name as the key:
+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.
-* `%links` lists the names of each page that a page links to, in an array
- reference.
-* `%destsources` contains the name of the source file used to create each
- destination file.
-* `%pagesources` contains the name of the source file for each page.
+ $pagesources{"foo"} = "foo.mdwn";
-Also, the `%IkiWiki::version` variable contains the version number for the
-ikiwiki program.
+### `%destsources`
-### Library functions
+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
+destination files. Do not modify this hash directly; call `will_render()`.
+
+ $destsources{"foo/index.html"} = "foo.mdwn";
+
+## Library functions
-#### `hook(@)`
+Several functions are exported to your plugin when you `use IkiWiki;`
+
+### `hook(@)`
Hook into ikiwiki's processing. See the discussion of hooks above.
will not override any existing hook with the same id. This is useful if
the id can be controled by the user.
-#### `debug($)`
+### `debug($)`
Logs a debugging message. These are supressed unless verbose mode is turned
on.
-#### `error($;$)`
+### `error($;$)`
Aborts with an error message. If the second parameter is passed, it is a
function that is called after the error message is printed, to do any final
dying on bad input when building a page, as that will halt
the entire wiki build and make the wiki unusable.
-#### `template($;@)`
+### `template($;@)`
+
+Creates and returns a [[!cpan HTML::Template]] object. (In a list context,
+returns the parameters needed to construct the obhect.)
-Creates and returns a [[!cpan HTML::Template]] object. The first parameter
-is the name of the file in the template directory. The optional remaining
+The first parameter is the name of the template file. The optional remaining
parameters are passed to `HTML::Template->new`.
-#### `htmlpage($)`
+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.
+
+### `template_depends($$;@)`
+
+Use this instead of `template()` if the content of a template is being
+included into a page. This causes the page to depend on the template,
+so it will be updated if the template is modified.
+
+Like `template()`, except the second parameter is the page.
+
+### `htmlpage($)`
Passed a page name, returns the base name that will be used for a the html
page created from it. (Ie, it appends ".html".)
Use this when constructing the filename of a html file. Use `urlto` when
generating a link to a page.
-#### `add_depends($$;@)`
+### `pagespec_match_list($$;@)`
+
+Passed a page name, and [[ikiwiki/PageSpec]], returns a list of pages
+in the wiki that match the [[ikiwiki/PageSpec]].
+
+The page will automatically be made to depend on the specified
+[[ikiwiki/PageSpec]], so `add_depends` does not need to be called. This
+is often significantly more efficient than calling `add_depends` and
+`pagespec_match` in a loop. You should use this anytime a plugin
+needs to match a set of pages and do something based on that list.
+
+Unlike pagespec_match, this may throw an error if there is an error in
+the pagespec.
+
+Additional named parameters can be specified:
+
+* `deptype` optionally specifies the type of dependency to add. Use the
+ `deptype` function to generate a dependency type.
+* `filter` is a reference to a function, that is called and passed a page,
+ and returns true if the page should be filtered out of the list.
+* `sort` specifies a sort order for the list. See
+ [[ikiwiki/PageSpec/sorting]] for the avilable sort methods. Note that
+ if a sort method is specified that depends on the
+ page content (such as 'meta(foo)'), the deptype needs to be set to
+ a content dependency.
+* `reverse` if true, sorts in reverse.
+* `num` if nonzero, specifies the maximum number of matching pages that
+ will be returned.
+* `list` makes it only match amoung the specified list of pages.
+ Default is to match amoung all pages in the wiki.
+
+Any other named parameters are passed on to `pagespec_match`, to further
+limit the match.
+
+### `add_depends($$;$)`
Makes the specified page depend on the specified [[ikiwiki/PageSpec]].
By default, dependencies are full content dependencies, meaning that the
page will be updated whenever anything matching the PageSpec is modified.
-This default can be overridden by additional named parameters, which can be
-used to indicate weaker types of dependencies:
+This can be overridden by passing a `deptype` value as the third parameter.
-* `presence` if set to true, only the presence of a matching page triggers
- the dependency.
-* `links` if set to true, any change to links on a matching page
- triggers the dependency. This includes when a link is added, removed,
- or changes what it points to due to other changes. It does not include
- the addition or removal of a duplicate link.
-
-#### `pagespec_match($$;@)`
+### `pagespec_match($$;@)`
Passed a page name, and [[ikiwiki/PageSpec]], returns a true value if the
[[ikiwiki/PageSpec]] matches the page.
PageSpec should match against. If not passed, relative PageSpecs will match
relative to the top of the wiki.
-#### `pagespec_match_list($$;@)`
+### `deptype(@)`
-Passed a reference to a list of page names, and [[ikiwiki/PageSpec]],
-returns the set of pages that match the [[ikiwiki/PageSpec]].
+Use this function to generate ikiwiki's internal representation of a
+dependency type from one or more of these keywords:
-Additional named parameters can be passed, to further limit the match.
-The most often used is "location", which specifies the location the
-PageSpec should match against. If not passed, relative PageSpecs will match
-relative to the top of the wiki.
+* `content` is the default. Any change to the content
+ of a page triggers the dependency.
+* `presence` is only triggered by a change to the presence
+ of a page.
+* `links` is only triggered by a change to the links of a page.
+ This includes when a link is added, removed, or changes what
+ it points to due to other changes. It does not include the
+ addition or removal of a duplicate link.
-Unlike pagespec_match, this may throw an error if there is an error in
-the pagespec.
+If multiple types are specified, they are combined.
-#### `bestlink($$)`
+### `bestlink($$)`
Given a page and the text of a link on the page, determine which
existing page that link best points to. Prefers pages under a
goes down the directory tree to the base looking for matching
pages, as described in [[ikiwiki/SubPage/LinkingRules]].
-#### `htmllink($$$;@)`
+### `htmllink($$$;@)`
Many plugins need to generate html links and add them to a page. This is
done by using the `htmllink` function. The usual way to call
* anchor - set to make the link include an anchor
* rel - set to add a rel attribute to the link
* class - set to add a css class to the link
+* title - set to add a title attribute to the link
-#### `readfile($;$)`
+### `readfile($;$)`
Given a filename, reads and returns the entire file.
A failure to read the file will result in it dying with an error.
-#### `writefile($$$;$$)`
+### `writefile($$$;$$)`
Given a filename, a directory to put it in, and the file's content,
writes a file.
the srcdir or destdir, and any subdirectories of this are included in the
filename parameter.
-#### `will_render($$)`
+### `will_render($$)`
Given a page name and a destination file name (not including the base
destination directory), register that the page will result in that file
the page that rendered them goes away or is changed to no longer render
them. will_render also does a few important security checks.
-#### `pagetype($)`
+### `pagetype($)`
Given the name of a source file, returns the type of page it is, if it's
a type that ikiwiki knowns how to htmlize. Otherwise, returns undef.
-#### `pagename($)`
+### `pagename($)`
Given the name of a source file, returns the name of the wiki page
that corresponds to that file.
-#### `pagetitle($)`
+### `pagetitle($)`
Give the name of a wiki page, returns a version suitable to be displayed as
the page's title. This is accomplished by de-escaping escaped characters in
the page name. "_" is replaced with a space, and '__NN__' is replaced by
the UTF character with code NN.
-#### `titlepage($)`
+### `titlepage($)`
This performs the inverse of `pagetitle`, ie, it converts a page title into
a wiki page name.
-#### `linkpage($)`
+### `linkpage($)`
This converts text that could have been entered by the user as a
[[ikiwiki/WikiLink]] into a wiki page name.
-#### `srcfile($;$)`
+### `srcfile($;$)`
Given the name of a source file in the wiki, searches for the file in
the source directory and the underlay directories (most recently added
be found. The second parameter can be set to a true value to make it return
undef instead.
-#### `add_underlay($)`
+### `add_underlay($)`
Adds a directory to the set of underlay directories that ikiwiki will
search for files.
If the directory name is not absolute, ikiwiki will assume it is in
the parent directory of the configured underlaydir.
-#### `displaytime($;$)`
+### `displaytime($;$$)`
Given a time, formats it for display.
The optional second parameter is a strftime format to use to format the
time.
-#### `gettext`
+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.
-#### `urlto($$;$)`
+### `ngettext`
+
+This is the standard ngettext function, although slightly optimised.
+
+### `urlto($$;$)`
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
If the third parameter is passed and is true, an absolute url will be
constructed instead of the default relative url.
-#### `newpagefile($$)`
+### `newpagefile($$)`
This can be called when creating a new page, to determine what filename
to save the page to. It's passed a page name, and its type, and returns
the name of the file to create, relative to the srcdir.
-#### `targetpage($$;$)`
+### `targetpage($$;$)`
Passed a page and an extension, returns the filename that page will be
rendered to.
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`.
{
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 => [
It's ok if this is not implemented, and throws an error.
+If the RCS cannot determine a ctime for the file, return 0.
+
+#### `rcs_getmtime($)`
+
+This is used to get the page modification time for a file from the RCS, by
+looking it up in the history.
+
+It's ok if this is not implemented, and throws an error.
+
+If the RCS cannot determine a mtime for the file, return 0.
+
#### `rcs_receive()`
This is called when ikiwiki is running as a pre-receive hook (or
attempted at all, for any page, it can instead return an
IkiWiki::ErrorReason object explaining why.
+When constructing these objects, you should also include information about
+of any pages whose contents or other metadata influenced the result of the
+match. Do this by passing a list of pages, followed by `deptype` values.
+
+For example, "backlink(foo)" is influenced by the contents of page foo;
+"link(foo)" and "title(bar)" are influenced by the contents of any page
+they match; "created_before(foo)" is influenced by the metadata of foo;
+while "glob(*)" is not influenced by the contents of any page.
+
+### Sorting plugins
+
+Similarly, it's possible to write plugins that add new functions as
+[[ikiwiki/pagespec/sorting]] methods. To achieve this, add a function to
+the IkiWiki::SortSpec package named `cmp_foo`, which will be used when sorting
+by `foo` or `foo(...)` is requested.
+
+The names of pages to be compared are in the global variables `$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