* Fix bug in deletion/move during edit code introduced in 1.44. Need to take

[git.ikiwiki.info.git] / doc / plugins / write.mdwn

diff --git a/doc/plugins/write.mdwn b/doc/plugins/write.mdwn
index 025a242a63c4318888225dd9975e7e680f8194f4..3030d65585d2cf4daea20dc74ee7d7d8348bb430 100644 (file)
--- a/doc/plugins/write.mdwn
+++ b/doc/plugins/write.mdwn
@@ -1,10 +1,37 @@
-ikiwiki [[plugins]] are written in perl. Each plugin is a perl module, in
-the `IkiWiki::Plugin` namespace. The name of the plugin is typically in
-lowercase, such as `IkiWiki::Plugin::inline`. Ikiwiki includes a
-`IkiWiki::Plugin::skeleton` that can be fleshed out to make a useful
-plugin. `IkiWiki::Plugin::pagecount` is another simple example.
+Ikiwiki's plugin interface allows all kinds of useful [[plugins]] to be
+written to extend ikiwiki in many ways. Despite the length of this page,
+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]].

 
 

-# Note
+[[toc levels=2]]
+
+## Types of plugins
+
+Most ikiwiki [[plugins]] are written in perl, like ikiwiki. This gives the
+plugin full access to ikiwiki's internals, and is the most efficient.
+However, plugins can actually be written in any language that supports XML
+RPC. These are called [[external]] plugins.
+
+A plugin written in perl is a perl module, in the `IkiWiki::Plugin`
+namespace. The name of the plugin is typically in lowercase, such as
+`IkiWiki::Plugin::inline`. Ikiwiki includes a `IkiWiki::Plugin::skeleton`
+that can be fleshed out to make a useful plugin.
+`IkiWiki::Plugin::pagecount` is another simple example. All perl plugins
+should `use IkiWiki` to import the ikiwiki plugin interface. It's a good
+idea to include the version number of the plugin interface that your plugin
+expects: `use IkiWiki 2.00`.
+
+An external plugin is an executable program. It can be written in any
+language. Its interface to ikiwiki is via XML RPC, which it reads from
+ikiwiki on its standard input, and writes to ikiwiki on its standard
+output. For more details on writing external plugins, see [[external]].
+
+Despite these two types of plugins having such different interfaces,
+they're the same as far as how they hook into ikiwiki. This document will
+explain how to write both sorts of plugins, albeit with an emphasis on perl
+plugins.
+
+## Considerations

 
 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
 
 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


@@ -15,207 +42,455 @@ 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.
 
 change until ikiwiki rebuilds the page for some other reason, like the page
 being edited.
 

-# Registering plugins
+## Registering plugins

 
 

-Plugins should, when imported, call IkiWiki::hook to hook into ikiwiki's
+Plugins should, when imported, call `hook()` to hook into ikiwiki's

 processing. The function uses named parameters, and use varies depending on
 processing. The function uses named parameters, and use varies depending on

-the type of plugin being registered. Note that a plugin can call the
-function more than once to register multiple hooks. All calls to
-IkiWiki::hook should be passed a "type" parameter, which gives the type of
+the type of hook being registered -- see below. Note that a plugin can call
+the function more than once to register multiple hooks. All calls to
+`hook()` should be passed a "type" parameter, which gives the type of

 hook, a "id" paramter, which should be a unique string for this plugin, and
 hook, a "id" paramter, which should be a unique string for this plugin, and

-a "call" parameter, which is a reference to a function to call for the
-hook.
-
-# Writing a [[PreProcessorDirective]]
-
-This is probably the most common use of a plugin.
-
-        IkiWiki::hook(type => "preprocess", id => "foo", call => \&preprocess);
-
-Replace "foo" with the command name that will be used inside brackers for
-the preprocessor directive.
-
-Each time the directive is processed, the referenced function (`preprocess`
-in the example above) is called, and is passed named parameters. A "page"
-parameter gives the name of the page that embedded the preprocessor
-directive, while a "destpage" parameter gices the name of the page the
-content is going to (different for inlined pages). All parameters included
-in the directive are included as named parameters as well. Whatever the
-function returns goes onto the page in place of the directive.
+a "call" parameter, which tells what function to call for the hook.

 
 

-## Error handing
+An optional "last" parameter, if set to a true value, makes the hook run
+after all other hooks of its type. Useful if the hook depends on some other
+hook being run first.

 
 

-While a plugin can call ikiwiki's error routine for a fatal error, for
-errors that aren't intended to halt the entire wiki build, including bad
-parameters passed to a [[PreProcessorDirective]], etc, it's better to just
-return the error message as the output of the plugin.
+## Types of hooks

 
 

-## Html issues
+In roughly the order they are called.

 
 

-Note that if the [[htmlscrubber]] is enabled, html in
-[[PreProcessorDirective]] output is sanitised, which may limit what your
-plugin can do. Also, the rest of the page content is not in html format at
-preprocessor time. Text output by a preprocessor directive will be passed
-through markdown (or whatever engine is used to htmlize the page) along
-with the rest of the page.
-
-# Other types of hooks
+### getopt

 
 

-Beyond PreProcessorDirectives, Other types of hooks that can be used by
-plugins include:
-
-## getopt
-
-       IkiWiki::hook(type => "getopt", id => "foo", call => \&getopt);
+       hook(type => "getopt", id => "foo", call => \&getopt);

 
 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
 not able to process on its own. The function should process any options it
 can, removing them from @ARGV, and probably recording the configuration
 
 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
 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 %IkiWiki::config. It should take care not to abort if it sees
+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.
 
 an option it cannot process, and should just skip over those options and
 leave them in @ARGV.
 

-## checkconfig
+### checkconfig

 
 

-       IkiWiki::hook(type => "checkconfig", id => "foo", call => \&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
 
 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

-IkiWiki::error if something isn't configured right.
+`error()` if something isn't configured right.
+
+### needsbuild
+
+       hook(type => "needsbuild", id => "foo", call => \&needsbuild);

 
 

-## filter
+This allows a plugin the 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 pages that will be rebuilt, and can modify the array, either
+adding or removing files from it.

 
 

-       IkiWiki::hook(type => "filter", id => "foo", call => \&filter);
+### filter
+
+       hook(type => "filter", id => "foo", call => \&filter);

 
 Runs on the raw source of a page, before anything else touches it, and can
 
 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` and
-`content` and should return the filtered content.
+make arbitrary changes. The function is passed named parameters "page",
+"destpage", and "content". It should return the filtered content.
+
+### preprocess

 
 

-## htmlize
+Adding a [[PreProcessorDirective]] is probably the most common use of a
+plugin.

 
 

-       IkiWiki::hook(type => "htmlize", id => "ext", call => \&filter);
+        hook(type => "preprocess", id => "foo", call => \&preprocess);
+
+Replace "foo" with the command name that will be used inside brackets for
+the preprocessor directive.
+
+Each time the directive is processed, the referenced function (`preprocess`
+in the example above) is called, and is passed named parameters. A "page"
+parameter gives the name of the page that embedded the preprocessor
+directive, while a "destpage" parameter gives the name of the page the
+content is going to (different for inlined pages), and a "preview"
+parameter is set to a true value if the page is being previewed. All
+parameters included in the directive are included as named parameters as
+well. Whatever the function returns goes onto the page in place of the
+directive.
+
+An optional "scan" parameter, if set to a true value, makes the hook be
+called during the preliminary scan that ikiwiki makes of updated pages,
+before begining to render pages. This parameter should be set to true if
+the hook modifies data in `%links`. Note that doing so will make the hook
+be run twice per page build, so avoid doing it for expensive hooks.
+
+Note that if the [[htmlscrubber]] is enabled, html in
+[[PreProcessorDirective]] output is sanitised, which may limit what your
+plugin can do. Also, the rest of the page content is not in html format at
+preprocessor time. Text output by a preprocessor directive will be
+linkified and passed through markdown (or whatever engine is used to htmlize
+the page) along with the rest of the page.
+
+### htmlize
+
+       hook(type => "htmlize", id => "ext", call => \&htmlize);

 
 Runs on the raw source of a page and turns it into html. The id parameter
 specifies the filename extension that a file must have to be htmlized using
 this plugin. This is how you can add support for new and exciting markup
 languages to ikiwiki.
 
 
 Runs on the raw source of a page and turns it into html. The id parameter
 specifies the filename extension that a file must have to be htmlized using
 this plugin. This is how you can add support for new and exciting markup
 languages to ikiwiki.
 

-## pagetemplate
+The function is passed named parameters: "page" and "content" and should
+return the htmlized content.
+
+### pagetemplate

 
 

-       IkiWiki::hook(type => "pagetemplate", id => "foo", call => \&pagetemplate);
+       hook(type => "pagetemplate", id => "foo", call => \&pagetemplate);

 
 

-Each time a page (or part of a blog page, or an rss feed) is rendered, a
-[[template|templates]] is filled out. This hook allows modifying that
-template. The function is passed named parameters. The "page" and
+[[Templates|wikitemplate]] 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

 "destpage" parameters are the same as for a preprocess hook. The "template"
 "destpage" parameters are the same as for a preprocess hook. The "template"

-parameter is a `HTML::Template` object that is the template that will be
-used to generate the page. The function can manipulate that template
-object.
+parameter is a [[cpan HTML::Template]] object that is the template that
+will be used to generate the page. The function can manipulate that
+template object.

 
 

-The most common thing to do is probably to call $template->param() to add
-a new custom parameter to the template. Note that in order to be robust,
-it's a good idea to check whether the template has a variable before trying
-to set it, as setting a variable that's not present is an error.
+The most common thing to do is probably to call `$template->param()` to add
+a new custom parameter to the template.

 
 

-       if ($template->query(name => 'foo')) {
-               $template->param("foo" => "bar");
-       }
+### templatefile

 
 

-## sanitize
+       hook(type => "templatefile", id => "foo", call => \&templatefile);

 
 

-       IkiWiki::hook(type => "sanitize", id => "foo", call => \&sanitize);
+This hook allows plugins to change the [[template|wikitemplate]] 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.
+
+### sanitize
+
+       hook(type => "sanitize", id => "foo", call => \&sanitize);

 
 Use this to implement html sanitization or anything else that needs to
 
 Use this to implement html sanitization or anything else that needs to

-modify the content of a page after it has been fully converted to html.
-The function is passed the page content and should return the sanitized
-content.
+modify the body of a page after it has been fully converted to html.
+
+The function is passed named parameters: "page" and "content", and 
+should return the sanitized content.
+
+### format
+
+       hook(type => "format", id => "foo", call => \&format);
+
+The difference between format and sanitize is that sanitize only acts on
+the page body, while format can modify the entire html page including the
+header and footer inserted by ikiwiki, the html document type, etc.
+
+The function is passed named parameters: "page" and "content", and 
+should return the formatted content.

 
 

-## delete
+### delete

 
 

-       IkiWiki::hook(type => "delete", id => "foo", call => \&dele);
+       hook(type => "delete", id => "foo", call => \&delete);

 
 Each time 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.
 
 
 Each time 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
+### change

 
 

-       IkiWiki::hook(type => "change", id => "foo", call => \&render);
+       hook(type => "change", id => "foo", call => \&render);

 
 Each time 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.
 
 
 Each time 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.
 

-## cgi
+### cgi

 
 

-       IkiWiki::hook(type => "cgi", id => "foo", call => \&cgi);
+       hook(type => "cgi", id => "foo", call => \&cgi);

 
 Use this to hook into ikiwiki's cgi script. Each registered cgi hook is
 called in turn, and passed a CGI object. The hook should examine the
 
 Use this to hook into ikiwiki's cgi script. Each registered cgi hook is
 called in turn, and passed a CGI object. The hook should examine the

-parameters, and if it will handle this CGI request, output a page and
+parameters, and if it will handle this CGI request, output a page (including the http headers) and

 terminate the program.
 
 terminate the program.
 

-## savestate
+### auth
+
+       hook(type => "auth", id => "foo", call => \&auth);
+
+This hook can be used to implement a different authentication method than
+the standard web form. When a user needs to be authenticated, each registered
+auth hook is called in turn, and passed a CGI object and a session object. 
+
+If the hook is able to authenticate the user, it should set the session
+object's "name" parameter to the authenticated user's name. Note that
+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
+
+       hook(type => "sessioncgi", id => "foo", call => \&sessioncgi);
+
+Unlike the cgi hook, which is run as soon as possible, the sessioncgi hook
+is only run once a session object is available. It is passed both a CGI
+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
+
+       hook(type => "canedit", id => "foo", call => \&pagelocked);
+
+This hook can be used to implement arbitrary access methods to control when
+a page can be edited using the web interface (commits from revision control
+bypass it). When a page is edited, each registered canedit hook is called
+in turn, and passed the page name, a CGI object, and a session object.
+
+If edit can proceed, the hook should return "". If the edit is not allowed
+by this hook, the hook should return an error message for the user to see.
+If the hook has no opinion about whether the edit can proceed, return
+`undef`, and the next plugin will be asked to decide.
+
+### formbuilder
+
+       hook(type => "formbuilder_setup", id => "foo", call => \&formbuilder_setup);
+       hook(type => "formbuilder", id => "foo", call => \&formbuilder);
+
+These hooks allow tapping into the parts of ikiwiki that use [[cpan
+CGI::FormBuilder]] to generate web forms. These hooks are passed named
+parameters: `cgi`, `session`, and `form`. These are, respectively, the
+`CGI` object, the user's `CGI::Session`, and a `CGI::FormBuilder`.
+
+Each time a form is set up, the `formbuilder_setup` hook is called.
+Typically the `formbuilder_setup` hook will check the form's title, and if
+it's a form that it needs to modify, will call various methods to
+add/remove/change fields, tweak the validation code for the fields, etc. It
+will not validate or display the form.

 
 

-       IkiWiki::hook(type => "savestate", id => "foo", call => \&savestate);
+Form validation and display can be overridden by the formbuilder hook.
+By default, ikiwiki will do a basic validation and display of the form,
+but if this hook is registered, it will stop that and let the hook take
+over. This hook is passed an additional named parameter: `buttons` is an
+array of the submit buttons for the form.
+
+### savestate
+
+       hook(type => "savestate", id => "foo", call => \&savestate);

 
 This hook is called wheneven ikiwiki normally saves its state, just before
 the state is saved. The function can save other state, modify values before
 they're saved, etc.
 
 
 This hook is called wheneven ikiwiki normally saves its state, just before
 the state is saved. The function can save other state, modify values before
 they're saved, etc.
 

-# Wiki configuration
+## Plugin interface
+
+To import the ikiwiki plugin interface:
+
+       use IkiWiki '1.00';

 
 

-A plugin can access the wiki's configuration via the `%IkiWiki::config`
+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.
+
+### %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
 [[ikiwiki.setup]], which sets the hash content to configure the wiki.
 
 hash. The best way to understand the contents of the hash is to look at
 [[ikiwiki.setup]], which sets the hash content to configure the wiki.
 

-# Wiki data
+### Other variables

 
 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:
 
 
 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:
 

-* `%IkiWiki::links` lists the names of each page
-  that a page links to, in an array reference.
-* `%IkiWiki::pagemtime` contains the last modification time of each page
-* `%IkiWiki::pagectime` contains the creation time of each page
-* `%IkiWiki::renderedfiles` contains the name of the file rendered by a
-  page
-* `%IkiWiki::pagesources` contains the name of the source file for a page.
-* `%IkiWiki::depends` contains a [[GlobList]] that is used to specify other
-  pages that a page depends on. If one of its dependencies is updated, the
-  page will also get rebuilt. 
-  
-  Many plugins will need to add dependencies to this hash; the best way to do
-  it is by using the IkiWiki::add_depends function, which takes as its
-  parameters the page name and a [[GlobList]] of dependencies to add.
-
-# A note on generating html links
+* `%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.
+
+Also, the %IkiWiki::version variable contains the version number for the
+ikiwiki program.
+
+### Library functions
+
+#### `hook(@)`
+
+Hook into ikiwiki's processing. See the discussion of hooks above.
+
+Note that in addition to the named parameters described above, a parameter
+named `no_override` is supported, If it's set to a true value, then this hook
+will not override any existing hook with the same id. This is useful if
+the id can be controled by the user.
+
+#### `debug($)`
+
+Logs a debugging message. These are supressed unless verbose mode is turned
+on.
+
+#### `error($;$)`
+
+Aborts with an error message. If the second parameter is passed, it is a
+function that is called after the error message is printed, to do any final
+cleanup.
+
+Note that while any plugin can use this for a fatal error, plugins should
+try to avoid dying on bad input, as that will halt the entire wiki build
+and make the wiki unusable. So for example, if a [[PreProcessorDirective]]
+is passed bad parameters, it's better to return an error message, which can
+appear on the wiki page, rather than calling error().
+
+#### `template($;@)`
+
+Creates and returns a [[cpan HTML::Template]] object. The first parameter
+is the name of the file in the template directory. The optional remaining
+parameters are passed to `HTML::Template->new`.
+
+#### `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".)
+
+#### `add_depends($$)`
+
+Makes the specified page depend on the specified [[PageSpec]].
+
+#### `pagespec_match($$;@)`
+
+Passed a page name, and [[PageSpec]], returns true if the [[PageSpec]]
+matches the page.
+
+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.
+
+#### `bestlink($$)`
+
+Given a page and the text of a link on the page, determine which
+existing page that link best points to. Prefers pages under a
+subdirectory with the same name as the source page, failing that
+goes down the directory tree to the base looking for matching
+pages, as described in [[SubPage/LinkingRules]].
+
+#### `htmllink($$$;@)`

 
 Many plugins need to generate html links and add them to a page. This is
 
 Many plugins need to generate html links and add them to a page. This is

-done by using the `IkiWiki::htmllink` function. The usual way to call
-htmlllink is:
+done by using the `htmllink` function. The usual way to call
+`htmlllink` is:

 
        htmllink($page, $page, $link)
 
 
        htmllink($page, $page, $link)
 

-Why is $page repeated? Because if a page is inlined inside another, and a
+Why is `$page` repeated? Because if a page is inlined inside another, and a

 link is placed on it, the right way to make that link is actually:
 
        htmllink($page, $destpage, $link)
 
 link is placed on it, the right way to make that link is actually:
 
        htmllink($page, $destpage, $link)
 

-Here $destpage is the inlining page. A destpage parameter is passed to some
-of the hook functions above; the ones that are not passed it are not used
+Here `$destpage` is the inlining page. A `destpage` parameter is passed to
+some of the hook functions above; the ones that are not passed it are not used

 during inlining and don't need to worry about this issue.
 
 during inlining and don't need to worry about this issue.
 

-# RCS plugins
+After the three required parameters, named parameters can be used to
+control some options. These are:
+
+* noimageinline - set to true to avoid turning links into inline html images
+* forcesubpage  - set to force a link to a subpage
+* linktext - set to force the link text to something
+* anchor - set to make the link include an anchor
+* rel - set to add a rel attribute to the link.
+
+#### `readfile($;$)`
+
+Given a filename, reads and returns the entire file.
+
+The optional second parameter, if set to a true value, makes the file be read
+in binary mode.
+
+A failure to read the file will result in it dying with an error.
+
+#### `writefile($$$;$$)`
+
+Given a filename, a directory to put it in, and the file's content,
+writes a file. 
+
+The optional fourth parameter, if set to a true value, makes the file be
+written in binary mode.
+
+The optional fifth parameter can be used to pass a function reference that
+will be called to handle writing to the file. The function will be called
+and passed a file descriptor it should write to, and an error recovery
+function it should call if the writing fails. (You will not normally need to
+use this interface.)
+
+A failure to write the file will result in it dying with an error.
+
+If the destination directory doesn't exist, it will first be created.
+
+#### `will_render($$)`
+
+Given a page name and a destination file name (not including the base
+destination directory), register that the page will result in that file
+being rendered. It's important to call this before writing to any file in
+the destination directory.
+
+#### `pagetype($)`
+
+Given the name of a source file, returns the type of page it is, if it's
+a type that ikiwiki knowns how to htmlize. Otherwise, returns undef.
+
+#### `pagename($)`
+
+Given the name of a source file, returns the name of the wiki page
+that corresponds to that file.
+
+#### `srcfile($)`
+
+Given the name of a source file in the wiki, searches for the file in
+the source directory and the underlay directory, and returns the full
+path to the first file found.
+
+#### `displaytime($)`
+
+Given a time, formats it for display.
+
+#### `gettext`
+
+This is the standard gettext function, although slightly optimised.
+
+#### `urlto($$)`
+
+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
+destination file, as registered by `will_render`.
+
+#### `targetpage($$)`
+
+Passed a page and an extension, returns the filename that page will be
+rendered to.
+
+## RCS plugins

 
 ikiwiki's support for revision control systems also uses pluggable perl
 modules. These are in the `IkiWiki::RCS` namespace, for example
 `IkiWiki::RCS::svn`. 
 
 
 ikiwiki's support for revision control systems also uses pluggable perl
 modules. These are in the `IkiWiki::RCS` namespace, for example
 `IkiWiki::RCS::svn`. 
 

-Each RCS plugin must support all the IkiWiki::rcs\_* functions.
+Each RCS plugin must support all the `IkiWiki::rcs_*` functions.

 See IkiWiki::RCS::Stub for the full list of functions. It's ok if
 See IkiWiki::RCS::Stub for the full list of functions. It's ok if

-rcs\_getctime does nothing except for throwing an error.
+`rcs_getctime` does nothing except for throwing an error.

 
 See [[about_RCS_backends]] for some more info.
 
 See [[about_RCS_backends]] for some more info.

+
+## PageSpec plugins
+
+It's also possible to write plugins that add new functions to
+[[PageSpecs|PageSpec]]. Such a plugin should add a function to the
+IkiWiki::PageSpec package, that is named `match_foo`, where "foo()" is
+how it will be accessed in a [[PageSpec]]. The function will be passed
+two parameters: The name of the page being matched, and the thing to match
+against. It may also be passed additional, named parameters. It should return
+a IkiWiki::SuccessReason object if the match succeeds, or an
+IkiWiki::FailReason object if the match fails.