Categories? (also known as "tags") are a way to organize and find related pages. Categories are implemented by default in PmWiki, and in most wikis they don't require any special code or markup, they're just a useful convention. The idea is that every page that falls into a particular subject area should have a link to a shared page containing links to other pages on that subject. These pages are created in the Category group, and thus these subject areas are called "categories".

Using categories

Getting categories to work requires a single step: adding links to each category. A category named Subject is created by adding a link to Category.Subject on any page. When you add the link to a page, the page can be described as being in the category "Subject".

There is a special markup for creating these links which makes categories work more smoothly: [[!Subject]] will create a link to Category.Subject. So [[!Subject]] is a kind of shortcut to the page Subject in the category group.

A Category.GroupFooter file is included in the PmWiki release that contains the line (:pagelist link={*$FullName} list=normal:) so that whenever a category page is displayed, it will show a list of links to pages that reference that page in the category group. Like any other page in wikilib.d you can modify this page and it will not get overwritten by another release.

It is worth noting that rather than using Category.GroupFooter, the pagelist directive can be added to Category.GroupHeader to similar effect; it just depends on whether you'd prefer to have the list of pages appear before or after any text that you add to the individual category pages (which can be edited just like normal pages).

Because we use the normal PageList link= markup, you can use it not only in the category group. If you want to show all pages belonging to the category Subject you can use on any wiki page (:pagelist link=Category.Subject list=normal:).

Similarly, there's no requirement that a "category page" has to be in the Category group -- any page can define a "category" of pages that link to it.

An administrator can override the default category group name of "Category" by setting the $CategoryGroup variable in config.php to another group name. (Normally a change such as this should be done during initial setup on a new wiki; changing this on a wiki with existing content can cause problems with pagelists unless each page with a category is re-saved.)

A page author can also link to a category list without adding the linking page to the category by using [[ {Category.Subject$PageUrl} | Subject ]]. This will create a link that looks like [[!Subject]] without adding the linking page to the category listing.

Recap

So, by adding the link [[!Subject]] to a page, a link to that page will automatically appear on the page Category.Subject, as long as Category.GroupFooter has been tweaked appropriately. Thus, you can create a page that automatically creates an alphabetized list of all movies discussed on your wiki by creating links to [[!Movies]] on each film's page; the resulting automatic list would be on the page Category.Movies .

authors (advanced)

Category nesting

Categories have the potential for even greater usefulness because Category.* pages can themselves be placed into categories! To follow an excellent example from John Rankin, let's suppose we have the following film pages in the categories listed to the right:

Film.ShaunOfTheDead   [[!Horror]] [[!Comedy]] [[!2003]]
Film.InMyFathersDen   [[!Drama]] [[!2004]]
Film.TheCorporation   [[!Documentary]] [[!2003]]

Now then, we can create Category.Horror, Category.Comedy, Category.Drama, and Category.Documentary, and in each one of those pages we put [[!Genre]]. In Category.2003 and Category.2004, we put [[!Year]].

So, what happens when we display Category.Genre ? We see links to "Comedy", "Drama", "Documentary", and "Horror", because they're in the Genre category. When we click on one of those links, we see all of the films listed in one of those categories. Similarly, if we click on Category.Year, we see links to "2003" and "2004", each of which in turn displays the list of films for that year.

Finally, in Category.Genre and Category.Year we can put [[!Category]], which makes them "top-level" categories reachable from the Category.Category page. Voila, we now have an instant "hierarchy":

Category.Category
    Category.Genre
        Category.Comedy
            Film.ShaunOfTheDead
        Category.Drama
            Film.InMyFathersDen
        Category.Documentary
            Film.TheCorporation
        Category.Horror
            Film.ShaunOfTheDead
    Category.Year
        Category.2003
            Film.ShaunOfTheDead
            Film.TheCorporation
        Category.2004
            Film.InMyFathersDen

Note however that this isn't a "strict" hierarchy--i.e., any page or category can appear simultaneously in multiple categories. For example, Category.Documentary could be a member of both the Genre and top-level category listings.

Each category page can have content text before the generated list, e.g., to give a generic description of things in the category. (Or it can be empty, which works fine.) It can also contain associations to related categories ("see also" references). For example, in a tourism wiki, the ''bed and breakfast" category might contain a see-also reference to the "self-catering" category.

administrators (intermediate)

The guts of the category markup

As mentioned, all of the necessary markup features for Categories are enabled by default in current releases of PmWiki 2.0, but here's how they work for those who are interested. The use of the Category group as the repository for all categories is determined by setting the $CategoryGroup variable, and the special [[!Subject]] markup is activated by a call to the Markup() function:

SDV($CategoryGroup,'Category');
Markup('[[!','<links','/\[\[!([^\|\]] ?)\]\]/',
  "<span class='category'>[[$CategoryGroup/$1]]</span>");

Coming up with good category schemes

The hard part about using categories is choosing a good vocabulary. Site content managers may wish to follow the Guidelines for the establishment and development of monolingual thesauri (ISO 2788-1986) and the Guidelines for the establishment and development of multilingual thesauri (ISO 5964-1985). Questions to think about include:

• whether a scheme already exists and can be reused
• number of levels in a multilevel scheme (not too shallow, not too deep -- e.g. 3)
• number of categories per page (not too many, not too few -- e.g. 3)
• consistent use of singular ([[Mercury]] is a [[!planet]]) or plural ([[Mercury]] is in the [[!planets]] category)
• disambiguation and use of phrases ([[!musical instruments]] and [[!medical instruments]]) or Cookbook:Subgroup Markup ([[!Instruments*Musical]] and [[!Instruments*Medical]])

Or you can just let people use whatever category terms they find meaningful. A vocabulary (or "folksonomy") will emerge over time.

Showing a list of categories

To show a list of categories we can use a pagelist for the pages in the category group. For instance the following will list pages in the Category group, put it on page Category.Category? for convenience, or on any other page:

But there is a problem: Just adding a category markup to a page will not create a corresponding category page, even though following the link will show the page with a list of pages linking to it!
To have category pages automatically created in group 'Category' add the following to config.php:

Change 'Category' to the name of your category group. You can also add more definitions for more category groups, useful if you use a recipe like Cookbook:Tagger which allows multiple category groups.

Linking = Categorizing

Note that categorizing a page (using the [[!category markup]]) cannot be distinguished from referring or linking to a category (using the normal [[link markup]]), i.e. pages referring to a category become part of that category. This is the subject of a long outstanding feature request that seems to be hard to implement without breaking other functionality. You can link to a category without categorizing the page by using an external link, such as: [[{Category.MyCategory$PageUrl}|MyCategory]]. Since the link is external, all pages (not just the category page) will ignore it when listing backlinks.

See also EditVariables#AutoCreate