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Question: is there a way to list all existing categories in pmwiki.org?
-malexim

You get this for free: http://www.pmwiki.org/wiki/Category/RecentChanges

though this is better organized (while it takes longer to process) http://www.pmwiki.org/wiki/Category/Category

-Radu?

Categories are a way to organise and find related pages

Categories are already implemented in PmWiki 2, and in most wikis they don't require any special code or markup, they're just a useful convention in Wiki pages. The idea is that every page that is in a given category should have a link to a shared category page.

There is also a special markup as a shortcut to category-entries: [[!category]] will create a link to Category/category

Since PmWiki has WikiGroups, I'll assume that category links go to the Category group. However, bear in mind that you can use any group or page naming scheme for this -- it's just a convention and doesn't require special programming support. TODO: special wiki-var for categories

A Category group holds all the category names

The key aspects of building a category are:

  1. On every page that belongs to a category "XYZ", create a link on that page to [[Category/XYZ]] or [[Category.XYZ]].
  2. Then, to see a list of all of the pages that belong to category XYZ, simply do a search to list all pages that have links to the Category.XYZ page.
note: all pages that contain the text "Category.XYZ" are displayed. A future version will allow (:pagelist linkto=Category.XYZ:) or (:pagelist backlink=Category.XYZ:) (I'm still working on the syntax), which will strictly search for links and not text. TODO: check implementation and change this docs

PmWiki 2 makes this second step incredibly simple, since you can get easily generate a list of all pages with links to Category.XYZ by doing 

    (:pagelist Category.XYZ:)

So, that's really all there is to categories. But wait, there's more!

PmWiki makes it easy to find categorised pages

As John Rankin pointed out in his excellent post, the above convention, combined with the fact that all of the category pages are in a single group, means that we can do much more. If we create a page called Category.GroupFooter, and put the following markup in that page 

    (:pagelist Category.{$Name}:)

then every page in the Category group, even empty/non-existent ones, will display a list of pages that are in the category named by the page.

For example, the Category.XYZ page display will automatically include Category.GroupFooter, and {$Name} in the GroupFooter will be replaced by the name of the current page (XYZ) to give us the (:pagelist Category.XYZ:) directive described above! So, we don't even need to write a bunch of (:pagelist Category.<Name>:) directives to create the page listings -- just creating a generic one in Category.GroupHeader or Category.GroupFooter will do it for all categories we might create.

It finds any instances of "Category.XYZ" in the text, and it also finds any page that has an outgoing link to Category.XYZ (regardless of whether that link was specified as [[Category.XYZ]], [[Category/XYZ]], [[Category.X(Y)Z]], or even [[x y z]]).
Also, pages such as RecentChanges show up in the category by default and probably should be filtered with list=normal:
(:pagelist Category.{$Name} list=normal:)
(it's not possible to add the non-existent pages as a link in a page, cause this link will automatically turned into an action=edit-link so you can add the link only in the URL-field of the browser)

So, what do we get? Every page belonging to Category.XYZ has a link to Category.XYZ, and following that link automatically displays a list of all pages in the XYZ category.

But that's not all...!

Category pages can themselves be categorised

The Category.* pages can themselves be placed into categories! So, to follow John's excellent example, let's suppose we have the following film pages in the categories listed to the right: 

 Film.SeanOfTheDead    [[!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.SeanOfTheDead
           Category.Drama
               Film.InMyFathersDen
           Category.Documentary
               Film.TheCorporation
           Category.Horror
               Film.SeanOfTheDead
       Category.Year
           Category.2003
               Film.SeanOfTheDead
               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.

A category markup makes it even easier

And all of this works "out of the box" without any modifications to PmWiki 2.0! John goes a step further and proposes that we create a special markup for "Category", so that one can write things like [[!Comedy]] and [[!Genre]] instead of the longer [[Category/Comedy]] and [[Category/Genre]], but at the moment I'm leaving this as a (one-line!) local customization until it's widely adopted or we have a good standard markup for it. TODO: This is implemented as a test since v2develop25(?)

When we choose a good markup, this is all that's needed: (uses [[!Comedy]] for illustration) 
  
    SDV($CategoryGroup,'Category');
    Markup('[[!','<links','/\[\[!([^\|\]]+?)\]\]/',
      "<span class='category'>[[$CategoryGroup/$1]]</span>");

Hope this helps. The advantage of a separate category markup is that authors can assign pages to categories independently and explicitly. It lets authors distinguish between a category reference and a reference to a page that happens to be a category page.

On an active wiki, a folksonomy emerges

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 -- eg 3)
  • number of categories per page (not too many, not too few -- eg 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 Subpage Markup? ([[!Instruments*Musical]] and [[!Instruments*Medical]])

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

(quotes of postings in pmwiki-user-maillist)

TODO: add the text of John Rankins post too ?

samples (created in groups SampleCategory and SampleFilm):

Subject: trial use of Category system of indexes

At the SolaRoof wiki I am looking to explore more dynamic navigation aids and one of the reasons for the 3 column format is to use a dynamic index in the right column in the SolaRoof group, which is a knowledge base group.

Other groups in the SolaRoof wiki are for Teams or Personal space (like SolaRoofGuy personal wiki(approve sites)) and they use the Calendar(approve sites) function on the right column to aid the navigation of a WikiBlog(approve sites) that belongs to that group.

However, the knowledge-base type of group like SolaRoof is strictly for working on content and organizing information for accessible DIY know-how. There is no need for a WikiBlog(approve sites) function in the community wide collaboration space (instead, we do have a community Forum(approve sites)) and therefore the right column is currently empty.

What I would like to have is a navigation aid in that right column that will help a user to go from the current page to the next page, which he can see in the right sidebar, which is a dynamic listing of pages in the same catagory and links to other related catagories. Now that I have read this page I appreciate that what I was developing at GroupName(approve sites) (this is our How To Wiki(approve sites)) where I have talked about Main Index and Sub Index pages should instead be talking about Category and Sub Catagory pages. These links can go on the right sidebar (3rd column) and the left group sidebar will provide the conventional navigation links to useful internal and external links.

Additonally, I would like to bring in further navigation aid that I believe will work well with the wiki groups structure and the Catagory organization since these provide information that is user (human rather than machine) designated that build relationships between pages and sub categories and Categories. It is the relationships that are mapped and structured that make the information so useful. Similar PageNames(approve sites) used in various WikiGroups are not just repetitious, they give depth of knowledge by presenting similar information but in a different context. So the ability to have groups, each with its own name space, will give richness and maintain diversity, avoiding "GroupThink"(approve sites).

The mapping of the context and relationship of any existing page to the other pages in that group and other groups and across categories would make the wiki much more effective and useful. You are not then "surfing blind" but can see the links and threads of connected information. I have recently received a very positive response to working on a "concept map" tool that could be a visual interface for the display of the relationship of Groups and Category and Sub Category structure of the SolaRoof wiki. This technology is available from CMapTools(approve sites). I would like to explore using this Concept Map interface as further naviagion aid.

In some ways I think that it is not possible to be over engaged in the devlopment of the Wiki Navigation interface - because as the wiki world grows it will be a significant advantage that it grows "intelligently" - that it is a manifestation of "Group Intelligence" - so we would not "surf blind" as we do on the internet - rather we would navigate a structured "Name Space" and have a good idea where we are going as we explore the wiki world.

This is long rambling edit but I hope that it fits here and that I will find some feedback to support the SolaRoof implementation to make it the best of the best. - SolaRoofGuy?

Category: Documentation ToDo

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