How WOM uses URLs

There are five types of URL:

Class URL

These contain the name of a class, and ask the server to create a new instance of it and send back its HTML.

Instance URL

These contain an instance identifier, and ask the server to locate the corresponding instance and send back its HTML.

Instance selection URL

These are like instance URLs, but also ask the server to make the specified instance the current page of a book (see Section 2.16).

State-change URL

These also contain an instance identifier. They ask for its HTML to be sent back, but only after the instance's state has been updated with data from the user. In general, a state-change URL will also point at a form or some other trigger.

Null URLs

These are mainly useful with client-side programs in, for example, JavaScript. They are used when the client program needs to send some information to the server but does not want a reply. The JavaScript code in Tow uses them to inform the server that the user has selected a new page, even if the client then withdraws it from the cache rather than asking the server for it.

Most of the time, the only type of URL the Web-page author will write explicitly is a class URL. Our Introduction to Tow page, for example, contains some explanatory material about the application and how to use it, which is followed at the end of the page by a link saying Go to Tow. In HTML, this is coded as

<A HREF="http://www6.ifs.org.uk/New$Tow">Go to Tow</A>

This represents a gate, if you like, from the non-WOM world to that of WOM, and since written outside WOM, the author has to know how it is represented. However, inside WOM source code, the author should always refer to parts of the application by WOM elements such as <InstanceLink>.

Each of these emits itself as a string which includes its type (Instance$, StateChange$ etc). It also contains the identifier for the session, so that the server can find the correct instance when it receives the URL back. It may also contain other information. For example, all URLs can contain instance paths to request HTML from any instance reachable by a path from the root.