Built-in classes and the run-time library

For author-defined classes, the emit method is generated by the compiler, as shown above. For built-in classes like TextField, it is defined by the implementor (me). TextField, for example, is specified to emit itself as an HTML data-entry field with size and value defined by its attributes, and with a name determined by its id. Typically, its HTML would look something like this:

<INPUT TYPE=TEXT SIZE=10 NAME="c137.Page.t" VALUE="0">

The WOM language is conceptually very sparse, most of the interesting behaviour lying in these built-ins. These include:

• TextField: as we just saw, this is displayed as a normal HTML data-entry field, for use within forms. It has a value attribute, which can be interrogated by its getValue method. This will contain the data last entered by the user.

• IntegerField, RealField, BooleanField: these are similar to TextField, but check the syntax of the user's data. If it is incorrect, they redisplay themselves with an error message by their side, and also set an error flag attribute.

• TextMenu: this displays a menu and transmits the user's selection to the server. The menu can hold a translation table, so that the user can select a string, but the server can translate this into a number before it is passed to a program. One can add and delete menu options dynamically.

• WOMForm: This name is used to avoid shielding the HTML FORM element. It denotes the built-in class which contains fields and other input elements and operates on these via an <OnSubmit> when submitted.

• ActionLink: This is a hypertext link which, when clicked on, initiates some action via an internal OnSubmit. It can be thought of as a form with no contents other than some static text and a Submit button.

• Imagemap: This is an imagemap whose attributes specify the point last clicked on.

• InstanceLink: This is another type of hypertext link. It points at an instance, and is mainly useful for larger applications which consist of several pages working together.

• Text: This is a slot for text which can be changed at run-time.

• Hider: This container element holds text and other objects, but displays them only if a condition attribute is set. This makes it easy to switch between displaying various different pieces of information on a page.

• File: A File's main attribute is the name of a text file. The File displays it. By using this class, one can write a page which runs a program whose output will go to temporary files, and which then displays that output.

• Region: This is like a File, but has extra attributes giving byte start- and end-positions within the file, for the reason given in Section 2.8. It displays the text within this region. WOM uses Regions so that the compiled code can refer to large chunks of the original non-WOM text in an input file without storing it all in memory as explicit string constants.