Chapter 8. Script Objects

A script object is a script within a script. In fact, a script really is a script object, though it also has a special status as the top-level script object. Script objects have certain remarkable features. They have variables of a special kind—I call these top-level entities—that belong to the script object, but can be retrieved and assigned to from elsewhere. They have a run handler, implicit or explicit; the code in this run handler can be executed. They are persistent over repeated executions of a script. They can be saved to disk as files, and a compiled script file can be turned back into a script object. And script objects can even implement a relationship of inheritance with polymorphism. You can think of a script object as a powerful, semi-autonomous package for some code and variables that go together. This chapter presents script objects and their distinctive features.

Script Object Definition

A script object is defined using a block with the keyword script:

script scriptName
    -- commands within the script object
end script

A script object definition may appear anywhere. If defined at the top level of a script object (or script), it is a top-level entity ("Top-Level Entities," later in this chapter). It functions as a scope block . (The rules of scope appear in Chapter 10.) Read Chapter 6 for an overview of how script object definitions fit into a script's overall structure.

A script object definition is just that—a definition. Merely encountering a script object definition in the course of execution does not cause of any of its commands to be executed. Rather, a script object definition is a form of variable initialization. So, for example:

script s
    display dialog "howdy"
end script

That code does not cause a dialog to display. It defines a script object with an implicit run handler; that run handler contains code that puts up a dialog, but the mere definition does not cause the run handler to run. The script object itself is the value of a variable. Here, that variable is s; when this code is encountered, the variable s is defined and initialized, and its initial value is the script object described by this block of code.