Section 10.8. Free Variables


10.8. Free Variables

A variable defined outside a handler or script object, but globally visible within it, and not overshadowed by a declaration of the same name, is called a free variable with respect to that handler or script object. A free variable in AppleScript is identified with a top-level entity or global through the downward effect of its declaration. For example:

 global x, y, z script myScript     property x : 1     local y     yy     z end script

Within myScript, x is explicitly defined as a property and y is explicitly defined as a local, so neither is a free variable. The variable yy isn't explicitly defined within myScript, but it isn't defined outside it either, so it is an implicit local and not a free variable. But the variable z is globally visible within myScript (from the global declaration at the start of the code), and the name is not redeclared within myScript, so z within myScript is a free variable, and is identified with the global z declared in the first line.

A free variable takes its value within the handler or script object at the time the code runs, not at the time the handler or script object is defined. For example:

 global x set x to 5 on myHandler( )     display dialog x -- x is a free variable end myHandler set x to 10 myHandler( ) -- 10 (not 5)

The dialog displays 10, not 5. It doesn't matter that x had been set to 5 when myHandler was defined; it only matters what its value is when the code inside myHandler actually runs. By that time, x has been set to 10.

Free variables' values are determined at runtime; but the identification of the fact that a variable is a free variable, and its association with some particular globally visible variable, happens during compilation. This way of resolving the meaning of free variable names is called lexical scoping .

So, in the previous example, it is important that x is declared global before myHandler is defined. If you move the global x declaration to after the myHandler definition, the script generates an error at runtime. Even if x is declared global before (temporally) myHandler runs, that's not good enough. We must declare x global before (physically) myHandler speaks of it; otherwise, myHandler's x isn't the global x, and when the code runs, myHandler's x isn't defined. This is another reason why you should declare your variables, and not only declare them but declare them early, even though AppleScript doesn't require you to do so.

Here's an example with two top-level entities as free variables. One is a property, the other is a script object:

 property x : 5 script myScript     display dialog x -- x is a free variable end script script myOtherScript     set x to 20 -- x is a free variable     run myScript -- myScript is a free variable end script set x to 10 run myScript -- 10 run myOtherScript -- 20




AppleScript. The Definitive Guide
AppleScript: The Definitive Guide, 2nd Edition
ISBN: 0596102119
EAN: 2147483647
Year: 2006
Pages: 267
Authors: Matt Neuburg

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