Section 10.4. Scope of Locals


10.4. Scope of Locals

An explicit local is a variable whose name appears in a local declaration . A local declaration can declare several variables at once, delimited by comma:

 local a local b, c, d

A local variable is visible only within the very same region of scope where it is declarednot outside it, and not at a deeper level. (But there's an exception, which we'll come to in a moment.)

Local variables completely disambiguate a name within their own scope, and local variables in other scopes can have the same name without conflict. Suppose a script object starts like this:

 script myScript     local x

The local declaration for x means that from now on when code in this script object's scope says x it means this local x and no other. What's more, other scopes may declare their own local x, they may declare a global x, they may bang the floor and have a temper tantrum, but they absolutely will not be able to have any effect upon myScript's x, nor will anything myScript does with its x have any effect upon them.

Here's an example of a local variable in action:

 local x set x to 5 script myScript     display dialog x -- error: The variable x is not defined end script run myScript

Observe how completely different this is from what would have happened if x had been a top-level property. Here, there is a variable called x and it is defined, but it is declared local and therefore is visible only within its own scope. That scope is the top-level script. The display dialog x command is in a different scope, that of the script object myScript. Therefore AppleScript takes this to be a different x, and this different x has never been assigned a value. (I'll explain later just what x AppleScript takes the x of display dialog x to be.)

A local declaration overshadows the visibility of a top-level entity from a higher level:

 property x : 5 script myScript     local x     display dialog x -- error: The variable x is not defined end script run myScript

But this overshadowing affects only the scope of the local declarationnot a deeper scope:

 property x : 5 script myScript     local x     on myHandler( )         set x to 10     end myHandler     myHandler( )     set x to 20 end script run myScript display dialog x -- 10 (not 20)

The dialog displays 10, not 20. Even though myScript overshadows the top-level property x with a local x declaration, this has no effect on myHandler, which still sees the top-level property x. (This makes sense, because it can't see myScript's local x.) When myHandler sets x to 10, that is the same x as at the top level. When myScript sets x to 20, that's its local x, and the value displayed in the last line is unaffected.

In a handler, the variable names used for parameters in the definition of the handler are local within the handler. Naturally, a handler may also declare further locals:

 on myHandler(what)     local x     set x to 7 end myHandler myHandler("howdy") display dialog what -- error: The variable what is not defined display dialog x -- error: The variable x is not defined

Now we come to the great exception: a script object defined in a handler can see the handler's local variables. To a script object in a handler, the handler's locals are like top-level entities: they are visible to the script object and to scopes nested within it. (We took advantage of this rule in "Power Handler Tricks" in Chapter 9.) Thus:

 on myHandler( )     local x     set x to 5     script myScript         on h( )             display dialog x         end h         h( )     end script     run myScript end myHandler myHandler( ) -- 5

I think the reason for this rule is that a handler can contain a nested scope (a script object) but has no top-level entities. A handler can't declare a property, so without this rule, it would have no encapsulated way to expose values to a nested script object. Similarly, without this rule, code in a script object in a handler would be unable to see a script object earlier in the same handler:

 on h( )     script s         display dialog "howdy"     end script     script ss         run s     end script     run ss end h h( ) -- howdy




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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