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