12.4. Identifying References

AppleScript goes to some lengths to hide the existence of references, making it remarkably difficult to find out that a value is a reference. Properly speaking, a reference should be a class, a datatype like string or integer (see "Class" in Chapter 11, and Chapter 13). If you ask a string about its class, it says string. If you ask an integer about its class, it says integer. But if you ask a reference about its class, it will never tell the truth and say reference.

set x to a reference to "hey"
set y to a reference to 9
tell application "Finder" to set z to folder 1
class of x -- string
class of y -- integer
class of z -- folder

Here are a couple of tricks you can use to learn that a value is a reference. (I don't guarantee any of them, but they do seem mostly to work.)

The reference coercion trick

The only thing that can be coerced to a reference is a reference. If you try to coerce anything else to a reference, you'll get a runtime error. So try to coerce a value to a reference, and if there's no error, it is a reference. For example:

tell application "Finder" to set x to folder 1
x as reference -- no error; it's a reference

The editor value trick

If a value, as shown in your script editor application, contains the word of, it is a reference. For example:

tell application "Finder" to set x to folder 1
x -- folder "Mannie" of...; it's a reference
set x to a reference to y
x -- y of «script»; it's a reference

When I'm debugging or developing a script, I like the second method best; I look at a variable's value and I can usually see right away whether it's likely to be a reference. If I'm writing code where the code itself needs to test whether something is a reference, I like the first method best. Here's a general handler that returns a boolean value telling whether its parameter is a reference:

on isRef(valueToTestAsRef)
    try
        valueToTestAsRef as reference
        return true
    on error
        return false
    end try
end isRef
-- and here's how to call it
tell application "Finder"
    set x to folder 1
end tell
isRef(x) -- true
set x to "haha"
isRef(x) -- false
isRef(a reference to x)) -- true