Chapter 15. Operators

An operator is a token that transforms a value or a pair of values to produce a new value. These transformations are operations, and the values operated upon are the operands. An operator with two operands is binary; an operator with one operand is unary. This chapter lists the AppleScript operators and explains what they do, with special attention to implicit coercions performed by the operators. It also talks about parentheses, because they help determine the effects of the operators. Finally, there's a section on the differences between what happens when AppleScript performs an operation and when a scriptable application performs it.

(For the coercion operator, as, see Chapter 14; for the object containment operator, of, see Chapter 11.)

Implicit Coercion

In Chapter 14, I explained coercion and described the coercions that are possible between built-in datatypes in the AppleScript language. Binary operators can (and will) perform coercion without your specifically asking for it. This is called implicit coercion , and is one of the most confusing aspects of AppleScript—and a frequent source of mistakes in scripts. If you are not prepared for what implicit coercions an operator will perform, you will be surprised when the result of an operation is not what you expected. That's why this chapter spends so much time on the implicit coercions performed by the various operators.

What coercions AppleScript will perform implicitly is not the same as what coercions you can get it to perform explicitly (with the as operator). AppleScript's error messages in this regard can add to the confusion. For example:

1 and 1 -- compile-time error: Can't make 1 into type boolean

That error message, on its face, is lying. AppleScript can make 1 into a boolean. What AppleScript really means by the error message here is: "In order for me to perform this operation, I would need to coerce the first 1 (before the and) to a boolean, implicitly; and I refuse to do that." Even weirder is what happens when you proceed to coerce the first 1 to a boolean explicitly:

1 as boolean and 1 -- true

It works—which means that even though AppleScript refused to coerce the first 1 to a boolean implicitly, it now happily coerces the second 1 to a boolean implicitly!

But do not imagine that there is some simple rule governing this behavior (such as that AppleScript never coerces the first operand implicitly). In this next example, AppleScript happily coerces both operands implicitly (to a number ):

"3" + "4" -- 7

You see now why the rules for implicit coercion need to be made explicit.