AppleScript: The Definitive Guide, 2nd Edition

Who Performs an Operation

Some operations within an interapplication communications context can be performed by the target application rather than by AppleScript. There are two cases to consider. The operation may appear as a bare expression (for example, the condition in an if clause); I will call this a direct operation . Or, the operation may be part of a boolean test element specifier.

Direct Operations

According to Apple's documentation, if the first operand of a direct operation is a reference to an object of the target application, the target application should perform the operation. So, for example:

tell application "Finder"
    if name of folder 1 contains "e" then

The object name of folder 1 is a Finder object, so in this case, according to Apple, the Finder should perform the operation. However, experimentation shows that the Finder does not perform the operation; AppleScript does try to get it to do so, but the target application replies with an error indicating that it doesn't wish to perform that sort of operation. AppleScript thereupon adopts a new strategy: it asks the target application for the values in question, and performs the operation itself.

So, here's how that example really works. AppleScript starts by sending the Finder a single Apple event that means: "Please tell me whether the name of your folder 1 contains "e"." The Finder replies with an error message. So then AppleScript tries the other strategy: it goes back to the Finder and sends it another Apple event ...