AppleScript: The Definitive Guide, 2nd Edition

Errors

An error is a message at runtime saying, in effect, that something bad has happened and execution cannot continue. The sender of such a message is said to throw an error. The message percolates up through the chain of handler calls (the call chain ), looking for an error-handling block surrounding the line currently being executed; such a block is said to catch the error. If no such block catches the error, it percolates all the way up to AppleScript, and the script terminates prematurely (possibly with an error dialog ).

This entire mechanism is extremely nice, because it provides a target application, or AppleScript itself, with a way to signal that it's impossible to proceed, interrupting the flow of code while leaving it up to the caller whether and how to recover. Your script can implement no error handling, in which case any runtime error will bring the script to a grinding halt. Or your script can implement error handling in certain areas where it expects an error might occur. It can recover from some errors and re-throw others, allowing them to terminate the script. It can even throw an error deliberately as a way of controlling the flow of code.

An error can be a positive thing, and can be built into the structure of a command's implementation. For example, display dialog throws an error if the user clicks the Cancel button in the dialog. This need not kill your script. It will if you let it, and this can be a good thing (because Cancel often means "stop"). But alternatively, ...