Beginning AppleScript®

9.2. The try...on error Statement

You are familiar with the try statement, which was first introduced in Chapter 3. You use it according to this general format:

try
    statement
    statement
    ...
end try

A statement in the try block that generates an error causes execution of the statements that follow, up to the end try, to be skipped. Execution of the program otherwise continues with whatever statement follows the try block.

One technique you learned to determine whether a statement in a try block fails is illustrated by the following code fragment from Chapter 4:

set validNumber to false
try
    set triangularNumber to text returned of result as integer
    set validNumber to true
end try

if not validNumber then
     display dialog "You didn't enter an integer" with icon note
end if

Here you set a flag to false before entering the try block and then set the flag to true after the last statement inside the try block is executed. This statement is reached only if none of the preceding statements—and here, you have only one such statement—in the try block generated an error. Outside the try block, you test the value of the flag to see if any errors occurred.

An easier way to accomplish this same logic involves using this form of the try statement:

try
    statement
    statement
    ...
on error
    statement
    statement
    ...
end try

If any of the statements between try and on error generates an error, the statements that follow the on error, up to the end try, get executed. If no errors are generated by the statements ...