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script:walkthrough:exceptions

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Exceptions

Try and catch

Like in C++, the try and catch keywords are used to begin blocks of code where exceptions can be thrown and processed.

If an exception is thrown outside try/catch, it will be uncaught and execution of the script to be aborted.

try: print 'Nothing thrown yet.'
catch: print 'Skipped.'

Throwing exceptions

Exceptions are thrown using a throw statement.

  • When given one argument, a new exception is thrown with the argument converted to a text string. The argument should be used for instance for error messages.
  • Without arguments, throw can be used inside a catch compound to rethrow the caught exception.
try
    throw "There was an error"
catch
    print 'Exception caught'
end

Handling exceptions

If the exception does not require any handling, one can use pass as the catch block.

try: throw 'Problem!'
catch: pass

The exception message can be assigned to a local variable so it can be accessed in the catch block.

try: throw "OMG!"
catch Error, er
    print 'Caught:', er
end

Output:

Caught: [Error] (in script) OMG!

Multiple catch blocks can be chained together to catch specific types of exceptions.

try
    throw 'This will be printed'
    
catch NullError: print 'Got NullError'
catch Error, er: print er
catch: print 'Never here.'

Try/catch can be nested, and an exception thrown in an inner try block can be caught in an outer block.

try
    try
        try
            throw 'Deep.'
        catch BogusError
        end
    catch BogusError
    end
    try
    catch Error, er
        print 'Should *not* be caught here.'
    end
catch Error, er
    print 'Got it:', er
end

In this example, BogusError does not match the type of error thrown in scripts (a generic Error), so the catch blocks are skipped. The output is:

Got it: [Error] (in script) Deep.

An exception thrown in a function unwinds the call stack to find a matching catch block.

def erroneous()
    throw "Dropped the ball"
end

try
    print "Calling erroneous()"
    erroneous()
    try
        print 'This will be skipped!'
    catch
        print 'I am on the wrong catch level.'
    end
catch
    print 'Fumbled it...'
end

Catching native errors

Native code may throw various kinds of exceptions that can then be caught in the script. In the example below, a NullError gets thrown because the code tries to access a deleted object via an alias.

try
    record temp
    reference = temp
    del temp
    print reference
catch NullError, er
    print 'Oh noes:', er
end

ReadOnlyError is thrown when trying to change a const variable.

const SOME_CONSTANT = 3.1415
try
    SOME_CONSTANT = 3
catch ReadOnlyError, er
    print 'Cannot do it.', er
end
try
    const SOME_CONSTANT = 3
catch ReadOnlyError, er
    print 'Will not work either:', er
end
script/walkthrough/exceptions.1574277548.txt.gz · Last modified: 2019-11-20 21:19 by skyjake