Doomsday Manual

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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

Any type of native exception can be caught in a script.
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