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--- script:walkthrough:exceptions [2017-03-20 20:25] – skyjake
+++ script:walkthrough:exceptions [2019-11-20 19:29] (current) – [Catching native errors] skyjake
@@ Line 1: / Line 1: @@
+<- Statements and compounds ^ ^ Records ->
+====== 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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