Perl 6 and Parrot Essentials, Second Edition

Exceptions and Exception Handlers

Exceptions provide a way of calling a piece of code outside the normal flow of control. They are mainly used for error reporting or cleanup tasks, but sometimes exceptions are just a funny way to branch from one code location to another one. The design and implementation of exceptions in Parrot isn’t complete yet, but this section will give you an idea where we’re headed.

Exceptions are objects that hold all the information needed to handle the exception: the error message, the severity and type of the error, etc. The class of an exception object indicates the kind of exception it is.

Exception handlers are derived from continuations. They are ordinary subroutines that follow the Parrot-calling conventions, but are never explicitly called from within user code. User code pushes an exception handler onto the control stack with the set_eh opcode. The system calls the installed exception handler only when an exception is thrown (perhaps because of code that does division by zero or attempts to retrieve a global that wasn’t stored.)

  newsub P20, .Exception_Handler, _handler
  set_eh P20                  # push handler on control stack
  null P10                    # set register to null
  find_global P10, "none"     # may throw exception
  clear_eh                    # pop the handler off the stack
   . . . 

_handler:                     # if not, execution continues here
  is_null P10, not_found      # test P10
   . . .

This example creates a new exception handler subroutine with the newsub opcode and installs it on the control stack with the set_eh ...