| paul@57 | 1 | Exception Handling
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| paul@232 | 2 | ==================
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| paul@232 | 3 |
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| paul@232 | 4 | Active Exceptions
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| paul@232 | 5 | =================
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| paul@232 | 6 |
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| paul@232 | 7 | When an exception is raised, an exception instance is defined as the active
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| paul@232 | 8 | exception. This active exception remains in force until either a new exception
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| paul@232 | 9 | is raised in any handling of the exception, or upon exit of a handler where
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| paul@232 | 10 | the active exception has been caught (thus resetting the active exception).
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| paul@232 | 11 |
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| paul@241 | 12 | NOTE: Currently, if classes are used with "raise" statements, no instantiation
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| paul@241 | 13 | NOTE: of such classes occurs. This should be remedied.
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| paul@241 | 14 |
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| paul@233 | 15 | Instructions
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| paul@233 | 16 | ------------
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| paul@233 | 17 |
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| paul@233 | 18 | StoreException records the current value reference using the exception
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| paul@233 | 19 | register.
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| paul@233 | 20 |
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| paul@233 | 21 | LoadException obtains the current exception and puts it in the value register.
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| paul@233 | 22 |
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| paul@233 | 23 | CheckException checks the current exception against a class referenced by the
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| paul@233 | 24 | current value.
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| paul@233 | 25 |
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| paul@233 | 26 | ClearException resets the exception register.
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| paul@233 | 27 |
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| paul@232 | 28 | Handlers
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| paul@232 | 29 | ========
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| paul@57 | 30 |
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| paul@109 | 31 | An exception handler stack is defined such that when a try...except or
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| paul@109 | 32 | try...finally block is entered, a new handler is defined.
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| paul@57 | 33 |
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| paul@109 | 34 | When an exception is raised, the program jumps to the most recently defined
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| paul@109 | 35 | handler. Inside the handler, the stack entry for the handler will be removed.
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| paul@57 | 36 |
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| paul@109 | 37 | Depending on the nature of the handler and whether the exception is handled,
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| paul@109 | 38 | the program may jump to the next most recent handler, and so on.
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| paul@57 | 39 |
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| paul@109 | 40 | If no handler is defined when an exception is raised or re-raised, the program
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| paul@109 | 41 | should terminate. This might be done by having a "handler #0" which explicitly
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| paul@109 | 42 | terminates the program.
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| paul@232 | 43 |
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| paul@232 | 44 | Instructions
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| paul@232 | 45 | ------------
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| paul@232 | 46 |
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| paul@232 | 47 | PushHandler(block) defines an active handler at the location indicated by the
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| paul@232 | 48 | given block.
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| paul@232 | 49 |
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| paul@398 | 50 | PopHandler(n) removes the n topmost active handlers. A single handler is
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| paul@398 | 51 | typically removed when leaving a try block, but potentially more handlers are
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| paul@398 | 52 | removed when such a block is exited using a return statement.
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