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Exception Handling

26th January 2010

These are lecture notes from my Computer Science course, not a general reference for "Exception Handling"

Ada & exception handling

The domain of the exception is the block (the begin).
Exception propagation.
No syntactic relationship between subprograms and exceptions; so the exceptions that might be raised are specified at a package level, not a procedure level. This can be a bit annoying.
Only strings can be passed to handlers.
Exceptions could (accidentally) be propagated to blocks that don’t know what those sorts of exceptions are.
Catch ‘other’ exceptions using when others. But that’s all or nothing, no hierarchy (I think).

Java & exception handling

Domain of exception is explicitly indicated (try statement). This block is then considered guarded.
Exception propagation.
Use catch (Exception E) to catch all exceptions. Can have hierarchy.
Supports finally clause as part of a try statement. This code is guaranteed to execute whether you caught an exception or not. Not easy to do in Ada; finalisation objects (will be in the practicals).

Erlang/OTP (ignore if revising)

try and catch too.
Exception propagation.
OTP supervisor deals with exceptions; restarts threads/terminates other threads if it is configured to. Very flexible in that regard.
Hierarchy possible too, as it’s just pattern patching on the exception.

Exception propagation

No handler associated with a block/procedure? Maybe consider it as programmer error & so report at compile time. May not be possible though as exception may be within a procedure so no idea if it needs to handle an exception or not (context sensitive).
Instead, use propagation.

Resumption versus termination

Resumption: Invoker continues after the exception has been handled. Difficult; might not be able to repair errors or backtrack. Arithmetic overflow in the middle of an expression would be difficult. However if you can do it; it’s asynchronous - it works a bit like a software interrupt.
Termination: Guess.

What about no exception handling?

Some languages (C?) return negative integers instead of having exceptions. An exception handler is a bit like a GOTO; which is Very Bad(tm). But in RTS we need error recovery and need to be able to cope with errors and so exception handling gives you a structured way of dealing with that :)