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I know that the implementation signals a MessageNotUnderstood exception, but how does that end up opening a debugger?

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1 Answer 1

Reset to default
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When an Exception remains unhandled after it has been signalled, its #defaultAction is invoked. MessageNotUnderstood>>defaultAction delegates to Error>>defaultAction, which signals an UnhandledError (another Exception). This exception, in turn, has a defaultAction whose code reads like this:

^ ToolSet debugError: self exception

...which opens the debugger if you use the StandardToolSet (which is the default in regular Squeak images).

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  • I'm wondering what invokes #defaultAction? I suspect that it's invoked from UndefinedObject>>handleSignal: when there are no more exception handlers in the call chain. Is this true? Or, is the VM sending that message when no more handlers remain?
    – rationalrevolt
    Commented Jul 16, 2018 at 15:46
  • @rationalrevolt Yes, it's true. The VM isn't involved in the way exceptions are handled.
    – Leandro Caniglia
    Commented Jul 16, 2018 at 19:09

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