12.13: Example — Halt Implementation

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As discussed in the Debugger chapter of Pharo By Example, the usual way of setting a breakpoint within a Smalltalk method is to insert the message-send self halt into the code. The method halt, implemented in Object, uses exceptions to open a debugger at the location of the breakpoint; it is defined as follows:

Object»halt
    "This is the typical message to use for inserting breakpoints during
    debugging. It behaves like halt:, but does not call on halt: in order to
    avoid putting this message on the stack. Halt is especially useful when
    the breakpoint message is an arbitrary one."
    Halt signal

Halt is a direct subclass of Exception. A Halt exception is resumable, which means that it is possible to continue execution after a Halt is signaled.

Halt overrides the defaultAction method, which specifies the action to perform if the exception is not caught (i.e., there is no exception handler for Halt anywhere on the execution stack):

Halt»defaultAction
    "No one has handled this error, but now give them a chance to decide
    how to debug it. If no one handles this then open debugger
    (see UnhandedError-defaultAction)"
    UnhandledError signalForException: self

This code signals a new exception, UnhandledError, that conveys the idea that no handler is present. The defaultAction of UnhandledError is to open a debugger:

UnhandledError»defaultAction
    "The current computation is terminated. The cause of the error should be logged or
        reported to the user. If the program is operating in an interactive debugging
        environment the computation should be suspended and the debugger activated."
    ^ UIManager default unhandledErrorDefaultAction: self exception

MorphicUIManager»unhandledErrorDefaultAction: anException
    ^ Smalltalk tools debugError: anException.

A few messages later, the debugger opens:

Process»debug: context title: title full: bool
    ^ Smalltalk tools debugger
        openOn: self
        context: context
        label: title
        contents: nil
        fullView: bool.