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   Re: error trapping

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  • From: james anderson <James.Anderson@mecomnet.de>
  • To: John Cowan <cowan@locke.ccil.org>
  • Date: Wed, 20 Oct 1999 11:47:09 +0200

interesting account re MESA.

well, resumption does require that the module which raises the exception
provide meaningfull and correctly operating provisions for
restarts/continuations, and that it provide exceptions which carry information
adequate to decide upon and undertake appropriate corrective action. the
necessary interfaces are on the same level as the principal functional
interface and, as such, would not appear to do anything to compromise encapsulation.

they may be harder to correctly program than simply exiting, but that's
another story.
as mr. bray notes, they may not be desirable, but that's also another story.

John Cowan wrote:
> james anderson scripsit:
> > one intrinsic problem with java parsers is that the java control model forces
> > exceptions to unwind from the initiator's dynamic  context before the handler
> > obtains control.
> The essential problem with resumption, IMHO, is that it breaks
> encapsulation: the resumptive handler is written by the author of the
> calling routine, but runs as a friend of the called routine.
> To work correctly, then, the caller must know everything about the
> callee.

not necessarily: yes, the caller must understand the causes and consequences
of the conditions raised, but they must also know and abide by the interface
provided by the exceptions and are constrained to the defined
restarts/continuations, both of which are specified by the callee. while this
form of encapsulation may have more facets, where's it broken?

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