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                     Imp Core Environment Standard


                      Section 2: Event Signalling

   Central to the IMP language and its environment is a mechanism for
reporting and handling exceptional circumstances which may occur during
the execution of a program.  These exceptions (called events by IMP) may
                                                      ______
be signalled in two ways.  Firstly, the programmer may use an explicit
   _________
signal statement: this technique is often used to report errors from
______
libraries and packages such as the EDWIN graphics package.  The
definition of the IMP environment in this standard contains several
examples of this: an event is signalled on an attempt to read characters
past the end of a file, for example.  The second possibility for an
event being signalled is as a result of an error condition being
detected by the underlying hardware or operating system.  Most
implementations of IMP attempt to translate such hardware exceptions as
floating-point overflow into a corresponding event.

   Note that this section of the core environment standard is intended
to define and standardise the event mechanism as it is currently
implemented.  Alternate proposals and extensions are being considered in
another document by Alan Culloch of Lattice Logic, Document_IMP_Signals.
Furthermore, details of the propagation of events and other
language-related issues are to be found in the language manual rather
than in this standard.




*       record format Event FM ( ... {see text} ... )
        ______ ______

           The pre-declared record format EVENT FM must contain the
        following field definitions:

         integer     Event
         _______
         integer     Sub
         _______
         integer     Extra
         _______
         string(255) Message
         ______

        Note that the exact definition of the EVENT FM record format is
        implementation defined (DEF0002; definition of EVENT FM).  Thus
        the order of these fields within the record format is also
        implementation defined.  In addition, a conforming
        implementation is free to include other fields within this
        record format for its own purposes, although if it does so a
        method must be available to specify that the implementation
        reject use of such identifiers as non-standard.  This condition
        is a special case of the general rule concerning "extension"
        identifiers given in section 1.3.2 on implementation
        conformance.




*       record(Event Fm) Event
        ______

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