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Ada '83 Rationale, Sec 16.6: Exceptions and Renaming

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CHAPTER 16: Input-Output

16.6 Exceptions and Renaming

Input-output can very easily be misused, and consequently there is a need for reporting errors. As might be expected, this is done through the raising of certain exceptions, which for convenience are declared together in the package IO_EXCEPTIONS, thus: 

package IO_EXCEPTIONS is
  STATUS_ERROR   :  exception;
  MODE_ERROR     :  exception;
  NAME_ERROR     :  exception;
  USE_ERROR      :  exception;
  DEVICE_ERROR   :  exception;
  END_ERROR      :  exception;
  DATA_ERROR     :  exception;
  LAYOUT_ERROR   :  exception;
end IO_EXCEPTIONS;

Note that it would be very inconvenient for these exceptions to be declared in each of the packages SEQUENTIAL_IO, DIRECT_IO, and TEXT_IO. Each instantiation of SEQUENTIAL_IO, for example, would then give rise to a different exception, and it would consequently be tedious to write a general purpose handler, since it would need to refer explicitly to all the instances.

For this reason the three packages all contain renaming declarations such as 

    STATUS_ERROR :  exception renames IO_EXCEPTIONS.STATUS_ERROR;

This has the advantage that the user (and indeed the writer of the IO package bodies) need not refer explicitly to IO_EXCEPTIONS.

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