11 Exceptions - Ada 95 Rationale
The changes to exception handling from Ada 83 are quite small. The main changes are
- The exception Numeric_Error is now a renaming of Constraint_Error and is also obsolescent.
- The notion of an exception occurrence is introduced. This refers to an instance of raising an exception. The package Ada.Exceptions contains procedures providing additional information regarding an exception occurrence.
- The interaction between exceptions and optimization is clarified.
- A minor improvement is that an accept statement may now directly have an exception handler.
11.1 Numeric Error
Those familiar with the early use of Ada 83 will recall that there was considerable confusion between Numeric_Error and Constraint_Error in a number of corner situations. As a consequence the ARG ruled that implementations should raise Constraint_Error in all situations for which Numeric_Error was originally intended. This was a non-binding interpretation with the intention of making it binding in due course.
Essentially all implementations of Ada 83 now raise Constraint_Error although for historic reasons some programs may contain dormant handlers for Numeric_Error.
The development of Ada 95 provided an opportunity to remove this historic wart once and for all. It was thus proposed that Numeric_Error be completely removed. However, many reviewers pointed out that those programs which had conformed to the advice of AI-387 by consistently writing
when Constraint_Error | Numeric_Error =>
would then become illegal. Accordingly, in Ada 95, Numeric_Error is simply a renaming of Constraint_Error. Such a change alone would still have made the above illegal because, in Ada 83, all the exceptions in a handler had to be distinct; a supplementary change is thus that an exception may appear more than once in a handler in Ada 95.
Allowing multiple instances of an exception in a given handler has benefits in other areas. It now allows sequences such as
when Text_IO.Data_Error | Integer_IO.Data_Error =>
where there may be documentation advantages in revealing the potential causes of the exception.
Of course if the user had deliberately relied upon a distinction between Numeric_Error and Constraint_Error then the program will now become incorrect. It may be simply incompatible but may also be inconsistent if the handlers are in different frames. For a more detailed discussion see A-4.5. Despite this possibility it was concluded that the perhaps safer alternative of completely removing Numeric_Error was not appropriate for this revision although it should be reconsidered at the next revision.
11.2 Exception Occurrences
It is important in many programs to be able to recover from all exceptions and to continue processing in some way. All exceptions including unexpected exceptions can be caught by an others handler. Unfortunately, however, Ada 83 provided no way of identifying the particular exception and it was thus not possible to log the details or take specific appropriate action.
This is overcome in Ada 95 by the introduction of the concept of an exception occurrence and a number of subprograms to access information regarding the occurrence. The type Exception_Occurrence and the subprograms are declared in the package Ada.Exceptions. The user can then declare a choice parameter in a handler through which the particular occurrence can be identified. For example a fragment of a continuous embedded system might take the form
with Ada.Exceptions; task body Control is ... begin loop begin ... -- main algorithm exception when Error: others => -- unhandled exception; log it Log("Unknown error in task Control" & Ada.Exceptions.Exception_Information(Error)); -- reset data structures as necessary end; -- loop around to restart the task end loop; end Control;
The choice parameter Error is a constant of the type Exception_Occurrence. The function Exception_Information returns a printable string describing the exception and details of the cause of the occurrence. The actual details depend on the implementation.
Two other functions in Ada.Exceptions are Exception_Name and Exception_Message. Exception_Name just returns the name of the exception (the expanded name) and Exception_Message returns a one-liner giving further details (it excludes the name). Thus Exception_Name, Exception_Message and Exception_Information provide a hierarchy of strings appropriate to different requirements.
The purpose of the three functions is to provide information suitable for output and subsequent analysis in a standard way. Although the details of the strings will depend upon the implementation nevertheless they should be appropriate for analysis on that system.
Exception occurrences can be saved for later analysis by the two subprograms Save_Occurrence. Note that the type Exception_Occurrence is limited; using subprograms rather than allowing the user to save values through assignment gives better control over the use of storage for saved exception occurrences (which could of course be large since they may contain full trace back information). The procedure Save_Occurrence may truncate the message to 200 characters whereas the function Save_Occurrence (which returns an access value) is not permitted to truncate the message. (Note that 200 corresponds to the minimum size of line length required to be supported, see 2.2.)
An occurrence may be reraised by calling the procedure Reraise_Occurrence. This is precisely equivalent to reraising an exception by a raise statement without an exception name and does not create a new occurrence (thus ensuring that the original cause is not lost). An advantage of Reraise_Occurrence is that it can be used to reraise an occurrence that was stored by Save_Occurrence.
It is possible to attach a specific message to the raising of an exception by the procedure Raise_Exception. The first parameter is a value of the type Exception_Id which identifies the exception; this value can be obtained by applying the attribute Identity to the identifier of the exception. The second parameter is the message (a string) which can then be retrieved by calling Exception_Message. This provides a convenient means of identifying the cause of an exception during program debugging. Consider
declare O_Rats: exception; begin ... Raise_Exception(O_Rats'Identity, "Hard cheese"); ... Raise_Exception(O_Rats'Identity, "Rat poison"); ... exception when Event: O_Rats => Put("O_Rats raised because of "); Put(Exception_Message(Event)); end;
Calling Raise_Exception raises the exception O_Rats with the string attached as the message. The second call of Put in the handler will output Hard cheese or Rat poison according to which occurrence of O_Rats was raised. See also the example in 9.6.1.
Note that the system messages do not include the name so user and system messages can be processed in a similar manner without the user having to insert the exception name in the message.
11.3 Exceptions and Optimization
The general objective is to strike a sensible balance between prescribing the language so rigorously that no optimizations are possible (which would make the language uncompetitive) and allowing so much freedom that the language semantics are almost non-existent (which would impact on portability and provability). A progressive approach is required that enables different degrees to be permitted in different circumstances.
Much of the difficulty lies with exceptions and ensuring that they are still raised in the appropriate frame. In particular we have ensured that calls to subprograms in other library units are not disrupted. For details of the approach taken the reader is referred to [RM95 11.6]. A more detailed discussion of the rationale will be found in [AARM].
11.4 Other Improvements
As mentioned in 1.3, the description of exception handling has been simplified by the introduction of a new syntactic category handled_sequence_of_statements which embraces a sequence of statements plus associated exception handlers and is used for all situations where handlers are allowed.
An incidental minor improvement following from this change to the syntax is that an accept statement can now have a handler without the necessity for an inner block, thus
accept E do ... ... exception ... end E;
A further practical improvement is that the concept of a current error file is introduced. This can conveniently be used to log error messages without cluttering other output. This is discussed in more detail in Part Three.
11.5 Requirements Summary
The specific requirement
- R 4.5-A(1) - Accessing an Exception Name
is met by the introduction of exception occurrences.
- R2.2-C(1) - Minimize Special Case Restrictions
mentions as an example the curious fact that an accept statement cannot have an exception handler. This has been rectified.
Laurent Guerby Ada 95 Rationale