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Appendix X Upward Compatibility

A major design goal of Ada 95 was to avoid or at least minimize the need for modifying the existing base of Ada 83 software to make it compatible with Ada 95. This involves not only pursuing upward compatibility but also preserving implementation-dependent behavior that can currently be relied upon. In common with the experience of revising other language standards, it is infeasible to guarantee 100% compatibility.

Other languages have been more or less successful in meeting this goal. For example, COBOL 83 has been very successful in achieving upward compatibility with COBOL 74. Nevertheless some minor incompatibilities were introduced which affect existing programs. For example, IS_ALPHABETIC, accepts upper and lower case in the new COBOL standard. The transition from C to ANSI C and from there to C++ has also caused incompatibilities, for example C++ requires all procedure definitions in old-style C to be modified.

In the design of Ada 95, a very conservative approach has been adopted. The few incompatibilities that exist can be dealt with in a simple mechanical way. For example, the introduction of a small number of reserved words requires their replacement in any program using them as identifiers. Extensive surveys of existing code show few programs to be affected. Most of the other incompatibilities involve obscure or pathological programming styles which are expected to appear very infrequently in existing code.

The great majority of programs will not be significantly affected by these changes - the most likely incompatibilities being automatically detected at compilation time. Moreover, tools are being developed to aid in the reliable detection of any problems and thereby smooth the process of transition.

Only three incompatibilities are considered likely to occur in normal programs. They are as follows:

The following further two incompatibilities might occur in normal programs but are less likely:

These incompatibilities usually cause a legal Ada 83 program to be an illegal Ada 95 program and hence are detected at compile time. They are described in more detail in the ensuing sections. In each case we give an example of the incompatibility, an indication of how it can be avoided in existing Ada 83 programs and the possibility of its automatic detection and removal.

The remaining incompatibilities which are considered very unlikely to occur in normal programs are briefly considered in X.6.

The reader should note that we say that an incompatibility is consistent if the worst that can happen is that a legal Ada 83 program becomes illegal in Ada 95 and thus fails to compile. An incompatibility is said to be inconsistent if the program can remain legal but have a different meaning.

X.1 Reserved Words

Six new reserved words are introduced in Ada 95: abstract, aliased, protected, requeue, tagged, and until.

Two alternatives to new reserved words were considered: a new concept of unreserved keywords or the use of combinations of existing reserved words. Neither of these options was considered preferable to the transitory inconvenience caused by the introduction of the new reserved words.

An Ada 83 program that uses any of these words as identifiers is an illegal Ada 95 program. For example, the following fragment of Ada 83 will fail to compile in Ada 95 because it uses two of the new reserved words

   Protected: Boolean := False;

   procedure Requeue(The_Activity: Activity; On_Queue: Queue);

Avoidance is clearly straightforward - avoid use of these six words as identifiers. Detection of the incompatibility is also straightforward. Automatic correction is problematic - to ensure that a name change is valid requires significant analysis especially if the identifier is the name of a library unit, or occurs in a package specification for which use clauses occur.

X.2 Type Character

In Ada 95, the type Character has 256 positions. In Ada 83, it had 128 positions.

Although suitable for English-speaking nations, a character type based on ASCII is inappropriate for most of Europe. ISO has defined a number of 256 character standards such as Latin-1 and Latin-2. This change to the language thus accommodates non-English speaking nations.

An Ada 83 program could be an illegal Ada 95 program if it has a case statement or an array indexed by Character, but it could be a legal Ada 95 program with different semantics if it relies on the position number or value of Character'Last. For example

   type Char_Kind is (Numeric, Alphabetic, Other);

   Kind_Array: array (Character) of Char_Kind :=         -- (1)
                 ('0' .. '9' => Numeric,
                  'A' .. 'Z' | 'a' .. 'z' => Alphabetic,
                  others => Other);

   case Char is                                          -- (2)
      when Character'Val(0) .. Character'Val(63) => ...
      when Character'Val(64) .. Character'Val(127) => ...
   end case;

   I: Integer := Character'Pos(Character'Last);          -- (3)

Declaration (1) is legal in Ada 95 but probably does not achieve the desired effect. Statement (2) is illegal in Ada 95 and will be detected at compilation. Statement (3) illustrates a situation where the program will still execute but have a different effect in Ada 95 (it is inconsistent).

As it is likely that allowing for 256 characters is outside the scope of the original requirement for the program concerned, avoidance is not really the issue - a review of the requirements is necessary.

The inconsistency illustrated by the third example can be avoided by not depending on the position or value of Character'Last. Avoiding the other incompatibilities avoids the real issue of how the extra 128 characters are to be handled. Unless uniform behavior is acceptable for these extra characters, use of an others choice, whilst ensuring a legal (but bad style) Ada 95 program might cause unacceptable behavior.

Detection of the consistent incompatibilities is straightforward; detection that an inconsistency may arise is possible. Manual correction is necessary to determine whether the required semantics of the program are those defined by Ada 95.

Finally, it should be noted that the ISO Working Group with responsibility for maintaining the Ada standard, has decreed that this change can be introduced into Ada 83 compilers, so this will increasingly become an Ada 83 portability issue as more implementations support 256 characters.

X.3 Library Package Bodies

In Ada 95, library unit packages are allowed to have a body only if required by language rules. This avoids a nasty and not so rare error.

In Ada 83, a body need only be provided for a package that really needed one, such as where the specification contains subprogram or task declarations. If a body was provided for a library package that did not need a body (for performing initializations for example), then if the package specification was subsequently changed, the body became obsolete. However, since it was optional, subsequent builds incorporating the package would not incorporate the body, unless it was manually recompiled. This obviously affects packages, for example, that only declare types, constants and/or exceptions, a very common occurrence. As a trivial example in Ada 83 consider

   package Optional_Body is
      Global_Variable: Integer;
   end Optional_Body;


   with Integer_Function;
   package body Optional_Body is
      Global_Variable := Integer_Function;
   end Optional_Body;

The solution adopted in Ada 95 is to allow a body for a library unit package only when one is required by some language rule; the above example is therefore illegal in Ada 95. However, the pragma Elaborate_Body can be used to cause a body to be required.

Given the non-uniform functionality of program libraries and sublibraries, it is probably wise not to try to automatically detect, let alone correct, this incompatibility.

X.4 Indefinite Generic Parameters

Ada 95 provides new syntax for a generic formal private type to indicate that the actual subtype is allowed to be indefinite. The old syntax is retained, but the meaning is changed to require definite actual parameters.

In Ada 83, no indication was given in a generic formal type declaration as to whether the actual needed to be definite, for example because the body declared an uninitialized variable for the type. It was thus possible for a legal instantiation to become illegal if the body was changed.

An Ada 83 program, where an indefinite type is used as a generic actual parameter is an illegal Ada 95 program. For example the following legal Ada 83 program is illegal in Ada 95

      type Element_Type is private;
   package Stack is ...


   with Stack;
   package String_Stack is new Stack(Element_Type => String);

There is no way to avoid this incompatibility but an Ada 83 program can be annotated with an appropriate comment, thus

      type Element_Type is private;  -- !! (<>) in Ada 95
   package Stack ...

Detection of the incompatibility is straightforward. Manual correction is necessary to determine whether restricting the actual to being definite is acceptable.

It is interesting to note that some predefined library units in Ada 83 used this feature and so are changed. Examples are Unchecked_Conversion and Unchecked_Deallocation and also Sequential_IO.

Finally, it should be noted that the ISO Working Group has recommended that Ada 83 compilers be allowed to accept the new syntax in order to simplify transition.

X.5 Numeric Error

In Ada 95, the exception Numeric_Error is declared in the package Standard as a renaming of Constraint_Error.

The checks that could cause Numeric_Error to be raised in Ada 83 have all been reworded to cause Constraint_Error to be raised instead. Indeed, this change has been sanctioned by the Ada Rapporteur Group and encouraged in existing Ada 83 implementations.

However, the alternative of completely removing Numeric_Error was rejected because it would naturally have caused an incompatibility in programs using the construction

   when Numeric_Error | Constraint_Error => Some_Action;
which is the currently recommended way of avoiding the confusion between Numeric_Error and Constraint_Error in Ada 83.

This construction is still allowed in Ada 95 because of an additional rule that permits an exception to be mentioned more than once in the same handler.

Programs which do have distinct separate handlers for Numeric_Error and Constraint_Error such as

      when Constraint_Error => Action_1;
      when Numeric_Error => Action_2;
are illegal in Ada 95. Moreover, an inconsistency will arise if a frame has an explicit handler for Numeric_Error or Constraint_Error but not both as in the following
      when Constraint_Error => Action_1;
      when others => Action_2;
since Numeric_Error will be caught by the first handler in Ada 95 but by the second in Ada 83.

Detection of the incompatibility is straightforward but manual correction will be necessary in cases where Numeric_Error is treated differently.

X.6 Other Incompatibilities

It is considered that other incompatibilities will be unlikely to occur in normal programs - the Ada 83 semantics being known only to the most erudite of Ada programmers - and so only a brief description seems appropriate in this document. In the following summary, they are grouped according to whether they result in a legal Ada 95 program but with different semantics; whether they would be detectable by an Ada 95 compiler and so on.

X.6.1 Unlikely Inconsistencies

These incompatibilities might cause a change in the runtime behavior, but they are not thought likely to occur in normal programs.

X.6.2 Unlikely Incompatibilities

These incompatibilities cause a legal Ada 83 program to be an illegal Ada 95 program and hence are detectable at compile time. They are considered to be unlikely in normal programs.

X.6.3 Implementation Dependent Incompatibilities

These incompatibilities only arise with some implementations. They occur either as a result of tightening up Ada semantics or where an Ada 83 implementation has used an identifier now predefined in Ada 95. In the latter case, an inconsistency could occur if the Ada 83 use of the identifier is compatible with the Ada 95 use, though this is unlikely.

X.6.4 Error Incompatibilities

These incompatibilities only occur in programs containing runtime errors, either detectable (an exception is raised) or undetectable (the execution is erroneous).

X.7 Conclusion

This appendix has outlined the incompatibilities between Ada 83 and Ada 95. As we have seen, the small number that are likely to occur in practice are easily overcome. The remainder are unlikely to be encountered in normal programs but have been mentioned for completeness. For further details the reader should consult the comprehensive discussion in [Taylor 95] upon which this discussion has been based.

In conclusion, it is clear that there are unlikely to be significant transition issues for the vast majority of Ada 83 programs. Ada 95 has been carefully designed to minimize incompatibilities while meeting the overall goals of the requirements.

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Laurent Guerby