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8 Visibility Rules

This is an area of the language which is largely ignored by the normal programmer except when it produces surprising or frustrating consequences. The changes have thus been directed largely towards making the rules clear and consistent and with more helpful consequences. The changes are

8.1 Scope and Visibility

This is an area where there is no substitute for a precise description of the rules. Suffice it to say that the rules for Ada 83 were obscure and probably not truly understood by anybody; a consequence was subtle variation between compilers to the detriment of portability. We will not attempt to summarize the 95 rules but refer the reader to [RM95] for the details.

One important change is that character literals are now treated like other literals with regard to visibility. This means that they are always visible although the legality of their use will depend upon the context. So

   package P is
      type Roman_Digit is ('I', 'V', 'X', 'L', 'C', 'D', 'M');
   end P;

   with P;
   package Q is
      Five: constant P.Roman_Digit := 'V';
   end Q;
is allowed in Ada 95 although in Ada 83 we would have had to write P.'V' or alternatively supplied a use clause for P.

The visibility rules now also take account of the introduction of child packages. Most of the changes are straightforward but there is one interesting interaction with subunits. Consider

   package P is
   end P;

   package P.Q is
   end P.Q;

   package body P is
      Q: Integer;  -- legal
      procedure Sub is separate;
   end P;

   with P.Q;       -- illegal
   procedure Sub is
   end Sub;

The declaration of Q in the body of P is permitted because the body does not have a with clause for P.Q. But the with clause makes the subunit illegal because it could otherwise see both P.Q the child package and P.Q the variable in the body, and they are not overloadable.

8.2 Use Clauses

As mentioned in Part One the use of operators in Ada 83 caused problems. Many groups of users recognized that the use clause could make programs hard to understand because the origin of identifiers became obscure. Accordingly many organizations banned use clauses. This meant that either operators of user defined types had to be called with prefix notation such as Complex."+"(P, Q) or else had to be locally renamed.

This difficulty also occurred with predefined operators. Thus given an access type T declared in a package P, it was very annoying to find that one could not write

   with P;
   procedure Q is
      X: P.T;
      if X /= null then
   end Q;
but had to provide a use clause for P or a renaming for "=" or write the diabolical
   if P."/="(X, null) then

This problem is overcome in Ada 95 by the introduction of the use type clause which just provides visibility of the operators of a type and thereby allows them to be used in the natural infixed form. This ensures that a use package clause is not needed and hence the full name is still required for other identifiers.

The introduction of child units causes some extension to the rules for packages. As explained in Chapter 10, child units are treated like separately compiled but logically nested units. Like nested units, the name of a child mentioned in a with clause becomes directly visible when the logically enclosing parent package is specified in a use clause.

And so, using the example from II.8

   with OS.File_Manager;
   procedure Hello is
      use OS;  -- makes File_Manager directly visible
               -- as well as other declarations in package OS
      File: File_Descriptor :=
              File_Manager.Open("Hello.Txt", File_Manager.Write_Only);
      File_Manager.Write(File, "Hello world.");
   end Hello;

8.3 Renaming

To enhance the usefulness of renaming, the body of a subprogram may be provided by a renaming declaration.

If the subprogram declaration is in a package specification while the subprogram definition via a renaming is in a package body, the renaming must be of a subprogram that has subtype conformance (see 6.2) with the subprogram's declaration. This ensures that the caller of the subprogram will perform the correct constraint checks on the actual parameters, and pass the parameters following the correct calling convention seeing only the subprogram's specification.

A normal subprogram renaming requires only mode conformance. This kind of conformance is too weak for a renaming provided in the body. Given only mode conformance, the caller might perform constraint checks that were too stringent or too lax, and might pass parameters following the wrong calling conventions, putting them in the wrong place on the stack, or in the wrong register.

We considered requiring subtype conformance for all subprogram renaming. However, this introduces upward incompatibilities, particularly given the existing equivalence between generic formal subprogram matching and renaming. Furthermore, it is not always possible to statically match the subtype of a formal parameter of a subprogram, if the subprogram is implicitly declared as part of the type definition. In particular, if the subprogram is derived from a parent type, then the formal parameter subtypes have the constraints that were present on the parent type's subprogram. If the derived type definition itself imposes a constraint, then it is likely that the constraint on the formal parameter of the derived subprogram is actually looser than the constraint on the first subtype of the derived type. This means there is no nameable subtype that has constraints as loose as those on the formal parameter.

In the case of a primitive operation of a tagged type, renaming will cause a new slot in the dispatch table to be created if the renaming is itself primitive (that is in the same package specification as the type). If the original primitive operation is overridden then the renamed view will naturally depend upon whether renaming occurs before or after the overriding. Consider

   package P is
      type T is tagged ...;
      function Predefined_Equal(X, Y: T) return Boolean renames "=";
      function "="(X, Y: T) return Boolean:  -- overrides
      function User_Defined_Equal(X, Y: T) return Boolean renames "=";
   end P;
where we have renamed the predefined equality both before and after overriding it. Both renamings create new slots which are then initialized with the current meaning of equality. That for Predefined_Equal thus refers to the predefined equal whereas that for User_Defined_Equal refers to the overridden version. The consequence is that renaming can be used to hang on to an old primitive operation irrespective of whether that old operation is subsequently overridden. Such a renaming is itself a distinct primitive operation which could later be overridden for any subsequently derived type.

On the other hand a renaming which is not a primitive operation will not create a new slot but will simply refer to the operation at the point of the renaming. Thus if User_Defined_Equal is declared in a distinct package Q (after P), then it will not be primitive but will still refer to the overridden operation. This will occur even if the overriding is in the private part and thus not visible to Q. For a further discussion see [AARM 8.5.4].

This ability of renaming to create a new slot may be considered surprising because the general purpose of renaming is simply to create a new name for an existing entity; but there is of course no new entity being created but just a different way of accessing an existing entity.

Another very useful change is the ability to rename a library unit as a library unit. (It was possible to rename a library unit in Ada 83 but only as a local unit.) Library unit renaming is particularly important with the hierarchical library; this is discussed in detail in 10.1.2.

A related change is the ability to rename a generic unit. Curiously enough this was not permitted in Ada 83 although most other entities could be renamed. Thus we can write

   generic package Enum_IO renames Ada.Text_IO.Enumeration_IO;
as mentioned in [RM95 8.5.5].

In order to prevent difficulties with generic children, a child of a generic parent (such a child must be generic) can only be renamed if the renaming occurs inside the declarative region of its parent; it could be renamed as a child. This is consistent with the rules regarding the instantiation of such generic child units mentioned in II.8.

8.4 Other Improvements

The overload resolution rules of Ada 83 were confusing and unclear and this section of the reference manual has been completely rewritten.

An important new rule is the preference rule for operators and ranges of root numeric types (see 3.3). Briefly this says that an ambiguity can be resolved by preferring the operator of the root type. This rule coupled with automatic conversion from a universal type removes the need for the special rules in Ada 83 regarding universal convertible operands.

As an example, consider

   C: constant := 2 + 3;
which was allowed in Ada 83 because the expression was required to be universal and so no question of ambiguity arose. It is not ambiguous in Ada 95 either but for different reasons; the expression 2 + 3 is considered to be of type root_integer (it could otherwise be Integer or Long_Integer). The root_integer is then converted implicitly to universal_integer as required for the initial value.

The special rule regarding ranges in loops and for array indexes (which are in the distinct syntactic category discrete_subtype_definition) which result in them by treated as of type Integer if no specific subtype mark is specified is changed in Ada 95. The new rule is that if the range resolves to be of the type root_integer then it is taken to be of type Integer.

One outcome of all this is that we can now write

   for I in -1 .. 100 loop
as we have already mentioned in Part One. The interpretation is that -1 resolves to root_integer because of the preference rules and then the special rule just mentioned is used so that the range is finally treated as of type Integer.

8.5 Requirements Summary

Many of the changes in this area of the language are aimed at making the language more precise and easier to understand as requested by the general requirement

     R2.2-B(1) - Understandability
and we note in particular that the example in [DoD 90 A.2.3] concerning visibility of literals and operations has been addressed and satisfied.

The related requirement

     R2.2-C(1) - Minimize Special Case Restrictions
discussed in [DoD 90 A.3.12] contains the example of negative literals in loops which has also been satisfied.

The requirement

     R2.2-A(1) - Reduce Deterrents to Efficiency
is addressed by the elimination of the problem of returning task objects explicitly mentioned in the requirement in [DoD 90].

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Laurent Guerby