Annotated Ada Reference Manual — Legal Information
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7.2 Package Bodies

1
[In contrast to the entities declared in the visible part of a package, the entities declared in the package_body are visible only within the package_body itself. As a consequence, a package with a package_body can be used for the construction of a group of related subprograms in which the logical operations available to clients are clearly isolated from the internal entities.] 

Syntax

2
package_body ::= 
    package body defining_program_unit_name is
       declarative_part
   [begin
        handled_sequence_of_statements]
    end [[parent_unit_name.]identifier];
3
If an identifier or parent_unit_name.identifier appears at the end of a package_body, then this sequence of lexical elements shall repeat the defining_program_unit_name.

Legality Rules

4
A package_body shall be the completion of a previous package_declaration or generic_package_declaration. A library package_declaration or library generic_package_declaration shall not have a body unless it requires a body[; pragma Elaborate_Body can be used to require a library_unit_declaration to have a body (see 10.2.1) if it would not otherwise require one]. 
4.a
Ramification: The first part of the rule forbids a package_body from standing alone — it has to belong to some previous package_declaration or generic_package_declaration.
4.b
A nonlibrary package_declaration or nonlibrary generic_package_declaration that does not require a completion may have a corresponding body anyway.

Static Semantics

5
In any package_body without statements there is an implicit null_statement. For any package_declaration without an explicit completion, there is an implicit package_body containing a single null_statement. For a noninstance, nonlibrary package, this body occurs at the end of the declarative_part of the innermost enclosing program unit or block_statement; if there are several such packages, the order of the implicit package_bodies is unspecified. [(For an instance, the implicit package_body occurs at the place of the instantiation (see 12.3). For a library package, the place is partially determined by the elaboration dependences (see Section 10).)] 
5.a
Discussion: Thus, for example, we can refer to something happening just after the begin of a package_body, and we can refer to the handled_sequence_of_statements of a package_body, without worrying about all the optional pieces. The place of the implicit body makes a difference for tasks activated by the package. See also RM83-9.3(5).
5.b
The implicit body would be illegal if explicit in the case of a library package that does not require (and therefore does not allow) a body. This is a bit strange, but not harmful. 

Dynamic Semantics

6
For the elaboration of a nongeneric package_body, its declarative_part is first elaborated, and its handled_sequence_of_statements is then executed. 
NOTES
7
3  A variable declared in the body of a package is only visible within this body and, consequently, its value can only be changed within the package_body. In the absence of local tasks, the value of such a variable remains unchanged between calls issued from outside the package to subprograms declared in the visible part. The properties of such a variable are similar to those of a “static” variable of C.
8
4  The elaboration of the body of a subprogram explicitly declared in the visible part of a package is caused by the elaboration of the body of the package. Hence a call of such a subprogram by an outside program unit raises the exception Program_Error if the call takes place before the elaboration of the package_body (see 3.11). 

Examples

9
Example of a package body (see 7.1): 
10
package body Rational_Numbers is
11
   procedure Same_Denominator (X,Y : in out Rational) is
   begin
      --  reduces X and Y to the same denominator:
      ...
   end Same_Denominator;
12
   function "="(X,Y : Rational) return Boolean is
      U : Rational := X;
      V : Rational := Y;
   begin
      Same_Denominator (U,V);
      return U.Numerator = V.Numerator;
   end "=";
13
   function "/" (X,Y : Integer) return Rational is
   begin
      if Y > 0 then
         return (Numerator => X,  Denominator => Y);
      else
         return (Numerator => -X, Denominator => -Y);
      end if;
   end "/";
14
   function "+" (X,Y : Rational) return Rational is ... end "+";
   function "-" (X,Y : Rational) return Rational is ... end "-";
   function "*" (X,Y : Rational) return Rational is ... end "*";
   function "/" (X,Y : Rational) return Rational is ... end "/";
15
end Rational_Numbers;

Wording Changes from Ada 83

15.a
The syntax rule for package_body now uses the syntactic category handled_sequence_of_statements.
15.b
The declarative_part of a package_body is now required; that doesn't make any real difference, since a declarative_part can be empty.
15.c
RM83 seems to have forgotten to say that a package_body can't stand alone, without a previous declaration. We state that rule here.
15.d
RM83 forgot to restrict the definition of elaboration of package_bodies to nongeneric ones. We have corrected that omission.
15.e
The rule about implicit bodies (from RM83-9.3(5)) is moved here, since it is more generally applicable. 
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