Ada Reference Manual (Ada 2022 Draft 34)Legal Information
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12.7 Formal Packages

1
Formal packages can be used to pass packages to a generic unit. The formal_package_declaration declares that the formal package is an instance of a given generic package. Upon instantiation, the actual package has to be an instance of that generic package. 

Syntax

2/3
formal_package_declaration ::= 
    with package defining_identifier is new generic_package_name formal_package_actual_part
        [aspect_specification];
3/2
formal_package_actual_part ::= 
    ([others =>] <>)
  | [generic_actual_part]
  | (formal_package_association {, formal_package_association} [, others => <>])
3.1/2
formal_package_association ::= 
    generic_association
  | generic_formal_parameter_selector_name => <>
3.2/2
Any positional formal_package_associations shall precede any named formal_package_associations.

Legality Rules

4
The generic_package_name shall denote a generic package (the template for the formal package); the formal package is an instance of the template.
4.1/3
 The generic_formal_parameter_selector_name of a formal_package_association shall denote a generic_formal_parameter_declaration of the template. If two or more formal subprograms of the template have the same defining name, then named associations are not allowed for the corresponding actuals.
4.2/3
 A formal_package_actual_part shall contain at most one formal_package_association for each formal parameter. If the formal_package_actual_part does not include “others => <>”, each formal parameter without an association shall have a default_expression or subprogram_default.
4.3/3
 The rules for matching between formal_package_associations and the generic formals of the template are as follows: 
4.4/3
If all of the formal_package_associations are given by generic associations, the explicit_generic_actual_parameters of the formal_package_associations shall be legal for an instantiation of the template.
4.5/5
If a formal_package_association for a formal type T of the template is given by <>, then the formal_package_association for any other generic_formal_parameter_declaration of the template that mentions T directly or indirectly shall also be given by <>.
5/2
The actual shall be an instance of the template. If the formal_package_actual_part is (<>) or (others => <>), then the actual may be any instance of the template; otherwise, certain of the actual parameters of the actual instance shall match the corresponding actual parameters of the formal package, determined as follows:
5.1/2
If the formal_package_actual_part includes generic_associations as well as associations with <>, then only the actual parameters specified explicitly with generic_associations are required to match;
5.2/2
Otherwise, all actual parameters shall match, whether any actual parameter is given explicitly or by default. 
5.3/2
 The rules for matching of actual parameters between the actual instance and the formal package are as follows:
6/2
For a formal object of mode in, the actuals match if they are static expressions with the same value, or if they statically denote the same constant, or if they are both the literal null
7
For a formal subtype, the actuals match if they denote statically matching subtypes.
8
For other kinds of formals, the actuals match if they statically denote the same entity. 
8.1/1
 For the purposes of matching, any actual parameter that is the name of a formal object of mode in is replaced by the formal object's actual expression (recursively). 

Static Semantics

9
A formal_package_declaration declares a generic formal package.
10/2
The visible part of a formal package includes the first list of basic_declarative_items of the package_specification. In addition, for each actual parameter that is not required to match, a copy of the declaration of the corresponding formal parameter of the template is included in the visible part of the formal package. If the copied declaration is for a formal type, copies of the implicit declarations of the primitive subprograms of the formal type are also included in the visible part of the formal package.
11/2
For the purposes of matching, if the actual instance A is itself a formal package, then the actual parameters of A are those specified explicitly or implicitly in the formal_package_actual_part for A, plus, for those not specified, the copies of the formal parameters of the template included in the visible part of A.

Examples

12/2
Example of a generic package with formal package parameters:
13/2
with Ada.Containers.Ordered_Maps;  -- see A.18.6
generic
   with package Mapping_1 is new Ada.Containers.Ordered_Maps(<>);
   with package Mapping_2 is new Ada.Containers.Ordered_Maps
                                    (Key_Type => Mapping_1.Element_Type,
                                     others => <>);
package Ordered_Join is
   -- Provide a "join" between two mappings
14/2
   subtype Key_Type is Mapping_1.Key_Type;
   subtype Element_Type is Mapping_2.Element_Type;
15/2
   function Lookup(Key : Key_Type) return Element_Type;
16/2
   ...
end Ordered_Join;
17/2
Example of an instantiation of a package with formal packages:
18/2
with Ada.Containers.Ordered_Maps;
package Symbol_Package is
19/5
   subtype Key_String is String(1..5);
   type String_Id is ...
20/2
   type Symbol_Info is ...
21/5
   package String_Table is new Ada.Containers.Ordered_Maps
           (Key_Type => Key_String,
            Element_Type => String_Id);
22/2
   package Symbol_Table is new Ada.Containers.Ordered_Maps
           (Key_Type => String_Id,
            Element_Type => Symbol_Info);
23/2
   package String_Info is new Ordered_Join(Mapping_1 => String_Table,
                                           Mapping_2 => Symbol_Table);
24/2
   Apple_Info : constant Symbol_Info := String_Info.Lookup("Apple");
25/2
end Symbol_Package;

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