12.5 Formal Types

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A generic formal subtype can be used to pass to a generic unit a subtype whose type is in a certain class of types.


formal_type_declaration ::=
    type defining_identifier[discriminant_part] is formal_type_definition;

formal_type_definition ::=
  | formal_derived_type_definition
  | formal_discrete_type_definition
  | formal_signed_integer_type_definition
  | formal_modular_type_definition
  | formal_floating_point_definition
  | formal_ordinary_fixed_point_definition
  | formal_decimal_fixed_point_definition
  | formal_array_type_definition
  | formal_access_type_definition

Legality Rules

For a generic formal subtype, the actual shall be a subtype_mark; it denotes the (generic) actual subtype.

Static Semantics

A formal_type_declaration declares a (generic) formal type, and its first subtype, the (generic) formal subtype.

The form of a formal_type_definition determines a class to which the formal type belongs. For a formal_private_type_definition the reserved words tagged and limited indicate the class (see 12.5.1). For a formal_derived_type_definition the class is the derivation class rooted at the ancestor type. For other formal types, the name of the syntactic category indicates the class; a formal_discrete_type_definition defines a discrete type, and so on.

Legality Rules

The actual type shall be in the class determined for the formal.

Static Semantics

The formal type also belongs to each class that contains the determined class. The primitive subprograms of the type are as for any type in the determined class. For a formal type other than a formal derived type, these are the predefined operators of the type. For an elementary formal type, the predefined operators are implicitly declared immediately after the declaration of the formal type. For a composite formal type, the predefined operators are implicitly declared either immediately after the declaration of the formal type, or later in its immediate scope according to the rules of 7.3.1. In an instance, the copy of such an implicit declaration declares a view of the predefined operator of the actual type, even if this operator has been overridden for the actual type. The rules specific to formal derived types are given in 12.5.1.


7  Generic formal types, like all types, are not named. Instead, a name can denote a generic formal subtype. Within a generic unit, a generic formal type is considered as being distinct from all other (formal or nonformal) types.

8  A discriminant_part is allowed only for certain kinds of types, and therefore only for certain kinds of generic formal types. See 3.7.


Examples of generic formal types:

type Item is private;
type Buffer(Length Natural) is limited private;

type Enum  is (<>);
type Int   is range <>;
type Angle is delta <>;
type Mass  is digits <>;

type Table is array (Enum) of Item;

Example of a generic formal part declaring a formal integer type:

    type Rank is range <>;
    First  : Rank := Rank'First; 
    Second : Rank := First + 1;  --  the operator "+" of the type Rank

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Copyright © 2000 The MITRE Corporation, Inc. Ada Reference Manual