3.2.2 Subtype Declarations

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A subtype_declaration declares a subtype of some previously declared type, as defined by a subtype_indication.


subtype_declaration ::=
  subtype defining_identifier is subtype_indication;

subtype_indication ::=  subtype_mark [constraint]

subtype_mark ::= subtype_name

constraint ::= scalar_constraint | composite_constraint

scalar_constraint ::=
  range_constraint | digits_constraint | delta_constraint

composite_constraint ::=
  index_constraint | discriminant_constraint

Name Resolution Rules

A subtype_mark shall resolve to denote a subtype. The type determined by a subtype_mark is the type of the subtype denoted by the subtype_mark.

Dynamic Semantics

The elaboration of a subtype_declaration consists of the elaboration of the subtype_indication. The elaboration of a subtype_indication creates a new subtype. If the subtype_indication does not include a constraint, the new subtype has the same (possibly null) constraint as that denoted by the subtype_mark. The elaboration of a subtype_indication that includes a constraint proceeds as follows:

  • The constraint is first elaborated.
  • A check is then made that the constraint is compatible with the subtype denoted by the subtype_mark.

The condition imposed by a constraint is the condition obtained after elaboration of the constraint. The rules defining compatibility are given for each form of constraint in the appropriate subclause. These rules are such that if a constraint is compatible with a subtype, then the condition imposed by the constraint cannot contradict any condition already imposed by the subtype on its values. The exception Constraint_Error is raised if any check of compatibility fails.


4  A scalar_constraint may be applied to a subtype of an appropriate scalar type (see 3.5, 3.5.9, and J.3), even if the subtype is already constrained. On the other hand, a composite_constraint may be applied to a composite subtype (or an access-to-composite subtype) only if the composite subtype is unconstrained (see 3.6.1 and 3.7.1).


Examples of subtype declarations:

 subtype Rainbow   is Color range Red .. Blue;        --  see 3.2.1
 subtype Red_Blue  is Rainbow;
 subtype Int       is Integer;
 subtype Small_Int is Integer range -10 .. 10;
 subtype Up_To_K   is Column range 1 .. K;            --  see 3.2.1
 subtype Square    is Matrix(1 .. 10, 1 .. 10);       --  see 3.6
 subtype Male      is Person(Sex => M);               --  see 3.10.1

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