3.8.1 Variant Parts and Discrete Choices

From OC Systems Wiki!
< Guide:95lrm
Revision as of 23:37, 4 May 2019 by imported>WikiVisor (1 revision imported)
(diff) ← Older revision | Latest revision (diff) | Newer revision → (diff)
Jump to: navigation, search

A record type with a variant_part specifies alternative lists of components. Each variant defines the components for the value or values of the discriminant covered by its discrete_choice_list.


variant_part ::=
   case discriminant_direct_name is
   end case;

variant ::=
   when discrete_choice_list =>

discrete_choice_list ::= discrete_choice {| discrete_choice}

discrete_choice ::= expression | discrete_range | others

Name Resolution Rules

The discriminant_direct_name shall resolve to denote a discriminant (called the discriminant of the variant_part) specified in the known_discriminant_part of the full_type_declaration that contains the variant_part. The expected type for each discrete_choice in a variant is the type of the discriminant of the variant_part.

Legality Rules

The discriminant of the variant_part shall be of a discrete type.

The expressions and discrete_ranges given as discrete_choices in a variant_part shall be static. The discrete_choice others shall appear alone in a discrete_choice_list, and such a discrete_choice_list, if it appears, shall be the last one in the enclosing construct.

A discrete_choice is defined to cover a value in the following cases:

  • A discrete_choice that is an expression covers a value if the value equals the value of the expression converted to the expected type.
  • A discrete_choice that is a discrete_range covers all values (possibly none) that belong to the range.
  • The discrete_choice others covers all values of its expected type that are not covered by previous discrete_choice_lists of the same construct.

A discrete_choice_list covers a value if one of its discrete_choices covers the value.

The possible values of the discriminant of a variant_part shall be covered as follows:

  • If the discriminant is of a static constrained scalar subtype, then each non-others discrete_choice shall cover only values in that subtype, and each value of that subtype shall be covered by some discrete_choice (either explicitly or by others);
  • If the type of the discriminant is a descendant of a generic formal scalar type then the variant_part shall have an others discrete_choice;
  • Otherwise, each value of the base range of the type of the discriminant shall be covered (either explicitly or by others).

Two distinct discrete_choices of a variant_part shall not cover the same value.

Static Semantics

If the component_list of a variant is specified by null, the variant has no components.

The discriminant of a variant_part is said to govern the variant_part and its variants. In addition, the discriminant of a derived type governs a variant_part and its variants if it corresponds (see 3.7) to the discriminant of the variant_part.

Dynamic Semantics

A record value contains the values of the components of a particular variant only if the value of the discriminant governing the variant is covered by the discrete_choice_list of the variant. This rule applies in turn to any further variant that is, itself, included in the component_list of the given variant.

The elaboration of a variant_part consists of the elaboration of the component_list of each variant in the order in which they appear.


Example of record type with a variant part:

type Device is (Printer, Disk, Drum);
type State  is (Open, Closed);

type Peripheral(Unit Device := Disk) is 
        Status : State;
        case Unit is
            when Printer =>
                 Line_Count : Integer range 1 .. Page_Size;
            when others =>
                 Cylinder   : Cylinder_Index;
                 Track      : Track_Number;
        end case;
      end record;

Examples of record subtypes:

subtype Drum_Unit is Peripheral(Drum);
subtype Disk_Unit is Peripheral(Disk);

Examples of constrained record variables:

Writer   : Peripheral(Unit  => Printer); 
Archive  : Disk_Unit;

Copyright © 1992,1993,1994,1995 Intermetrics, Inc.
Copyright © 2000 The MITRE Corporation, Inc. Ada Reference Manual