Variant Parts and Discrete Choices
Author: JTC1/SC22/WG9/ARG, by Randall Brukardt, ARG Editor

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# 3.8.1 Variant Parts and Discrete Choices

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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.

#### Syntax

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variant_part ::=
case discriminant_direct_name is
variant
{variant}
end case;
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variant ::=
when discrete_choice_list =>
component_list
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discrete_choice_list ::= discrete_choice {| discrete_choice}
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discrete_choice ::= expression | discrete_range | others

#### Name Resolution Rules

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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

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The discriminant of the variant_part shall be of a discrete type.
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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.
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A discrete_choice is defined to cover a value in the following cases:
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- A discrete_choice that is an expression covers a value if the value equals the value of the expression converted to the expected type.

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- A discrete_choice that is a discrete_range covers all values (possibly none) that belong to the range.

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- The discrete_choice others covers all values of its expected type that are not covered by previous discrete_choice_lists of the same construct.

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A discrete_choice_list covers a value if one of its discrete_choices covers the value.
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The possible values of the discriminant of a variant_part shall be covered as follows:
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- 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);

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- 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;

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- Otherwise, each value of the base range of the type of the discriminant shall be covered (either explicitly or by others).

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

#### Static Semantics

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

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

#### Examples

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Example of record type with a variant part:
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`type Device is (Printer, Disk, Drum);
type State is (Open, Closed);`
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`type Peripheral(Unit : Device := Disk) is
record
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;`
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Examples of record subtypes:
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`subtype Drum_Unit is Peripheral(Drum);
subtype Disk_Unit is Peripheral(Disk);`
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Examples of constrained record variables:
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`Writer : Peripheral(Unit => Printer);
Archive : Disk_Unit;`

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