Variants

Variant types are used to create inheritance tree. Variant is an abstract type that other variants and records can derive from, inheriting all ancestor fields.

Root variants have one special enum field, called tag field. All descendent records are bound to one of possible tag values.

Generic variants are not supported.

Warning

Variant type and all its descendents must be declared in the same module.

Variant Declaration

Syntax:

variant AncestorVariant.VariantName : ImplementsInterface1, ImplementsInterface2, ...
{
    tag TagFieldType TagFieldName = TagDefaultValue;
    FieldType1 FieldName1 = DefaultValue1;
    FieldType2 FieldName2 = DefaultValue2;
    ...
}

where

VariantName is the valid identifier of the variant type being declared
AncestorVariant (optional) is the existing name of parent variant type
ImplementsInterfaceX (otional) is the existing interface name, see Interfaces
FieldType, FieldName and DefaultValue (optional) have the same meaning as for records, see Records.

Example:

enum ItemType
{
    sword;
    bow;
    shield;
}

variant Item
{
    int id;
    tag ItemType type; // Tag field
    string name;
}

// Nested variant
variant Item.Weapon
{
    float damage;
}

record Weapon.Sword[sword]
{
    float arc;
}

record Weapon.Bow[bow]
{
    float range;
}

record Item.Shield[shield]
{
    float armor;
}

See Field Order to understand order in which records are serialized (but tag field always goes first).

Tag Field

Note

All records deriving from variants should have tag value (e.g. [sword] in example above).

Warning

It is an error to define a variant record without tag value, or to define a non-variant record with tag value. It is also an error to have two variant records derived from the same variant with the same tag value.

Only enum tag field types are supported.

When deserializing, tag value is read first, and the relevant descendent record is instantiated and deserialized. When serializing, tag value is serialized first.

Field Redeclaration

The parent variant field may be redeclared in ancestor variant or record by declaring the field with the same name and type. It is an error to use another type for the redeclared field. But the default value and attributes may be overriden.