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traits.md

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Traits

A trait defines an interface for a type. Traits can't have fields, but they can define behavior. That behavior is then inherited by all classes that are subtypes of the trait. A class can implement multiple traits by listing them in the base type list. Traits can be subtypes of other types. Trait members that do not provide implementations are not marked "abstract".

public trait Example: SomeClass
{
}

trait_declaration
    : access_modifier "closed"? ("const"|"drop")? "trait" identifier generic_parameters? base_types trait_body
    ;

trait_body
    : "{" trait_member* "}"
    ;

trait_member
    : named_function
    | get_function
    | set_function
    | operator_overload
    ;

Const Traits

A trait may be marked const. This indicates any subtype of it must be const. This allows the compiler to assume all references to such a trait are const. See constant classes and structs for more information.

Drop Traits

As with classes, structs and values, a trait may be declared as a drop type. This requires that all subtypes of the trait be a drop class, struct, value, or trait.

Closed Traits

A trait marked closed can only be directly subtyped in the same package. This enables exhaustiveness checking when matching on the trait. For more information see Closed Classes.