Traits

Traits: declaring one, implementing it with with or extend, supertraits, bounds, intersections, delegation with by, coherence and object safety.

What belongs here#

What does not belong here: type parameters themselves, which are in generics/. Every page in this folder is a reference page: an example first, then the syntax, then numbered rules, then what the construct is not.

Pages#

• Traits - A trait is a capability a type comes with. A single-method trait is named after its method, there is no inheritance, and operators are traits.
• extend - extend adds constants and functions to a type after its declaration, with a trait or without one, never adds a field or a case, and is named by the file that uses it when it targets a type of another package.
• Supertraits - A trait declared with a supertrait requires every implementing type to also implement that supertrait, and a default member can call the supertrait's members directly.
• Traits as types - A trait can stand wherever a type can, a value coerces to it automatically, and that coercion is the only subtyping the language has, with no variance for the types built from it.
• Trait intersections - The & operator combines two or more traits into one type, in a parameter, a field or a bound, and only traits can be combined this way.
• Delegation with by - by forwards a trait's required members to the one field of a single-field type, binding only to the trait or parenthesised group written directly in front of it.
• Coherence and blanket implementations - A package may implement a trait for a type only if it owns the type, the trait, or a type named as an argument of the trait, and two implementations of one trait may never overlap.
• Operators are traits - An operator is a trait exactly when it is a method call, so writing one on your own type means implementing the trait it stands for - and the three that are no method call are the three that are not traits.
• Object safety - A member that mentions Self in a parameter or its result, or that has no self, cannot be called on a trait-typed value, even though the trait stays a legal type.