Re:The Vale Programming Language
by wwright on Friday November 20, 2020 @12:20PM
(#25162571)
You may be thinking of Java's idea of an interface, which is a type (with specific subtyping behaviors). Traits in Rust are not types. There are two different ways to reference an object which implements a trait indirectly (actually more, but these are the dominant ones):
- a `Box<dyn Trait>` is like a Java object referenced by an interface. Everything uses dynamic dispatch, and it is an actual concrete type (though the actual "underlying" type is type-erased).
- an `-> impl Trait` is an existential type which uses a trait as a bound, which should be equivalent to D's "Voldemort" types, except that it can still satisfy Trait requires for other functions. For example, if you `-> impl Iterator<Item = u32>`, you can pass that result to a function expecting an iterator of `u32`. However, the type is fully defined by the callee and can't be instantiated/inspected externally.
- a `Box<dyn Trait>` is like a Java object referenced by an interface. Everything uses dynamic dispatch, and it is an actual concrete type (though the actual "underlying" type is type-erased).
- an `-> impl Trait` is an existential type which uses a trait as a bound, which should be equivalent to D's "Voldemort" types, except that it can still satisfy Trait requires for other functions. For example, if you `-> impl Iterator<Item = u32>`, you can pass that result to a function expecting an iterator of `u32`. However, the type is fully defined by the callee and can't be instantiated/inspected externally.