Is GAT actually as powerful as HRTB?

language design
14

This could be alleviated with some very basic GAT helper trait (actually, a set of trait to cover arities):

trait TypeConstructor1 { type T<_0> : ?Sized; }
trait TypeConstructor2 { type T<_0, _1> : ?Sized; }
// etc.

// and some ergonomic type aliases
type Feed1<T_, A0    > = <T_ as TypeConstructor1>::T<A0    >;
type Feed2<T_, A0, A1> = <T_ as TypeConstructor2>::T<A0, A1>;
// ...

and then have something like

#[derive(TypeConstructor)]
struct Arc<T>...

doing the expected thing for some type Arc_.

From there, any downstream library could write their own type aliases:

trait RProvider : TypeConstructor1 {
    type R<T> : Deref<Target = T> + Clone;
}
impl<RC_> RProvider for RC_
where
    RC_ : TypeConstructor1,
    for<T> // <- no `for<T>` yet...
        Feed1<RC_, T> : Deref<Target = T> + Clone
    ,
{
    type R<T> = Feed1<RC_, T>;
}

If we had:

  • those for<T> higher order bounds (that is, HRTB but with types rather than just lifetimes),
  • potentially trait aliases for these versions with added bounds to show better intent (but does not seem paramount),
  • the #[derive(TypeConstructor)] automagically applied to all type definitions (with a way to handle any arity),
  • and the sugar to be able to:
    • write Rc rather than Rc_,
    • write Thing<T> rather than Feed1<Thing, T>,

then we'd have HKTs. When you stare at that list, besides the type-generic HRTB (for<T> ...), everything is mostly sugar and convenience (but for the impossibility to forget to derive(TypeConstructor)). This last point does indeed hurt in practice because of coherence: should a user library try to provide this, there should be only one such canonical library all the crates ought to turn to, based on cooperation and stuff, à la serde.

So I am mostly agreeing with you :grinning_face_with_smiling_eyes:, but I just wanted to mention that regarding coherence, there could be a decoupling between some universal serde-like crate providing the TypeConstructor definitions, and then freedom for downstream crates to provide their own specialized aliases :slightly_smiling_face:

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