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[benrimmington]

Is it at the end of:

Casting a type to a Protocol with Self requirements

Existentials, Protocols with Associated Types (PATs), Generic Protocols (GPs), Higher Kinded Types (HKTs) are disjoint language features aiming to solve different problem spaces. Existentials: Allow for implementing things like Collection<Element = Int>. They allow for this by making the generic <T> concrete via <Element = Int> and applying some kind of type erasure. As such their instances are heap-allocated and make use of dynamic dispatch. You can only receive the concrete type from an exi…

Which you recently quoted in:

Questions about the future of Protocols with Associated Types

In that other post, there is a problem with this section: Because it should be more like: protocol Bindable<T> { // … associatedtype Flattenable: Bindable = Self func flatMap(_ transform: (Self.Element) throws -> Flattenable) rethrows → Output<Flattenable.Element> }