I quite like this idea and have written my thoughts on this topic down here on a similar thread:
The Function protocol looks very interesting. If we add an associated error type we can support throwing functions as well e.g.:
protocol Function<each Parameters, Error, Result> {
associatedtype each Parameters
associatedtype Error
associatedtype Result
func callAsFunction(_: repeat each Parameters) throws(Error) -> Result
}
In addition, we would want to make in ~Escable to make it non-escaping by default and likely ~Copyable or inherit from AnyObject to have reference semantics.
Async functions would likely just need a separate AsyncFunction protocol.