(Improved) refinement types

[rockofox]

struct Person = (age: Int)
let can_drink (it: Person) => Bool = it.age > 21

let drink (person: Person{can_drink} beverage: String) => IO = ...

let main => IO = do
	drink Person {age: 23} # Fine
	drink Person {age: 16} # Error: 16 fails refinement `can_drink`
	
	println "How old are you?"
	let age = ($getLine) as Int
	drink Person{age: age} # Error: cannot validate refinement `can_drink`


	assert (can_drink person) do
		drink person
	else
		println "Under drinking age!"
	end
end