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