DistributionUpToMap.swift

struct Distribution<A> {
    var get: () -> A?
    
    func sample(n: Int) -> [A] {
        return (1...n).map { x in get()! }
    }
    
    func map<B>(f: A -> B) -> Distribution<B> {
        var d = Distribution<B>(get: {() -> Optional<B> in return nil})
        d.get = {
            (Void) -> B in return f(self.get()!)
        }
        return d
    }
    
    //...
}