KeyPath_Map.swift

prefix operator ~

prefix func ~<A, B>(_ keyPath: KeyPath<A, B>) -> (A) -> B {
    return { $0[keyPath: keyPath] }
}

// list of names for each person
people.map(~\.name)
// => ["Tom", "Dick", "Harry", "Alice", "Bob", "Charles"]

// list of cities for each person
people.map(~\.address.city)
// => ["London", "Nottingham", "London", "Edinburgh", "Nottingham", "Leicester"]

// list of IDs for each person's address, where present
people.compactMap(~\.address.id)
// => ["1b0d", "0xg2"]