Generic predicates (in Swift 4)

Predicates.swift

extension Sequence {
	func find<Property, Argument>(where predicate: (propertyPath: KeyPath<Self.Element, Property>, comparator: (Property, Argument)->Bool, argument: Argument)) -> [Self.Element] {
		return filter { predicate.comparator($0[keyPath: predicate.propertyPath], predicate.argument) }
	}
  
	// A more specialised version:
	func find(where characteristic: KeyPath<Self.Element, Bool>) -> [Self.Element] {
		return filter { $0[keyPath: characteristic] }
	}
}

// Usage
struct Car {
	let model: String
	let gears: UInt8
}
var cars = [Car]()

cars.find(where: (\.model, ==, "A9"))
cars.find(where: \.model.isEmpty)