A port of Kotlin-stdlib's associate method to Swift 4.

Sequence+associate.swift

extension Sequence {
    
    func associate<K, V>(_ transform: (Element) -> (K, V)) -> [K: V] {
        return self.reduce(into: [:]) { (dict, element) in
            let (key, value) = transform(element)
            dict[key] = value
        }
    }
    
    func associateBy<K>(_ keySelector: (Element) -> K) -> [K: Element] {
        return self.reduce(into: [:]) { (dict, element) in
            dict[keySelector(element)] = element
        }
    }
    
    func associateBy<K, V>(_ keySelector: (Element) -> K, _ valueTransform: (Element) -> V) -> [K: V] {
        return self.reduce(into: [:]) { (dict, element) in
            dict[keySelector(element)] = valueTransform(element)
        }
    }
    
}