HKTs

HKTs.swift

protocol Functor {
    associatedtype Tag
    associatedtype E //Element or _Element clashes on Dictionary :(
}
struct Functoring<F: Functor> {
    let _map: (F, (F.E) -> Any) -> Any
    func map<FB: Functor>(_ functor: F, transform: (F.E) -> FB.E) -> FB where F.Tag == FB.Tag {
        return _map(functor, transform) as! FB
    }
}

enum ArrayTag {}
extension Array: Functor {
    typealias Tag = ArrayTag
    typealias E = Element
}
enum DictionaryTag {}
extension Dictionary: Functor {
    typealias Tag = DictionaryTag
    typealias E = Value
}

func functoring<Element>() -> Functoring<[Element]> { return .init { $0.map($1) } }
func functoring<Key, Element>() -> Functoring<[Key: Element]> { return .init { $0.mapValues($1) }}

let array: [Int] = functoring().map(["foo"]) { $0.count }
let dict: [String: Int] = functoring().map(["bar": "foo"]) { $0.count }
let fail: [String: Int] = functoring().map(["foo"]) { $0.count }