Higher-kinded types encoded as path-dependent types

hk.swift

// Based off of https://gist.github.com/runarorama/33986541f0f1ddf4a3c7

protocol λ {
  typealias α
}

struct K<L: λ>: λ {
  typealias α = L.α
}

protocol Each: λ {
  typealias α: λ
  typealias A = K<α>.α

  static func foreach(x: α, f: A -> Void)
}

extension Optional: λ {
  typealias α = Wrapped
}

struct OptionalEach<Wrapped>: Each {
  typealias α = Optional<Wrapped>

  static func foreach(x: α, f: Wrapped -> Void) {
    if let e = x {
      f(e)
    }
  }
}

extension Array: λ {
  typealias α = Element
}

struct ArrayEach<Element>: Each {
  typealias α = Array<Element>

  static func foreach(x: α, f: Element -> Void) {
    x.forEach(f)
  }
}

ArrayEach<Int>.foreach([1,2,3]) { print($0) }

OptionalEach<String>.foreach("a") { print($0) }
OptionalEach<String>.foreach(nil) { print($0) }


protocol HasEach: λ {
  typealias E: Each
}

extension Array: HasEach {
  typealias E = ArrayEach<Element>
}

// Can't figure out how to get the typeclass to resolve for global functions.
func foreach<X: HasEach>(x: X, f: X.α -> Void) {
  X.E.foreach(x, f: f)
}

// Compiler Error:
// error: cannot convert value of type 'X.α -> Void' to expected argument type '_ -> Void'
// X.E.foreach(x, f: f)