LensType.swift

struct Lens<A,B> {
    let get: (A) -> B
    let set: (B,A) -> A
}

extension Lens {
    func compose<C>(other: Lens<B, C>) -> Lens<A, C> {
        return Lens<A, C>(
            get: { whole in
                let part = self.get(whole)
                let subpart = other.get(part)
                
                return subpart
            },
            set: { (newSubpart, whole) in
                let part = self.get(whole)
                let newPart = other.set(newSubpart, part)
                let newWhole = self.set(newPart, whole)
                
                return newWhole
            }
        )
    }
}