Avoid hard-coded KVC collection operators

CollectionOperator.swift

import Foundation


enum CollectionOperator: String {

    case avg
    case count
    case max
    case min
    case sum
    case unionOfObjects
    case distinctUnionOfObjects
    case distinctUnionOfArrays
    case unionOfArrays
    case distinctUnionOfSets

}

precedencegroup Additive {
    associativity: left
}

precedencegroup Multiplicative {
    associativity: left
    higherThan: Additive
}
infix operator => : Multiplicative

func => (lhs: CollectionOperator, rhs: String) -> String {
    return "@" + lhs.rawValue + "." + rhs
}


let a = ["2.1", "-10", "2.1", "5", "2.5", "hello"]
let b = a as NSArray

// @avg.doubleValue
b.value(forKeyPath: .avg => #keyPath(String.doubleValue))

// @distinctUnionOfObjects.length
b.value(forKeyPath: .distinctUnionOfObjects => #keyPath(String.length))