A proof-of-concept that attempts to create a syntax like "left < center < right", which ensures that "center" is both greater than "left" and less than "right".

Swift range conformance check.swift

//
//  Swift range conformance check.swift
//  Requires Swift 4
//
//  Created by Ben Leggiero on 2017-11-06.
//  Copyright © 2017 Ben Leggiero using BH-1-PS license
//
// This is a proof-of-concept that attempts to create a syntax like "left < center < right", which ensures that
// "center" is both greater than "left" and less than "right".
//
// This was created to address the idea put forth by Sid Mani in this SwifterSwift GitHub issue:
// https://github.com/SwifterSwift/SwifterSwift/issues/274
//



infix operator <: AdditionPrecedence



public struct PartialComparison_LeftAndCenter<T : Comparable> {
    
    // In case you want to save the original left side for later, uncomment this line:
    //let left: T
    let center: T
    
    let leftIsLessThanCenter: Bool
    
    init(left: T, center: T) {
        // In case you want to save the orignal left side for later, uncomment this line:
        //self.left = left
        self.center = center
        self.leftIsLessThanCenter = left < center
    }
    
    
    public static func <(partialLeft: PartialComparison_LeftAndCenter<T>, right: T) -> Bool {
        return partialLeft.leftIsLessThanCenter && partialLeft.center < right
    }
}



public extension Comparable {
    
    fileprivate func isLowerBound_creatingPartialLeftSide(of center: Self) -> PartialComparison_LeftAndCenter<Self> {
        return PartialComparison_LeftAndCenter(left: self, center: center)
    }
    
    
    public static func <(lowerBound: Self, center: Self) -> PartialComparison_LeftAndCenter<Self> {
        return lowerBound.isLowerBound_creatingPartialLeftSide(of: center)
    }
}



assert((1 < 2 < 3) == true, "1 < 2 < 3 ==  true")
assert((3 < 2 < 1) == false, "3 < 2 < 1 == false")
assert((1 < 1 < 3) == false, "1 < 1 < 3 == false")
assert((1 < 3 < 3) == false, "1 < 3 < 3 == false")
assert((1 < 1.5 < 2) == true, "1 < 1.5 < 3 == true")