RoundHole.swift

protocol Square {
    var size : Float{ get }
}

protocol Circle {
    var radius : Float{ get }
}


class SquarePeg : Square {
    let _size : Float
    init( size : Float ) { _size = size }
    
    var size : Float { get{ return _size } }
}


class RoundHole {
    func fit( _ circleShapedThing : Circle ) {
        print( "fit, with a radius of: \(circleShapedThing.radius)")
    }
}

// ----------

extension SquarePeg : Circle {
    var radius : Float{ get { return (_size*size*2).squareRoot() } }
}

let squarePeg = SquarePeg( size: 42.0 )
let hole = RoundHole()

hole.fit( squarePeg )