To break up the expression into distinct sub-expressions, you can try separating out each part of the calculation into its own intermediate variable. Here is one example.way you could do this: internal static let EaseOutBack = { (elapsed: TimeInterval, duration: TimeInterval) -> Double in let s: TimeInterval = 1.70158 var position: TimeInterval = elapsed / duration position -= 1.0 let a = position * position let b = (s + 1.0) * position let c = a * (b + s) let result = c + 1.0 return Double(result) }