protocols_vs_generics.swift

import Cocoa

protocol Shape { }

struct Triangle: Shape { }

// Function signature requires a concrete type T, it is specialized by T
// Think of it as having a unique function per type: 
// - `protoflip(_ s: Triangle) -> Shape`
// - `protoflip(_ s: Square) -> Shape`
// - `protoflip(_ s: Circle) -> Shape`
//
func protoFlip<T: Shape>(_ shape: T) -> Shape {
    return shape
}

let triangle = Triangle()

// Error: protocol type 'Shape' cannot conform to 'Shape'
//        because only concrete types can conform to protocols
// Does not work because you receive T but return Shape
// T is more specific than Shape
// Think: subclass vs superclass
protoFlip(protoFlip(triangle))

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

// Function does not require a concrete type
func protoFlip2(_ shape: Shape) -> Shape {
    return shape
}

// Works
// function receives and returns Shape
protoFlip2(protoFlip2(triangle))


// This would also work, because you return a specific T
func protoFlip3<T: Shape>(_ shape: T) -> T {
    return shape
}

// Works
// function receives and returns T
// in this case T = Triangle
protoFlip3(protoFlip3(triangle))