Today, I want to talk about a curious case I discovered while playing with generic programming with Swift.

To illustrate, let’s start by writing a simple function.

func Min(x:Int, y:Int) -> Int {
    println("Using Min<Int>")
    return (x < y) ? x : y
}

let a = Min(-1, 1) // Using Min<Int>

And now let’s make it generic.

func Min<T:Comparable>(x:T, y:T) -> T {
    println("Using Min<T>")
    return (x < y) ? x : y
}

let a = Min(-1, 1) // Using Min<T>

What if we’ve both the implementations? The compiler automatically picks the specialized version.

func Min<T:Comparable>(x:T, y:T) -> T {
    println("Using Min<T>")
    return (x < y) ? x : y
}

func Min(x:Int, y:Int) -> Int {
    println("Using Min<Int>")
    return (x < y) ? x : y
}

let a = Min(-1, 1) // Using Min<Int>

The swift standard library already provides a min and max functions.

func min<T : Comparable>(x: T, y: T) -> T

Suppose we use that as the generic version and override our specialized one for Int? The compiler still picks the specialized one.

func min(x:Int, y:Int) -> Int {
    println("Using Min<Int>")
    return (x < y) ? x : y
}

let a = min(-1, 1) // Using min<Int>

This is really convenient, isn’t it? Let’s expand our example to something you might face in real life. Let’s work with a Vector2 type.

struct Vector2 {
    var x : Float = 0.0
    var y : Float = 0.0

    init(x:Float = 0,y:Float = 0) {
        self.x = x
        self.y = y
    }
}

How about making use of standard min and max functions with Vector2?

let lowerBound = Vector2(x: -1, y: -1)
let upperBound = Vector2(x: 1, y: 1)

let a = min(lowerBound, upperBound)

This is going to throw an error, as the standard min and max functions need the type to conform to the comparable protocol. So let’s do that first.

func ==(lhs: Vector2, rhs: Vector2) -> Bool {
   return (lhs.x == rhs.x) && (lhs.y == rhs.y)
}

func < (lhs: Vector2, rhs: Vector2) -> Bool {
    return (lhs.x < rhs.x) && (lhs.y < rhs.y)
}

extension Vector2 : Comparable {}

let lowerBound = Vector2(x: -1, y: -1)
let upperBound = Vector2(x: 1, y: 1)

let a = min(lowerBound, upperBound)

This works as expected. This is a great feature, in my opinion, one of the best things to switch from Objective-C to Swift. Moving forward, let’s write a generic clamp function.


func clamp<T:Comparable>(value:T, lowerBound:T, upperBound:T) -> T {
    return min(max(lowerBound, value), upperBound)
}

let b = clamp(10, -1, 1) // prints 1

This is awesome! Let’s try our clamp function with Vector2.

let lowerBound = Vector2(x: -100, y: -100)
let upperBound = Vector2(x: 100, y: 100)

let test1 = clamp(Vector2(x: 200, y: 200), lowerBound, upperBound) // {x:100, y:100}
let test2 = clamp(Vector2(x: -200, y: -200), lowerBound, upperBound) // {x:-100, y:-100}
let test3 = clamp(Vector2(x: -10, y: -134), lowerBound, upperBound) // {x:-10, y:-134}
let test4 = clamp(Vector2(x: 10, y: 134), lowerBound, upperBound) // {x:10, y:134}

At first this might look bad, because the test3 and test4 are not correct. But, this is not the compiler’s fault. The standard min and max use the overloaded comparison operators and they are not correct. We can test this with

let test5 = min(Vector2(x: -10, y: -134), lowerBound) // {x:-10, y:-134}
let test6 = max(Vector2(x: 10, y: 134), upperBound) // {x:10, y:134}

Let’s fix them by providing our own specialized versions.

func min(lhs: Vector2, rhs: Vector2) -> Vector2 {
    return Vector2(x: min(lhs.x, rhs.x), y: min(lhs.y, rhs.y))
}

func max(lhs: Vector2, rhs: Vector2) -> Vector2 {
    return Vector2(x: max(lhs.x, rhs.x), y: max(lhs.y, rhs.y))
}

let test5 = min(Vector2(x: -10, y: -134), lowerBound) // {x:-100, y:-134}
let test6 = max(Vector2(x: 10, y: 134), upperBound) // {x:100, y:134}

This looks better in the sense that the min and max is calculated per component. But, something is still wrong with our clamp tests, as they still print the same old value. Turns out the min and max within clamp function still use the standard generic function, rather than our provided specialized version.

The only way to make this work is if I provide a specialized clamp function for Vector2.

func clamp(value:Vector2, lowerBound:Vector2, upperBound:Vector2) -> Vector2 {
    return min(max(lowerBound, value), upperBound)
}

let test1 = clamp(Vector2(x: 200, y: 200), lowerBound, upperBound) // {x:100, y:100}
let test2 = clamp(Vector2(x: -200, y: -200), lowerBound, upperBound) // {x:-100, y:-100}
let test3 = clamp(Vector2(x: -10, y: -134), lowerBound, upperBound) // {x:-10, y:-100}
let test4 = clamp(Vector2(x: 10, y: 134), lowerBound, upperBound) // {x:10, y:100}

Note that the specialized clamp implementation for Vector2 is exactly the same as the generic one. And this is not good, as now if we want the compiler to automatically pick the right version, we have to implement the entire chain down to every function. So, it comes down to either using the generic functions all the way up or implementing the entire chain.

To compare, here’s a C++ version of the same functionality that works great with a single clamp generic implementation.

#include <iostream>

struct Vector2 {
    float x;
    float y;

    Vector2(float x = 0, float y = 0) {
        this->x = x;
        this->y = y;
    }
};

template <typename T>
T min(T a, T b) {
    return a < b ? a : b;
}

template <>
Vector2 min(Vector2 a, Vector2 b) {
    return Vector2(min(a.x, b.x), min(a.y, b.y));
}

template <typename T>
T max(T a, T b) {
    return a > b ? a : b;
}

template <>
Vector2 max(Vector2 a, Vector2 b) {
    return Vector2(max(a.x, b.x), max(a.y, b.y));
}

template <typename T>
T clamp(T value, T lowerBound, T upperBound) {
    return min(max(lowerBound, value), upperBound);
}

std::ostream &operator<<(std::ostream &os, const Vector2 &v) {
    os << v.x << ", " << v.y;
    return os;
}

int main() {
    Vector2 lowerBound(-100, -100);
    Vector2 upperBound(100, 100);

    std::cout << clamp(Vector2(200, 200), lowerBound, upperBound) << std::endl;
    std::cout <<  clamp(Vector2(-200, -200), lowerBound, upperBound) << std::endl;
    std::cout << clamp(Vector2(-10, -134), lowerBound, upperBound) << std::endl;
    std::cout <<  clamp(Vector2(10, 134), lowerBound, upperBound) << std::endl;

    std::cout <<  min(Vector2(-10, -134), lowerBound) << std::endl;
    std::cout <<  max(Vector2(10, 134), upperBound) << std::endl;
}

In conclusion, generic programming with Swift is great and definitely a step forward than Objective-C, but somebody coming from C++ would be a little disappointed. The bright side is that the Swift language is rapidly evolving and maybe this would get improved in the future version.

The entire code for this rant is available at C++ and Swift.