Types & Functions Are Equivalent

June 10, 2026

It just occurred to me that types and functions are equivalent in the sense that one can declare a type in the form of a function. I’d never thought of it this way.

struct Circle {
    let radius: Double

    func area() -> Double {
        .pi * radius * radius
    }

    func perimeter() -> Double {
        2 * .pi * radius
    }
}

let circle = Circle(radius: 5)
circle.radius       // 5.0
circle.area()       // 78.54
circle.perimeter()  // 31.42

func makeCircle(radius: Double) -> (
    radius: () -> Double,
    area: () -> Double,
    perimeter: () -> Double
) {
    func getRadius() -> Double { radius }
    func area() -> Double { .pi * radius * radius }
    func perimeter() -> Double { 2 * .pi * radius }

    return (getRadius, area, perimeter)
}

let circle = makeCircle(radius: 5)
circle.radius()     // 5.0
circle.area()       // 78.54
circle.perimeter()  // 31.42