function composition.swift

func compose<A, B, C>(
    f: @escaping (A) -> B, // f: A -> B
    g: @escaping (B) -> C  // g: B -> C
) -> (A) -> C {            // h: A -> C = g • f
    return { a in
        g(f(a)) // g • f
    }
}

// ... then
compose(f: Double.init(_:), g: String.init(_:))(1)

// which under the hood is the same that:
String(Double(Int(1)))