QuadraticFormula implemented in Swift

QuadraticFormula.swift

//
//  QuadraticFormula.swift
//  Convenience around the QuadraticFormula
//
//  Created by Valentin Walter on 11/12/20.
//

import Foundation

/// Provides convenience around the [Quadratic Formula](https://en.wikipedia.org/wiki/Quadratic_formula).
///
/// Given a general quadratic equation of the form
///
///     ax² + bx + c = 0
///
/// with x representing an unknown, a, b and c representing
/// constants with a ≠ 0, the quadratic formula is:
///
///     x = (-b ± √(b² - 4ac)) / 2a
///
struct QuadraticFormula: CustomStringConvertible {
    /// The `a` in `ax² + bx + c = 0`. Default is 1.
    var a: Double = 1
    /// The `b` in `ax² + bx + c = 0`. Default is 0.
    var b: Double = 0
    /// The `c` in `ax² + bx + c = 0`. Default is 0.
    var c: Double = 0
    
    /// The first solution to the formula, if any.
    var x1: Double? { roots.x1 }
    /// The second solution to the formula, if any.
    var x2: Double? { roots.x2 }
    
    /// A tuple containing the two solutions `x1` and `x2`.
    /// If there's only one solution, `x2` is `nil`.
    /// If there's no solution both values are `nil`.
    var roots: (x1: Double?, x2: Double?) {
        let discriminant = ((b * b) - (4 * a * c)).squareRoot()
        
        if discriminant > 0 {
            let x1 = (-b + discriminant) / (2 * a)
            let x2 = (-b - discriminant) / (2 * a)
            return (x1, x2)
        } else if discriminant == 0 {
            return (0, nil)
        } else {
            return (nil, nil)
        }
    }
    
    // CustomStringConvertible implementation
    // Inherits documentation
    var description: String {
        switch calculate() {
        case let (.some(x1), .some(x2)):
            return "x1 is \(x1), x2 is \(x2)"
        case let (.some(x), nil):
            return "x is \(x)"
        default:
            return "No solution."
        }
    }
}

// --------- Terminal application loop ---------
func main() {
    var formula = QuadraticFormula()
    print("a:")
    formula.a = Double(readLine()!) ?? 0
    print("b:")
    formula.b = Double(readLine()!) ?? 0
    print("c:")
    formula.c = Double(readLine()!) ?? 0

    print(formula)
    
    print("Repeat? (y/n)")
    if readLine() == "y" {
        main()
    } else {
        exit(0)
    }
}

main()
// --------- Terminal application loop ---------