Try It Yourself - SyntaxStudy Editor

HTML / CSS / JS

import Foundation

enum ParseError: Error {
    case invalidInput(String)
    case outOfRange(value: Int, min: Int, max: Int)
}

func parseAge(_ input: String) throws -> Int {
    guard let age = Int(input) else {
        throw ParseError.invalidInput("'\(input)' is not a number")
    }
    guard (0...150).contains(age) else {
        throw ParseError.outOfRange(value: age, min: 0, max: 150)
    }
    return age
}

// do-catch
do {
    let age = try parseAge("25")
    print("Age: \(age)")
} catch ParseError.invalidInput(let msg) {
    print("Invalid: \(msg)")
} catch ParseError.outOfRange(let val, let min, let max) {
    print("\(val) must be between \(min) and \(max)")
} catch {
    print("Unexpected error: \(error)")
}

// try? - returns optional
let age1 = try? parseAge("abc")  // nil
let age2 = try? parseAge("30")   // Optional(30)

// Propagating errors
func validateUser(name: String, ageStr: String) throws -> String {
    let age = try parseAge(ageStr)
    return "\(name) is \(age) years old"
}