Error Handling

Throwing Functions

Functions marked with throws can stop normal execution and report an error.

Reject invalid input

throwing_functions.swift 
enum ScoreError: Error {
    case belowMinimum
}

func checkedScore(_ score: Int) throws -> String {
    if score < 0 {
        throw ScoreError.belowMinimum
    }
    return "score=\(score)"
}

let rawScore = 
let message: String

do {
    message = try checkedScore(rawScore)
} catch ScoreError.belowMinimum {
    message = "score rejected"
}

print(message)

enum ScoreError: Error {
    case belowMinimum
}

func checkedScore(_ score: Int) throws -> String {
    if score < 0 {
        throw ScoreError.belowMinimum
    }
    return "score=\(score)"
}

let rawScore = 
let message: String

do {
    message = try checkedScore(rawScore)
} catch ScoreError.belowMinimum {
    message = "score rejected"
}

print(message)

enum ScoreError: Error {
    case belowMinimum
}

func checkedScore(_ score: Int) throws -> String {
    if score < 0 {
        throw ScoreError.belowMinimum
    }
    return "score=\(score)"
}

let rawScore = 
let message: String

do {
    message = try checkedScore(rawScore)
} catch ScoreError.belowMinimum {
    message = "score rejected"
}

print(message)
throwing function A throwing function uses `throw` to leave the normal path when it cannot produce a valid result.