Error Handling

Try Optional

try? turns a thrown error into nil so the caller can branch with optionals.

Convert failure to nil

try_optional.swift 
enum ParseError: Error {
    case notNumber
}

func parseCount(_ text: String) throws -> Int {
    if let value = Int(text) {
        return value
    }
    throw ParseError.notNumber
}

let rawCount = 
let count = try? parseCount(rawCount)
let label = count == nil ? "missing" : "count=\(count!)"

print(label)

enum ParseError: Error {
    case notNumber
}

func parseCount(_ text: String) throws -> Int {
    if let value = Int(text) {
        return value
    }
    throw ParseError.notNumber
}

let rawCount = 
let count = try? parseCount(rawCount)
let label = count == nil ? "missing" : "count=\(count!)"

print(label)

enum ParseError: Error {
    case notNumber
}

func parseCount(_ text: String) throws -> Int {
    if let value = Int(text) {
        return value
    }
    throw ParseError.notNumber
}

let rawCount = 
let count = try? parseCount(rawCount)
let label = count == nil ? "missing" : "count=\(count!)"

print(label)
try optional `try?` is useful when a missing value is enough information for the caller.