Error Handling

Try Optional

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

Convert failure to nil

rawCount
try_optional.swift
Replay: real traced execution (multi-file project)
enum ParseError: Error {
    case notNumber
}

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

let rawCount = "12"
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 = "bad"
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 = "4"
let count = try? parseCount(rawCount)
let label = count == nil ? "missing" : "count=\(count!)"

print(label)

  1. rawCount ← 12

    12let rawCount→ 12 = "12"  //@rawCount="bad", "4"13let count = try? parseCount(rawCount12)14let label = count == nil ? "missing" : "count=\(count!)"

  2. func parseCount(_ text: String) throws -> Int

    5func parseCount(_ text12: String) throws -> Int {6    if let value = Int(text) {

  3. if let value = Int(text)

    5func parseCount(_ text: String) throws -> Int {6    if let value12 = Int(text12) {7        return value128    }

  4. count ← Optional(12), label ← count=12

    12let rawCount = "12"  //@rawCount="bad", "4"13let count→ Optional(12) = try? parseCount(rawCount12)14let label→ count=12 = countOptional(12) == nil ? "missing" : "count=\(count!)"1516print(labelcount=12)
    outputcount=12

  1. rawCount ← bad

    12let rawCount→ bad = "bad"13let count = try? parseCount(rawCountbad)14let label = count == nil ? "missing" : "count=\(count!)"

  2. func parseCount(_ text: String) throws -> Int

    5func parseCount(_ textbad: String) throws -> Int {6    if let value = Int(text) {7        return value8    }9    throw ParseError.notNumber10}

  3. count ← nil, label ← missing

    12let rawCount = "bad"13let count→ nil = try? parseCount(rawCountbad)14let label→ missing = countnil == nil ? "missing" : "count=\(count!)"1516print(labelmissing)
    outputmissing

  1. rawCount ← 4

    12let rawCount→ 4 = "4"13let count = try? parseCount(rawCount4)14let label = count == nil ? "missing" : "count=\(count!)"

  2. func parseCount(_ text: String) throws -> Int

    5func parseCount(_ text4: String) throws -> Int {6    if let value = Int(text) {

  3. if let value = Int(text)

    5func parseCount(_ text: String) throws -> Int {6    if let value4 = Int(text4) {7        return value48    }

  4. count ← Optional(4), label ← count=4

    12let rawCount = "4"13let count→ Optional(4) = try? parseCount(rawCount4)14let label→ count=4 = countOptional(4) == nil ? "missing" : "count=\(count!)"1516print(labelcount=4)
    outputcount=4
try optional `try?` is useful when a missing value is enough information for the caller.