Error Handling

Result Values

Result stores either a success value or a failure value for later handling.

Store success or failure

userId
result_values.swift
Replay: real traced execution (multi-file project)
enum LookupError: Error {
    case missing
}

func lookupName(_ id: Int) -> Result<String, LookupError> {
    if id == 1 {
        return Result<String, LookupError>.success("Ada")
    }
    return Result<String, LookupError>.failure(LookupError.missing)
}

let userId = 1
let result = lookupName(userId)
let name = try? result.get()
let message = name == nil ? "missing user" : "user=\(name!)"

print(message)

enum LookupError: Error {
    case missing
}

func lookupName(_ id: Int) -> Result<String, LookupError> {
    if id == 1 {
        return Result<String, LookupError>.success("Ada")
    }
    return Result<String, LookupError>.failure(LookupError.missing)
}

let userId = 2
let result = lookupName(userId)
let name = try? result.get()
let message = name == nil ? "missing user" : "user=\(name!)"

print(message)

  1. userId ← 1

    12let userId→ 1 = 1  //@userId=213let result = lookupName(userId1)14let name = try? result.get()

  2. func lookupName(_ id: Int) -> Result<String, LookupError>

    5func lookupName(_ id1: Int) -> Result<String, LookupError> {6    if id == 1 {

  3. if id == 1

    5func lookupName(_ id: Int) -> Result<String, LookupError> {6    if id1 == 1 {7        return Result<String, LookupError>.success("Ada")8    }

  4. result ← success("Ada"), name ← Optional("Ada"), message ← user=Ada

    12let userId = 1  //@userId=213let result→ success("Ada") = lookupName(userId1)14let name→ Optional("Ada") = try? resultsuccess("Ada").get()15let message→ user=Ada = nameOptional("Ada") == nil ? "missing user" : "user=\(name!)"1617print(messageuser=Ada)
    outputuser=Ada

  1. userId ← 2

    12let userId→ 2 = 213let result = lookupName(userId2)14let name = try? result.get()

  2. func lookupName(_ id: Int) -> Result<String, LookupError>

    5func lookupName(_ id2: Int) -> Result<String, LookupError> {6    if id == 1 {7        return Result<String, LookupError>.success("Ada")8    }9    return Result<String, LookupError>.failure(LookupError.missing)10}

  3. result ← failure(main.LookupError.missing), name ← nil, message ← missing user

    12let userId = 213let result→ failure(main.LookupError.missing) = lookupName(userId2)14let name→ nil = try? resultfailure(main.LookupError.missing).get()15let message→ missing user = namenil == nil ? "missing user" : "user=\(name!)"1617print(messagemissing user)
    outputmissing user
result `Result` lets code pass an error outcome as data instead of throwing immediately.