Error protocol conformance in Swift - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Swift error handling code?

Consider the following Swift code that defines an error and uses do-catch to handle it. What will be printed?

Swift

enum MyError: Error {
    case badInput
}

func testError() throws {
    throw MyError.badInput
}

do {
    try testError()
    print("No error")
} catch MyError.badInput {
    print("Caught badInput error")
} catch {
    print("Caught some other error")
}

A Caught badInput error

B Compilation error

C Caught some other error

D No error

💡 Hint

Think about which error is thrown and which catch block matches it.

The function testError() throws MyError.badInput. The do-catch block catches this specific error in the first catch clause and prints "Caught badInput error".

❓ Predict Output

intermediate

What error does this Swift code raise?

Given this Swift code, what error will be raised at runtime?

Swift

struct NotAnError {}

func throwSomething() throws {
    throw NotAnError()
}

try? throwSomething()

A Type 'NotAnError' does not conform to protocol 'Error'

B No error, code runs fine

C Runtime error: unexpected non-Error thrown

D Compilation error: missing throws keyword

💡 Hint

Check if the thrown type conforms to the Error protocol.

In Swift, only types conforming to the Error protocol can be thrown. Since NotAnError does not conform, the compiler will raise an error.

🔧 Debug

advanced

Why does this Swift code fail to compile?

Examine the code below. Why does it fail to compile?

Swift

enum CustomError: Error {
    case errorOne
}

func mightThrow(_ flag: Bool) throws {
    if flag {
        throw "Error occurred"
    }
}

try? mightThrow(true)

A Missing throws keyword in function signature

B String does not conform to Error protocol

C Cannot throw from non-throwing function

D Enum cases must have associated values

💡 Hint

Look at the type being thrown inside the function.

The code tries to throw a String, but String does not conform to Error. Only types conforming to Error can be thrown.

🧠 Conceptual

advanced

Which Swift type correctly conforms to the Error protocol?

Which of the following Swift types correctly conforms to the Error protocol?

A enum MyError {}

B class MyError {}

C struct MyError: CustomStringConvertible {}

D struct MyError: Error {}

💡 Hint

Remember that to be throwable, a type must explicitly conform to Error.

Only option D explicitly declares conformance to Error. The others do not conform and thus cannot be thrown.

❓ Predict Output

expert

What is the output of this Swift error handling with pattern matching?

Analyze the following Swift code. What will it print?

Swift

enum NetworkError: Error {
    case timeout
    case serverError(code: Int)
}

func fetchData() throws {
    throw NetworkError.serverError(code: 500)
}

do {
    try fetchData()
} catch NetworkError.timeout {
    print("Timeout occurred")
} catch NetworkError.serverError(let code) where code >= 500 {
    print("Server error with code \(code)")
} catch {
    print("Other error")
}

A Timeout occurred

B Other error

C Server error with code 500

D Compilation error

💡 Hint

Check the thrown error and the pattern matching in the catch clauses.

The function throws NetworkError.serverError(code: 500). The second catch matches this case with a where clause checking code >= 500, so it prints "Server error with code 500".