Async functions declaration in Swift - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of an async function call with await

What is the output of this Swift code when run in an async context?

Swift

func fetchNumber() async -> Int {
    return 42
}

Task {
    let result = await fetchNumber()
    print(result)
}

A Compilation error: missing await

B 42

C Runtime error: cannot call async function

D 0

💡 Hint

Remember to use 'await' when calling async functions inside async contexts.

The async function 'fetchNumber' returns 42. Using 'await' inside the Task closure correctly waits for the result, so it prints 42.

❓ Predict Output

intermediate

Return type of async function

What is the type of the variable 'value' after this code runs?

Swift

func getString() async -> String {
    return "Hello"
}

Task {
    let value = await getString()
    print(type(of: value))
}

A Void

B Optional<String>

C Task<String, Never>

D String

💡 Hint

The 'await' keyword unwraps the async result to its return type.

The async function returns a String. Awaiting it gives a String, so 'value' is of type String.

❓ Predict Output

advanced

Async function with throwing error

What will be printed when this code runs?

Swift

enum MyError: Error {
    case failed
}

func riskyTask() async throws -> String {
    throw MyError.failed
}

Task {
    do {
        let result = try await riskyTask()
        print(result)
    } catch {
        print("Error caught")
    }
}

A Runtime crash

B Compilation error: missing try

C Error caught

D No output

💡 Hint

Async functions that throw must be called with 'try await' inside a do-catch block.

The function throws an error, which is caught by the catch block, so it prints 'Error caught'.

🧠 Conceptual

advanced

Correct async function declaration

Which of the following is the correct way to declare an async function in Swift that returns an integer?

A func compute() async -> Int { return 10 }

B async func compute() -> Int { return 10 }

C func async compute() -> Int { return 10 }

D func compute() -> async Int { return 10 }

💡 Hint

The 'async' keyword goes after the function parentheses.

In Swift, the 'async' keyword is placed after the parameter list and before the return arrow.

❓ Predict Output

expert

Async function call order and output

What is the output of this Swift code?

Swift

func first() async -> String {
    return "First"
}

func second() async -> String {
    return "Second"
}

Task {
    async let a = first()
    async let b = second()
    let result = await a + " & " + await b
    print(result)
}

A First & Second

B Second & First

C Compilation error: cannot concatenate async let results

D Runtime error: deadlock

💡 Hint

async let allows concurrent execution; awaiting each unwraps the result.

Both async lets run concurrently. Awaiting them unwraps their results, concatenating to 'First & Second'.