Throwing functions with throws in Swift - Mini Project: Build & Apply

Throwing functions with throws

📖 Scenario: Imagine you are building a simple app that checks if a password is strong enough. If the password is too short, the app should tell the user with an error.

🎯 Goal: You will create a throwing function that checks password length and throws an error if the password is too short. Then you will call this function and handle the error.

📋 What You'll Learn

Create an enum called PasswordError that conforms to Error with a case tooShort

Create a throwing function called checkPassword that takes a String parameter password

Inside checkPassword, throw PasswordError.tooShort if the password length is less than 8

Call checkPassword inside a do-catch block with the password "abc123"

Print "Password is strong enough" if no error is thrown

Print "Password is too short" if PasswordError.tooShort is caught

💡 Why This Matters

🌍 Real World

Throwing functions are used in apps to handle unexpected problems like invalid input or failed network calls. This helps apps respond gracefully instead of crashing.

💼 Career

Understanding how to write and handle throwing functions is important for Swift developers to build robust and user-friendly applications.

1

Create the PasswordError enum

Create an enum called PasswordError that conforms to Error with a case called tooShort.

Swift

# Your code here

Hint

Use enum PasswordError: Error { case tooShort } to define the error type.

2

Create the throwing function checkPassword

Create a throwing function called checkPassword that takes a String parameter named password. Inside the function, throw PasswordError.tooShort if password.count is less than 8.

Swift

enum PasswordError: Error {
    case tooShort
}

# Your code here

Hint

Define the function with throws and use throw PasswordError.tooShort inside an if check.

3

Call checkPassword inside a do-catch block

Write a do-catch block that calls checkPassword with the password "abc123". In the do block, call checkPassword("abc123"). In the catch block, catch PasswordError.tooShort specifically.

Swift

enum PasswordError: Error {
    case tooShort
}

func checkPassword(_ password: String) throws {
    if password.count < 8 {
        throw PasswordError.tooShort
    }
}

# Your code here

Hint

Use do { try checkPassword("abc123") ... } catch PasswordError.tooShort { ... } to handle the error.

4

Print the result

Run the program to print the result of the password check. The output should be "Password is too short" because the password "abc123" has less than 8 characters.

Swift

enum PasswordError: Error {
    case tooShort
}

func checkPassword(_ password: String) throws {
    if password.count < 8 {
        throw PasswordError.tooShort
    }
}

do {
    try checkPassword("abc123")
    print("Password is strong enough")
} catch PasswordError.tooShort {
    print("Password is too short")
}

# Run the program to see the output

Hint

Run the code and check the console output.

Practice

1. What does the throws keyword indicate when used in a Swift function declaration?

easy

A. The function cannot be called outside its module.

B. The function always returns a Boolean value.

C. The function can produce an error that must be handled.

D. The function runs asynchronously.

Solution

1. Step 1: Understand the role of throws

   The throws keyword marks a function that can produce an error during execution.

2. Step 2: Recognize error handling requirement

   Calling a throws function requires handling the possible error with try and do-catch.

3. Final Answer:

   The function can produce an error that must be handled. -> Option C

4. Quick Check:

   throws means function can throw error [OK]

Hint: Throws means function can fail and needs error handling [OK]

Common Mistakes:

• Thinking throws means async function
• Assuming throws means function returns Bool
• Confusing throws with access control keywords

2. Which of the following is the correct way to declare a throwing function in Swift?

easy

A. func fetchData() -> throws String

B. func fetchData() throw -> String

C. func fetchData() throws String

D. func fetchData() throws -> String

Solution

1. Step 1: Recall correct syntax for throwing functions

   In Swift, throws comes after the parentheses and before the return type.

2. Step 2: Match syntax with options

   func fetchData() throws -> String matches the correct syntax: func name() throws -> ReturnType.

3. Final Answer:

   func fetchData() throws -> String -> Option D

4. Quick Check:

   Correct syntax is throws after () [OK]

Hint: Put throws right after () before return type [OK]

Common Mistakes:

• Writing throw instead of throws
• Placing throws after return type
• Omitting throws keyword

3. What will be the output of the following Swift code?

enum FileError: Error {
    case notFound
}

func readFile() throws -> String {
    throw FileError.notFound
}

do {
    let content = try readFile()
    print(content)
} catch {
    print("Error occurred")
}

medium

A. Compilation error

B. Error occurred

C. No output

D. notFound

Solution

1. Step 1: Analyze the throwing function behavior

   The function readFile() always throws FileError.notFound error.

2. Step 2: Understand the do-catch block

   The try calls readFile(), which throws, so control moves to catch block printing "Error occurred".

3. Final Answer:

   Error occurred -> Option B

4. Quick Check:

   Thrown error triggers catch printing message [OK]

Hint: Thrown error triggers catch block output [OK]

Common Mistakes:

• Expecting thrown error value to print
• Ignoring that error causes catch block
• Thinking no output occurs

4. Identify the error in this Swift code snippet:

func riskyOperation() throws -> Int {
    return 42
}

let result = riskyOperation()
print(result)

medium

A. Missing try keyword when calling riskyOperation()

B. Function should not return Int

C. Throws keyword is not allowed here

D. Missing catch block after function call

Solution

1. Step 1: Check calling syntax for throwing functions

   Calling a function marked with throws requires using try keyword.

2. Step 2: Identify missing try keyword

   The code calls riskyOperation() without try, causing a compile error.

3. Final Answer:

   Missing try keyword when calling riskyOperation() -> Option A

4. Quick Check:

   Throwing functions need try when called [OK]

Hint: Use try when calling throws functions [OK]

Common Mistakes:

• Calling throws function without try
• Assuming throws means no return value
• Forgetting to handle errors with do-catch

5. You want to write a function parseNumber(from: String) that throws an error if the string is not a valid integer. Which of the following implementations correctly uses throws and error handling?

hard

A. enum ParseError: Error { case invalidFormat }
func parseNumber(from text: String) throws -> Int {
guard let number = Int(text) else {
throw ParseError.invalidFormat
}
return number
}

B. func parseNumber(from text: String) -> Int {
if Int(text) == nil {
print("Invalid format")
}
return Int(text) ?? 0
}

C. enum ParseError: Error { case invalidFormat }
func parseNumber(from text: String) -> Int throws {
if Int(text) == nil {
throw ParseError.invalidFormat
}
return Int(text)!
}

D. func parseNumber(from text: String) throws -> Int {
if Int(text) == nil {
return 0
}
return Int(text)!
}

Solution

1. Step 1: Check function signature and error throwing

   enum ParseError: Error { case invalidFormat } func parseNumber(from text: String) throws -> Int { guard let number = Int(text) else { throw ParseError.invalidFormat } return number } correctly declares throws after parameters and before return type, and throws error if parsing fails.

2. Step 2: Verify error throwing logic

   enum ParseError: Error { case invalidFormat } func parseNumber(from text: String) throws -> Int { guard let number = Int(text) else { throw ParseError.invalidFormat } return number } uses guard let to safely unwrap Int, throwing ParseError.invalidFormat if conversion fails.

3. Final Answer:

   Correctly implements throwing function with error handling. -> Option A

4. Quick Check:

   Throws declared and error thrown on invalid input [OK]

Hint: Use guard and throw error inside throws function [OK]

Common Mistakes:

• Not marking function with throws
• Throwing error outside throws function
• Returning default instead of throwing error