Why Throwing functions with throws in Swift? - Purpose & Use Cases

The Big Idea

What if your program could catch mistakes before they cause crashes, all by itself?

The Scenario

Imagine you are writing a program that reads a file or fetches data from the internet. Without special tools, you have to check every step manually to see if something went wrong, like the file missing or the internet being down.

The Problem

Manually checking for errors everywhere makes your code long, confusing, and easy to break. You might forget to check an error, causing your program to crash or behave strangely without explanation.

The Solution

Throwing functions with throws let you mark functions that can fail. This way, Swift forces you to handle errors clearly and cleanly, keeping your code safe and easy to read.

Before vs After

✗ Before

func readFile() -> String? {
  // returns nil if file not found
  // caller must check for nil
}

✓ After

func readFile() throws -> String {
  // throws error if file not found
  // caller must handle error
}

What It Enables

It enables writing safer programs that clearly separate normal work from error handling, making bugs easier to find and fix.

Real Life Example

When downloading a photo from the internet, a throwing function can signal if the download failed, so your app can show a friendly message instead of crashing.

Key Takeaways

Manual error checks clutter code and cause bugs.

throws marks functions that can fail, forcing error handling.

This leads to safer, clearer, and more reliable programs.

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