Switch must be exhaustive in Swift

Introduction

Swift requires switch statements to cover all possible cases. This helps avoid mistakes by making sure you handle every option.

When you want to check all possible values of an enum.

When you want to make sure no case is forgotten in decision making.

When you want your code to be safer and easier to maintain.

When you want the compiler to warn you if a new case is added but not handled.

When you want to handle different types of input clearly and completely.

Syntax

Swift

switch value {
case pattern1:
    // code
case pattern2:
    // code
// ... all cases must be covered
}

All possible cases must be covered, either explicitly or with a default case.

If you use an enum, the switch must handle every enum case or include default.

Examples

This switch covers all enum cases, so it is exhaustive.

Swift

enum Direction {
    case north, south, east, west
}

let dir = Direction.north

switch dir {
case .north:
    print("Going north")
case .south:
    print("Going south")
case .east:
    print("Going east")
case .west:
    print("Going west")
}

Using default covers all other values, making the switch exhaustive.

Swift

let number = 3

switch number {
case 1:
    print("One")
case 2:
    print("Two")
default:
    print("Other number")
}

Sample Program

This program uses a switch on an enum with all cases handled. It prints the correct action for each light color.

Swift

enum Light {
    case red, yellow, green
}

func action(for light: Light) {
    switch light {
    case .red:
        print("Stop")
    case .yellow:
        print("Get ready")
    case .green:
        print("Go")
    }
}

action(for: .red)
action(for: .yellow)
action(for: .green)

Important Notes

If you forget a case in a switch on an enum, Swift will give a compile error.

You can use default to cover all other cases, but it's better to list all enum cases explicitly for clarity.

Exhaustive switches help catch errors early and make your code safer.

Summary

Swift switch statements must cover all possible cases.

Use explicit cases or a default to be exhaustive.

This rule helps prevent bugs and makes code easier to understand.

Practice

1. What does it mean when we say a Swift switch statement must be exhaustive?

easy

A. It must handle all possible values of the variable being switched on.

B. It can skip some cases if they are unlikely to happen.

C. It only needs to handle the first case it matches.

D. It must have at least two cases.

Solution

1. Step 1: Understand the meaning of exhaustive switch

   In Swift, a switch must cover every possible value of the variable it checks to avoid missing cases.

2. Step 2: Compare options to definition

   Only It must handle all possible values of the variable being switched on. states that all possible values must be handled, which matches the requirement.

3. Final Answer:

   It must handle all possible values of the variable being switched on. -> Option A

4. Quick Check:

   Exhaustive means all cases covered = A [OK]

Hint: Remember: switch covers all cases or uses default [OK]

Common Mistakes:

• Thinking switch can skip cases
• Believing only first match matters
• Assuming minimum number of cases

2. Which of the following Swift switch statements is syntactically correct and exhaustive for an enum Direction with cases north, south, east, and west?

easy

A. switch dir { case .north: print("North") case .south: print("South") }

B. switch dir { case .north: print("North") case .south: print("South") case .east: print("East") case .west: print("West") }

C. switch dir { case .north: print("North") case .south: print("South") case .east: print("East") }

D. switch (dir) { case .north: print("North") case .south: print("South") default: print("Other") }

Solution

1. Step 1: Check all enum cases covered

   The enum has four cases: north, south, east, west. The switch must cover all or use default.

2. Step 2: Analyze each option

   switch dir { case .north: print("North") case .south: print("South") case .east: print("East") case .west: print("West") } covers all four cases explicitly, making it exhaustive and syntactically correct.

3. Final Answer:

   switch with all four cases covered explicitly -> Option B

4. Quick Check:

   All enum cases covered = D [OK]

Hint: Cover all enum cases or use default to be exhaustive [OK]

Common Mistakes:

• Missing some enum cases
• Not using default when cases are incomplete
• Assuming partial cases are enough

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

enum Mood { case happy, sad, neutral }
let todayMood = Mood.sad
switch todayMood {
 case .happy: print("Feeling great!")
 case .sad: print("Feeling down.")
 case .neutral: print("Just okay.")
}

medium

A. Feeling great!

B. Just okay.

C. Feeling down.

D. Compile error: switch not exhaustive

Solution

1. Step 1: Identify the value of todayMood

   todayMood is set to Mood.sad.

2. Step 2: Match the switch case

   The switch has a case for .sad that prints "Feeling down." so this case runs.

3. Final Answer:

   Feeling down. -> Option C

4. Quick Check:

   Value sad matches case sad = B [OK]

Hint: Match variable value to switch case to find output [OK]

Common Mistakes:

• Confusing cases and printing wrong output
• Thinking switch is not exhaustive here
• Ignoring the actual value assigned

4. Identify the error in this Swift switch statement for enum Color with cases red, green, and blue:

switch color {
 case .red: print("Red")
 case .green: print("Green")
}

medium

A. Cannot use enum cases without raw values.

B. Syntax error: missing parentheses after switch.

C. No error, this switch is correct.

D. Missing case for .blue or default case, so switch is not exhaustive.

Solution

1. Step 1: Check all enum cases handled

   The enum has three cases: red, green, blue. The switch only handles red and green.

2. Step 2: Identify missing coverage

   Since .blue is missing and no default case is provided, the switch is not exhaustive, causing a compile error.

3. Final Answer:

   Missing case for .blue or default case, so switch is not exhaustive. -> Option D

4. Quick Check:

   All cases must be covered or default = C [OK]

Hint: Always cover all enum cases or add default [OK]

Common Mistakes:

• Ignoring missing enum cases
• Thinking parentheses needed after switch
• Confusing raw values with case coverage

5. Given this enum and switch, how can you make the switch exhaustive without listing all cases explicitly?

enum Status { case success, failure, unknown }
let currentStatus = Status.unknown
switch currentStatus {
 case .success: print("Success")
 case .failure: print("Failure")
}

hard

A. Add a default case to handle all other values.

B. Remove the switch and use if-else instead.

C. Add a case for .unknown explicitly.

D. Change enum to have only two cases.

Solution

1. Step 1: Understand the problem with current switch

   The switch handles .success and .failure but misses .unknown, so it's not exhaustive.

2. Step 2: Find a way to cover missing cases without listing all explicitly

   Adding a default case covers any cases not explicitly handled, making the switch exhaustive.

3. Final Answer:

   Add a default case to handle all other values. -> Option A

4. Quick Check:

   Use default to cover missing cases = A [OK]

Hint: Use default to cover all unlisted cases quickly [OK]

Common Mistakes:

• Thinking you must list every case explicitly
• Removing switch instead of fixing it
• Changing enum just to fix switch