Switch must be exhaustive in Swift - Step-by-Step Execution

Concept Flow - Switch must be exhaustive

Start switch with value

↓

Check each case

↓

Execute case body

↓

All cases covered?

No→Error: Must be exhaustive

Yes↓

End

The switch checks each case for a match and must cover all possible values, otherwise it causes an error.

Execution Sample

Swift

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

let dir = Direction.north

switch dir {
case .north: print("Up")
case .south: print("Down")
case .east: print("Right")
case .west: print("Left")
}

This code switches over all Direction enum cases and prints a direction word.

Execution Table

Step	Switch Value	Case Checked	Match?	Action	Output
1	Direction.north	.north	Yes	Execute print("Up")	Up
2	Direction.north	.south	No	Skip
3	Direction.north	.east	No	Skip
4	Direction.north	.west	No	Skip
5	-	All cases checked	Yes	Switch ends

💡 All enum cases are covered, so switch is exhaustive and ends normally.

Variable Tracker

Variable	Start	After Step 1	After Step 5
dir	Direction.north	Direction.north	Direction.north

Key Moments - 3 Insights

Why does Swift require all enum cases to be handled in a switch?

Swift wants to make sure you don't forget any case, so the switch must be exhaustive as shown in execution_table row 5.

What if I leave out one case in the switch?

The code will not compile because the switch is not exhaustive, meaning not all enum cases are covered.

Can I use a default case to make the switch exhaustive?

Yes, a default case covers all remaining cases and makes the switch exhaustive if you don't want to list all explicitly.

Visual Quiz - 3 Questions

Look at the execution table, which case matches the switch value at step 1?

A .south

B .north

C .east

D .west

💡 Hint

Check the 'Case Checked' and 'Match?' columns at step 1 in the execution_table.

At which step does the switch confirm it is exhaustive?

A Step 1

B Step 3

C Step 5

D Step 4

💡 Hint

Look at the 'All cases checked' row in the execution_table.

If the case .west was missing, what would happen?

A The switch would not compile due to missing case.

B The switch would cause a runtime error.

C The switch would still compile and run.

D The switch would print nothing.

💡 Hint

Refer to key_moments about missing cases causing compile errors.

Concept Snapshot

Switch statements in Swift must cover all possible values of the switched variable.
For enums, this means all cases must be handled.
If not all cases are listed, the compiler gives an error.
You can use a default case to cover any missing cases.
This ensures safe and predictable code execution.

Full Transcript

In Swift, when you use a switch statement with an enum, you must cover every possible case. The switch checks each case one by one. If the value matches a case, it runs that case's code. If you forget a case, the compiler will stop you because the switch is not exhaustive. You can add a default case to handle any cases you don't list explicitly. This rule helps prevent bugs by making sure you think about every possible value.