Autoclosures (@autoclosure) in Swift - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a function parameter as an autoclosure.

Swift

func logIfTrue(_ predicate: [1]) {
    if predicate() {
        print("Condition is true")
    }
}

Drag options to blanks, or click blank then click option'

A() -> Bool

BBool

C@escaping () -> Bool

D@autoclosure () -> Bool

Attempts:

3 left

💡 Hint

Common Mistakes

Forgetting to add '@autoclosure' before the closure type.

Using just 'Bool' instead of a closure type.

Using '@escaping' which is not needed here.

The @autoclosure attribute automatically wraps an expression into a closure, so the parameter type must be '@autoclosure () -> Bool'.

2fill in blank

medium

Complete the code to call the function with an autoclosure parameter.

Swift

logIfTrue([1] > 5)

Drag options to blanks, or click blank then click option'

A3 + 4

B3 > 4

C7

Dtrue

Attempts:

3 left

💡 Hint

Common Mistakes

Passing a Boolean literal directly without an expression.

Passing a value that is not a Boolean expression.

You can pass an expression like '3 + 4' directly to an autoclosure parameter; it will be wrapped automatically.

3fill in blank

hard

Fix the error in the function declaration to correctly use @autoclosure.

Swift

func check(_ condition: [1]) {
    if condition() {
        print("Checked true")
    }
}

Drag options to blanks, or click blank then click option'

A@autoclosure () -> Bool

BBool

C() -> Bool

D@escaping () -> Bool

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'Bool' instead of a closure type.

Using '@escaping' which is not needed here.

The parameter must be declared as '@autoclosure () -> Bool' to use autoclosure correctly.

4fill in blank

hard

Fill both blanks to declare a function with an autoclosure parameter and call it correctly.

Swift

func evaluate(_ expression: [1]) {
    print("Result is \(expression())")
}
evaluate([2])

Drag options to blanks, or click blank then click option'

A@autoclosure () -> Int

B5 * 3

C5 + 3

DInt

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'Int' instead of a closure type for the parameter.

Passing a value without an expression.

The parameter must be '@autoclosure () -> Int' and the call passes an expression like '5 * 3' which is wrapped automatically.

5fill in blank

hard

Fill all three blanks to create a function with an autoclosure and escaping parameter, then call it.

Swift

func storeCondition(_ condition: [1], completion: @escaping () -> Void) {
    if condition() {
        completion()
    }
}

let conditionClosure: [2] = [3] > 10

Drag options to blanks, or click blank then click option'

A@autoclosure () -> Bool

B() -> Bool

C15

D20

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'Bool' instead of closure types.

Assigning a raw value instead of an expression to the closure variable.

The function parameter uses '@autoclosure () -> Bool', the variable is a closure '() -> Bool', and the expression '20 > 10' is assigned to it.

Practice

1. What does the @autoclosure attribute do in Swift?

easy

A. Converts a closure into a normal function

B. Forces immediate execution of a closure parameter

C. Automatically wraps an expression into a closure to delay its execution

D. Prevents a closure from capturing variables

Solution

1. Step 1: Understand the purpose of @autoclosure

   @autoclosure automatically wraps an expression passed as an argument into a closure, delaying its execution until called.

2. Step 2: Compare with other options

   Options B, C, and D describe behaviors unrelated to @autoclosure's purpose.

3. Final Answer:

   Automatically wraps an expression into a closure to delay its execution -> Option C

4. Quick Check:

   @autoclosure delays execution = A [OK]

Hint: Autoclosure delays code execution by wrapping it automatically [OK]

Common Mistakes:

• Thinking @autoclosure runs code immediately
• Confusing @autoclosure with normal closures
• Assuming it changes closure syntax

2. Which of the following is the correct way to declare a function parameter using @autoclosure in Swift?

easy

A. func check(_ condition: @autoclosure () -> Bool) { }

B. func check(@autoclosure condition: Bool) { }

C. func check(condition: @autoclosure Bool) { }

D. func check(_ condition: Bool @autoclosure) { }

Solution

1. Step 1: Recall correct syntax for @autoclosure

   The correct syntax places @autoclosure before the closure type: @autoclosure () -> Bool.

2. Step 2: Analyze each option

   func check(_ condition: @autoclosure () -> Bool) { } matches the correct syntax. Options A, B, and C misuse the attribute placement or type declaration.

3. Final Answer:

   func check(_ condition: @autoclosure () -> Bool) { } -> Option A

4. Quick Check:

   @autoclosure syntax = func check(_ condition: @autoclosure () -> Bool) { } [OK]

Hint: Remember: @autoclosure before closure type () -> Type [OK]

Common Mistakes:

• Placing @autoclosure after the type
• Using @autoclosure on non-closure types
• Omitting the closure arrow syntax

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

func logIfTrue(_ predicate: @autoclosure () -> Bool) {
    if predicate() {
        print("True")
    } else {
        print("False")
    }
}

logIfTrue(3 > 5)

medium

A. True

B. Runtime crash

C. Compile-time error

D. False

Solution

1. Step 1: Understand the function behavior

   The function takes a condition as an autoclosure and calls it inside the if statement.

2. Step 2: Evaluate the passed expression

   The expression 3 > 5 is false, so predicate() returns false, triggering the else branch.

3. Final Answer:

   False -> Option D

4. Quick Check:

   3 > 5 is false, so output = False [OK]

Hint: Evaluate the expression inside autoclosure when called [OK]

Common Mistakes:

• Assuming autoclosure runs immediately before function call
• Confusing true/false logic
• Expecting compile errors due to autoclosure

4. Identify the error in this Swift function using @autoclosure:

func execute(_ action: @autoclosure () -> Void) {
    action
}

execute(print("Hello"))

medium

A. Missing parentheses to call the closure inside the function

B. Incorrect use of @autoclosure on a Void return type

C. Cannot pass print statement as autoclosure

D. Function parameter should not be marked @autoclosure

Solution

1. Step 1: Check how the closure is called inside the function

   The parameter action is a closure and must be called with parentheses: action().

2. Step 2: Identify the error

   The code uses action without parentheses, so the closure is not executed, causing no output.

3. Final Answer:

   Missing parentheses to call the closure inside the function -> Option A

4. Quick Check:

   Closure must be called with () = Missing parentheses to call the closure inside the function [OK]

Hint: Call autoclosure with () to execute it inside function [OK]

Common Mistakes:

• Forgetting to call the closure with parentheses
• Thinking @autoclosure forbids Void closures
• Assuming print can't be autoclosure

5. Consider this Swift code using @autoclosure to log messages only when needed:

func log(_ message: @autoclosure () -> String, onlyIf condition: Bool) {
    if condition {
        print(message())
    }
}

log("Debug info", onlyIf: false)

What is the main benefit of using @autoclosure here?

hard

A. The function runs faster because closures are always synchronous

B. The message string is constructed only if the condition is true, saving resources

C. The message is printed twice automatically

D. The condition parameter becomes optional

Solution

1. Step 1: Understand how @autoclosure delays evaluation

   The message string expression is wrapped in a closure and only evaluated when message() is called.

2. Step 2: Analyze the condition effect

   Since condition is false, message() is never called, so the string is not constructed, saving computation.

3. Final Answer:

   The message string is constructed only if the condition is true, saving resources -> Option B

4. Quick Check:

   @autoclosure delays message creation = The message string is constructed only if the condition is true, saving resources [OK]

Hint: Autoclosure delays expensive expressions until needed [OK]

Common Mistakes:

• Thinking message is always created regardless of condition
• Assuming autoclosure prints automatically
• Confusing condition with message parameter