@resultBuilder attribute in Swift - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Swift code using @resultBuilder?

Consider this Swift code using a custom @resultBuilder named StringBuilder. What will be printed when buildString() is called?

Swift

@resultBuilder
struct StringBuilder {
    static func buildBlock(_ components: String...) -> String {
        components.joined(separator: ", ")
    }
}

func buildString(@StringBuilder content: () -> String) -> String {
    content()
}

let result = buildString {
    "Hello"
    "World"
    "Swift"
}
print(result)

A "[Hello, World, Swift]"

B "HelloWorldSwift"

C "Hello, World, Swift"

D "Hello World Swift"

💡 Hint

Look at how buildBlock joins the strings with a comma and space.

The buildBlock function joins all string components with ", ". So the output is the three strings separated by commas and spaces.

🧠 Conceptual

intermediate

What is the main purpose of the @resultBuilder attribute in Swift?

Choose the best description of what @resultBuilder does in Swift.

A It marks a function as asynchronous and able to run in the background.

B It allows building complex values from multiple statements in a closure, combining them into one result.

C It automatically generates getter and setter methods for properties.

D It defines a protocol for handling errors in Swift code.

💡 Hint

Think about how SwiftUI uses @ViewBuilder to combine views.

@resultBuilder lets you write multiple statements inside a closure and combines them into a single value, making code more readable and declarative.

🔧 Debug

advanced

Why does this Swift code using @resultBuilder cause a compile error?

Examine this code snippet. Why does it fail to compile?

Swift

@resultBuilder
struct IntBuilder {
    static func buildBlock(_ components: Int...) -> Int {
        components.reduce(0, +)
    }
}

func sumInts(@IntBuilder content: () -> Int) -> Int {
    content()
}

let total = sumInts {
    1
    "2"
    3
}
print(total)

A Because "2" is a String, not an Int, causing a type mismatch in the builder.

B Because buildBlock must return an array, not a single Int.

C Because the function sumInts is missing the @escaping attribute.

D Because the @resultBuilder attribute can only be used with String types.

💡 Hint

Check the types of all values inside the closure.

The closure includes a String "2" where an Int is expected, causing a type mismatch error during compilation.

📝 Syntax

advanced

Which option correctly defines a minimal @resultBuilder in Swift?

Select the code snippet that correctly defines a minimal @resultBuilder named BoolBuilder that combines multiple Bool values using logical AND.

A

@resultBuilder
struct BoolBuilder {
    static func buildBlock(_ components: Bool...) -> Bool {
        components.reduce(true) { $0 && $1 }
    }
}

B

@resultBuilder
struct BoolBuilder {
    func buildBlock(_ components: Bool...) -> Bool {
        components.allSatisfy { $0 }
    }
}

C

struct BoolBuilder {
    static func buildBlock(_ components: Bool...) -> Bool {
        components.reduce(true) { $0 && $1 }
    }
}

D

@resultBuilder
class BoolBuilder {
    static func buildBlock(_ components: Bool...) -> Bool {
        components.reduce(false) { $0 || $1 }
    }
}

💡 Hint

Remember @resultBuilder must be applied to a struct or enum with static buildBlock method.

Option A correctly uses @resultBuilder on a struct with a static buildBlock method that combines Bool values with AND. Option A's buildBlock is not static. Option A lacks @resultBuilder. Option A uses a class and OR logic, which is not minimal AND builder.

🚀 Application

expert

How many items are in the resulting array from this @resultBuilder usage?

Given this Swift code using a custom @resultBuilder named ArrayBuilder, how many elements does the result array contain?

Swift

@resultBuilder
struct ArrayBuilder {
    static func buildBlock<T>(_ components: [T]...) -> [T] {
        components.flatMap { $0 }
    }
    static func buildExpression<T>(_ expression: T) -> [T] {
        [expression]
    }
    static func buildExpression<T>(_ expression: [T]) -> [T] {
        expression
    }
}

func buildArray<T>(@ArrayBuilder content: () -> [T]) -> [T] {
    content()
}

let result = buildArray {
    1
    [2, 3]
    4
    []
    5
}

A 4

B 6

C 7

D 5

💡 Hint

Count how many elements each line contributes to the final array.

Each single value is wrapped into an array of one element by buildExpression. The array [2, 3] is passed as is. The empty array contributes zero elements. So total elements: 1 + 2 + 1 + 0 + 1 = 5 elements.