BuildBlock for combining results in Swift - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

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

Consider the following Swift code that uses a custom @resultBuilder named ResultCombiner. What will be printed when combineResults() is called?

Swift

import Foundation

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

func combineResults() -> String {
    @ResultCombiner var result: String {
        "Hello"
        "World"
        "Swift"
    }
    return result
}

print(combineResults())

A "Hello, World, Swift"

B "HelloWorldSwift"

C "[Hello, World, Swift]"

D "Hello World Swift"

💡 Hint

Look at how the buildBlock function joins the components.

The buildBlock function receives all components as an array of strings and joins them with a comma and space separator. So the output is the three strings joined by ", ".

🧠 Conceptual

intermediate

What does the buildBlock function do in a Swift result builder?

In Swift's result builder, what is the main role of the buildBlock static function?

A It initializes the result builder instance.

B It combines multiple components into a single result.

C It validates the types of components passed.

D It handles errors during building.

💡 Hint

Think about how multiple parts are combined into one output.

buildBlock takes multiple components and combines them into one final value that the result builder returns.

🔧 Debug

advanced

Why does this buildBlock implementation cause a compile error?

Examine this buildBlock function inside a Swift result builder. Why does it cause a compile error?

static func buildBlock(_ components: String) -> String {
    components + "!"
}

Swift

static func buildBlock(_ components: String) -> String {
    components + "!"
}

A buildBlock cannot be static.

B You cannot use + operator on String in buildBlock.

C buildBlock must return an Int, not String.

D buildBlock must accept a variadic parameter, not a single String.

💡 Hint

Check the parameter type expected by buildBlock.

buildBlock is called with multiple components, so it must accept a variadic parameter (e.g., String...) instead of a single String.

📝 Syntax

advanced

Which buildBlock implementation correctly combines Int values into their sum?

You want to create a buildBlock function that sums all Int components passed to it. Which option is correct?

A static func buildBlock(_ components: Int...) -> Int { components.add() }

B static func buildBlock(components: Int...) -> Int { components.reduce(0, +) }

C static func buildBlock(_ components: Int...) -> Int { components.reduce(0, +) }

D static func buildBlock(_ components: [Int]) -> Int { components.sum() }

💡 Hint

Check parameter syntax and method names for summing arrays.

Option C correctly uses a variadic parameter and the standard reduce method to sum the integers.

🚀 Application

expert

How to combine optional String results safely in buildBlock?

You want to combine multiple optional String components in buildBlock, ignoring nil values and joining non-nil with ", ". Which implementation achieves this?

A

static func buildBlock(_ components: String?...) -> String {
    components.compactMap { $0 }.joined(separator: ", ")
}

B

static func buildBlock(_ components: String?...) -> String {
    components.map { $0 ?? "" }.joined(separator: ", ")
}

C

static func buildBlock(_ components: [String?]) -> String {
    components.joined(separator: ", ")
}

D

static func buildBlock(_ components: String?...) -> String {
    components.reduce("") { $0 + $1! + ", " }
}

💡 Hint

Consider how to safely unwrap optionals and skip nils.

Option A uses compactMap to remove nils and then joins the remaining strings with ", ". Option A includes empty strings for nils, C is invalid because joined cannot be called on [String?], and D force unwraps optionals causing runtime error if nil.