@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.

Practice

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

easy

A. To combine multiple expressions into a single result in a clean way

B. To declare a new type of variable

C. To handle errors automatically

D. To create a new thread for concurrent execution

Solution

1. Step 1: Understand the role of @resultBuilder

   @resultBuilder is used to combine multiple code lines or expressions into one result, making code cleaner and easier to write.

2. Step 2: Compare options with the purpose

   Options B, C, and D describe unrelated features. Only To combine multiple expressions into a single result in a clean way correctly describes the purpose of @resultBuilder.

3. Final Answer:

   To combine multiple expressions into a single result in a clean way -> Option A

4. Quick Check:

   @resultBuilder purpose = combine expressions [OK]

Hint: Remember: @resultBuilder combines code lines into one result [OK]

Common Mistakes:

• Confusing @resultBuilder with error handling
• Thinking it creates new variables
• Assuming it manages concurrency

2. Which of the following is the correct way to declare a custom result builder in Swift?

easy

A. @resultBuilder class MyBuilder { func buildBlock(_ components: Int...) -> Int { components.reduce(0, +) } }

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

C. @resultBuilder func MyBuilder(_ components: Int...) -> Int { components.reduce(0, +) }

D. struct MyBuilder { func buildBlock(_ components: Int...) -> Int { components.reduce(0, +) } }

Solution

1. Step 1: Identify correct @resultBuilder declaration

   A custom result builder must be declared as a struct or enum with the @resultBuilder attribute and static buildBlock method.

2. Step 2: Check each option

   @resultBuilder struct MyBuilder { static func buildBlock(_ components: Int...) -> Int { components.reduce(0, +) } } correctly uses @resultBuilder on a struct and static buildBlock method. struct MyBuilder { func buildBlock(_ components: Int...) -> Int { components.reduce(0, +) } } misses @resultBuilder and static keyword. @resultBuilder func MyBuilder(_ components: Int...) -> Int { components.reduce(0, +) } wrongly applies @resultBuilder to a function. @resultBuilder class MyBuilder { func buildBlock(_ components: Int...) -> Int { components.reduce(0, +) } } uses a class, which is not allowed.

3. Final Answer:

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

4. Quick Check:

   Custom builder = struct + @resultBuilder + static buildBlock [OK]

Hint: Use @resultBuilder on struct with static buildBlock method [OK]

Common Mistakes:

• Forgetting static keyword on buildBlock
• Applying @resultBuilder to functions
• Using class instead of struct or enum

3. Given the following code, what will be printed?

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

func sum(@SumBuilder _ content: () -> Int) -> Int {
    content()
}

let result = sum {
    2
    3
    5
}
print(result)

medium

A. Error: Missing return

B. 235

C. 0

D. 10

Solution

1. Step 1: Understand buildBlock behavior

   The buildBlock method receives all integers (2, 3, 5) and sums them using reduce(0, +), resulting in 10.

2. Step 2: Check function call and output

   The sum function calls content(), which triggers the builder and returns 10. print(result) outputs 10.

3. Final Answer:

   10 -> Option D

4. Quick Check:

   Sum of 2+3+5 = 10 [OK]

Hint: Sum builder adds all values inside closure [OK]

Common Mistakes:

• Thinking values concatenate as strings
• Expecting a syntax error
• Missing that buildBlock returns sum

4. Identify the error in this custom result builder code:

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

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

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

medium

A. Missing return statement in buildBlock

B. Cannot use String type in result builder

C. buildBlock must be static

D. Function buildString should not have @MyBuilder

Solution

1. Step 1: Check buildBlock method declaration

   In a result builder, buildBlock must be a static method. Here it is an instance method (missing static).

2. Step 2: Verify other parts

   Return statement is present, String type is allowed, and function parameter correctly uses @MyBuilder.

3. Final Answer:

   buildBlock must be static -> Option C

4. Quick Check:

   buildBlock static keyword required [OK]

Hint: Always make buildBlock static in result builders [OK]

Common Mistakes:

• Omitting static keyword on buildBlock
• Confusing return statement presence
• Thinking @resultBuilder restricts types

5. You want to create a custom result builder that builds a dictionary from key-value pairs written inside a closure. Which of the following implementations correctly uses @resultBuilder to achieve this?

hard

A. @resultBuilder
struct DictBuilder {
static func buildBlock(_ components: (String, Int)...) -> [String: Int] {
var dict = [String: Int]()
for (key, value) in components {
dict[key] = value
}
return dict
}
}

func buildDict(@DictBuilder _ content: () -> [String: Int]) -> [String: Int] {
content()
}

let dict = buildDict {
("a", 1)
("b", 2)
("c", 3)
}
print(dict)

B. @resultBuilder
struct DictBuilder {
static func buildBlock(_ components: [String: Int]...) -> [String: Int] {
components.reduce([:]) { $0.merging($1) { $1 } }
}
}

func buildDict(@DictBuilder _ content: () -> [String: Int]) -> [String: Int] {
content()
}

let dict = buildDict {
["a": 1]
["b": 2]
["c": 3]
}
print(dict)

C. @resultBuilder
struct DictBuilder {
static func buildBlock(_ components: String...) -> [String: Int] {
[:]
}
}

func buildDict(@DictBuilder _ content: () -> [String: Int]) -> [String: Int] {
content()
}

let dict = buildDict {
"a:1"
"b:2"
"c:3"
}
print(dict)

D. struct DictBuilder {
static func buildBlock(_ components: (String, Int)...) -> [String: Int] {
var dict = [String: Int]()
for (key, value) in components {
dict[key] = value
}
return dict
}
}

func buildDict(@DictBuilder _ content: () -> [String: Int]) -> [String: Int] {
content()
}

let dict = buildDict {
("a", 1)
("b", 2)
("c", 3)
}
print(dict)

Solution

1. Step 1: Check for @resultBuilder attribute and static buildBlock

   @resultBuilder struct DictBuilder { static func buildBlock(_ components: (String, Int)...) -> [String: Int] { var dict = [String: Int]() for (key, value) in components { dict[key] = value } return dict } } func buildDict(@DictBuilder _ content: () -> [String: Int]) -> [String: Int] { content() } let dict = buildDict { ("a", 1) ("b", 2) ("c", 3) } print(dict) correctly declares @resultBuilder on a struct and has a static buildBlock method that combines tuples into a dictionary.

2. Step 2: Verify code correctness and usage

   @resultBuilder struct DictBuilder { static func buildBlock(_ components: [String: Int]...) -> [String: Int] { components.reduce([:]) { $0.merging($1) { $1 } } } } func buildDict(@DictBuilder _ content: () -> [String: Int]) -> [String: Int] { content() } let dict = buildDict { ["a": 1] ["b": 2] ["c": 3] } print(dict) uses arrays of dictionaries which works but is less direct than tuples; @resultBuilder struct DictBuilder { static func buildBlock(_ components: String...) -> [String: Int] { [:] } } func buildDict(@DictBuilder _ content: () -> [String: Int]) -> [String: Int] { content() } let dict = buildDict { "a:1" "b:2" "c:3" } print(dict) returns empty dictionary ignoring input; struct DictBuilder { static func buildBlock(_ components: (String, Int)...) -> [String: Int] { var dict = [String: Int]() for (key, value) in components { dict[key] = value } return dict } } func buildDict(@DictBuilder _ content: () -> [String: Int]) -> [String: Int] { content() } let dict = buildDict { ("a", 1) ("b", 2) ("c", 3) } print(dict) misses @resultBuilder attribute, so it won't work as a builder.

3. Final Answer:

   Option A correctly implements a dictionary builder using @resultBuilder -> Option A

4. Quick Check:

   @resultBuilder + static buildBlock + tuple input = correct dictionary builder [OK]

Hint: Use tuples and static buildBlock with @resultBuilder for dictionary [OK]

Common Mistakes:

• Forgetting @resultBuilder attribute
• Returning empty or wrong type
• Using non-static buildBlock