Custom result builder declaration in Swift - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a custom result builder named Builder.

Swift

resultBuilder [1] Builder {
    static func buildBlock(_ components: String...) -> String {
        components.joined(separator: ", ")
    }
}

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

A@

Bclass

Cstruct

Denum

Attempts:

3 left

💡 Hint

Common Mistakes

Forgetting the '@' symbol before 'resultBuilder'.

Using 'struct' or 'class' instead of '@resultBuilder'.

In Swift, a custom result builder is declared using the '@resultBuilder' attribute before the builder's name.

2fill in blank

medium

Complete the code to define the buildBlock method that combines components into a single string.

Swift

static func buildBlock(_ components: [1]) -> String {
    components.joined(separator: ", ")
}

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

AString

BInt

C[String]

DString...

Attempts:

3 left

💡 Hint

Common Mistakes

Using '[String]' instead of 'String...'.

Using a single 'String' instead of variadic.

The buildBlock method takes a variadic parameter of type String, written as 'String...'.

3fill in blank

hard

Fix the error in the buildIf method to correctly handle optional components.

Swift

static func buildIf(_ component: [1]) -> String {
    component ?? ""
}

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

AString?

BString

COptional<String>

DInt?

Attempts:

3 left

💡 Hint

Common Mistakes

Using non-optional 'String' instead of 'String?'.

Using 'Int?' which is unrelated.

The buildIf method takes an optional String parameter, written as 'String?'.

4fill in blank

hard

Fill both blanks to define a buildEither method for the first case in the result builder.

Swift

static func buildEither(first component: [1]) -> String {
    [2]
}

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

AString

Bcomponent

CString?

Dreturn component

Attempts:

3 left

💡 Hint

Common Mistakes

Using optional type 'String?' instead of 'String'.

Omitting the 'return' keyword.

The buildEither method takes a String and returns it using 'return component'.

5fill in blank

hard

Fill all three blanks to define a buildArray method that combines an array of components.

Swift

static func buildArray(_ components: [[1]]) -> [2] {
    components.[3](separator: ", ")
}

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

AString

Cjoined

Djoined(separator:)

Attempts:

3 left

💡 Hint

Common Mistakes

Using incorrect types for the array or return type.

Using 'joined(separator:)' as a method name instead of 'joined'.

The buildArray method takes an array of Strings and returns a String by joining components with ', '.

Practice

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

easy

A. To handle errors automatically in functions

B. To create a new data type like a class or struct

C. To optimize memory usage during runtime

D. To define how multiple code parts combine into a single result

Solution

1. Step 1: Understand the role of @resultBuilder

   A @resultBuilder is used to customize how multiple code blocks combine into one result, making code cleaner.

2. Step 2: Compare with other options

   Options A, B, and D describe unrelated concepts like error handling, data types, and optimization, which are not the purpose of result builders.

3. Final Answer:

   To define how multiple code parts combine into a single result -> Option D

4. Quick Check:

   Result builder = combine code parts [OK]

Hint: Result builders combine code parts into one result [OK]

Common Mistakes:

• Confusing result builders with classes or structs
• Thinking result builders handle errors
• Assuming result builders optimize memory

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

easy

A. @resultBuilder struct MyBuilder { static func buildBlock() -> String { "" } }

B. @resultBuilder class MyBuilder { func buildBlock() {} }

C. class MyBuilder { func buildBlock() -> Int { 0 } }

D. struct MyBuilder { static func buildBlock() {} }

Solution

1. Step 1: Identify correct syntax for result builder

   A custom result builder must be declared with @resultBuilder attribute on a struct, and have static methods like buildBlock returning a value.

2. Step 2: Check each option

   @resultBuilder struct MyBuilder { static func buildBlock() -> String { "" } } correctly uses @resultBuilder, declares a struct, and has a static buildBlock method returning a String. The other options miss the attribute, use class instead of struct, or have non-static buildBlock.

3. Final Answer:

   @resultBuilder struct MyBuilder { static func buildBlock() -> String { "" } } -> Option A

4. Quick Check:

   Result builder = @resultBuilder + struct + static methods [OK]

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

Common Mistakes:

• Using class instead of struct for result builder
• Forgetting the @resultBuilder attribute
• Making buildBlock non-static

3. Given this custom result builder and usage, what is the output?

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

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

let result = sum {
    5
    10
    3
}
print(result)

medium

A. 5103

B. 18

C. Error: buildBlock requires array

D. 0

Solution

1. Step 1: Understand buildBlock behavior

   The buildBlock method takes multiple Int values and sums them using reduce(0, +).

2. Step 2: Evaluate the sum call

   The closure passed to sum returns three Int values: 5, 10, and 3. The builder sums them: 5 + 10 + 3 = 18.

3. Final Answer:

   18 -> Option B

4. Quick Check:

   Sum of 5, 10, 3 = 18 [OK]

Hint: buildBlock sums all Int inputs passed in closure [OK]

Common Mistakes:

• Thinking values concatenate as strings
• Expecting buildBlock to receive an array instead of variadic
• Confusing output with zero or error

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

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

medium

A. buildBlock must be static

B. buildBlock must return Int

C. struct cannot be used with @resultBuilder

D. buildBlock cannot take variadic parameters

Solution

1. Step 1: Check method requirements in result builder

   Methods like buildBlock inside a @resultBuilder must be declared as static so they can be called without an instance.

2. Step 2: Analyze the given code

   The buildBlock method is missing the static keyword, which causes a compile error.

3. Final Answer:

   buildBlock must be static -> Option A

4. Quick Check:

   buildBlock method = static [OK]

Hint: buildBlock must always be static in result builders [OK]

Common Mistakes:

• Omitting static keyword on buildBlock
• Thinking buildBlock must return Int always
• Believing struct can't be used with @resultBuilder

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 buildBlock implementations correctly combines multiple (String, Int) tuples into a single dictionary?

hard

A. static func buildBlock(_ pairs: (String, Int)...) -> [String: Int] { pairs.map { [$0.0: $0.1] } }

B. static func buildBlock(_ pairs: [(String, Int)]) -> [String: Int] { pairs.reduce([:]) { $0 + $1 } }

C. static func buildBlock(_ pairs: (String, Int)...) -> [String: Int] { Dictionary(uniqueKeysWithValues: pairs) }

D. static func buildBlock(_ pairs: (String, Int)...) -> [(String, Int)] { pairs }

Solution

1. Step 1: Understand the goal

   The builder should combine multiple key-value pairs into a single dictionary.

2. Step 2: Analyze each option

   static func buildBlock(_ pairs: (String, Int)...) -> [String: Int] { Dictionary(uniqueKeysWithValues: pairs) } uses Dictionary(uniqueKeysWithValues:) which correctly creates a dictionary from tuples. static func buildBlock(_ pairs: [(String, Int)]) -> [String: Int] { pairs.reduce([:]) { $0 + $1 } } tries to reduce with + which is invalid for dictionaries. static func buildBlock(_ pairs: (String, Int)...) -> [String: Int] { pairs.map { [$0.0: $0.1] } } maps to an array of dictionaries, not a single dictionary. static func buildBlock(_ pairs: (String, Int)...) -> [(String, Int)] { pairs } returns an array of tuples, not a dictionary.

3. Final Answer:

   static func buildBlock(_ pairs: (String, Int)...) -> [String: Int] { Dictionary(uniqueKeysWithValues: pairs) } -> Option C

4. Quick Check:

   Use Dictionary(uniqueKeysWithValues:) to combine tuples [OK]

Hint: Use Dictionary(uniqueKeysWithValues:) to build dictionary from tuples [OK]

Common Mistakes:

• Returning array of dictionaries instead of one dictionary
• Using invalid + operator to combine dictionaries
• Returning array of tuples instead of dictionary