@resultBuilder attribute in Swift - Time & Space Complexity

Time Complexity: @resultBuilder attribute

O(n)

Understanding Time Complexity

We want to understand how the time it takes to build results grows when using the @resultBuilder attribute in Swift.

Specifically, how does the number of steps change as we add more parts to the builder?

Scenario Under Consideration

Analyze the time complexity of this simple result builder usage.

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

func makeList(@SimpleBuilder content: () -> String) -> String {
  content()
}

let result = makeList {
  "Apple"
  "Banana"
  "Cherry"
}

This code combines multiple strings into one using a result builder.

Identify Repeating Operations

Look for repeated actions that take time.

• Primary operation: Joining all string components into one string.
• How many times: The join operation processes each component once.

How Execution Grows With Input

As you add more strings, the join operation must handle each one.

Input Size (n)	Approx. Operations
10	About 10 string joins
100	About 100 string joins
1000	About 1000 string joins

Pattern observation: The work grows directly with the number of items added.

Final Time Complexity

Time Complexity: O(n)

This means the time to build the result grows in a straight line as you add more parts.

Common Mistake

[X] Wrong: "Using a result builder makes building results instant no matter how many parts there are."

[OK] Correct: Even with a result builder, each part must be processed, so time grows with the number of parts.

Interview Connect

Understanding how result builders work helps you explain how Swift handles complex code blocks efficiently.

Self-Check

What if the builder combined nested builders instead of just strings? How would that affect the time complexity?

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