@resultBuilder attribute in Swift - Mini Project: Build & Apply

Building a Simple HTML Builder with @resultBuilder

📖 Scenario: Imagine you want to create HTML code using Swift in a clean and readable way. Swift's @resultBuilder attribute helps you build complex structures like HTML by writing code that looks like regular Swift code but produces HTML strings.

🎯 Goal: You will build a simple HTML builder using @resultBuilder that lets you write HTML elements in Swift code and get the final HTML string as output.

📋 What You'll Learn

Create a HTMLBuilder result builder to combine strings

Write a function buildHTML that uses @HTMLBuilder

Use the builder to create multiple HTML elements inside buildHTML

Print the final combined HTML string

💡 Why This Matters

🌍 Real World

Using <code>@resultBuilder</code> helps developers write clean and readable code for building complex data like HTML, JSON, or UI layouts.

💼 Career

Understanding <code>@resultBuilder</code> is useful for Swift developers working on SwiftUI, DSLs, or any code that benefits from declarative syntax.

1

Create the HTMLBuilder result builder

Create a struct called HTMLBuilder with the @resultBuilder attribute. Inside it, write a static function buildBlock that takes a variadic parameter of String and returns a single String by joining all parts.

Swift

// Your code here

Hint

Use @resultBuilder before the struct name. The buildBlock function combines all strings into one.

2

Create the buildHTML function using @HTMLBuilder

Write a function called buildHTML that uses the @HTMLBuilder attribute. The function should return a String and have no parameters. Inside the function, return multiple HTML element strings like "

Hello

" and "

Welcome to Swift

" on separate lines.

Swift

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

// Your code here

Hint

Use @HTMLBuilder before the function. Return multiple strings inside the function body.

3

Call buildHTML and store the result

Create a constant called html and assign it the result of calling buildHTML().

Swift

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

@HTMLBuilder
func buildHTML() -> String {
    "Hello"
    "Welcome to Swift"
}

// Your code here

Hint

Use let html = buildHTML() to store the result.

4

Print the final HTML string

Write a print statement to display the value of the html constant.

Swift

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

@HTMLBuilder
func buildHTML() -> String {
    "Hello"
    "Welcome to Swift"
}

let html = buildHTML()

// Your code here

Hint

Use print(html) to show the final HTML string.

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