Why result builders enable DSLs in Swift - See It in Action

Why result builders enable DSLs

📖 Scenario: Imagine you want to create a simple way to build HTML code using Swift. Writing HTML tags by hand can be long and confusing. Result builders help you write HTML in a clean, easy style that looks like a mini language inside Swift.

🎯 Goal: You will build a small HTML builder using Swift's result builders. This will show how result builders let you create your own little language (DSL) inside Swift code.

📋 What You'll Learn

Create a simple data structure to hold HTML content

Define a result builder called HTMLBuilder

Use the result builder to build HTML tags in a clean way

Print the final HTML string

💡 Why This Matters

🌍 Real World

Result builders let developers write code that looks like a small language inside Swift. This makes building UI, HTML, or other structured data easier and clearer.

💼 Career

Understanding result builders helps you work with SwiftUI and other modern Swift frameworks that use DSLs for clean, readable code.

1

Create a struct called HTML with a content string

Create a struct called HTML with one property content of type String. Add an initializer that sets content.

Swift

struct HTML {
    // Your code here
}

Hint

Use struct to create HTML. Add a property content and an initializer that takes a String.

2

Define a result builder called HTMLBuilder

Define a result builder called HTMLBuilder using @resultBuilder. Add a static function buildBlock that takes multiple HTML parameters and returns a single HTML by joining their content strings.

Swift

struct HTML {
    let content: String
    init(_ content: String) {
        self.content = content
    }
}

// Define the result builder here
// Your code here

Hint

Use @resultBuilder before struct HTMLBuilder. The buildBlock function combines all HTML contents into one.

3

Create a function html using HTMLBuilder to build HTML content

Create a function called html that takes a closure marked with @HTMLBuilder and returns HTML. Inside the function, call the closure and return its result.

Swift

struct HTML {
    let content: String
    init(_ content: String) {
        self.content = content
    }
}

@resultBuilder
struct HTMLBuilder {
    static func buildBlock(_ components: HTML...) -> HTML {
        let combined = components.map { $0.content }.joined(separator: "")
        return HTML(combined)
    }
}

// Create the html function here
// Your code here

Hint

Define html with a closure parameter marked @HTMLBuilder. Return the result of calling the closure.

4

Use the html function to build and print HTML content

Use the html function to build HTML with two tags: HTML("

Hello

") and HTML("

Welcome to DSLs!

"). Store the result in page. Then print page.content.

Swift

struct HTML {
    let content: String
    init(_ content: String) {
        self.content = content
    }
}

@resultBuilder
struct HTMLBuilder {
    static func buildBlock(_ components: HTML...) -> HTML {
        let combined = components.map { $0.content }.joined(separator: "")
        return HTML(combined)
    }
}

func html(@HTMLBuilder content: () -> HTML) -> HTML {
    return content()
}

// Build the HTML page and print the content
// Your code here

Hint

Call html with two HTML lines inside. Print the combined content.

Practice

1. What is the main reason result builders enable Domain Specific Languages (DSLs) in Swift?

easy

A. They replace all Swift syntax with a new language.

B. They automatically optimize code for faster execution.

C. They enforce strict type checking at compile time.

D. They allow combining multiple code statements into a single, readable result.

Solution

1. Step 1: Understand what result builders do

   Result builders combine multiple code pieces into one result, making code easier to read.

2. Step 2: Connect to DSL creation

   This combining ability lets developers write mini languages (DSLs) inside Swift that look clean and simple.

3. Final Answer:

   They allow combining multiple code statements into a single, readable result. -> Option D

4. Quick Check:

   Result builders combine code = C [OK]

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

Common Mistakes:

• Thinking result builders change Swift syntax completely
• Confusing result builders with performance optimizers
• Believing result builders only check types

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

easy

A. @resultBuilder struct MyBuilder { ... }

B. resultBuilder MyBuilder { ... }

C. @resultBuilder func MyBuilder() { ... }

D. builderResult MyBuilder { ... }

Solution

1. Step 1: Recall result builder declaration syntax

   In Swift, result builders are declared using the attribute @resultBuilder before a struct or class.

2. Step 2: Match options with correct syntax

   @resultBuilder struct MyBuilder { ... } uses @resultBuilder before struct, which is correct syntax.

3. Final Answer:

   @resultBuilder struct MyBuilder { ... } -> Option A

4. Quick Check:

   @resultBuilder + struct = A [OK]

Hint: Result builders use @resultBuilder before struct [OK]

Common Mistakes:

• Using @resultBuilder on functions instead of structs
• Writing resultBuilder instead of @resultBuilder
• Mixing keywords builderResult or resultBuilder incorrectly

3. Given this Swift code using a result builder, what will be printed?

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

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

let result = makeList {
    "Apple"
    "Banana"
    "Cherry"
}
print(result)

medium

A. Apple, Banana, Cherry

B. List: Apple Banana Cherry

C. List: Apple, Banana, Cherry

D. List: ["Apple", "Banana", "Cherry"]

Solution

1. Step 1: Understand buildBlock behavior

   The buildBlock function joins all string components with ", ". So the strings "Apple", "Banana", "Cherry" become "Apple, Banana, Cherry".

2. Step 2: Analyze makeList output

   makeList prepends "List: " to the joined string, so the final output is "List: Apple, Banana, Cherry".

3. Final Answer:

   List: Apple, Banana, Cherry -> Option C

4. Quick Check:

   Joined strings with commas + prefix = A [OK]

Hint: Look for join separator in buildBlock to find output format [OK]

Common Mistakes:

• Ignoring the join separator and printing without commas
• Forgetting the "List: " prefix
• Assuming output is an array instead of a string

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

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

func makeSum(@NumberBuilder content: () -> Int) -> Int {
    content()
}

let total = makeSum {
    1
    "2"
    3
}
print(total)

medium

A. Function makeSum should not have a parameter.

B. Cannot mix Int and String in result builder components.

C. Result builder must return an array, not Int.

D. Missing return statement in buildBlock function.

Solution

1. Step 1: Check types in result builder components

   The buildBlock expects Int components, but the closure includes a String "2" which is invalid.

2. Step 2: Understand type mismatch error

   Mixing Int and String causes a compile-time error because the builder cannot combine different types.

3. Final Answer:

   Cannot mix Int and String in result builder components. -> Option B

4. Quick Check:

   Type mismatch in components = D [OK]

Hint: Check all builder inputs have the same type [OK]

Common Mistakes:

• Ignoring type mismatch between Int and String
• Thinking missing return causes error here
• Assuming result builder must return array

5. How can result builders be used to create a simple HTML DSL in Swift? Choose the best approach.

hard

A. Define a result builder that combines HTML element strings and use it to build nested HTML code.

B. Use result builders to automatically convert Swift structs into HTML tags without any code.

C. Result builders replace the need for HTML by generating Swift UI views only.

D. Result builders only work with numbers and cannot handle string-based DSLs like HTML.

Solution

1. Step 1: Understand result builders for DSLs

   Result builders let you combine multiple pieces of code into one result, perfect for building nested structures like HTML strings.

2. Step 2: Apply to HTML DSL creation

   By defining a result builder that joins HTML element strings, you can write clean Swift code that looks like HTML markup.

3. Final Answer:

   Define a result builder that combines HTML element strings and use it to build nested HTML code. -> Option A

4. Quick Check:

   Result builders combine strings for DSL = B [OK]

Hint: Use result builders to combine nested strings for DSLs [OK]

Common Mistakes:

• Thinking result builders auto-convert structs to HTML
• Believing result builders replace HTML completely
• Assuming result builders only work with numbers