BuildOptional and buildEither in Swift - Mini Project: Build & Apply

Using BuildOptional and BuildEither in Swift

📖 Scenario: You are creating a simple Swift function that builds a message based on optional and conditional values.

🎯 Goal: Build a Swift function that uses BuildOptional and BuildEither to conditionally create parts of a message.

📋 What You'll Learn

Create an optional string variable called optionalPart with the value "Optional part included."

Create a boolean variable called includeFirst set to true

Use BuildOptional to include optionalPart only if it is not nil

Use BuildEither to choose between two strings based on includeFirst

Print the final combined message

💡 Why This Matters

🌍 Real World

Result builders like <code>BuildOptional</code> and <code>BuildEither</code> help Swift developers write clean, readable code for building UI views, DSLs, or complex strings with conditional parts.

💼 Career

Understanding these concepts is useful for Swift developers working on SwiftUI apps or any code that benefits from declarative and conditional building patterns.

1

Create the optional string variable

Create an optional string variable called optionalPart and set it to "Optional part included."

Swift

# Create the optional string variable 'optionalPart'
# Your code here

Hint

Use let optionalPart: String? = "Optional part included." to create the optional string.

2

Create the boolean variable

Create a boolean variable called includeFirst and set it to true

Swift

let optionalPart: String? = "Optional part included."
# Create the boolean variable 'includeFirst'
# Your code here

Hint

Use let includeFirst: Bool = true to create the boolean variable.

3

Use BuildOptional and BuildEither to build message parts

Use BuildOptional to include optionalPart only if it is not nil, and use BuildEither to choose between "First part included." and "Second part included." based on includeFirst. Store the combined message in a variable called message.

Swift

let optionalPart: String? = "Optional part included."
let includeFirst: Bool = true
# Use BuildOptional and BuildEither to build the message
# Your code here

Hint

Define a @resultBuilder called MessageBuilder with buildOptional and buildEither methods. Use it in buildMessage() to conditionally add parts.

4

Print the final message

Write a print statement to display the variable message

Swift

let optionalPart: String? = "Optional part included."
let includeFirst: Bool = true

@resultBuilder
struct MessageBuilder {
    static func buildBlock(_ components: String...) -> String {
        components.joined(separator: " ")
    }
    static func buildOptional(_ component: String?) -> String {
        component ?? ""
    }
    static func buildEither(first component: String) -> String {
        component
    }
    static func buildEither(second component: String) -> String {
        component
    }
}

@MessageBuilder
func buildMessage() -> String {
    if includeFirst {
        "First part included."
    } else {
        "Second part included."
    }
    optionalPart
}

let message = buildMessage()
# Print the message
# Your code here

Hint

Use print(message) to display the final combined message.

Practice

1. What is the main purpose of buildOptional in Swift's result builders?

easy

A. To choose between two code paths explicitly

B. To wrap optional code blocks so they can be included or skipped

C. To force unwrap optionals in code blocks

D. To create loops inside result builders

Solution

1. Step 1: Understand buildOptional's role

   buildOptional is designed to handle optional code blocks, allowing them to be included if present or skipped if nil.

2. Step 2: Compare with other builder functions

   buildEither chooses between two paths, but buildOptional specifically wraps optional code, making it flexible for optional UI parts.

3. Final Answer:

   To wrap optional code blocks so they can be included or skipped -> Option B

4. Quick Check:

   buildOptional wraps optionals = B [OK]

Hint: Remember: buildOptional handles optionals, buildEither picks sides [OK]

Common Mistakes:

• Confusing buildOptional with buildEither
• Thinking buildOptional forces unwrapping
• Assuming buildOptional creates loops

2. Which of the following is the correct syntax to use buildEither for choosing the first branch in a Swift result builder?

easy

A. buildEither(option: 1) { Text("First") }

B. buildEither(second: { Text("First") })

C. buildEither(first: { Text("First") })

D. buildEither(branch: .first, content: { Text("First") })

Solution

1. Step 1: Recall buildEither syntax

   The correct syntax uses named parameters first or second to specify the branch.

2. Step 2: Identify the correct usage

   buildEither(first: { Text("First") }) uses buildEither(first: { ... }), which matches the official Swift syntax for choosing the first branch.

3. Final Answer:

   buildEither(first: { Text("First") }) -> Option C

4. Quick Check:

   buildEither first branch = A [OK]

Hint: Use named parameters first or second with buildEither [OK]

Common Mistakes:

• Using incorrect parameter names like option or branch
• Confusing first and second parameters
• Trying to pass branch as enum instead of named argument

3. What will be the output of this Swift code using buildEither?

enum ResultBuilder {
  static func buildEither(first: String) -> String { "First: \(first)" }
  static func buildEither(second: String) -> String { "Second: \(second)" }
}

let result = ResultBuilder.buildEither(first: "Hello")
print(result)

medium

A. Compilation error

B. Second: Hello

C. Hello

D. First: Hello

Solution

1. Step 1: Identify which buildEither is called

   The code calls buildEither(first: "Hello"), so the first overload is used.

2. Step 2: Understand the returned string

   The first overload returns the string "First: Hello" which is printed.

3. Final Answer:

   First: Hello -> Option D

4. Quick Check:

   buildEither first branch returns "First: Hello" = A [OK]

Hint: Check which named parameter is used: first or second [OK]

Common Mistakes:

• Assuming second branch runs when first is called
• Expecting just the input string without prefix
• Thinking this causes a compile error

4. Given this Swift code snippet, what is the error and how to fix it?

struct Builder {
  @resultBuilder
  struct MyBuilder {
    static func buildEither(first: String) -> String { first }
    static func buildEither(second: String) -> String { second }
  }
}

let value = Builder.MyBuilder.buildEither(third: "Oops")

medium

A. Error: No 'third' parameter; fix by using 'first' or 'second'

B. Error: Missing return statement; fix by adding return

C. Error: buildEither cannot be static; remove static keyword

D. No error; code runs fine

Solution

1. Step 1: Identify the method call

   The code calls buildEither(third: "Oops"), but only first and second parameters exist.

2. Step 2: Understand the fix

   To fix, replace third with either first or second to match the defined overloads.

3. Final Answer:

   Error: No 'third' parameter; fix by using 'first' or 'second' -> Option A

4. Quick Check:

   buildEither only accepts first or second = C [OK]

Hint: Use only first or second parameters with buildEither [OK]

Common Mistakes:

• Trying to add unsupported parameter names
• Ignoring static keyword requirements
• Assuming code runs without error

5. How can you use buildOptional and buildEither together to conditionally build a SwiftUI view that shows "Logged In" if a user is logged in, "Guest" if not logged in but guest mode is enabled, and nothing otherwise?

hard

A. Use buildOptional to wrap guest mode view, and buildEither to choose between logged in and guest

B. Use buildEither to wrap guest mode view, and buildOptional to choose between logged in and guest

C. Use only buildOptional for all conditions

D. Use only buildEither for all conditions

Solution

1. Step 1: Understand the conditions

   We have three cases: logged in, guest mode, and neither. buildEither can choose between logged in and guest mode views.

2. Step 2: Use buildOptional for the third case

   The third case (neither) means no view, so wrap the guest mode view in buildOptional to include it only if guest mode is enabled.

3. Step 3: Combine both builders

   Use buildEither to select logged in or guest mode, and buildOptional to skip guest mode if disabled, resulting in no view if neither condition is true.

4. Final Answer:

   Use buildOptional to wrap guest mode view, and buildEither to choose between logged in and guest -> Option A

5. Quick Check:

   buildOptional wraps optional guest, buildEither picks logged in or guest = D [OK]

Hint: buildEither picks branches; buildOptional skips optional parts [OK]

Common Mistakes:

• Using buildEither alone for optional skipping
• Using buildOptional alone for branching
• Not handling the 'no view' case properly