Why BuildOptional and buildEither in Swift? - Purpose & Use Cases

The Big Idea

What if you could replace tangled if-else checks with clean, simple building blocks for choices and options?

The Scenario

Imagine you are writing code to handle different choices or optional values manually, like checking if a value exists or deciding between two options with many if-else statements.

The Problem

This manual approach quickly becomes messy and hard to read. You might forget a case, write repetitive code, or introduce bugs because the logic is scattered and unclear.

The Solution

BuildOptional and buildEither let you cleanly and clearly express optional and either-or choices in your code. They help you build these decisions in a structured way, making your code easier to write, read, and maintain.

Before vs After

✗ Before

if let value = optionalValue {
    // handle value
} else {
    // handle no value
}

if condition {
    // do A
} else {
    // do B
}

✓ After

buildOptional { value }
buildEither(first: A)
buildEither(second: B)

What It Enables

You can express complex optional and choice logic in a simple, clear, and reusable way that scales well as your code grows.

Real Life Example

Think about a form where a user can optionally enter a phone number, or choose between email or phone contact methods. BuildOptional and buildEither help you handle these choices cleanly in your code.

Key Takeaways

Manual checks for options and choices get messy fast.

BuildOptional and buildEither provide a neat way to express these decisions.

This leads to clearer, safer, and easier-to-maintain code.

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