An simple example of using resultBuilder to create an Array.

ArrayBuilder.swift

@resultBuilder
struct ArrayBuilder<Element> {
    static func buildBlock(_ components: [Element]...) -> [Element] {
        components.flatMap { $0 }
    }
    
    static func buildExpression(_ expression: Element) -> [Element] {
        [expression]
    }
    
    static func buildEither(first component: [Element]) -> [Element] {
        component
    }
    
    static func buildEither(second component: [Element]) -> [Element] {
        component
    }
    
    static func buildArray(_ components: [[Element]]) -> [Element] {
        components.flatMap { $0 }
    }
    
    static func buildOptional(_ component: [Element]?) -> [Element] {
        component ?? []
    }
    
    static func buildLimitedAvailability(_ component: [Element]) -> [Element] {
        component
    }
}

extension Array {
    init(@ArrayBuilder<Element> content: () -> [Element]) {
        self = content()
    }
    
    static func build(@ArrayBuilder<Element> content: () -> [Element]) -> Self {
        content()
    }
}

NotUsingResultBuilder.swift

// An Example of not using the result builder to make and array.

let shouldWave = true
let isDaytime = true
let names = ["Alice", "Bob"]

var greetingParts = [
    "Hello",
    "World"
]

if shouldWave {
    greetingParts.append("👋")
}

if isDaytime {
    greetingParts.append("☀️")
} else {
    greetingParts.append("🌙")
}

for name in names {
    greetingParts.append(name)
}

if #available(iOS 14.0, *) {
    greetingParts.append("🆕")
}

print(greetingParts) // ["Hello", "World", "👋", "☀️", "Alice", "Bob", "🆕"]

UsingResultBuilder.swift

// An Example of using the result builder to make an array.

let shouldWave = true
let isDaytime = true
let names = ["Alice", "Bob"]

let greetingParts = Array {
    "Hello"
    "World"
        
    if shouldWave {
        "👋"
    }
    
    if isDaytime {
        "☀️"
    } else {
        "🌙"
    }
    
    for name in names {
        name
    }
    
    if #available(iOS 14.0, *) {
        "🆕"
    }
}

print(greetingParts) // ["Hello", "World", "👋", "☀️", "Alice", "Bob", "🆕"]