ArrayBuilder.swift

import Foundation

@resultBuilder
public struct ArrayBuilder<T> {
  public static func buildBlock() -> [T] { [] }
  public static func buildExpression(_ expression: T) -> [T] { [expression] }
  public static func buildExpression(_ expression: T...) -> [T] { expression }
  public static func buildExpression(_ expression: T?...) -> [T] { expression.compactMap { $0 } }
  public static func buildExpression(_ expression: T?) -> [T] { [expression].compactMap { $0 } }
  public static func buildExpression(_ expression: [T]) -> [T] { expression }
  public static func buildBlock(_ components: [T]...) -> [T] { components.flatMap { $0 } }
  public static func buildArray(_ components: [[T]]) -> [T] { components.flatMap { $0 } }
  public static func buildOptional(_ component: [T?]) -> [T] { component.compactMap { $0 } }
  public static func buildOptional(_ component: [T]?) -> [T] { component ?? [] }
  public static func buildEither(first component: T) -> [T] { [component] }
  public static func buildEither(second component: T) -> [T] { [component] }
  public static func buildEither(first component: [T]) -> [T] { component }
  public static func buildEither(second component: [T]) -> [T] { component }
  public static func buildPartialBlock(first: [T]) -> [T] { first }
  public static func buildPartialBlock(accumulated: [T], next: [T]) -> [T] { accumulated + next }
  public static func buildLimitedAvailability(_ component: [T]) -> [T] { component }
  public static func buildFinalResult(_ component: [T]) -> [T] { component }
}

extension Array {
  public init(@ArrayBuilder<Element> builder: () -> [Element]) {
    self = builder()
  }
}

let flag = true

let array = Array {
  if flag {
    "value 0"
  }
  
  "value 1"
  
  ["value 2", "value 3"]
  
  nil
}

print(array) // ["value 0", "value 1", "value 2", "value 3"]