ResultBuilder is a free @resultBuilder (as in https://wiki.haskell.org/Free_structure)

ResultBuilder.swift

@resultBuilder
enum ResultBuilder<Expression> {
	case block([Self])
	static func buildBlock(_ components: Self...) -> Self {
		.block(components)
	}
	
	case expression(Expression)
	static func buildExpression(_ expression: Expression) -> Self {
		.expression(expression)
	}
	
	indirect case optional(Self?)
	static func buildOptional(_ component: Self?) -> Self {
		.optional(component)
	}
	
	indirect case either(first: Self)
	static func buildEither(first component: Self) -> Self {
		.either(first: component)
	}
	indirect case either(second: Self)
	static func buildEither(second component: Self) -> Self {
		.either(second: component)
	}
	
	case array([Self])
	static func buildArray(_ components: [Self]) -> Self {
		.array(components)
	}
	
	indirect case limitedAvailability(Self)
	static func buildLimitedAvailability(_ component: Self) -> Self {
		.limitedAvailability(component)
	}
}