Swift attributed strings with @resultBuilder

AttributedStrings.swift

import UIKit

public protocol AttributedStringConvertible {
    var attributedString: NSMutableAttributedString { get }
}

extension String: AttributedStringConvertible {
    public var attributedString: NSMutableAttributedString {
        NSMutableAttributedString(string: self)
    }
}

extension NSMutableAttributedString: AttributedStringConvertible {
    public var attributedString: NSMutableAttributedString {
        self
    }
}

extension AttributedStringConvertible {
    public func font(_ font: UIFont) -> NSMutableAttributedString {
        let attr = attributedString
        attr.addAttribute(.font, value: font, range: NSRange(location: 0, length: attr.length))
        return attr
    }
    
    public func color(_ color: UIColor) -> NSMutableAttributedString {
        let attr = attributedString
        attr.addAttribute(.foregroundColor, value: color, range: NSRange(location: 0, length: attr.length))
        return attr
    }
    
    public static func attributed(@AttributedStringBuilder _ data: () -> NSMutableAttributedString) -> NSMutableAttributedString {
        data()
    }
}

@resultBuilder
public struct AttributedStringBuilder {
    public static func buildBlock(_ components: AttributedStringConvertible...) -> NSMutableAttributedString {
        components.map(\.attributedString).joined()
    }
    
    public static func buildEither(first component: AttributedStringConvertible) -> AttributedStringConvertible {
        component
    }
    
    public static func buildEither(second component: AttributedStringConvertible) -> AttributedStringConvertible {
        component
    }
    
    public static func buildOptional(_ component: AttributedStringConvertible?) -> AttributedStringConvertible {
        component ?? ""
    }
    
    public static func buildArray(_ components: [AttributedStringConvertible]) -> AttributedStringConvertible {
        components.map(\.attributedString).joined()
    }
}

extension Array where Element == NSMutableAttributedString {
    fileprivate func joined() -> NSMutableAttributedString {
        reduce(NSMutableAttributedString()) {
            $0.append($1)
            return $0
        }
    }
}

Usage

someLabel.attributedText = String.attributed {
    "up to "
    "$543,000".font(.bold14).color(.primary6)
    " lifetime impact"
}