Stringのextensionにcomputed propertyをつくる

before :

extension String {
    func numeralsToNumber() -> String {
        enum Chartype {
            case numerical
            case exp
            case normal
            
            init(_ number : Int) {
                switch number {
                case 0,1:
                    self = .numerical
//                case 1:
//                    self = .exp
                default:
                    self = .normal
                }
            }
        }
        
        let japaneseNumericalChars : [String : String] = [
            "〇": "0",
            "一": "1",
            "二": "2",
            "三": "3",
            "四": "4",
            "五": "5",
            "六": "6",
            "七": "7",
            "八": "8",
            "九": "9",
            ]
        let japaneseChars : Set = Set(japaneseNumericalChars.keys)

        let japaneseNumericalExpChars : [String : String] = [
            "十": "1",
            "百": "2",
            "千": "3",
            "万": "4",
            "億": "8",
            ]
        let japaneseExpChars : Set = Set(japaneseNumericalExpChars.keys)
        
        var splitedStr : [String] = []
        // 以下略

after :

extension String {
    var japaneseNumericalChars : [String : String] {
        return  [
            "〇": "0",
            "一": "1",
            "二": "2",
            "三": "3",
            "四": "4",
            "五": "5",
            "六": "6",
            "七": "7",
            "八": "8",
            "九": "9",
        ]
    }
    
    var japaneseChars : CharacterSet {
        return CharacterSet(charactersIn: japaneseNumericalChars.keys.joined())
    }
    
    private func convertCharToStr1To9(_ char : Character) -> String {
        return japaneseNumericalChars[char.description]!
    }
    
    var japaneseNumericalExpChars : [String : String] {
        return  [
            "十": "1",
            "百": "2",
            "千": "3",
            "万": "4",
            "億": "8",
        ]
    }
    
    var japaneseExpChars : CharacterSet {
        return CharacterSet(charactersIn: japaneseNumericalExpChars.keys.joined())
    }
    
    private func convertExpStrToNumStr(_ str : String) -> String {
        return japaneseNumericalChars[str]!
    }
    
    enum Chartype {
        case numerical
        case exp
        case normal
        
        init(_ number : Int) {
            switch number {
            case 0,1:
                self = .numerical
            default:
                self = .normal
            }
        }
    }
    
    func numeralsToNumber() -> String {
        
        var splitedStr : [String] = []
        // 以下略

computed propertyはletでできないとかdictionaryプロパティに直接アクセスできないから専用のメソッドを作ったりしないといけないとか色々あったけど大きな問題はなく実装できた。