How to set object parameters

Question

I am rewriting a lexer class that has several "parameters" like whitespaceSplit.

So it goes this way:

lexer = new Lexer();
lexer.whitespaceSplit = true;
lexer.otherParameter = ...;
lexer.parse(file);

The trouble is that a user of my library could change the parameters such as whitespaceSplit in the middle of the parsing process and allowing this looks like a code smell for me.

What to do?

Answer 1

"several parameters" means you will likely forget which position is which if you set these with a constructor. There are ways around this problem but they depend on your language.

If you use a language with named arguments like C# you can fix that problem:

new Lexer(
    whitespaceSplit:true,
    otherParameter:"other"
);

If you use a language without them like Java you can simulate them with the Joshua Block Builder Pattern.

new Lexer.Builder
    .whitespaceSplit(true)
    .otherParameter("other")
    .build()
;

Either way you get an immutable object and the parameters are clearly labeled.

Answer 2

What about letting those parameters which change the parsers behavior only to be passed with the construction of the instance?

class Lexer {
public:
    constructor Lexer(whitespaceSplit : boolean; otherParameter : ...) 
       : whitespaceSplit_(whitespaceSplit ), otherParameter_(otherParameter) {
    }

    property whitespaceSplit : boolean read whitespaceSplit_;
    property otherParameter : ... read otherParameter_;
private:
    whitespaceSplit_ : boolean;
    otherParameter_ : ...;
};

lexer = new Lexer(true,whatever);

The access to these attributes can be made read only for further access.

The trouble is that a user of my library could change the parameters such as whitespaceSplit in the middle of the parsing process and allowing this looks like a code smell for me.

Thus this can't happen at all.