Error in functions overload in TypeScript : Visitor Pattern

Question

I m trying to create a Visitor Design pattern but i'm facing a compilation error that i was unable to solve despites the hours I tried finding a solution ...

Visitor interface :

export interface Visitor {
    visit(a: A): X;
    visit(b: B): Y;
}

Visitor implementation :

export class VisitorImp implements Visitor {

    visit(a: A): X {
      return a.getX();
    }

    visit(b: B): Y{
     return b.getY();
    }
}

With that, I have the following compilation Error :

Property 'visit' in type 'VisitorImp' is not assignable to the same property in base type 'Visitor'.
  Type '(a: A) => X' is not assignable to type '{ (a: A): X; (b: B): Y; }'.
    Type 'X' is missing the following properties from type Y

I really hope someone can help me on that because it is driving me crazy right now !

Vodis · Accepted Answer · 2021-03-24 00:31:05Z

2

I haven't tested this but you could try

export interface Visitor {
    visit: ((a: A) => X) | ((b: B) => Y);
}

Got that from skimming this site.

EDIT:

So it appears that this is not prettily implementable. This should do the trick:

interface Visitor {
    visit(a: String): Number;
    visit(b: Number): String;
}

class VisitorImp implements Visitor {

    visit(a: String): Number;
    visit(b: Number): String;
    visit(a: String | Number): Number | String {
        if(a instanceof String) {
            return 0;
        } else {
            return "";
        }
    }
}

where String/Number are A/B. As seen here.

edited Mar 24, 2021 at 0:31

answered Mar 23, 2021 at 23:21

Vodis

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2 Comments

Aion Over a year ago

When I use this, i have the following error : Duplicate function implementation on each : visit(a: A): X { return a.getX()}

Aion Over a year ago

Your comment do the trick, thank you a lot for that. Unfortunatly and as you said, it is not a pretty solution. So what I think I'll do is that i will change the visitor pattern a bit by changing the name of the Visitor function : visitA(a: A): X and visitB(b: B): Y

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