Issue with TypeScript function overloading: Visitor Pattern not working as expected

Question

Issue with TypeScript function overloading: Visitor Pattern not working as expected

I've been working on implementing a Visitor Design pattern, but I keep running into a compilation error that I just can't seem to resolve no matter how many hours I spend trying to find a solution...

Here's my Visitor interface:

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

And here's my Visitor implementation:

export class VisitorImp implements Visitor {

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

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

Unfortunately, I'm encountering 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'm really hoping someone can assist me with this issue because it's starting to drive me crazy!

typescript interface overloading visitor-pattern

Answer 1

Answer №1

I haven't actually tried this out, but you might want to consider experimenting with the following code:

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

I came across this snippet while browsing this website.

UPDATE:

Upon further examination, it seems that the initial implementation might not be very elegant. In that case, you can try this alternative:

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 "";
        }
    }
}

Here, String/Number represent A/B. More details can be found here.

Answer 2

I haven't actually tried this out, but you might want to consider experimenting with the following code:

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

I came across this snippet while browsing this website.

UPDATE:

Upon further examination, it seems that the initial implementation might not be very elegant. In that case, you can try this alternative:

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 "";
        }
    }
}

Here, String/Number represent A/B. More details can be found here.

Issue with TypeScript function overloading: Visitor Pattern not working as expected

Answer №1

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