Error with generics in type constraint in TypeScript

Question

I've created a short sample code to demonstrate the problem I'm facing:

class List<T> { }

class StringList extends List<string> { }

class NumberList extends List<number> { }

class Iterator<T extends List<any>> {
    list: T;

    constructor() {
        this.list = new StringList();
    }
}

(on playground)

This code results in Cannot convert 'StringList' to 'T'.

Any idea why and how to avoid it? Thanks.

Edit

Here's an example of why it's wrong (thanks to @RyanCavanaugh for pointing it out):

var iterator: Iterator<List<any>> = new Iterator<NumberList>();

Where you'd expect iterator to have list of type List<number> but the ctor tried to assign List<string> to it.

Ryan Cavanaugh · Accepted Answer · 2014-03-15 01:50:47Z

2

This is an error because it's wrong:

class MagicList extends StringList {
    magic = 'shazam';
}

var x = new Iterator<MagicList>();
console.log(x.list.magic); // x.list does not have member 'magic'

Remember that a constraint only establishes a lower bound on the generic type.

answered Mar 15, 2014 at 1:50

Ryan Cavanaugh

223k59 gold badges280 silver badges235 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

Nitzan Tomer Over a year ago

Well yeah, it's pretty obvious ha? Now I feel retarded. Your example wasn't dead on (for one thing you did not put magic on the list member, just added a member, so you have x.magic) but I got the point and edited my question with an example. Thanks!

Collectives™ on Stack Overflow

Error with generics in type constraint in TypeScript

Edit

1 Answer 1

1 Comment

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

Edit

1 Answer 1

1 Comment

Your Answer

Sign up or log in

Post as a guest

Related