Suggestion: Interface extend generic type

[andreasbotsikas]

Hi all,

I was hoping that typescript could support interface extending generic types like the following example:

    interface IHttpPromise<T extends {}> extends T{
        $resolved: boolean;
        $error: boolean;
    }

    interface IHttpPost {
        <T extends {}>(actionUrl: string, postData: any): IHttpPromise<T>;
    }

The use case is that an http request is initiated and the IHttpPost method returns an object which gets populated when the request completes. In angular this pattern is being used by the $resource service (see definitelyTyped IResource declaration here).

Is this feasible?

Thanks in advance,

Andreas

PS: I have seen this issue in codeplex but couldn't find it here.

[pgrm]

I think it is actually quite common in JS to take an object, add properties or functions to it and return it. And I wouldn't know how else to define the return as a generic except

getRemoteObject<T>(id: string): IRemoteObject<T>;

interface IRemoteObject<T> extends T { save(); getRawObject(): T; ... }

I've just written a definition file for such a similar case and had to discribe in the comments the workaround to actually create an interface yourself which merges IRemoteObject<T> and T. For instance:

interface IUser {email: string; name: string; ...}
interface IRemoteUser extends IRemoteObject<IUser>, IUser { }

var user: IRemoteUser = getRemoteObject<IUser>('id');

Or did I miss any other way to implement this?

[richardhuaaa]

+1! I think this is the best way to use TypeScript with Immutable js records. Also generally when you want to use the decorator pattern on generic types, such as with Radium.enhancer().

[eggers]

Here's another example:

interface Animal {
  numberOfLegs: number;
}

interface MaleAnimal<A extends Animal> extends A {}

interface FemaleAnimal<A extends Animal, M extends MaleAnimal<A>> extends A {
  mate(father:M): Promise<A[]>;
}

This has two errors:

• An interface may only extend a class or another interface.
• Constraint of a type parameter cannot reference any type parameter from the same type parameter list.

[mhegazy]

Why not use intersection types to model this? e.g.

interface Controller {
    name: string;
}

interface EventHandler {
    handel(event: string): void;
}

var MyEventHandelingControler: Controller & EventHandler;

MyEventHandelingControler.name;
MyEventHandelingControler.handel("event");

[pgrm]

Is this a suggestion his it could be implemented, or already a working feature?

[mhegazy]

@pgrm, intersection types is part of TypeScript 1.6, here is the release blog

[ronzeidman]

Will I be able to do something like this:

function addMetadata<T>(data: T): T & MetaData {
     ....
}

[DanielRosenwasser]

@ronzeidman yes, that should work.

[pgrm]

Yes the example seems to suggest that it should work also with generics:

function extend<T, U>(first: T, second: U): T & U {
    let result = <T & U> {};
    for (let id in first) {
        result[id] = first[id];
    }
    for (let id in second) {
        if (!result.hasOwnProperty(id)) {
            result[id] = second[id];
        }
    }
    return result;
}

var x = extend({ a: "hello" }, { b: 42 });
var s = x.a;
var n = x.b;

That solves what I wanted. So from my side this issue could be closed. thx @mhegazy

[mhegazy]

@andreasbotsikas, any other scenarios not captured in the previous discussion?