Polymorphic "this" for static members

[xealot]

When trying to implement a fairly basic, but polymorphic, active record style model system we run into issues with the type system not respecting this when used in conjunction with a constructor or template/generic.

I've posted before about this here, #5493, and #5492 appears to mention this behavior also.

And here is an SO post this that I made:
http://stackoverflow.com/questions/33443793/create-a-generic-factory-in-typescript-unsolved

I have recycled my example from #5493 into this ticket for further discussion. I wanted an open ticket representing the desire for such a thing and for discussion but the other two are closed.

Here is an example that outlines a model Factory which produces models. If you want to customize the BaseModel that comes back from the Factory you should be able to override it. However this fails because this cannot be used in a static member.

// Typically in a library
export interface InstanceConstructor<T extends BaseModel> {
    new(fac: Factory<T>): T;
}

export class Factory<T extends BaseModel> {
    constructor(private cls: InstanceConstructor<T>) {}

    get() {
        return new this.cls(this);
    }
}

export class BaseModel {
    // NOTE: Does not work....
    constructor(private fac: Factory<this>) {}

    refresh() {
        // get returns a new instance, but it should be of
        // type Model, not BaseModel.
        return this.fac.get();
    }
}

// Application Code
export class Model extends BaseModel {
    do() {
        return true;
    }
}

// Kinda sucks that Factory cannot infer the "Model" type
let f = new Factory<Model>(Model);
let a = f.get();

// b is inferred as any here.
let b = a.refresh();

Maybe this issue is silly and there is an easy workaround. I welcome comments regarding how such a pattern can be achieved.

[Think7]

The size and shape of my boat is quite similar! Ahoy!

A factory method in a superclass returns new instances of its subclasses. The functionality of my code works but requires me to cast the return type:

class Parent {
    public static deserialize(data: Object): any { ... create new instance ... }
    // Can't return a this type from statics! ^^^ :(
}

class Child extends Parent { ... }

let data = { ... };
let aChild: Child = Child.deserialize(data);
//           ^^^ Requires a cast as type cannot be inferred.

[LPGhatguy]

I ran into this issue today too!

A fixup solution is to pass the child type in as a generic extending the base class, which is a solution I apply for the time being:

class Parent {
    static create<T extends Parent>(): T {
        let t = new this();

        return <T>t;
    }
}

class Child extends Parent {
    field: string;
}

let b = Child.create<Child>();

[paul-go]

Is there a reason this issue was closed?

The fact that polymorphic this doesn't work on statics basically makes this feature DOA, in my opinion. I've to date never actually needed polymorphic this on instance members, yet I've needed every few weeks on statics, since the system of handling statics was finalized way back in the early days. I was overjoyed when this feature was announced, then subsequently let down when realizing it only works on instance members.

The use case is very basic and extremely common. Consider a simple factory method:

class Animal
{
    static create(): this
    {
        return new this();
    }
}

class Bunny extends Animal
{
    hop()
    {
    }
}

Bunny.create().hop() // Type error!! Come on!!

At this point I've been either resorting to ugly casting or littering static create() methods in each inheritor. Not having this feature seems like a fairly large completeness hole in the language.

[RyanCavanaugh]

@paul-go the issue is not closed... ?

[Think7]

@paul-go I've been frustrated with this issue also but the below is the most reliable workaround i've found. Each Animal subclass would need to call super.create() and just cast the result to it's type. Not a big deal and it's a one liner that can easily be removed with this is added.

The compiler, intellisense, and most importantly the bunny are all happy.

class Animal {
    public static create<T extends Animal>(): T {
        let TClass = this.constructor.prototype;
        return <T>( new TClass() );
    }
}

class Bunny extends Animal {    
    public static create(): Bunny {
        return <Bunny>super.create();
    }

    public hop(): void {
        console.log(" Hoppp!! :) ");
    }
}

Bunny.create().hop();

         \\
          \\_ " See? I am now a happy Bunny! "
           (')   " Don't be so hostile! "
          / )=           " :P "
        o( )_

[paul-go]

@RyanCavanaugh Oops ... for some reason I confused this with #5862 ... sorry for the battle axe aggression :-)

@Think7 Yep ... hence the "resorting to ugly casting or littering static create() methods in each inheritor". It's pretty hard though when you're a library developer and you can't really force end users to implement a bunch of typed static methods in the classes that they inherit from you.

[Think7]

lawl. Totally missed everything under your code :D

Meh was worth it, Got to draw a bunny.

[xealot]

👍 bunny

[RyanCavanaugh]

🐰 ❤️

[nathan-rice]

+1, would definitely like to see this

[LPGhatguy]

Have there been any discussion updates on this topic?

[RyanCavanaugh]

It remains on our enormous suggestion backlog.