Allow a module to implement an interface

[ivogabe]

It would be useful when a module can implement an interface using the implements keyword. Syntax: module MyModule implements MyInterface { ... }.

Example:

interface Showable {
 show(): void;
}
function addShowable(showable: Showable) {

}

// This works:
module Login {
 export function show() {
 document.getElementById('login').style.display = 'block';
 }
}
addShowable(Login);

// This doesn't work (yet?)
module Menu implements Showable {
 export function show() {
 document.getElementById('menu').style.display = 'block';
 }
}
addShowable(Menu);

[sophiajt]

How would this work with external modules? It's likely once people can use it for internal, they'll also want to use it with external.

[ivogabe]

That's a good question. I don't know which syntax would be the best, but here are a few suggestions:

implements Showable; // I would prefer this one.
module implements Showable;
export implements Showable;

It should only be allowed on external modules that don't use an export assignment, since if you use an export assignment, the thing that you export can already have an implements on another place.

[RyanCavanaugh]

Approved. We prefer the syntax

export implements Showable;

and agreed that this is unneeded for files export = assignments.

[mhegazy]

Some more questions:

• Since we are allowing modules to have types at declaration sites, should not we allow them to have types at use sites as well. e.g.:

declare module "Module" implements Interface { }

import i : Interface = require("Module");

• What do you do with merged declarations, should you enforce the interface on the aggregate of all declarations? and what happens if they do not match in visibility?
  e.g.:

module Foo {
 export interface IBar {
 (a:string): void;
 }

 export module Bar implements IBar { // should this be an error?
 export interface Interface {}
 } 

 function Bar(a: string) : void { } // not exported
}

var bar: Foo.IBar = Foo.Bar;

[ivogabe]

It should be allowed on ambient external modules. For these modules two syntaxes should be allowed in my opinion:

declare module "first" implements Foo { }
declare module "second" {
 interface Bar { }
 export implements Bar; // this syntax is necessary, with the first syntax you can't reference Bar. 
}

Or should Bar be in the scope in an implements clause before the opening {?

Adding type info to an import statement isn't really useful in my opinion, since you can add the type info to the module itself.

And for merged declarations, I'd say that the module block that contains the implements clause should implement the interface. That also prevents issues with visibility.

[jbondc]

How would this be related to #2159? A namespace implements an interface?