extendable schema type

[newash]

With the existing type definition it was not possible to add extra fields to the schema (such as "default").
Using generics now anything can be added.

With my current project it's kinda necessary, so it would be nice of you if you could pull it in. Thanks!

[awwright]

@newash Could you provide a quick code example of how this would be used, and maybe how it runs for you with and without the patch?

Thanks!

[newash]

@awwright Sure, this is my code for generating defaults:

export interface MySchema extends Schema<MySchema> {
  default: any;
}

const validator = new Validator<MySchema>();

function addDefaults(instance: any, schema: MySchema): any {
  if (schema.default !== undefined && instance === undefined) {
    return schema.default;
  }
  return instance;
}

function validate(instance: any,  uri: string, extension: RewriteFunction<MySchema>): ValidatorResult<MySchema> {
  return validator.validate(instance, uri, { rewrite: extension });
}

export function createDefaults(instance: any,  uri: string): any {
  let result: ValidatorResult<MySchema> = validate(instance, uri, addDefaults);
  if (!result.valid)
    throw "something went wrong with model creation";
  return result.instance;
}

Without the generics, the schema.default expression in the addDefaults function throws a compilation error saying there's no "default" attribute of Schema. And I had to decorate all interfaces for the RewriteFunction to get the proper tying.

I know it's not a small change and I'd be the happiest if you knew a neater solution, but I'm afraid this is the neat one.