Convert isPattern functions to TS user-defined type guards

[Swahvay]

Instead of simply doing

export interface ISomePattern {
  someID: ID;
}

export function SomePatternMixin<T extends Constructor>(
  BaseClass: T,
) {
  return class SomePatternMixin extends BaseClass {
    readonly someID: ID;
    constructor(...args: any[]) {
      super(...args);
      const { data } = extractFromArgs(args);
      this.someID = data.some_id;
    }

    isSomePattern() {
      return true;
    }
  };
}

The isSomePattern can type the variable when it is called by changing the call signature.

isSomePattern(): this is ISomePattern {
  return true;
}

This would then allow you to do someEnt?.isSomePattern() && someEnt.someID without type errors.

[Swahvay]

Actually, I guess for that to work, you'd also have to default every ent to include isSomePattern to return false. Is there a good way then to get if an ent is an instance of a pattern?

[lolopinto]

I think https://github.com/lolopinto/ent/compare/fix-1732?expand=1 is the best I can do (expand the PR to see manual changes)

and then you'll have to call isSomePattern(ent) and trust that if it meets the criteria, that's good enough

if you somehow have a different pattern or an ent which has that same shape, it'll pass and then fail in a different way...

we can cheat and add a new field that's only unique to objects which have that pattern name which should heavily increase the odds of it returning the right thing

[Swahvay]

What if, instead of

const o = (obj as unknown as IWithAddress);
return 'addressId' in o && typeof o.addressId === 'string' || typeof o.addressId === 'number' || o.addressId === null;

You kept the isWithAddress function on the pattern/ent, then you could do

const o = (obj as unknown as IWithAddress);
return o.isWithAddress?.();

[lolopinto]

we can do that but it doesn't guarantee anything

would still have this same issue

and then you'll have to call isSomePattern(ent) and trust that if it meets the criteria, that's good enough

happy to make that change tho cos it's easier :)

[Swahvay]

Yeah, I get that it's never possible to do this with 100% safety. Figured this was at least a little more direct about it though.