4 ms·

FWIW, classes in TypeScript become nominal types when they have a non-public member. But I definitely do feel real FOMO over Flow’s opaque types.

• mirekrusin 2y ago
  Yes, ts is full of this kind kind of adhoc-ifs-like glued together, also exactness check only when it's literal object etc. Flow is more principled.
  • eyelidlessness 2y ago
    I mean. I think this specific case is a lot more principled than you seem to think. It’s certainly well reasoned from a perspective of structural typing by default.
    • mirekrusin 2y ago
      Yes, they have their reasons. They always do, tradeoff etc, I know. It doesn't change the fact that ie. adding private member is breaking change in your library which is kind of funny (until it's not funny of course). Also stuff like: class Foo { private foo = 1 } class Bar {} const a: Bar = new Foo ...typechecks so that's it for nominality. It's all ifs all the way down.
      • eyelidlessness 2y ago
        It’s only a breaking change if you’ve been using a class to stand in for an interface (happens to the best of us I’m sure!). You can still avoid it being one after the fact by introducing a non-breaking interface consistent with what you were already accepting/expecting. And yeah, I’m not a fan of that class instance assignability case. Not to make excuses for it, but I have other reasons I generally prefer to expose interfaces (distinct from classes, even if they’re identical in shape and even have the same internal purpose) at most API boundaries; avoiding that particular footgun just turns out to be a nice happy side effect of the preference.