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Comment by esprehn | Hacker News Reader

The TypeScript comment is wrong, the author needs to use a const enum: https://www.typescriptlang.org/docs/handbook/enums.html#cons...

Then it generates code that's better than Flow. It's not as aggressive as Reason for sure, but it's still pretty good.

https://www.typescriptlang.org/play/#src=const%20enum%20Scre...

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Not just that, the initial Flow example can actually be translated 1-to-1 to TypeScript (without using enums at all):

https://www.typescriptlang.org/play/index.html#src=type%20Ap...

Ah! Thanks for pointing this out, I'll be sure update the post. (I haven't used TypeScript all that extensively, so I didn't even know about `const enum`.)

FYI here's a variant that eliminates the `impossible` function and instead uses an assignment to the same effect:

https://www.typescriptlang.org/play/#src=const%20enum%20Scre...

This is what uglify makes of that:

    var needsCancelButton = function (e) {
      switch (e) {
        case 0:
        case 1:
          return!0;
        case 2:
          return!1
      }
      return e
    };

EDIT: Just using `return screen` without the reassignment also works because the return type of the function and the type of the argument are incompatible, but that seems a lot more error prone and less widely applicable that explicitly reassigning to a `never` variable and returning that.

I hope the author adds this correction to his article because there’s no way to comment on it directly.

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