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0 pointsint_19h1y ago0 comments

TS does have sum types, so you can already do something like:

   type Color = "red" | "green";

What GP is doing is some scaffolding on top to make the values more discoverable and allow associating arbitrary Color-specific metadata with them.

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davorak1y ago

I thought there were edge cases where sum types were not fully supported, nesting[1] is the example found first, but I thought there were others.

[1] https://github.com/microsoft/TypeScript/issues/18758

int_19hOP1y ago

Per the comments there, it still works if you use an explicit type guard function for checks, as opposed to checking the nested property directly, so I'd argue that the type itself is still supported in general, just not this particular way of testing for one of the options.

davorak1y ago

Maybe I am doing something wrong but when I fill in a concrete value for `aOrB` I still get a type error in that example code:

Playground Link: https://www.typescriptlang.org/play/?#code/C4TwDgpgBAglC8UDe...

I tried with 5.8.2 and nightly and the results were the same.

Interestingly, the playground reports aOrB(from github comment with concrete value) and aOrB2(modified) as the same type, `A | B`, but aOrB will give an error in the typeguarded if block but aOrB2 does not trigger an error. I do not know what is going on there either they do not really have the same type despite the playground reporting both as `A | B` or there is different bug going on.

So the solution presented in github does not look like a full solution as is.

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