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A comment on From TypeScript to ReScript. Parent comment

Re:From TypeScript to ReScript by ragnese on Wednesday January 12, 2022 @02:24PM (#29911604)
Yes and no. I do think that there are only two correct ways for a type system to do generic user-defined types: you either give us variance markers or you make all user-defined generic types invariant (Swift does this).

Personally, I hate the latter, but not as much as I hate unsoundness.

But, what I'm talking about is even more basic: https://www.typescriptlang.org/play?#code/MYGwhgzhAECCB2BLAt...

The code I wrote there is actually wrong for two reasons:

1. Type variance

2. Readonly doesn't do fuck-all in TypeScript for object types

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Re:From TypeScript to ReScript by inbx0 on Wednesday January 12, 2022 @05:06PM (#29913383)
Yeah I agree, that's not what I'd consider good variance rules either.

I guess my point was more about the reasons behind these weird rules. AFAIK interop with existing JavaScript was not the primary reason for doing them this way, if that's any consolation.

FWIW your example wouldn't fly in Flow

https://flow.org/try/#0PQKgBAAgZgNg9gdzCYAoVBjGBDAzrsAQQDsBL...

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Re:From TypeScript to ReScript by ragnese on Thursday January 13, 2022 @08:14AM (#29920394)
> I guess my point was more about the reasons behind these weird rules. AFAIK interop with existing JavaScript was not the primary reason for doing them this way, if that's any consolation.

Are you sure? Because there was an essay written by the TypeScript devs when they released whatever version that introduced the strict-function-variance flag explaining why they "couldn't" make it also apply to class methods, and it boiled down to breaking too much existing JavaScript code.

That's not proof that the original variance choice was also due to JavaScript compatibility, but I feel like it's a very strong suggestion that it was probably a big motivator.

I'm glad that it wouldn't work in Flow. I'm sure it wouldn't work in most/all of the other statically typed compile-to-JavaScript languages. I just hate that none of them won out over TypeScript, so they're basically all irrelevant- Flow included.

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