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replyVery interesting. I’m shocked the typescript creators built a system with this failure mode. I guess the solution here is to have tsc change the type of “a” after the call to mutateArray, unless the arr argument is marked as readonly.
replyIs there a name for this failure mode?
In TypeScript it's called "bivariance", which sounds very programming language theory like, but is not a term typically used in academic contexts, since it is unsound almost by default. It's described here: https://www.typescriptlang.org/docs/handbook/type-compatibil...
replyI think the problem is the union argument type - intuitively we read "array of strings OR numbers", but actually it means "array of strings AND numbers". Probably generics would be more appropriate here, with the type param constrained to string or number. Then tsc would also complain about pushing a number without checking the item type before.
replyIf it's an "array of strings AND numbers", then it should not be allowed to call the function with a string[], because those are different types.
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replyI'm not aware of a name, but I'm also curious if there is one because I had a hard time searching for it.
replyI came across it on ThePrimeagen's YouTube channel: https://youtu.be/u1WmiqlrqL0
I asked an LLM and it described the problem as "covariant typing of mutable collections" or "unsound covariance". I'm not mathematically educated in this area but that sounds right?
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