Very 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.

Is there a name for this failure mode?

▲ sally_glance 4 days ago | parent | next [-]

I 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.

▲ fainpul 4 days ago | parent [-]

If 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.

▲ saghm 3 days ago | parent | next [-]

That would be the sound way to do things, but it's also surprisingly common for arrays for some reason. It also doesn't get checked compile time in Java, although it does throw an exception because the arrays are type-enforced at runtime at least.

This compiles fine:

    class A {}

    class B1 extends A {}
    class B2 extends A {}

    public class MyClass {
        public static void main(String args[]) {
            A[] array = new B1[1];
            array[0] = new B2();
        }
    }

▲ clownstrikelol 3 days ago | parent | prev [-]

That’s barely scratching the surface.

In TypeScript the following is also valid:

class Something{

    value: Number

}

class SomethingElse{

    value: Number

    foo: String

    bar: SomeOtherThing[]

}

function AddSomething(v: Something)

{

    v.value += 1;

}

var ex = new SomethingElse{

    value: 3,

    foo: “TypeScript is fake types”,

    bar: []

};

AddSomething(ex);

Why does it work? Because in TypeScript as long as you have a “shape” that fits, it’s the “same type.”

Complete and utter insanity to me, to pretend there’s any real type checking, when types are entirely fake and made up.

▲

saghm 3 days ago | parent [-]

That's because it's structurally typed (as opposed to nominally typed). I don't happen to prefer it, but I don't think it's fair to conflate that with unsoundness like the example given above; it's totally possible to have a sound structural type system. TypeScript doesn't happen to be sound, but it's not because of that.

▲

teaearlgraycold 3 days ago | parent [-]

Structural typing is great. It's the verified version of duck typing.

	▲	LelouBil 3 days ago \| parent [-]
		You can even get nominal typing with branded types if you need it. (Like for the newtype pattern)

▲ rav 3 days ago | parent | prev | next [-]

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...

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drzaiusx11 3 days ago | parent [-]

Key sentence in their justification seems to be:

"allowing this enables many common JavaScript patterns"

Honestly at this point they should make a new strict "no js" mode, as the ecosystem likely has reached a tipping point where you can get by without mixing typed and untyped js at compile time. Wonder if targeting wasm directly would help ensure those boundaries are ensured...

	▲	drzaiusx11 3 days ago \| parent [-]
		https://www.assemblyscript.org/ is pretty darn close, but it would probably be better to converge and not split communities

▲ sargunv 4 days ago | parent | prev | next [-]

I'm not aware of a name, but I'm also curious if there is one because I had a hard time searching for it.

I came across it on ThePrimeagen's YouTube channel: https://youtu.be/u1WmiqlrqL0

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teaearlgraycold 4 days ago | parent [-]

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?

	▲	saghm 3 days ago \| parent [-]
		Yes, that's correct. If you're curious for a slightly more concrete source, Wikipedia covers this reasonably well (although citing one often decried source to validate another might not be particularly convincing to some people): https://en.wikipedia.org/wiki/Type_variance#Arrays

▲ 4 days ago | parent | prev [-]

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