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> A piece of code that accepts a reference to some type A and only ever reads from it can correctly accept a reference to a subtype of A. The same is true of a

> A piece of code that accepts a reference to some type A and only ever reads from it can correctly accept a reference to a subtype of A. The same is true of a piece of code that writes through the reference or returns it. That's how sub-typing works. > A piece of code that accepts a reference to a type A and only ever writes to it can correctly accept a reference to a supertype of A. Have you ever programmed in a language with subtyping? Let me show you an example from Java (a popular object oriented programming language). class Parent {} class Child { int x; } class Example { static void writeToChild(Child c) { c.x = 20; } static void main() { writeToChild(new Parent()); } } This code snippet doesn't compile, but suppose the compiler allowed us to do so, do you think it could work? No. The function writeToChild cannot accept a reference to the supertype even though it only writes through the reference. I've seen a lot of people in this comment section talking about read and write which I find really odd. They have nothing to do with variance. The contravariant property is a property of function values and their parameters. It is entirely unrelated to the body of the function. A language without higher order functions will actually never have a contravariant type within it. This is why many popular OOP languages do not have them.

kannanvijayan 1y ago
> The same is true of a piece of code that writes through the reference or returns it. That's how sub-typing works. But it is not true that it is correctly typed with respect to a a supertype of A (it is not valid to call the code with a reference to a supertype of A). Code that only writes through the reference is correctly typed with respect to a super-type of A (it is valid to call the code with a reference to a supertype of A). > Have you ever programmed in a language with subtyping? Sigh, keep that snark for the twitter battles. I don't care enough about this to get snippy about it or to deal with folks who do.
- James_K 1y ago
  > Code that only writes through the reference is correctly typed with respect to a super-type of A (it is valid to call the code with a reference to a supertype of A). I'm not trying to be snarky, I genuinely want to know what you think of that code snippet I showed you. You claim it should work, but anyone with an understanding of programming would tell you it shouldn't. You can't write to a member variable that doesn't exist. Have you encountered inheritance before? Do you know what a “super-type” is? The mistake you're making here is very basic and I should like to know your level of experience.

kannanvijayan 1y ago

> The same is true of a piece of code that writes through the reference or returns it. That's how sub-typing works. But it is not true that it is correctly typed with respect to a a supertype of A (it is not valid to call the code with a reference to a supertype of A). Code that only writes through the reference is correctly typed with respect to a super-type of A (it is valid to call the code with a reference to a supertype of A). > Have you ever programmed in a language with subtyping? Sigh, keep that snark for the twitter battles. I don't care enough about this to get snippy about it or to deal with folks who do.

James_K 1y ago
> Code that only writes through the reference is correctly typed with respect to a super-type of A (it is valid to call the code with a reference to a supertype of A). I'm not trying to be snarky, I genuinely want to know what you think of that code snippet I showed you. You claim it should work, but anyone with an understanding of programming would tell you it shouldn't. You can't write to a member variable that doesn't exist. Have you encountered inheritance before? Do you know what a “super-type” is? The mistake you're making here is very basic and I should like to know your level of experience.