[branko_d]

Why? It's just a property of type transformation.

Assuming Parent <- Child ("<-" denotes inheritance):

- If Generic<Parent> <- Generic<Child>: it's covariant.

- If Generic<Parent> -> Generic<Child>: it's contravariant.

- Otherwise: it's invariant.

Or at least it's that straightforward in C#. Are there complications in Rust?

[kannanvijayan]

The difficulty is that even trivial generic types aren't cleanly one or the other. A mutable reference type is covariant on read, and contra on write.

Scala was the first language in my exposure to try to simplify that away by lifting the variance annotations into the type parameter directly. It reduced some of the power but it made things easier to understand for developers. A full variance model would annotate specific features (methods) of a type as co/contra/invariant.

I'm not sure what approach C# takes - I haven't looked into it.

Rust doesn't expose variances for data structures at all. It exposes them for traits (type classes) and lifetimes, but neither of those are accessible in a higher order form. Trait usage is highly constrained and so is lifetime usage.

Traits mainly can be used as bounds on types. Some subset of traits, characterized as object traits, can be used as types themselves, but only in an indirect context. These are highly restricted. For example, if you have traits T and U where U extends T, and you have a reference `&dyn U`, you can't convert that to a `&dyn T` without knowledge of the underlying concrete type. You _can_ convert `&A where A: U` to `&B where B: T`, but that just falls out of the fact that the compiler has access to all the concrete type and trait definitions there and can validate the transform.

Rust's inheritance/variance story is a bit weird. They've kept it very minimal and constrained.

[James_K]

> A mutable reference type is covariant on read, and contra on write.

No it isn't. The type is covariant because a reference is a subtype of all parent types. The the function read must have it's argument invariant because it both takes and returns an instance of the type. I think you're confusing the variance of types for the variance of instances of those types. Read is effectively a Function(t: type, Function(ref t, t)). If it was covariant as you suggest, we would have a serious problem. Consider that (read Child) works by making a memory read for the first (sizeof Child) bytes of it's argument. If read were covariant, then that would imply you could call (read Child) on a Parent type and get a Parent back, but that won't work because (sizeof Child) can be less than (sizeof Parent). Read simply appears covariant because it's generic. (read Child) ≠ (read Parent), but you can get (read Parent). It also appears contravariant because you can get (read Grandchild).

Scala doesn't simplify anything, that's just how variance works.

[kannanvijayan]

Even in your own description, it is clear that with regards to _correctness_, the variance model bifurcates between the read and the write method.

The discussion about the type sizes is a red herring. If the type system in question makes two types of differing sizes not able to be subtypes of each other, then calling these things "Child" and "Parent" is just a labeling confusion on types unrelated by a subtyping relationship. The discussion doesn't apply at all to that case.

The variance is a property of the algorithm with respect to the type. 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.

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.

In an OO language, an instance method is covariant whenever the subject type occurs in return position (read analogue), and it's contravariant whenever the subject type occurs in a parameter position (write analogue). On instance methods where both are present, you naturally reduce to the intersection of those two sets, which causes them to be annotated as invariant.

[James_K]

> 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]

> 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]

> 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.

[zozbot234]

Note that the Rust folks are working on a "safe-transmute" facility that may end up introducing a kind of variance to the language.

[Sharlin]

It's not particularly easy to teach what that actually means and why it's a thing. It's quite easy to show why in general G<A> cannot be a subtype of G<B> even if A is a subtype of B, it's rather more involved pedagogically to explain when it can, and even more confusing when it's actually the other way around (contravariance).

Anyway, Rust has no subtyping except for lifetimes ('a <: 'b iff 'a lasts at least as long as 'b) , so variance only arises in very advanced use cases.

[smallstepforman]

I’m getting old. I can understand the words, but not the content. At this point, show me dissassembly so that I can understand what actually happens on the fundamental byte/cpu instruction level, then I can figure out what you’re trying to explain.

[Sharlin]

Nothing really happens on the instruction level because this is all type system logic.

[Grayskull]

I think this might be the issue. If something has zero effect in the end, why should I care about in the first place?

[bobbylarrybobby]

Variance doesn't affect generated code because it acts earlier than that: determining whether code is valid or not and in doing so preventing invalid code (UB) from being compiled in the first place.

The simplest example of incorrect variance → UB is that `&'a mut T` must be invariant in T. If it were covariant, you could take a `&'a mut &'static T`, write a `&'b T` into it for some non-static lifetime `'b` (since `'static: 'b` for all `'b`), and then... kaboom. 'b ends but the compiler thought this was a `&'a mut &'static T`, and you've got a dangling reference.

`&'a mut T` can't be covariant in T for a similar reason: if you start with a `&'a mut &'b T`, contravariance would let you cast it to a `&'a mut &'static T`, and then you'd have a `&'static T` derived from a `&'b T`, which is again kaboom territory.

So, variance’s effect is to guide the compiler and prevent dangling references from occurring at runtime by making code that produces them invalid. Neither of the above issues is observable at runtime (barring compiler bugs) precisely because the compiler enforces variance correctly.

[Sharlin]

It doesn't have zero effect. Like everything about type systems, it helps prevent incorrect, and possibly unsound, code from compiling. So I guess the giant runtime effect is that you either have a program to run or not.

[tialaramex]

Sure, you can keep telling me that and it doesn't stay. I'm completely happy writing Rust, and I am aware it needs variance to work in principle and when I do need that information I know the magic words to type into doc search.

It's like how I can hold in my head how classical DH KEX works and I can write a toy version with numbers that are too small - but for the actual KEX we use today, which is Elliptic Curve DH I'm like "Well, basically it's the same idea but the curves hurt my head so I just paste in somebody else's implementation" even in a toy.

Sorry?

[dwattttt]

One day the rote example finally made sense to me, and I go back to it every time I hear about variance.

Got a Container<Animal> and want to treat it as a Container<Cat>? Then you can only write to it: it's ok to put a Cat in a Container<Cat> that's really a Container<Animal>.

Reading from it is wrong. If you treat a Container<Animal> like a Container<Cat> and read from it, you might get a Dog instead.

The same works in reverse, treating a Container<Cat> like a Container<Animal> is ok for read, but not for write.

[zozbot234]

The usual way of phrasing this is, famously, https://en.wikipedia.org/wiki/A_white_horse_is_not_a_horse

[sesm]

It wad easy to teach to Java devs just by using one mnemonic: PECS = Producer Extends, Consumer Super