I am just going to answer the differences between declaration-site and use-site variance, since, while C# and Java generics differ in many other ways, those differences are mostly orthogonal to variance. First off, if I remember correctly use-site variance is strictly more powerful than declaration-site variance (although at the cost of concision), or at least … Read more
On the pure issue of contra-variance Adding contra-variance to a language opens a whole lot of potential problems or unclean solutions and offers very little advantage as it can be easily simulated without language support: struct A {}; struct B : A {}; struct C { virtual void f( B& ); }; struct D : … Read more
The problem is that GenericTraversableTemplate is used twice: once for mutable collections (where its type parameter should be invariant), and once for immutable collections (where covariance is invariably king). GenericTraversableTemplate’s typechecks assuming either covariance or invariance for the A type parameter. However, when we inherit it in a mutable trait, we have to pick invariance. … Read more
“out” means, roughly speaking, “only appears in output positions”. “in” means, roughly speaking, “only appears in input positions”. The real story is a bit more complicated than that, but the keywords were chosen because most of the time this is the case. Consider a method of an interface or the method represented by a delegate: … Read more
It’s certainly related to polymorphism. I wouldn’t say they’re just “another word” for polymorphism though – they’re about very specific situations, where you can treat one type as if it were another type in a certain context. For instance, with normal polymorphism you can treat any reference to a Banana as a reference to a … Read more
Variance has to do with how a generic type F<T> varies with respect to its type parameter T. If you know that T extends U, then variance will tell you whether you can conclude that F<T> extends F<U>, conclude that F<U> extends F<T>, or neither, or both. Covariance means that F<T> and T co–vary. That … Read more
Variance has to do with how a generic type F<T> varies with respect to its type parameter T. If you know that T extends U, then variance will tell you whether you can conclude that F<T> extends F<U>, conclude that F<U> extends F<T>, or neither, or both. Covariance means that F<T> and T co–vary. That … Read more