State of Valhalla

Re:State of Valhalla by kgeist on Friday December 24, 2021 @12:05AM (#29670448)

The main use case of generics is collections, and if my memory serves me right, in .NET they simply created a new collections library (System.Collections.Generics) leaving the original intact (System.Collections) allowing old programs to work as before. It lacked 100% compatibility because you couldn't freely interchange the classes (without writing adapters) but from what I gathered, it was a small price to pay compared to type erasure (in my opinion) which prevented more aggressive runtime optimizations/evolution. Today you usually find the old collections in ancient software which hasn't been updated for years.

[ Parent | on HN ]

Re:State of Valhalla by kaba0 on Friday December 24, 2021 @02:21AM (#29671212)

As it is usually brought up, not erasing types also comes with some potential cons: namely making CLR languages dependent on C#’s chosen variance model.

In Java, List<Cat> may or may not be the subclass of List<Animal>, but it is up to the List implementor. This way Scala/Kotlin/another JVM language is free to define their own variance model independent of the host language. C# did limit their language ecosystem with it quite a bit. (Afaik Scala for CLR stopped in part due to this).

[ Parent | on HN ]

Re:State of Valhalla by jayd16 on Friday December 24, 2021 @02:47PM (#29677083)

In C# I believe you can optionally mark generic classes/methods as covariant or contravariant. Is that not enough or does it not get exposed in the CLR or something?

https://docs.microsoft.com/en-us/dotnet/csharp/language-refe... https://docs.microsoft.com/en-us/dotnet/csharp/language-refe...

[ Parent | on HN ]

Re:State of Valhalla by kgeist on Friday December 24, 2021 @10:25AM (#29674324)

How do two languages with two different variance models interoperate in JVM if they share the same type but expect different behavior? Is it safe to share a list created in one language and pass into another if their variance models differ? Having an explicit variance model makes cross-language interoperability safer and easier (which was one of the main selling points of the Common Language Runtime), doesn't it?

[ Parent | on HN ]

Re:State of Valhalla by kaba0 on Friday December 24, 2021 @12:09PM (#29675291)

Simply from an empirical point of view, the CLR is pretty much a desert compared to the flourishing JVM one so while it can be attributed to many things, I am really sure that explicit variance is not that attractive for language developers.

[ Parent | on HN ]

Re:State of Valhalla by lokedhs on Friday December 24, 2021 @05:38AM (#29672432)

List<Cat> cannot be a subclass of List<Animal> as that would imply I can do list.add(dog) into that list.

You probably know this, but I'm replying in the hope it clarifies things to other readers. The way it's expressed in Java is List<? extends Animal>

[ Parent | on HN ]

Re:State of Valhalla by kaba0 on Friday December 24, 2021 @07:05AM (#29672924)

Yeah, java can change the variance model used by generics, but my point is that it is a language-level feature, not something fundamental at a JVM level, which is imo the correct decision.

Also, unfortunately arrays are covariant so Cat[] is a subclass of Animal[] (both in Java and C# actually), where your mentioned example indeed introduces a “poisoned” value, waiting for a classcastexception.

[ Parent | on HN ]