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Comment by cogman10 | Hacker News Reader

Part of the reason Java hasn't reified generics is because C# did and it was a real big headache that also limited non-C# languages on the C# runtime (CLI?). Everything had to be recompiled to work with newer C# runtimes. While it's pretty easy to run a bunch of language on the JVM (Javascript, python, ruby, clojure) doing the same for C# is somewhat a nightmare, particularly for non-type aware languages.

For example, Imagine you have an api like `void do(List<Foo> foos)`. In the erasure environment of the JVM that looks like `void do(List foos)`. From python it's pretty easy to call with a `foos = [Foo()]`. But not so much if your python implementation needs to figure out how and if it can coarse it's `List` type into a `List<Foo>` type.

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I don’t think that’s the case. You can absolutely implement a type-erased language on top of the CLR. Your language will just have the same constraints of a type-erased language like Java.

Having reified generics in the CLR just lets you store more type information. There isn’t much of a trade off for CLR end-users.

Compare this to the constraints and workarounds that Kotlin and Scala have due to type-erasure on the JVM.

You CAN do it, but it's much more difficult.

And as far as I'm aware, both kotlin and Scala don't really suffer due to type erasure.

I mean, the language is what it is. But, it definitely constrains the language developers. Especially when considering interop with other JVM languages.

That being said, it is easier to write a language on top of the JVM with good interop, since there are less ways to implement features. Essentially, your language has to interop with Java.

And it is harder to have good interop between CLR languages because there are more ways to implement features. Essentially, your language has to interop with C#.

> Compare this to the constraints and workarounds that Kotlin and Scala have due to type-erasure on the JVM.

The creator of Scala disagrees: https://youtu.be/Xn_YpUtXWT4?t=850

Not necessarily. You can ignore the reified generic system in the CLR and monomorphize it in the CIL output for your language. Debugging for users is usually a nightmare though due to the monomorphization. The benefit of a type-erased runtime is the interop between the languages built on the runtime.

The monomorphization of CLR generics is what NativeAOT does, though it doesn't support some C# features.

TypeScript is essentially C#, but with type-erasure and lacking the low-level struct & pass-by-reference features.

I do think the C#/CLR struct implementation is better though.

IronPython did just fine with reified generics.

There's also https://github.com/ikvmnet/ikvm that converts Java bytecode to CIL; essentially Java on CLR.

Actually no it didn't, DLR was created exactly to support dynamic languages on the CLR.

Nowadays largely abandoned, and I think not everything survived the transition from .NET Framework into modern .NET.

Yeah, that’s a different issue. Statically-typed languages, including type-erased languages, are fine on the CLR. Dynamically-typed languages are a different beast.

I suppose DLR would be comparable to GraalVM/Truffle.

The difficulty of implementing a dynamically-typed language directly on the CLR and JVM are about the same. Though, it would probably be more efficient on the CLR with access to lower level operations for memory management.

I think an interesting project would be to implement a CIL interpreter on GraalVM.

DLR inspired the addition of invokedynamic opcode on the JVM, which is actually the foundation for how lambdas get generated, many still think nested anonymous classes are used.

GraalVM already handles LLVM bitcode, much cooler than plain MSIL.

And here there is another example where Java ecosystem ends up being better.

MSR had a compiler framework similar to GraalVM, called Phoenix, it was going to replace VS, LLVM style, instead it died and what is left are a couple of research papers.

> many still think nested anonymous classes are used.

Anonymous classes are still used (sometimes). It simply depends on the circumstance of the lambda.

For example, this will result in a new anonymous class being generated.

    void foo(String s) {
      stream.filter(i->s.equals(i));
    }

The class gets generated to capture the `s` variable. Indy gets used in the `filter` method because the incoming lambda or method reference could be several types of method calls. For example, a constructor, an instance method, or a static method (I believe there are few more in the JVM bytecode).

What won't generate a new anonymous class is this sort of lambda

    stream.filter(i->"foo".equals(i));

That will generate a new method on the parent class which ultimately gets called. Since nothing is captured it can be directly called without a new instance being created.

I had another idea given Brian's talk on the matter.

Never bothered to actually look into the generated bytecodes.

Surprisingly hard to find good docs on this. [1]

I was wrong, it looks like the case I gave is one which doesn't result in a new anonymous class being generated. Instead the lambda metafactory gets involved to avoid that allocation.

I apparently didn't see what I thought I saw. I thought that I had seen new `lambda.$1` classes being created in call stacks when debugging. Maybe I did, but the lambda was serializable (we have that in some unfortunate places in our code base).

There are still cases where new classes get generated, but that's pretty much just for serialization.

[1] https://cr.openjdk.org/~briangoetz/lambda/lambda-translation...

The DLR is just a shared library. It never changed anything about the CLR under the hood nor did it change anything about how Generics worked in the CLR (being the most relevant part of the thread here). IronPython could instantiate Generics just fine, and did just fine working with them.

The DLR is not "abandoned" so much as "complete". Everything did survive the transition into modern .NET and IronPython 3.4.2 runs just fine on .NET 6+ [1]. (For those trapped on the ugly side of the Python 2/3 split, even IronPython 2.7.12 runs on .NET 6.) Most of the "magic" of the DLR is shared with C# Linq in interesting ways (the System.Linq.Expressions AST) and sort of "has to survive" if only to properly support IQueryable<T> (and wilder relatives like IQbservable<T>, the Q is not a typo) even if most people aren't actively using DLR languages today (nor that much usage of C#'s `dynamic` keyword).

IronPython is a community supported (truly open source) language so doesn't get as much attention now as it did in the brief "first party" support era, so some may call that "abandoned" but F# has always lived in that gray space where it is primarily "community supported" more than "officially supported" by a dedicated team at Microsoft.

[1] https://github.com/IronLanguages/ironpython3/releases/tag/v3...

The addition of dynamic, and JIT being aware of IDynamicMetaObjectProvider also mattered.

Even though most of the communication around dynamic was about COM support and Excel.

IDynamicMetaObjectProvider mostly just caches and returns System.Linq.Expressions for various asks, so again most of the JIT awareness is still just System.Linq.Expressions awareness.

The COM support is by implementing IDynamicMetaObjectProvider in a cross-language reusable base class.

The C# `dynamic` keyword adds some smarts about IDynamicMetaObjectProvider to the C# compiler, but those smarts were never seen to be needed as a low level tool in the CLR itself. And again, once the C# compiles the IDynamicMetaObjectProvider calls, those mostly just result in System.Linq.Expressions for the JIT compiler to optimize in the same way it optimizes other usages of Linq.

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