I can’t speak to Rust but there have been plenty of C# repls in the past.

https://fuqua.io/CSharpRepl/ For one

I've seen comments like this a few times, and I think there's a misunderstanding at play. I think people who are used to using REPLs look at people who aren't and conclude that, since they don't have a REPL in their toolbox, they don't have a tool in there that does what a REPL does. But I think most of the time, they are accomplishing the same tasks with a different workflow.

For instance, I use throwaway files instead of REPLs, even when I have access to one. I'm still running small and isolated experiments to build my understanding, but I get to use the editor I'm comfortable with and it's easy to copy the code into my project afterwards.

The other thing I used to use REPLs for was introspection/looking up documentation, which my LSP handles nicely. You can think of the LSP as a sort of REPL integrated seamlessly into the editor; when I have a question, I'll query the LSP by writing a line of dummy code that causes the LSP to generate an annotation with the answer. So for instance if I want to know if `foo` implements a certain trait/interface, I can write a variable declaration like `let x: Box<dyn MyTrait> = Box::new(foo)` and see if my LSP annotated that as a type error. I can do this very quickly and without leaving my editor, so it helps me stay in the flow of things.

There's a really nice VS Code extension called Polyglot Notebooks that's great for running small C# "scripts"

Is there a contradiction? I'm not aware about Rust or C# REPLs (a lot of results comes up, but no clue how serious are those), but GHCi surely proves that you can have both.