Performance is only half the point. The other half is correctness. And I think this pattern of "important correctness and performance sensitive core, surrounded

Performance is only half the point. The other half is correctness. And I think this pattern of "important correctness and performance sensitive core, surrounded by simple but flexible glue code" is common enough that such a language makes sense. It describes pretty much all of the code that I write at work in fact. Being able to simply add type annotations and have the compiler generate the runtime type checks would be wonderful.