Yeah this is my absolute dream language. Something that lets you prototype as easily as Python but then compile as efficiently and safely as Rust. I thought Rust might actually fit the bill here and it is quite good but it's still far from easy to prototype in - lots of sharp edges with say modifying arrays while iterating, complex types, concurrency. Maybe Rust can be something like this with enough unsafe but I haven't tried. I've also been meaning to try more Typescript for this kind of thing.
You should give Julia a shot.
That’s basically that. You can start with super dynamic code in a REPL and gradually hammer it into stricter and hyper efficient code. It doesn’t have a borrow checker, but it’s expressive enough that you can write something similar as a package (see BorrowChecker.jl).
Some Common Lisp implementations like SBCL have supported this style of development for many years. Everything is dynamically typed by default but as you specify more and more types the compiler uses them to make the generated code more efficient.
I quite like common lisp but I don't believe any existing implementation gets you anywhere near the same level of compile time safety. Maybe something like typed racket but that's still only doing a fraction of what rust does.
