I would like to have a language that has a builtin dial for strictness. The lowest setting (for use on a REPL or when prototyping something) would use dynamic t

I would like to have a language that has a builtin dial for strictness. The lowest setting (for use on a REPL or when prototyping something) would use dynamic typing with implicit coercions like e.g. JavaScript. But then, as the safety/performance/stability of your code becomes more critical, you can turn the dial higher and higher to get static type checking, then borrow checking, etc. Sort of like how the NPM guys rewrite some critical-path components in Rust, but without having to hop to a different language. Sort of like how a lot of JS devs move to TypeScript halfway through the project, but then again, TypeScript can only go so far because it's shoehorned onto the existing JS, rather than designed in from the get go.