Statically typed languages that employ type inference (eg. Hindley–Milner) do exactly this, ie. you just write your function definition and the inference engine

Statically typed languages that employ type inference (eg. Hindley–Milner) do exactly this, ie. you just write your function definition and the inference engine figures out its type. Doing this in the repl gives you comparable flow to doing it in dynamic langs but with full type safety.