[keithlfrost]

Clojure allows one to start with essentially untyped code, then add type declarations for efficiency and safety.

[kyllo]

core.typed never really caught on though, what seems more popular is Schema which focuses on annotating and validating the structure of lists and maps: https://github.com/plumatic/schema

[Scarbutt]

And Schema died in favor of Spec, which is still in flux.