Java and Go today are much better to work with than Java was in, say, the early 2000s, but to realize the productivity boost of a dynamic language within a statically typechecked language, you need type inference and a REPL.

You still don't get this with Java or Go, but you certainly do with ML-like languages (Haskell, SML, OCaml, F# etc.)

A lot of Haskell developers will tell you that writing Haskell feels a lot like a functional Python, and they use Haskell for scripting as well as application development.