You are conflating FP with statically typed FP. There are dynamic typed FPLs too (Scheme, Elixir etc.,). Yes, I agree that Go could've been better if it had simple generics and discriminated unions.

By the way, generics are not at all exclusive to FP. Both C++ and D proved advanced generic/meta-programming facilities like template template parameters(HKTs) and constant generics which Rust currently lacks. But I'd certainly not use these if there is no absolute need.

What I objected to was abstracting and generalizing too much to the point of over-engineering. Some type systems, like those of Rust and Haskell, provide more room for such abuse and it takes some discipline to keep it simple. In contrast, OCaml is a very good sweet spot. ML modules are as powerful as type classes and lead to much cleaner and simpler APIs. OCaml compiles even faster than Go!