> For example, functions that return a different type depending on the values passed in are a useful pattern, but not allowed in staticly typed languages, except those with dependent types (Idris) or runtime downcasting (Go). There's always a way to acheive the same thing, but usually at the cost of safety or much more verbosity.

Look at this verbose OCaml:

    type my_return_type = AFloat of float | AnInt of int | AString of string;;

    let my_fun n =
        if n < 0 then AString "negative"
        else if n == 0 then AFloat 0.0
        else AnInt n

    # my_fun (-4);;
    - : my_return_type = AString "negative"
    # my_fun 0;;
    - : my_return_type = AFloat 0.
    # my_fun 42;;
    - : my_return_type = AnInt 42

(But yes, to use these values you have to do pattern matching, and if you're in a FUD mood, that is "runtime downcasting" and incurs a "cost of safety".)