funcall isn't needed, because in your snippet defun-ed f.

funcall for defvar lambdas in CL.

Scheme is a Lisp-1 -> variables and procedures are defined in one namespace.

In CL (Lisp-2) variables and functions separated.

I tried hash-lang and switched back to the Melpa racket mod. there is no syntax highlighting in the rhombus-prototype source code, or I can't configure the Racket-Hash-Lang mode correctly. The rhombus is not highlighted even in DrRacket8.10