Question

Is there a way to find out at runtime, how many inputs (arguments, parameters) a function has?

Say, I want to:

(define (my-function unknown-function)
  ...
  (number-of-necessary-arguments unknown-function)
  ...)

Answer 1

You can use procedure-arity.

(procedure-arity expt)                     ; => 2

Note that when using procedure-arity with variadic functions or case-lambda or the like, the results are more complicated:

(procedure-arity apply)                    ; => (arity-at-least 2)
(procedure-arity (case-lambda
                  ((x) x)
                  ((x y z) z)
                  ((a b c d e f . g) g)))  ; => `(1 3 ,(arity-at-least 6))