random function in scheme

I am trying to get a random string from a list of strings in scheme. Example List 开发者_运维技巧( "this" "that" "today" "yesterday") So based on length of the list a random number is created and that word is output. But keep getting error!

I tried it like this:

;; produces random number that should be input to the random-function

(define (random-num list)

(random-function ((random (length (list))) list)))

;; loops the number of times till random number is 0 and outputs the list value

(define (random-function num list )
  (cond 
    [(zero? num) (car list)]
    [else (random-function (- num 1) (cdr list))]))

Error:

procedure application: expected procedure, given: 
("this" "that" "today" "yesterday") (no arguments)

When I try to do :

(random-function (random (length list)) 

on the console I get a random number.

Do not understand why it is crashing here when done inside my program???

Could I do this in a better way rather than looping so many times. In Java I would have used an array and given the position directly. Anyway to do it in scheme too ?

---

(define (random-element list)
  (list-ref list (random (length list))))