(define (maybe fn #!rest args #!key (pred (lambda (x) x)) (otherwise '()))
  (if (pred args)
    (apply fn args)
    otherwise))

(maybe list-ref '(1 2 3 4 5) 2 pred: (lambda (x) #t))
; => Error: bad argument count - received 4 but expected 2: #<procedure (list-ref lst i)>