(define (something x)
  (let ((x (cond
             ((string? x) x)
             ((char? x) (string x))
             ((pair? x) (list->string x))
             (else (error "invalid string" x)))))
    (do-something-with x)))

;;; The following won't work because `x` is bound to an unspecified
;;; value during execution.
(define (something x)
  (define x
    (cond
      ((string? x) x)
      ((char? x) (string x))
      ((pair? x) (list->string x))
      (else (error "invalid string" x))))
  (do-something-with x))