(define largest-in-list
  (lambda (ls)
    (if (null? ls)
	(errorf 'largest-in-list "list cannot be empty")
	(largest-in-non-empty ls))))

(define largest-in-non-empty
  (lambda (ls)
    (if (null? (cdr ls)) 
	(car ls)
	(let ([largest-in-cdr (largest-in-non-empty (cdr ls))])
	  (if (> (car ls) largest-in-cdr)
	      (car ls)
	      largest-in-cdr)))))
(largest-in-list '())
(largest-in-list '(3 1 4 2))
(largest-in-list '(5 1 4 2))
(largest-in-list '(5 1 4 7))
(largest-in-list '(7))
(largest-in-list '(3 4 a))
; not the greatest error message.  Fix that in next version.

; Now define another version with an accumulator (that is also more robust)

(define largest-in-list
  (lambda (ls)
    (if (null? ls)
	(errorf 'largest-in-list "list cannot be empty")
	(largest-in-list-acc (cdr ls) (car ls)))))

; cond is like if ... else if ... else

(define largest-in-list-acc
  (lambda (ls largest-so-far)
    (cond [(null? ls) largest-so-far]
	  [(not (number? (car ls)))
	   (errorf 'largest-in-list "everything in the list must be a number")]
	  [(> (car ls) largest-so-far) (largest-in-list-acc (cdr ls) (car ls))]
	  [else (largest-in-list-acc (cdr ls) largest-so-far)])))