(define letter-to-number
  (lambda (letter)
    (if (eq? letter 'A)
	4.0
	(if (eq? letter 'B+)
	    3.5
	    3.0))))
(letter-to-number 'A)
(letter-to-number 'B+)
(letter-to-number 'B)


(define letter-to-number
  (lambda (letter)
    (cond [(eq? letter 'A) 4.0]
	  [(eq? letter 'B+) 3.5]
	  [else 3.0])))
(define letter-to-number
  (lambda (letter)
    (case letter
      [(A) 4.0]
      [(B+) 3.5]
      [else 3])))

(define largest-in-list
  (lambda (lon)
    (if (null? lon)
	(errorf 'largest-in-list "empty list")
	(largest-in-nonempty (car lon) (cdr lon)))))

(define largest-in-nonempty
  (lambda (largest-so-far rest-of-list)
    (if (null? rest-of-list)
	largest-so-far
	(largest-in-nonempty
	 (max largest-so-far (car rest-of-list))
	 (cdr rest-of-list)))))

(largest-in-list '())
(largest-in-list '(3 6 2 4))
(largest-in-list '(-4 -1))

(define make-adder
  (lambda (n)
    (lambda (m)
      (+ n m))))

(make-adder 5)
(define add5 (make-adder 5))
(add5 6)
((make-adder 5) 6)
 (((lambda (n)
    (lambda (m)
      (+ n m)))
  5)
6)