(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 3.0])))

(define largest-in-list
  (lambda (lon)
    (if (null? lon)
	(errorf 'largest-in-list "empty list")
	(largest-in-non-empty (car lon)(cdr lon)))))
(define largest-in-non-empty
  (lambda (largest-so-far unseen)
    (if (null? unseen)
	largest-so-far
	(largest-in-non-empty
	 (max largest-so-far (car unseen))
	 (cdr unseen)))))

(largest-in-list '())
(largest-in-list '(-3 -1))
(largest-in-list '(1 5 3 2 0))

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

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

(positives '(3 -1 2 6 -7 0 9 2) --> (3 2 6 9 2)