sol-2.39.tex

\begtt\scm
(define (reverse sequence)
  (fold-right (lambda (x y) (append y (list x))) nil sequence))

(define (reverse sequence)
  (fold-left (lambda (x y) (cons y x)) nil sequence))
\endtt
Notice that the `reverse` procedure defined in terms of `fold-right` is very inefficient, because it generates a process that requires a number of steps proportional to the square of the length of the given sequence;  instead the procedure defined in terms of `fold-left` generates a linear iterative process.