2.22.tex

\documentclass[a4paper,12pt]{article}
\usepackage{listings}
\lstset{language=Lisp}

\begin{document}
\begin{lstlisting}
(define (square-lst items)
  (define (iter things answer)
    (if (null? things)
        answer
        (iter (cdr things)
              (cons (square (car things))
                    answer))))
  (iter items null))
\end{lstlisting}
The newly computed square at the head of things is added to the
beginning of answer.  Thus when \lstinline!iter! terminated, the
answer is inverted.

\begin{lstlisting}
(define (square-list items)
  (define (iter things answer)
    (if (null? things)
        answer
        (iter (cdr things)
              (cons answer
                    (square (car things))))))
  (iter items null))
\end{lstlisting}
This doesn't work because it doesn't return a list.  After each
iteration the value of \lstinline!answer! is a dotted-pair whose
second element is the newly computed square from the list.
\end{document}