Summary of Rules

1.

When recurring on a list, do three things:

a.

check for the termination condition;

b.

use the first element of the list;

c.

recur with the ``rest'' of the list.

2.

If a function builds a list using ``cons,'' return () at the terminating line.

3.

When recurring on a nested list, do three things:

a.

check for the termination condition;

b.

check if the first element of the list is an atom or a list;

c.

recur with the ``first'' and the ``rest'' of the list.

4.

When evaluating an expression, do three things:

a.

check for the termination condition;

b.

identify operator;

c.

apply operator to recursive calls on the operands.

5.

When building a number value using +, return 0 at the terminating line. When building a number value using *, return 1 at the terminating line.

6.

When recurring on a number, do three things:

a.

check for the termination condition;

b.

use the number in some form;

c.

recur with a changed form of the number.

7.

To ensure proper termination do two things:

a.

make sure that you are changing at least one argument in your recursive call;

b.

make sure that your test for termination looks at the arguments that change in the recursive call.

8.

Two simple cases may occur when changing an argument in a recursive call:

a.

if you are using ``rest'' to change an argument which is a list, use ``null'' in the test for termination;

b.

if you are decreasing an argument which is a number, compare it with 0 in the test for termination.

9.

Use ``let'' to reduce the number of function calls.

10.

Encapsulate program fragments into new functions to improve clarity.

11.

Encapsulate repeated program fragments into new functions to reduce program size.