Chapter 6, Problem 34E

Explanation of Solution

Generating code for uptoby function:

• This problem uses lazy evaluation to implement iterator objects.
• In other words, let an iterator be either null list or a pair of an element.
• The values of expressions are not calculated until they are required, this is known as lazy evaluation.
• There are two different functions in lazy evaluation. They are delay function and force function.
• The intermediate states are called promise.
• The delay operator is used indicate that an expression evaluated in a lazily manner.
• A force function computes the final values based on promise.
• The following gives the lazy evaluation.

    $\begin{array}{l}\text{(define uptoby}\\ \text{(lambda (low high step)}\\ \text{(letrec ((iter (lambda (n)}\\ \text{(cons n (delay}\end{array}$

    $\begin{array}{l}\text{(let ((m (+ n step)))}\\ \text{(if (x}\end{array}$