CS150 Notes 5 (2 September 2005)

Assignments Due

• Monday, 5 September: Read GEB, Little Harmonic Labyrinth and GEB, Chapter 5
• Friday, 9 September: PS2

In SICP, they use nil. In MzScheme (DrScheme's language), they use null. The Revised⁵ Report on the Algorithmic Language Scheme mentions nil but uses the empty list (denoted by '()) instead. As the footnote on page 101 explains, the exact meaning and name for the empty list is cause for lots of dispute and confusion. We will use null in class to be consistent with MzScheme, and reserve nil for reporting soccer scores.

If instead of using the definitions we used in class, we defined cons as:

(define (cons a b)
 (lambda (w) (if (w) b a)))

A list is either (1) null or (2) a pair where the second part of the pair is a list. To define procedures on lists, we need to handle both cases: what do we do when the list is null and what do we do when the list is a pair.

Define length a procedure that takes a list as its operand and evaluates to the number of elements in that list.