Notes: Wednesday 5 February 2003

Schedule

• 5 February: Read SICP, 2.1 and 2.2 (through page 127). You don't need to read the example section 2.2.4.
• Wednesday, 12 February: Problem Set 3
• Before 10 March: Read rest of GEB part I (Chapters 2-4 and 6-9, in addition to Chapters 1 and 5 you have already read).

Null/Nil Confusion

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.

Code

(define (gauss-sum n)
  (if (= n 0) 0 (+ n (gauss-sum (- n 1)))))

(define (insertl lst f stopval)
  (if (null? lst) stopval
      (f (car lst) 
	 (insertl (cdr lst) f stopval))))

(define (intsfrom n)
  (if (= n 0) null
      (cons n (intsfrom (- n 1)))))

(define (intsto n)
  (if (= n 0) null
      (append (intsto (- n 1)) (list n))))

(define (gauss-sum n)
  (insertl (intsto n) + 0))

(define (factorial n)
  (insertl (intsto n) * 1))

(define (map f lst)
  (if (null? lst) null 
      (cons (f (car lst)) 
	    (map f (cdr lst))))

(define (map f lst)
  (insertl lst (lambda (a b) (cons (f a) b)) null))

    def f ≡ isposint → iszero → g1 ; g2 o [f o dec, g3] ; error
     ::= def f ≡ isposint → (/l:(g2 o [s1, g3 o s2])) o al o [g1 o ~0, intsto] ; error
              FL Rewrite Rule (/l is insertl), shown for amusement purposes only.
FL is a language John Backus (the B in BNF, and the designer of Fortran) designed.
It was designed to make manipulating functions very compact and powerful, but very few people ever used it and no one does anymore.
When I was an intern at IBM Almaden, I worked on developing rules like this to make FL programs run faster.

	CS 200: Computer Science Department of Computer Science University of Virginia	cs200-staff@cs.virginia.edu Using these Materials