Com S 342 meeting -*- Outline -*-

*  logical connectives  (SICP, p. 19; Little Schemer, chapter 2)

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     LOGICAL CONNECTIVES

problem: verbosity

(deftype member?
  (forall (T)
    (-> (T (list-of T)) boolean)))
(define member?
  (lambda (e ls)
    (if (not (null? ls))
        (if (equal? e (car ls))
            #t
            (member? e (cdr ls)))
	#f)))

solution:

(define member?
  (lambda (e ls)





		TERMS
short-circuit evaluation
conditional-evaluation
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...
    (and (not (null? ls))
         (or (equal? e (car ls))
             (member? e (cdr ls))))))

	Q: Can or evaluate all its arguments?  What would happen?
		no, it would loop
		bring out importance of short-circuit evaluation

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	SYNTAX OF LOGICAL CONNECTIVES

<expression> ::=



		SEMANTICS








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	...      (or {<expression>}*)
	       | (and {<expression>}*)
	       | ...

	...
(or ) = #f
(or e1 e2 ...) =  (if e1 #t (or e2 ...))

(and ) = #t
(and e1 e2 ...) =  (if e1 (and e2 ...) #f)

	Actually, in Scheme anything not #f is true,
		so or is a bit more complex in reality, see the book

	Q: What's this like in other languages?  in C?  in Pascal?
		|| and && in C, not | and &
		Pascal has no equivalent
		cor, cand in Algol 68?
		and_then, or_else in Ada

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		FOR YOU TO DO

Rewrite the following using "and" and "or"

(deftype foo? (-> (datum) boolean))
(define foo?
  (lambda (x)
     (if (pair? x)
         (if (number? (car x))
             #t
             (number? (cdr x)))
         #f)))






Define the predicate
  (deftype two-elem-list?
                    (-> (datum) boolean)
to test whether a datum is a list
of length two.




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