[plt-scheme] 12.4.2 word definition; is this OK?

On May 10, 2009, at 10:25 AM, Carl Eastlund wrote:

> ... you must use the contract Any -> Boolean.
> Otherwise you can't sensibly ask this question any time you might not
> already know the answer.
>
> ... if the contract is Word -> Boolean, this simplifies even
> further to a trivial function.  But if the contract is Any -> Boolean
> as with most predicates, your last version will most definitely crash
> on inputs such as 5, "hello", and (make-posn 100 200).  The use of
> cons? before applying first and rest is critical.

Oops: you're right.  That should have been

(define (word? arg)
    (or (empty? arg)
          (and (cons? arg) (symbol? (first arg)) (word? (rest arg)))))

relying on the short-circuit behavior of both "or" and "and".

Stephen Bloch
sbloch at adelphi.edu