# [plt-scheme] Checking to see if a key is present in a hash table

There doesn't seem to be a simple way to do it. The problem is that
an error is signaled if the key is not present, so if I use hash-
table-get, I need to deal with it somehow. The following code does
not work
(define (bound? t s)
(call/cc
(lambda (k)
(let ((x
(hash-table-get t s (lambda () k #f))))
k #t))))
(define t (make-hash-table))
(hash-table-put! t 'x #f)
(bound? t 'x)
(bound? t 'y)
I get #t in both cases. That seems odd because if the failure thunk
is ever invoked, I'd expect bound? to return #f immediately.
Now, the odd (to me, anyway) thing is that I use the following code
to (say) take the symbol '+ as input and return a function. It works
just fine
;;get builtin (or #<void>)
;;symbol -> procedure + void
(define get-builtin
(let ((t (make-hash-table)))
(hash-table-put! t '+ (lambda (x y) (+ x y)))
(hash-table-put! t '- (lambda (x y) (- x y)))
(hash-table-put! t '* (lambda (x y) (* x y)))
(hash-table-put! t '/ (lambda (x y) (unless (zero? y) (/ x y))))
(hash-table-put! t 'and (lambda (x y) (and x y)))
(hash-table-put! t 'or (lambda (x y) (or x y)))
(hash-table-put! t 'not (lambda (x) (not x)))
(hash-table-put! t '< (lambda (x y) (< x y)))
(hash-table-put! t '> (lambda (x y) (> x y)))
(hash-table-put! t '= (lambda (x y) (= x y)))
(lambda (op)
(call/cc
(lambda (k)
(hash-table-get t op (lambda () k (void))))))))
What's the difference?
===
Gregory Woodhouse
gregory.woodhouse at sbcglobal.net
"Doubt may be uncomfortable,
but certainty is absurd." --Voltaire