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

FWIW, with-handlers is implemented using escaping continuations (and
thread cells and continuation marks and call-with-values and ...).

Robby

At Sun, 26 Feb 2006 00:31:17 -0500, Dave Herman wrote:
> It's easier than you're making it out to be. Don't bother with 
> continuations. Just use an exception handler.
> 
> (define (bound? t s)
>    (with-handlers ([exn? (lambda (exn) #f)])
>      (hash-table-get t s)
>      #t))
> 
> Dave
> 
> Gregory Woodhouse wrote:
> > 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
> > 
> > 
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> 
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