1. RE: Short-cut evaluation Question
It might be thinking: o = (0 or TypesMatch(o, "QWidget") or
IsBaseClass(o,"QWidget")))
So Try:
global type Widget(object o)
if not atom(o) then
return false
else
return ((o = 0) or TypesMatch(o, "QWidget") or
IsBaseClass(o, "QWidget"))
end if
end type
> -----Original Message-----
> From: Travis Beaty [mailto:twbeaty at osage.net]
> Sent: Sunday, February 29, 2004 3:01 PM
> To: EUforum at topica.com
> Subject: Short-cut evaluation Question
>
>
> Hello everyone!
>
> I've got a question here about short-cut evaluation. As reference here,
> I'm
> using the Linux version of the 2.4 interpreter. I'm pretty sure this is a
> bug between my ears as opposed to a bug in the interpreter, but I'd like
> to
> get some clarification on this.
>
> I have the following type declaration:
>
>
> global type Widget(object o)
>
> if not atom(o) then
> return false
> else
> return (o = 0 or TypesMatch(o, "QWidget") or IsBaseClass(o,
> "QWidget"))
> end if
>
> end type
>
>
> Now then, in this particular instance I have confirmed via trace that o is
> equal to zero. It seems to me that since we are testing using "or" in the
> return statement, if any expression in the test evaluates to "true," then
> the
> entire expression would be true. If that is the case, then short-cut
> evaluation should stop processing the expression at that point and return
> true.
>
> Thus, in the case of the return statement here, the interpreter should see
> that, indeed, o = 0, and then return true. But it is not ... it in
> instead
> evaluating TypesMatch(o, "QWidget"), which is causing my library to become
> quite upset, since o is a NULL pointer.
>
> Now, it would be easy for me to add the line
>
> elsif o = 0 then
> return true
>
> But apparently I'm not understanding something about short-cut evaluation
> here. Could anyone enlighten me?
>
> Thanks,
>
> Travis W. Beaty
> Osage, Iowa.
>
> --
> Infancy, n.:
> The period of our lives when, according to Wordsworth, "Heaven lies
> about us." The world begins lying about us pretty soon afterward.
> -- Ambrose Bierce
>
2. RE: Short-cut evaluation Question
> -----Original Message-----
> From: Travis Beaty [mailto:twbeaty at osage.net]
> Subject: Re: Short-cut evaluation Question
>
>
>
> Hello again!
>
> On Sunday 29 February 2004 02:47 pm, Juergen Luethje wrote:
>
> > Current Euphoria versions only use short-circuit evaluation in
> > conditions tested by "if", "elsif" or "while". I.e. if you want
> > short-circuit evaluation, write the code above like this:
> >
> > global type Widget(object o)
> > if not atom(o) then
> > return false
> > elsif (o = 0 or TypesMatch(o, "QWidget") or
> IsBaseClass(o, "QWidget"))
> > then return true
> > else
> > return false
> > end if
> > end type
> >
> > Regards,
> > Juergen
>
>
> Yes, your way of doing it does work. However, now I get
> warning messages that
> "call to TypesMatch() might be short-circuited." Hmm ...
> wouldn't this
> typically be the desirable outcome? I suppose the only time
> that it wouldn't
> be is in someone were depending upon a side effect, and such
> a place would be
> a rather bad place to do that, since Euphoria uses short-cut
> evaluation.
>
> I've opted for this rather wordy set up instead.
>
>
> global type Widget(object o)
>
> if not atom(o) then
> return false
> elsif o = 0 then
> return true
> else
> return (TypesMatch(o, "QWidget") or
> IsBaseClass(o, "QWidget"))
> end if
>
> end type
>
> It seems to work, and without warnings.
For what its worth, I prefer this method below because it means that the
routine only has a single exiting point, but sometimes one must compromise
if not efficient enough.
global type Widget(object o)
integer rc
rc = false
if atom(o) then
if o = 0 then
rc = true
elsif TypesMatch(o, "QWidget")
rc = true
elsif IsBaseClass(o, "QWidget")
rc = true
end if
end if
return rc
end type
> As far as my previous set up ...
>
> return (o = 0 or TypesMatch(o, "QWidget") or IsBaseClass(o,
> "QWidget"))
>
> Why would short-cut evaluation not be a normal, or even
> "Good" thing in this
> instance?
Well, it could be argued that Euphoria cannot know that there is a
dependancy between TypesMatch() and IsBaseClass(). This is why you need to
explictly encode this in your application. However, in some cases euphoria
could be made smarter...
return (i >= 1) and (i <= length(s)) and (s[i] = somevalue)
--
Derek
