variable existence checking

[Tony Balinski]

Quoting "A.V.Kuznetsov" <kuzn at umps.mephi.ru>:

> Hi,
>
> >
> > I would actually prefer an existence operator, but NOT !!. This has a
> definite
> > meaning: "is true" (in macro !!x is equivalent to x!=0). I have been
> thinking
> > along the lines of an "exists" operator "??" instead:
>
> You are right as always.
>
> >     ??var: return true (1) if var has a value, false (0) otherwise
> >     ??array[]: return true if array is an array
> >     ??array[key]: return (key in array)
> >     ??array[key1][key2]: return ((key1 in array) && (key2 in array))
> > (with my array.key language extension, also: array??.key)
> >
>
> ??array[key1][key2]: return ((key1 in array) && (key2 in array[key1]))

Oops, well spotted!

BTW I wanted to avoid the ? as the operator so that we could at some point
have C's ?: ternary operator, which I find very useful.

Another route would be to allow undefined as a pseudo-value in variables (an
actual value in simple variables, possibly just a sign of a missing index in
arrays) that you could pass to functions etc, test against and assign using.
This would move things a little closer to Perl, say, where undefined values do
more than cause failure.

As far as I'm concerned, this is an open issue.

Tony