On Sat, Dec 11, 2021 at 11:28 AM Ralph Mellor <ralphdjmel...@gmail.com>
wrote:
> On Fri, Dec 10, 2021 at 9:49 PM Sean McAfee <eef...@gmail.com> wrote:
> >
> > @array[-1]:exists is a syntax error, even though it looks like
> > it should just evaluate to True.
>
> In Raku `foo[-N]` is invalid. At compile-time if N is statically
> known, otherwise at run-time.
>
It's not, though, if certain adverbs such as :exists are present, as in my
original message.
> @array[-1]:exists <-- compile time error
> my $i = -1; @array[$i]:exists <-- evaluates to False
To me it appears that the first case should also evaluate to False, but
that the compiler is trying to aggressively catch negative-indexing errors
without considering whether any adverbs are present that might make the
expression sensible.
Another example is the :v adverb. I was recently delighted to discover
that I could easily have a sliding window over an array, where the window
might hang over either end of the array.
> my @a = <a b c d e>
[a b c d e]
> say @a[^3 + $_]:v for -2 .. 4
(a)
(a b)
(a b c)
(b c d)
(c d e)
(d e)
(e)
But anyway, my original use case was driven by a desire to simplify a
routine similar to the following. Given a 2-D array and an x- and
y-coordinate, I want to increment the number at the given coordinates, but
only if the coordinates are valid. My first effort went like this:
sub increment(@array, $x, $y) {
++@array[$y][$x] if 0 <= $y < @array && 0 <= $x < @array[$y];
}
Then I remembered semicolon subscripting and :exists, and tried changing
the body of the routine to this:
++@array[$y; $x] if @array[$y; $x]:exists;
I thought that was a vast improvement; let Raku figure out if the indices
are valid! But while it does work--that is, I get the right answers at the
end--it results in a slew of "unhandled Failure detected in DESTROY"
warnings. I haven't observed this ever happening when using :exists with a
single index, only with subscripts that use semicolons. So for now, I'm
getting by with:
++@array[$y; $x] if @array[$y]:exists and @array[$y][$x]:exists;
[snipped a bunch of comments about the pitfalls of using negative numbers
to index from the end of arrays, with which I am in total agreement]
