TL;DR Raku does not treat a negative index as valid. To specify the index of the last element, write `*-1`. What you're seeing is Raku trying to inform you that you are using invalid indices.
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. You must be used to PLs that treat a negative index as an offset from the end. Designers of older PLs considered this a feature. And indeed, for *literal* values it's arguably a reasonable choice, saving a character compared to Raku's alternative (`*-1`). But a mass of empirical evidence has shown that when this idiom is applied to *variables* it becomes a bug magnet. So Raku avoided that mistake, and instead carefully treats any negative indices as errors. These errors are reported at compile-time if the indices are literal, and at run-time if they're variables or expressions. By default run-time errors are reported via `Failure`s that are returned as the (error) value(s) of the failed indexing. `Failure`s are wrapped `Exceptions` that serve as (error) values. If you handle them, they are disarmed, but if you don't, or try to use them as if they were non-error values, they blow up at the first sensible time to do so. That is to say, immediately if you try to use them when unhandled or at their `DESTROY` time if you don't. > Actually, I get a bunch of those errors, as if they were stored > up by all my previous evaluations of @a[$i;$i]:exists. Yes. They are stored up because you ignored them, or at least didn't handle them. If you reread the error messages you'll see that that's what they convey. > If I continue to evaluate the expression repeatedly, I get > a run of Falses, then another batch of errors at some later > point. Exactly as they should. > I wonder if I'm somehow invoking undefined behavior No. You're "just" getting errors, suitably reported given Raku's design. -- love, raiph
