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Re: "no snails"; or having signatured subs complain about @_

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Dave Mitchell
December 9, 2021 18:47
Re: "no snails"; or having signatured subs complain about @_
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On Thu, Dec 09, 2021 at 01:41:29PM -0500, Felipe Gasper wrote:
> > On Dec 9, 2021, at 13:37, Dave Mitchell <> wrote:
> > 
> > On Thu, Dec 09, 2021 at 05:36:06PM +0000, Paul "LeoNerd" Evans wrote:
> >> This was the problem though - I couldn't work out how to keep @DB::args
> >> working in that situation.
> > 
> > A more radical proposition would be to declare that @DB::args will always
> > return empty for signature subs.
> Would that break stack traces--or at least the argument-listing part of
> them?


> If so, that would seem to be an unfortunate disincentive to adopt signatures.

Here's the current disclaimer in caller()'s pod:

    Be aware that setting C<@DB::args> is I<best effort>, intended for
    debugging or generating backtraces, and should not be relied upon.  In
    particular, as L<C<@_>|perlvar/@_> contains aliases to the caller's
    arguments, Perl does not take a copy of L<C<@_>|perlvar/@_>, so
    C<@DB::args> will contain modifications the subroutine makes to
    L<C<@_>|perlvar/@_> or its contents, not the original values at call
    time.  C<@DB::args>, like L<C<@_>|perlvar/@_>, does not hold explicit
    references to its elements, so under certain cases its elements may have
    become freed and reallocated for other variables or temporary values.
    Finally, a side effect of the current implementation is that the effects
    of C<shift @_> can I<normally> be undone (but not C<pop @_> or other
    splicing, I<and> not if a reference to L<C<@_>|perlvar/@_> has been
    taken, I<and> subject to the caveat about reallocated elements), so
    C<@DB::args> is actually a hybrid of the current state and initial state
    of L<C<@_>|perlvar/@_>.  Buyer beware.

@DB::args should never have been added. This gives us a chance to remove it
without breaking backwards compatibility.

A major Starfleet emergency breaks out near the Enterprise, but
fortunately some other ships in the area are able to deal with it to
everyone's satisfaction.
    -- Things That Never Happen in "Star Trek" #13

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