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Re: Salvaging lexical $_ from deprecation

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Ricardo Signes
February 19, 2013 23:23
Re: Salvaging lexical $_ from deprecation
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* Rafael Garcia-Suarez <> [2013-02-19T16:07:08]
> I notice that the lexical topicalizer "my $_" is deprecated in blead.
> At this point I assume that the reasons for the deprecation given in
> perl5177delta are exhaustive. All those reasons are pointing to
> implementation problems, notably at the XS interface level, which can
> be solved; there are no reasons given that pertain to language design
> issues.

The reasons I find more compelling are those that Jesse L. mentioned.  How do
you suppose that the language is supposed to work, there?

> In conclusion: think first about language design; think about having a
> vision for the future. Don't throw away language features just because
> they introduce difficult bugs you'd rather sweep under the rug.
> Deprecating $_ is laziness and impatience of the worst kind; do not
> forget to also display hubris, because only hubris makes the future
> promises shine.

I *am* thinking about language design.  The bugs that interest me are the ones
that will be written by users, in Perl, because of the language design.  I
don't see how it can be made to make sense or become valuable.

What *should* this program print?

  use 5.12.0;
  my ($A, $B, $C, $D, $E);

    my $_ = "foo";

    $A = sub { say "A" if       /foo/ };
    $B = sub { say "B" if $_ =~ /foo/ };
    $C = sub { say "C" if grep { /foo/ } qw(bar) };
    $D = sub { say "D" if grep { (sub { $_ =~ /foo/})->() } qw(bar) };
    $E = sub { say "E" if (sub { $_ =~ /foo/ })->() };

  local $_ = "bar";

> In my opinion, the more important problem here is the impression that
> P5P is throwing away without much thought of a perfectly nice and
> modern language feature (for some value of modern that means
> "post-FORTRAN"). This could give the impression of a the lack of
> vision for Perl 5 (and reinforce the "perl is dead" death spiral as
> perceived by the outside world -- the Perl users).

You may have that impression, but I believe you have it entirely backward.  I
do not think that lexical $_ is nice or modern.  I think it is half-baked,
confused, and confusing.  I don't see how it can be made useful or
straightforward, at the language level.  If ejecting constructs that cannot be
salvaged makes someone think that the language is dead, I can live with that.

On the other hand, I am absolutely willing to give you the benefit of the
doubt, here.  Can you tell me how this feature can be made to make sense
going forward?  (My understanding of the lexicality of the topic in Perl 6 is
that it makes sense because it is also propagated dynamically by their calling
conventions.  I am looking into this.)

If you feel certain that it can be salvaged, and can give some idea of how it
would work sensibly in the future, maybe we'll mark it experimental instead
to see if the plan can be enacted.

> So, to be clear: $_ is still global by default due to the need for
> backwards compatibility. If something should be deprecated, it should
> be plain $_, not "my $_".

How would we get there from here without breaking "everything"?  Right now, a
subroutine can rely on the $_ from its caller being available without that
subroutine having to do anything.

  lather, rinse, reap, eat for @data;

If we can't get there, how will $::_ and my $_ interoperate without causing
great confusion?


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