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Sat Dec 22 00:30:00 2012

Slides for the talk the-devils-repl at lpw-2010

The
Devil's
REPL

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5 minutes
65 slides
1 crack pipe

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Devel::REPL

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written for
a series
of blog posts

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Moose
MooseX::Object::Pluggable

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Works
great

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FATTER
THAN
CARTMAN

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Shitloads
of XS

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FAIL

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Minimalist
repl?

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Pure perl
repl!

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But I need
lexical
persistence

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Lexical::Persistence

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PadWalker
Devel::LexAlias

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Hmm ...

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 eval q{sub {
   my $x; $x++; $x
 }}

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 use B;
 my $obj = svref_2object($sub);

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 $obj->PADLIST

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 $obj->PADLIST->ARRAY

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  [
    'B::SPECIAL=SCALAR(0x1a47510)',
    'B::PVNV=SCALAR(0x1a47570)',
    'B::SPECIAL=SCALAR(0x1a475d0)'
  ],
  [
    'B::AV=SCALAR(0x1a47678)',
    'B::IV=SCALAR(0x1a476a8)',
    'B::NULL=SCALAR(0x1a47708)'
  ]

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 [ \@names,
   \@values ]

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  map $_->PV, grep $_->isa('B::PV'),
    $obj->PADLIST->ARRAYelt(0)->ARRAY;

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Values are
useless

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Values are
already
DESTROYed

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Hmm ...

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 BEGIN { our $y = \$x; }

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Works at
toplevel

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 sub {
   ...
   BEGIN { our $y = \$x; }
 }

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Variable "$x"
is not
available

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:(

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  sub {
    ...
    capture_pads()
  }

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Breaks
return
values

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Hmm ...

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  sub {
    my $guard = bless({}, 'HasADESTROY');
    ...
  }

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Lexicals
are freed
in reverse
order of
declaration

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So $guard
is DESTROYed
too late

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:(

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Hmm ...

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  sub {
    ...
    sub aha { our $y = \$x }
  }

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Variable $x
will not
stay shared

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... *stay* ...

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I only need
to share it
the once!

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Now how do I
get the list?

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  sub {
    ...
    sub inside { }
    BEGIN { inspect_inside() }
    ...

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  my $obj = svref_2object(
    \&inside
  );

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  map $_->PV, grep $_->isa('B::PV'),
    $obj->PADLIST->ARRAYelt(0)->ARRAY;

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But how do
I inject the
capture code?

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  sub {
    ...
    sub inside { }
    BEGIN { inspect_inside() }
    __NOW_CAPTURIZE__

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  s{__NOW_CAPTURIZE__}
   {$capture_code}

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Source
filter?

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Source filters
don't work
in string eval

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:(

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But ...

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coderef
in @INC

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... opening
a string
filehandle

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  local @INC = (sub {
    if ($_[1] eq '/eval_do') {
      open my $fh, '<', $text_ref;
      return $fh;
    }
  }, @INC);

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The BEGIN
happens
before that
line's read

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And my s{}{}
will work!

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Or even an
append

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  $code .= '+{ '
    .join(', ',
      map "'$_' => \\$_", @names
    )
  .' };'

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  my $x = ${$_[2]->{"\$x"}};
  sub Eval::WithLexicals::Cage::current_line {
    package Eval::WithLexicals::Scratchpad;
    ++$x
    ;sub Eval::WithLexicals::Cage::pad_capture { }
    BEGIN { Eval::WithLexicals::Util::capture_list() }
    sub Eval::WithLexicals::Cage::grab_captures {
      no warnings 'closure';
      package Eval::WithLexicals::Cage;+{ '$x' => \$x };
    }
  }

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  my $eval = Eval::WithLexicals->new;
  my $read = Term::ReadLine->new('Perl REPL');
  while (1) {
    my $line = $read->readline('re.pl$ ');
    exit unless defined $line;
    my @ret; try {
      local $SIG{INT} = sub { die "Caught SIGINT" };
      @ret = $eval->eval($line);
    } catch {
      @ret = ("Error!", $_);
    };
    print Dumper @ret;
  }

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  re.pl$ my $x = 1
  1
  re.pl$ ++$x
  2
  re.pl$ $x
  2

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Eval::WithLexicals
went to CPAN during
the LPW hackathon

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  $ cpanm Eval::WithLexicals
  $ tinyrepl
  re.pl$

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Thank you.