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Message from discussion Autovivi
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Nicholas Clark  
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 More options Aug 13 2002, 4:48 pm
Newsgroups: perl.perl6.language
From: n...@unfortu.net (Nicholas Clark)
Date: Tue, 13 Aug 2002 21:33:32 +0100
Local: Tues, Aug 13 2002 4:33 pm
Subject: Re: Autovivi
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On Tue, Aug 13, 2002 at 03:06:40PM -0400, Deven T. Corzine wrote:
> The only accurate way to know if the code modifies the variables is to do
> some sort of dataflow analysis, and it can't be 100% accurate even then.  
> (Suppose a "shift" may or may not happen, depending on the parameters, then
> $_[2] is modified?)  Of course, it should often be possible (at least in
> principle) to determine that it's impossible for a particular parameter to
> be modified, and play it safe by assuming "is rw" for undeterminable cases.

Well, perl5 does already manage to avoid auto-vivifying hash keys when they
are used as subroutine arguments. It uses magic, rather than dataflow
analysis:

$ cat autovivi.pl
#!/usr/local/bin/perl -w
use strict;
use Devel::Peek;

sub rval {
  my $a = $_[0];

}

sub lval {
  $_[0] = undef;

}

sub dumpit {
  Dump ($_[0]);

}

my %a = (camel => 2);

rval ($a{llama});

print "rval: @{[keys %a]}\n";

lval ($a{alpaca});

print "lval: @{[keys %a]}\n\n";

dumpit ($a{dromedary});

__END__
$ ./autovivi.pl
rval: camel
lval: camel alpaca

SV = PVLV(0x1134d8) at 0xf4368
  REFCNT = 1
  FLAGS = (GMG,SMG)
  IV = 0
  NV = 0
  PV = 0
  MAGIC = 0x113420
    MG_VIRTUAL = &PL_vtbl_defelem
    MG_TYPE = PERL_MAGIC_defelem(y)
    MG_FLAGS = 0x02
      REFCOUNTED
    MG_OBJ = 0xf43a4
    SV = PVIV(0xf4990) at 0xf43a4
      REFCNT = 1
      FLAGS = (POK,pPOK)
      IV = 0
      PV = 0x1138a0 "dromedary"\0
      CUR = 9
      LEN = 10
  TYPE = y
  TARGOFF = 0
  TARGLEN = 1
  TARG = 0x1329ec
SV = PVHV(0x114820) at 0x1329ec
  REFCNT = 2
  FLAGS = (PADBUSY,PADMY,SHAREKEYS)
  IV = 2
  NV = 0
  ARRAY = 0x1133f8  (0:6, 1:2)
  hash quality = 125.0%
  KEYS = 2
  FILL = 2
  MAX = 7
  RITER = -1
  EITER = 0x0
  Elt "camel" HASH = 0x787f96b8
  SV = IV(0x112ee4) at 0xf4284
    REFCNT = 1
    FLAGS = (IOK,pIOK)
    IV = 2
  Elt "alpaca" HASH = 0x5af8b041
  SV = PVNV(0x12a6d8) at 0x111b40
    REFCNT = 1
    FLAGS = ()
    IV = 0
    NV = 0
    PV = 0

As you can see, the ingredients for "magic" are quite cumbersome, but do get
the job done.

Nicholas Clark
--
Even better than the real thing:        http://nms-cgi.sourceforge.net/


 
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