in reply to Re^5: Changing array by changing $_?
in thread Changing array by changing $_?

chomp (my $module = shift);
require $module or die;
for my $foo (@array) {
    func($foo);
}

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How are you going to determine at compile time whether $foo gets modified? Because if func changes its $_[0], the elements of @array get modified.

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Re^7: Changing array by changing $_?
by ikegami (Patriarch) on Oct 13, 2008 at 17:17 UTC
    I knew I phrased it as a question for a reason :D
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Re^7: Changing array by changing $_?
by gone2015 (Deacon) on Oct 13, 2008 at 17:42 UTC

    There's a mechanism to cope with:

      func('Hello Sailor') ;

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    which might be extended to passing of aliases, under suitable strictness...

    You are right. This particular trap is deeper, darker and altogether more ghastly than I thought !

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      Said mechanism is to make the SV readonly. 
      >perl -MDevel::Peek -e"Dump('Hello Sailor')"
SV = PV(0x228e68) at 0x1831670
  REFCNT = 1
  FLAGS = (PADBUSY,PADTMP,POK,READONLY,pPOK)
                              ^^^^^^^^
  PV = 0x1833f3c "Hello Sailor"\0
  CUR = 12
  LEN = 16

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      Obviously that mechanism won't help you here.

      In fact, a flag alone won't work since you'd need to know how many times a variable has been aliased to know when to remove the flag. (Upd: I'm not sure about the preceding statement. Can't think straight right now. )

      You could add magic to the variable like taint does. 

      >perl -T -MDevel::Peek -e"Dump($ARGV[0])" foo
SV = PVMG(0x1822dcc) at 0x226ba0
  REFCNT = 1
  FLAGS = (GMG,SMG,pPOK)
  IV = 0
  NV = 0
  PV = 0x182d324 "foo"\0
  CUR = 3
  LEN = 4
  MAGIC = 0x182d33c
    MG_VIRTUAL = &PL_vtbl_taint
    MG_TYPE = PERL_MAGIC_taint(t)
    MG_LEN = 1

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      But now you're getting into costly.

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        OK, the penny is dropping...

        So the @_ is an array of SV*. To distinguish aliases as arguments requires the pointer, not the target, to be marked -- it's only assignments via the alias that should be trapped. That would add overhead.

        Mind you, the compiler knows when it's assigning to $_[x], and only then does it need to worry about copies of aliases -- I think the propagation of aliases is limited -- so I guess the overhead could be contained... assuming one could figure out a way of passing the information with the pointer. Hmmm. I'm starting to feel queasy.

        Well, nothing's perfect... if the 99% cases (lexically inside the foreach/map/etc) were trapped at compile-time it would be a step in the right direction.

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