Detecting constant arguments passed to subroutines

dcd has asked for the wisdom of the Perl Monks concerning the following question:

perltrap.pod has an example that shows how perl5 will trap modification of a constant.

  $foo = "x";
  &mod($foo);
  for ($x = 0; $x < 3; $x++) {
      &mod("a");
  }
  sub mod {
    print "before: $_[0]";
    $_[0] = "m";
    print "  after: $_[0]\n";
  }
[download]

perl5 reports this error as

  Modification of a read-only value attempted...
[download]

Short of doing an eval { } like:

  eval { $_[0] .= '' };
  if ($@ =~ /Modification of a read-only value attempted/) {
[download]

to try to modify to the constant ahead of time, what other ways does perl provide detect that a parameter to a sub is a constant? I've seen that Devel::Peek can report that the flags of a variable are READONLY.

$ perl -MDevel::Peek -le 'Dump("hello")'
SV = PV(0x8121a3c) at 0x812b36c
  REFCNT = 1
  FLAGS = (POK,READONLY,pPOK)
  PV = 0x812f498 "hello"\0
  CUR = 5
  LEN = 6
[download]

Does a module that returns the FLAGS exist?

Comment on Detecting constant arguments passed to subroutines Select or Download Code

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Re: Detecting constant arguments passed to subroutines by japhy (Canon) on May 17, 2001 at 00:42 UTC
Well, I'd do: `sub is_const { !eval { ($_[0]) = $_[0]; 1; } } for (...) { if (is_const($_)) { ... } else { ... } }` [download] `japhy` -- Perl and Regex Hacker	[reply] [d/l]
Re: Re: Detecting constant arguments passed to subroutines by dcd (Scribe) on May 17, 2001 at 01:24 UTC
The is_const code doesn't seem to detect undef as a constant, although Devel::Peek reports it as READONLY `perl -w -le 'sub is_const { !eval { ($_[0]) = $_[0]; 1; } } print "undef is ",(is_const(undef)?"":"not")," a constant"' undef is not a constant perl -MDevel::Peek -le 'Dump(undef)' SV = NULL(0x0) at 0x8120d38 REFCNT = 2147483622 FLAGS = (READONLY)` [download]	[reply] [d/l]
Re: Re: Re: Detecting constant arguments passed to subroutines by dcd (Scribe) on May 17, 2001 at 01:39 UTC
I just noticed the REFCNT the Devel::Peek::Dump(undef) printed That can't be right, can it?	[reply]
Re: Re: Re: Re: Detecting constant arguments passed to subroutines by japhy (Canon) on May 17, 2001 at 01:44 UTC
Re: Re: Re: Re: Detecting constant arguments passed to subroutines by chipmunk (Parson) on May 17, 2001 at 01:43 UTC
Re: Re: Detecting constant arguments passed to subroutines by dcd (Scribe) on May 17, 2001 at 10:32 UTC
After more research I found a function that I was looking for `sub is_const {use B;B::SVf_READONLY & B::svref_2object(\$_[0])->FLAGS} my $m="va"; our $n=3; my $u=undef; our $u2=undef; for $x ($m, $n, $u, $u2, 4, "a", undef) { print "$x ";$x="VAR" unless is_const($x); print $x,"\n\n" }` [download] but it too doesn't catch undef being passed it. For what I'm doing now, I don't think I'll encounter this condition. I think that japhy's is_const is 'better' than the flags code that I came across - at least it's easier to read. What do you think?	[reply] [d/l]
Re: Detecting constant arguments passed to subroutines by dcd (Scribe) on May 29, 2001 at 10:30 UTC
I just found the real functions I was looking for `readonly` in `Scalar::Util`. I haven't figured out yet why calling it indirectly via the is_const function doesn't detect that undef is a constant, as perl dies with an error when trying to modify the undef constant, but at least the `readonly` is easier to read and comprehend. If anyone can explain the undetect undef constant in is_const I'd be greatful. `use Scalar::Util 'readonly'; sub is_const { readonly($_[0]); } print "undef is readonly\n" if readonly(undef); print "undef not detected in is_const\n" unless is_const(undef); for $x (4, "a", undef) { print "$x ";$x="VAR" unless readonly($x); print $x,"\n"; } for $x (4, "a", undef) { print "$x ";$x="VAR" unless is_const($x); print $x,"\n"; }` [download]	[reply] [d/l] [select]
Re^2: Detecting constant arguments passed to subroutines by Anonymous Monk on Sep 14, 2005 at 00:24 UTC
Re: Re: Detecting constant arguments passed to subroutines by Anonymous Monk on May 17, 2001 at 01:02 UTC
Thanks, assigning the value to itself would avoid stringifying	[reply]


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