SPUG: Strange code effect

[Skylos]

Ooh good point Ben!

What is there was something else that hit the bool functionality of the
object though?  It occurs to me that sorting can be a complex operation -
maybe something happened when it was sorted, like this mod of your lil test
script?


  1 #!/usr/bin/perl
  2
  3 use strict;
  4 use warnings;
  5 use Carp;
  6
  7 my @objects = sort reverse map{Snoop->new()} 1..3;
  8
  9 # Implicit boolean context here?
 10 my $first = shift @objects;
 11
 12 # explicit boolean context here
 13 if ($first) {
 14   # nothing to do here.
 15 }
 16
 17 package Snoop;
 18
 19 use overload (
 20   q{bool} => sub {
 21         my $self = shift;
 22         Carp::confess("Boolean context on objnum $self->{objnum}");
 23         return $self;
 24       },
 25   q{cmp} => sub {
 26         my $self = shift;
 27         my $other = shift;
 28         if ($self) {
 29           Carp::confess "Did I bool in the cmp?";
 30         }
 31         return $self->{objnum} <=> $other->{objnum};
 32      },
 33 );
 34
 35 my $objnum = 0;
 36
 37 sub new {
 38   my $class = shift;
 39   return bless {objnum => $objnum++}, $class;
 40 }
 41 __END__

That outputs for me:

Boolean context on objnum 2 at testbool line 22
        Snoop::__ANON__('Snoop=HASH(0x8924c8c)', 'undef', '') called at
testbool line 28
        Snoop::__ANON__('Snoop=HASH(0x8924c8c)', 'Snoop=HASH(0x8918258)',
'') called at testbool line 7


One thing I've learned for sure now is that if you're going to use the
override pragma, you probably should define something for *all* of the
operators/contexts - otherwise, you risk the undefined operation error all
over the place!  I couldn't sort the array without defining the cmp
operation...

But seriously, maybe thats how it happened, due to the lack of actual
feedback about where the error was, it perhaps *seemed* to be on the shift
when it was somewhere down in the sort?

An idea at any rate,

Skylos


"If only I could get rid of hunger by rubbing my belly" - Diogenes


On Wed, Dec 23, 2009 at 4:07 PM, BenRifkah Bergsten-Buret <
mail.spammagnet at gmail.com> wrote:

> On Wed, Dec 23, 2009 at 3:27 PM, Joseph Werner <telcodev at gmail.com> wrote:
>
>> On Wed, Dec 23, 2009 at 2:52 PM, Skylos <skylos at gmail.com> wrote:
>> > ...
>> > Which makes me think "something about that value when a shift is done
>> uses
>> > the value in a boolean context"
>> > ...
>>
>> Thanks for the feedback Skylos, This is exactly the same conclusion
>> that we have come too; but I STILL cannot place a boolean context in
>> the a simple shift assignment. The powers that be crack the whip and
>> say "Problem solved, move on".   Code that does not behave the way I
>> expect it to bothers me...
>>
>>
> I was considering that shift had an implicit boolean context as well so did
> some digging.  Based on my test implementation it appears that shift doesn't
> have an implicit boolean context.  Perhaps the boolean context is occuring
> after the $choosenO is returned?
>
> Here's my test script is_shift_boolean.pl:
>       1 #!/usr/bin/perl
>       2
>       3 use strict;
>       4 use warnings;
>       5 use Carp;
>       6
>       7 my @objects = map{Snoop->new()} 1..3;
>       8
>       9 # Implicit boolean context here?
>      10 my $first = shift @objects;
>      11
>      12 # explicit boolean context here
>      13 if ($first) {
>      14   # nothing to do here.
>      15 }
>      16
>      17 package Snoop;
>      18
>      19 use overload (
>      20   q{bool} => sub {
>      21         my $self = shift;
>      22         Carp::confess("Boolean context on objnum $self->{objnum}");
>      23         return $self;
>      24       },
>      25 );
>      26
>      27 my $objnum = 0;
>      28
>      29 sub new {
>      30   my $class = shift;
>      31   return bless {objnum => $objnum++}, $class;
>      32 }
>      33 __END__
>
> This uses the overload pragma to do the operator overloading so if you're
> using something else the results may be different.
>
> Upon execution I got the following output:
> Boolean context on objnum 0 at is_shift_boolean.pl line 22
>         Snoop::__ANON__('Snoop=HASH(0x814ccd4)', 'undef', '') called at
> is_shift_boolean.pl line 13
>
> This reports only one boolean context in the if statement at line 13.
> There is no report of boolean context from line 10 where the shift is.
>
> A mystery for the ages,
>
> --
> Ben
>
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