What does @_[OBJECT, ARG0, ARG1] mean?

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

Consider this sub from Bot::BasicBot

sub _fork_said {
    my ($self, $body, $wheel_id) = @_[OBJECT, ARG0, ARG1];
    chomp $body;    # remove newline necessary to move data;

    # pick up the default arguments we squirreled away earlier
    my $args = $self->{forks}{$wheel_id}{args};
    $args->{body} = $body;

    $self->say($args);
    return;
}
[download]

What does the first line in the sub do? It would seem to me to set off a strict error. Original code here.

Neil Watson
watson-wilson.ca

Comment on What does @_[OBJECT, ARG0, ARG1] mean? Download Code

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Re: What does @_[OBJECT, ARG0, ARG1] mean? by toolic (Bishop) on Feb 27, 2016 at 21:47 UTC
My guess is that one of the modules it uses exports three constants, like: `use constant { OBJECT => 0, ARG0 => 1, ARG1 => 2, };` [download] Then, your line is essentially like: `my ($self, $body, $wheel_id) = @_[0, 1, 2];` [download] UPDATE: see POE::Session	[reply] [d/l] [select]
Re^2: What does @_[OBJECT, ARG0, ARG1] mean? by neilwatson (Priest) on Feb 27, 2016 at 22:15 UTC
The constants were confusing me. Thanks. Neil Watson watson-wilson.ca	[reply]
Re: What does @_[OBJECT, ARG0, ARG1] mean? by AnomalousMonk (Archbishop) on Feb 27, 2016 at 22:08 UTC
Further to toolic's reply: the expression `@_[0, 1, 2]` is an array slice (see Slices in perldata). Don't be thrown by the `_` (underscore): that's a perfectly valid (special) variable name. Give a man a fish: `<%-{-{-{-<`	[reply] [d/l] [select]
Re: What does @_[OBJECT, ARG0, ARG1] mean? by thomas895 (Deacon) on Feb 28, 2016 at 00:53 UTC
When you encounter things like this in the future, the Debugger is very useful. In particular, look at the `X vars...` (or `V Package::Name vars...`) commands. Try, for example, `DB<1> X OBJECT $OBJECT = 123` [download] There's nothing special about constants. All that `constant.pm` does is create a method in the package that returns the value you gave, by cleverly using import through use. Here's the relevant parts from `constant.pm` in 5.12.3: #===================================================================== +== # import() - import symbols into user's namespace # # What we actually do is define a function in the caller's namespace # which returns the value. The function we create will normally # be inlined as a constant, thereby avoiding further sub calling # overhead. #===================================================================== +== sub import { #... #note: $full_name is set to "Package_I_Was_Called_From::ConstantNa +me", for each constant you provide #if you have a scalar, it basically does $full_name = sub () { $scalar }; #and if you have an array: $full_name = sub () { @list }; #... } [download] Note: In the debugger output above, you may have noticed that it appeared as a regular variable. Indeed, in Perl 5.8 (as I understand it), `constant.pm` had some magic added to it so that scalars are simply added into the symbol table, appearing to be a package variable. Still, you can't really use them as variables, as you can observe here: `DB<6> x &OBJECT 0 123 DB<7> x $OBJECT 0 undef` [download] -Thomas "Excuse me for butting in, but I'm interrupt-driven..."	[reply] [d/l] [select]


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