in reply to Re: Perl module symbol import versus direct invocation
in thread Perl module symbol import versus direct invocation

Calling a function using the -> syntax calls it as an object method.
Nope. -> has a double purpose:
  1. Object method call: $obj->meth(@params)
  2. Class method call: Class->meth(@params)
What the OP did, is applying the latter: invoking a function intended to be used as a function, as a class method. The result is that an extra parameter is unshifted, by perl, in the argument list, @_, before control is passed to it: the Class, which is a string.

He should try, with a higher chance of success, to call 

Date::Calc::Add_Delta_Days($year,$month,$day, -3);

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Re: Re: Re: Perl module symbol import versus direct invocation
by JayBonci (Curate) on Feb 26, 2003 at 09:57 UTC
    So I guess my next logical question is, why have a Class->meth() call at all, if it's not quite an Object call, and it's still not calling the function with the intended arguments? It uses a weak reference to the classname, and is likely to break things for non-OO modules.

    I suppose you could intend for your module to be used that way, but is there a sane way to test for how your module is being used, other than to always check the first parameter to make sure that it's not your classname? Or is this just a matter of "read the documentation before use"?

        --jaybonci
[reply]
      I agree it's very annoying that on the callee side, i.e. in the sub, it's virtually impossible to distinguish between being called as a function or as a method. Looks at the mess CGI.pm made of it — look into the subs "self_or_default" and "self_or_CGI".

      A reason to have calls as class methods instead of as functions, is the simple fact that now, inheritance works. It doesn't when you call a sub as a function.

      Two rather well known class methods are import, automatically invoked by use — and often inherited :-) from Exporter, and new, a name people often choose for a sub that serves to create a new instance of a class.

[reply]
        I agree it's very annoying that on the callee side, i.e. in the sub, it's virtually impossible to distinguish between being called as a function or as a method.
        {
    package Foo;
    use Devel::Caller qw(called_as_method);
    
    sub new { bless {}, shift }
    
    sub foo {
        my $arg = shift;
        if (called_as_method(0)) {
            ref($arg) ? "object method" : "class method";
        } else {
            "subroutine";
        }
    }
}

use Test::More tests => 3;

my $foo = Foo->new;
is( Foo->foo,       "class method" );
is( $foo->foo,      "object method" );
is( Foo::foo($foo), "subroutine" );

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        (although wanting to do this in real code is nearly always a sign that you need to rethink your design :-)

[reply]
[d/l]

      You can use Devel::Caller's called_as_method to determine when a particular subroutine is being called as an object/class method or as a normal subroutine.

      However, in my opinion, mixing function/procedural and OO APIs in the same subroutines is almost always a bad idea :-)

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[d/l]

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