Phoenix.pm: Attribute::Handlers and BEGIN blocks -- question(s)

[Shay Harding]

This got so long I'm going to post up top so you don't have to scroll...

After some investigation it seems both of you are kind of correct. The

$test : Test;

was getting parsed before the BEGIN blocks, as it is checked at compile time 
since this is valid Perl syntax and is something I did not even know existed.

The ':' is used to set attributes for subroutines as 'locked' or 'method'. 
>From perldelta, perl 5.6:

---------------------------------------------------------
New syntax for declaring subroutine attributes
 
Formerly, if you wanted to mark a subroutine as being a method call or as 
requiring an automatic lock() when it is entered, you had to declare that 
with a `use attrs' pragma in the body of the subroutine.  That can now be 
accomplished with declaration syntax, like this:
 
           sub mymethod : locked method ;
           ...
           sub mymethod : locked method {
               ...
           }
 
           sub othermethod :locked :method ;
           ...
           sub othermethod :locked :method {
               ...
           }
 
(Note how only the first `:' is mandatory, and whitespace surrounding 
the `:' is optional.)
 
AutoSplit.pm and SelfLoader.pm have been updated to keep the attributes with 
the stubs they provide.  See the attributes manpage.
--------------------------------------------------------------

>From the attrs.pm pod:

-----------------------------------------------------
=head1 DESCRIPTION
 
NOTE: Use of this pragma is deprecated.  Use the syntax
 
    sub foo : locked method { }
 
to declare attributes instead.  See also L<attributes>.
 
This pragma lets you set and get attributes for subroutines.
Setting attributes takes place at compile time; trying to set
invalid attribute names causes a compile-time error. Calling
C<attrs::get> on a subroutine reference or name returns its list
of attribute names. Notice that C<attrs::get> is not exported.
------------------------------------------------------

So it seems these attributes were orignally entered in for use with threads. 
Learn something new everyday :)

Attribute::Handlers seems to take this a step further and can be applied to 
subroutined, scalars, hashes, arrays, etc.


Shay


On Sunday 23 December 2001 20:15, you wrote:
> At 03:46 PM 12/22/01 -0700, you wrote:
> >Hello all,
> >
> >Have a question for anyone that might be familiar with Attribute::Handlers
> >module. Even if you're not familiar maybe you can provide insight into why
> >the following is happening...
> >
> >Ok, first BEGIN blocks should be evaluated before the rest of the script,
> >FIFO order. So the following:
> >
> >
> >#!/usr/bin/perl
> >
> >print "Top of script\n";
> >
> >BEGIN{
> >     print "BEGIN 1\n";
> >}
> >
> >print "Middle\n";
> >
> >BEGIN{
> >     print "BEGIN 2\n";
> >}
> >
> >
> >will print:
> >
> >BEGIN 1
> >BEGIN 2
> >Top of script
> >Middle
> >
> >
> >Now take the following code:
> >
> >#!/usr/bin/perl
> >
> >   my $test : Test;  #set an attribute for variable
> >
> >   print "Top\n";
> >
> >    BEGIN{
> >        print "In BEGIN 1\n";
> >        use Attribute::Handlers;
> >        sub Test($;) : ATTR(SCALAR, BEGIN) {
> >            print "In profiled sub\n";
> >        }
> >    }
> >
> >   print "Middle\n";
> >
> >   BEGIN{
> >       print "In BEGIN 2\n";
> >   }
> >
> >
> >This gives the following error:
> >
> >Invalid SCALAR attribute: Test at t.pl line 3
> >BEGIN failed--compilation aborted at t.pl line 3.
> >
> >
> >which I really don't understand as it seems it is parsing '$test : Test;'
> >before the BEGIN blocks? I try to assign $test right after the attribute
> >setting and display it within the BEGIN blocks but under -w gives the
> >following (which I would expect):
> >
> >Use of uninitialized value in concatenation (.) at t.pl line 9.
> >
> >because $test should not exist within the BEGIN block meaning it hasn't
> > yet been looked at by the compiler.
> >
> >Why then do I get the 'Invalid SCALAR arttribute' error? The order of
> >processing in the above code is:
> >
> >Attribute::Handlers::import
> >BEGIN 1
> >BEGIN 2
> >script
> >
> >Which means the atribute handler Test should be parsed before the
> > assignment of $test...
> >
> >I know I can fix it by moving the assignment but I'm just curoius as to
> > why it is happening.
>
> I'm not familiar with Attribute::Handlers, so take this with a grain of
> salt...
>
> I checked Programming Perl 3rd Ed., and thus spake the camel (on page 465):
>
>    BEGIN blocks are handed off to the interpreter to be RUN as as(sic) soon
> as they are
>    parsed, which effectively runs them in FIFO order (first in, first
> out).  (Emphasis mine)
>
> I would guess that Attribute::Handlers is playing some magic with Perl's
> syntax, and if that syntax is used before the "use Attribute::Handlers"
> (which is treated like a BEGIN), the perl compiler will barf on the
> unrecognized syntax. So, compile wise, it should be this:
>
> script
> BEGIN 1
> Attribute::Handlers::import
> script
> BEGIN 2
>
> I'm in over my head, so I could be wrong.  Does anyone else want to confirm
> or deny this?