[pm-h] Using Moose Attributes In Regular Expressions - Question Regarding Magic ${\( ... )}

[Zakariyya Mughal]

On 2016-08-23 at 14:35:38 -0500, Robert Stone via Houston wrote:
> Greetings,
> 
> You are correct in the sense that $object->foo actually calls a method
> "foo" created via Moose Magic that returns the value of the foo attribute.
> Good call!
> 
> Are you thinking that given the above, the \( ) business makes $object->foo
> a coderef, and then ${ } executes it because it dereferences it?  Given
> that, why does this not match:
> 
> if( $some_value =~ m/\Q$object->foo\E/ ) {
> 
> While this does:
> 
> if( $some_value =~ m/\Q${\( $object->foo )}\E/ ) {
> 
> Is that ${ } "protecting" the $object->foo so that the regex engine doesn't
> see it as regex-y?

Not a coderef, but a scalar ref. It is similar to the approach used by
the baby cart operator: <https://metacpan.org/pod/perlsecret#Baby-cart>.

  @{[ ]}

Reading inside out:

 - [ ... ]  : turn into an ArrayRef
 - @{ ... } : circumfix ArrayRef dereference

So that approach for the regexp does:

 - \ ... : turn into a reference (unary \ operator)
 - ${ ... } : circumfix ScalarRef dereference

I use the baby cart operator enough that I have added a line in my vimrc
to make it work with surround.vim: <https://github.com/zmughal/vimrc/commit/ad7894dcbe273b0ecb9703db88b51aa9a33d7f0c>.

 Cheers,
 - Zaki Mughal

> 
> Best Regards,
> Robert Stone
> 
> On Tue, Aug 23, 2016 at 2:19 PM, Julian Brown via Houston <houston at pm.org>
> wrote:
> 
> > I am not familiar with Moose, but am not convinced this is specific to
> > Moose.
> >
> > I assume $object->foo is really a method call that returns the foo
> > attribute?  Or is it like a hash value?
> >
> >
> > • [root at julian64:~/work]# cat doit.pl
> > #!/usr/local/cpanel/3rdparty/bin/perl
> >
> > use strict;
> > use warnings;
> >
> > my $var = "A";
> > my $var_ref = \$var;
> >
> > print "VAR :${var}:\n";
> > print "VAR VAR :${$var_ref}:\n";
> >
> > Output is:
> >
> > VAR :A:
> > VAR VAR :A:
> >
> > I think we are in the same realm perhaps the parens inside the ${} is
> > necessary to execute the method?
> >
> > Julian
> >
> >
> >
> > On Tue, Aug 23, 2016 at 2:02 PM, Robert Stone via Houston <houston at pm.org>
> > wrote:
> >
> >> Greetings,
> >>
> >> I find myself needing to use the value of a Moose Attribute in a regular
> >> expression every now and then.  Typically I accomplish this via (warning
> >> that all examples are very contrived and may contain bugs):
> >>
> >> *Sample Using Variable Assignment First*
> >>
> >> my $value = $my_moose_object->attribute;
> >> if( $some_value =~ m/\Q$value/ ) {
> >>
> >> Needless to say this isn't the most efficient/easiest to work with.
> >> Given that fact, I've come across a way of using Moose Attributes directly
> >> in regular expressions:
> >>
> >> *MyObject*
> >>
> >> package MyObject;
> >>
> >> use Moose;
> >>
> >> has foo => ( is => 'ro', isa => 'Str' );
> >>
> >> ...;
> >>
> >> *Script That Consumes MyObject*
> >>
> >> my $object = MyObject->new( foo => 'Value' );
> >> if( $some_value =~ m/\Q${\( $object->foo )}\E/ ) {
> >>
> >>
> >> This works, but frankly I'm not entirely certain why.  From the
> >> documentation at http://perldoc.perl.org/perlre.html#Regular-Expressions
> >> I have:
> >>
> >> \Q          quote (disable) pattern metacharacters until \E
> >> \E          end either case modification or quoted section, think vi
> >>
> >> Great, that makes sense.
> >>
> >> But what magic is the ${\( ... )} doing here?  I'd be most grateful if
> >> anyone had some insight and could share it with us!
> >>
> >> Best Regards,
> >> Robert Stone
> >>
> >>
> >>
> >> _______________________________________________
> >> Houston mailing list
> >> Houston at pm.org
> >> http://mail.pm.org/mailman/listinfo/houston
> >> Website: http://houston.pm.org/
> >>
> >
> >
> > _______________________________________________
> > Houston mailing list
> > Houston at pm.org
> > http://mail.pm.org/mailman/listinfo/houston
> > Website: http://houston.pm.org/
> >

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