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

[Robert Stone]

Greetings,

Wow, https://metacpan.org/pod/distribution/perlsecret/lib/perlsecret.pod is
a treasure trove of goodies!  Thanks for sharing that.

I believe this is the part of the documentation that stands out to me as
most relevant:

        *This is a container, or circumfix operator. The expression inside
the [] is run in list context, stored in an anonymous array, which is
immediately dereferenced by @{}.*

The "trick" here, as you pointed out, is the ${ } circumfix ScalarRef
dereference.  However, in order to use it we have to have, unsurprisingly,
a ScalarRef, hence the need for \( ) to convert the return value of
$object->foo first into a ScalarRef and then usage of ${ } for both
dereferencing it back and that tasty circumfixing action (which is higher
in precedence then the regex-ing, which is why this code snippet works)!

Does that sound about right?

Best Regards,
Robert Stone

On Tue, Aug 23, 2016 at 2:57 PM, Zakariyya Mughal <zaki.mughal at gmail.com>
wrote:

> 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/
> > >
>
> > _______________________________________________
> > Houston mailing list
> > Houston at pm.org
> > http://mail.pm.org/mailman/listinfo/houston
> > Website: http://houston.pm.org/
>
>
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