[pm-h] Using Moose Attributes In Regular Expressions - Question Regarding Magic ${\( ... )}
Julian Brown
jlbprof at gmail.com
Tue Aug 23 15:01:43 PDT 2016
I need to know if I am an "old" dog trying to learn new tricks.
--------------------------------------
$ cat ./doit.pl
#!/usr/bin/perl
use strict;
use warnings;
my $ref = \( "a", "b" );
print ":$ref:\n";
print ":${$ref}:\n";
my $other = ("a", "b");
print "other :$other:\n";
$ ./doit.pl
Useless use of a constant (a) in void context at ./doit.pl line 11.
:SCALAR(0xaf9998):
:b:
other :b:
--------------------------------------
This is what confused me \( ).
I think I am learning a new thing (not a trick).
The "use warnings" is enlightening.
I never really ever used a list in a scalar context before, in a scalar
context it always returns the last item. The warning is telling me that I
am essentially throwing away the value of "a".
Julian
On Tue, Aug 23, 2016 at 4:08 PM, Zakariyya Mughal via Houston <
houston at pm.org> wrote:
> On 2016-08-23 at 15:16:42 -0500, Robert Stone wrote:
> > 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 ${ } for both
> > dereferencing it back and that tasty circumfixing action (which is higher
> > in precedence then the regex-ing, which is why this code snippet works)!
> >
> > Does that sound about right?
>
> Yep! However, I believe that the `${}` evaluation inside of `m//` is due
> to interpolation rather than precedence. In `perldoc perlop`, Perl puts
> terms
> (e.g., quote-like operators and variables) higher than anything else and
> these get processed left to right.
>
> You can also used B::Concise to see how Perl parses the expression:
>
> perl -MO=Concise -e 'qr/before\Q${\( $object->foo )}\Eafter/;'
>
> Really fascinating stuff to play around with!
>
> Cheers,
> - Zaki Mughal
>
> >
> > 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/
> > > > >>
> > > > >
> > > > >
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> > > > >
> > >
> > > > _______________________________________________
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> > >
> > >
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