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

Tue Aug 23 19:04:41 PDT 2016

This blog post by mst covers this "madness" in some detail:
http://shadow.cat/blog/matt-s-trout/madness-with-methods/

The code blocks seem to be borked cause of the new theme his site is using
though...

On Tue, Aug 23, 2016 at 5:01 PM Julian Brown via Houston <houston at pm.org>
wrote:

> 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/
>> > > > >>
>> > > > >
>> > > > >
>> > > > > _______________________________________________
>> > > > > 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
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>>
>
> _______________________________________________
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