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Re: Altering the inheritance path of an object

by tobyink (Abbot)
on Sep 27, 2012 at 15:22 UTC ( #996029=note: print w/ replies, xml ) Need Help??


in reply to Altering the inheritance path of an object

Yes. The basic technique is to create a new package which inherits from the old one but adds some new methods, then rebless the object into that new package. Here's a quick example:

use 5.010; use Math::BigInt; # quite big!! my $seven = Math::BigInt->new('7'); # $seven cannot "speak", so this warns eval { $seven->speak } or warn $@; { package Math::BigInt::Speaker; our @ISA = qw(Math::BigInt); use Scalar::Util qw(blessed); use Carp qw(confess); sub upgrade { my ($class, $instance) = @_; confess "can only upgrade Math::BigInt objects" unless blessed $instance && $instance->isa('Math::BigInt') +; bless $instance => $class; } sub speak { CORE::say($_[0]); } } # Swaps $seven into our new class. Math::BigInt::Speaker->upgrade($seven); # $seven can now "speak" eval { $seven->speak } or warn $@;

If you use Moose you can do this in a much nicer and more organised way. (Though it's basically the same thing happening under the hood.) You'd just create a new anonymous role, and apply that role to the object.

use 5.010; use Math::BigInt; use Moose (); # quite big!! my $seven = Math::BigInt->new('7'); # $seven cannot "speak", so this warns eval { $seven->speak } or warn $@; my $speaker = Moose::Meta::Role->create_anon_role( methods => { speak => sub { CORE::say($_[0]) }, }, ); Moose::Util::apply_all_roles($seven, $speaker); # $seven can now "speak" eval { $seven->speak } or warn $@;
perl -E'sub Monkey::do{say$_,for@_,do{($monkey=[caller(0)]->[3])=~s{::}{ }and$monkey}}"Monkey say"->Monkey::do'


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Re^2: Altering the inheritance path of an object
by remiah (Hermit) on Sep 28, 2012 at 12:47 UTC

    Hello.

    I read Moose::Manual::Roles.pod and saw the car which applied Breakable roll on the fly, like you demonstrated. I heard several times Ruby people talks something like this... I mean instance extended on the fly.

    I , who have little experience for OO programing, thinks why people do like these, I would do ... say

    #!/usr/bin/perl { package TalkativeInt; use 5.010; use strict;use warnings; use base qw/Math::BigInt/; sub talk { say "I say ". $_[0]->value; } sub value { $_[0]->bstr(); } 1; } use 5.010; use strict;use warnings; my $p=TalkativeInt->new(7); $p->talk;
    with old school perl and extend with Moose,
    #!/usr/bin/perl { package TalkativeInt; use 5.010; use Moose; extends 'Math::BigInt'; sub talk { say "I say ". $_[0]->value; } sub value { $_[0]->bstr(); } 1; } use 5.010; use strict;use warnings; my $p=TalkativeInt->new(7); $p->talk;
    Do you have opportunity to use apply_all_roles in real world? This is just my curiosity.

    I really enjoy your Moose, ternary tree, lazy worker, I learned a lot.

    regards

      A project I'm working on now is a system for data transformation pipelines. Kind of like a makefile for data. So your pipeline might be something like:

      • Output the following tabular data as Excel:
        • Run the following query on the following database, resulting in tabular data.
          • SELECT person.name, ... FROM ...
          • Create a temporary database with these tables in it:
            • Create a table named "person" and load it with tabular data:
              • Parse this data as CSV:
                • Download data from http://example.com/people.csv
            • Create a table named "companies" and load it with tabular data:
              • Parse this data as tab-delimited data:
                • Download data from http://example.com/client-companies.tab

      So you'd load up the pipeline, and tell the pipeline that you want the Excel file. And it thinks: OK, so I need to query this database, but first I need to build the database, how do I do that... I need to create it and load these tables... how do I do that... etc. You get the picture.

      And in fact, some details of the pipeline are inferred during processing. We often don't want to hard-code file formats like CSV into the pipeline, but instead infer them from HTTP content-type headers and so forth.

      It's still at a fairly early stage, but it's progressing quite nicely, and Moose roles are an important part of the implementation.

      The reason being that at many stages in the process you need mix-and-match transformations. You need an object which accepts CSV and outputs Excel, or accepts tab-delimited data and outputs Excel, or accepts CSV and outputs an SQL table, or accepts an SQL table and outputs an HTML table.

      OK, so each of these could be a class, but it's far easier to do as a role. When you come to a stage in processing, you instantiate a very generic object (which just has a few debugging methods, a unique identifier string, a link back to the pipeline object, etc) and compose it with the roles it needs to be able to handle (read CSV, write Excel).

      The technique which you showed in your example is to make a subclass of Math::BigInt, and stuff the new methods into that. It's worth noting that actually, this is exactly how Moose role composition works internally - it creates a new class for you, and reblesses the object into that class. But it's all done on the fly at run-time, so you don't need to create modules for each class, choose names for them, etc. All that is abstracted away.

      perl -E'sub Monkey::do{say$_,for@_,do{($monkey=[caller(0)]->[3])=~s{::}{ }and$monkey}}"Monkey say"->Monkey::do'

        update: my ascii drawing corrupt with threaded view. please have a look from single article view.

        Thanks for reply. This is too interesting for me.

        I used extends here, not with, like you always do. Roles, mixins are new to me.
        I guess this picture vaguely like this.

        
                         request from client
                     +------------------------->            pipeline
                                +------------------------------------------------------------------+
                                |                                                                  |
                                |                                                                  |
                                |                            r                                     |
                                |                    +-------------+                               |
                                |        +-----------+-+           v            +----+             |
                                |        |  master db  |        +--+--+              |             |
                                |        +-------------++--+    | csv |+--+          v             |
                                |                     r|  r|    +-----+   |       r r  r           |
                                |        +-------------|---|---------+    |r      r r  r r         |
                                |        |  tempral db |   |         |    |          +             |
            client              |        |             v   v         <----+          v             |
                                |        |         person    company |             generic object  |
                                |        +-------------+----+--+---+-+             with            |
                                |     r  +-------------|----|--|---|--+            various         |
                                |    +-----+ csv  <----+r   |  |   |  |            combination     |
                                |    |r  |                  |r |r  |r |            of roles        |
                                |    +-----> tab delimited <+  |   |  |              +             |
                                |    |r  |                     |   |  |              |             |
                                |    +-----> html     <--------+   |  |              |             |
                                |    |r  |                         |  |              |             |
                                |    +-----> excel    <------------+  |          <---+             |
                                |        +----------------------------+                            |
                      <-------+ |                                                                  |
                                |                                                                  |
        
        So, your generic object should know the dependencies of roles to use, plugging them , create the output. Dependencies varies from case by case, you have to use roles dynamically, with apply_all_roles.

        From client, it will just look like google api, tell the pipe line parameters needed with HTML header, or sending XML.

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