Raku question: "Moose is Perl", but it sure ain't Raku.

[Cow1337killr]

I have been studying OOP and Moose.

I stumbled upon the RJBS presentation https://cdn.oreillystatic.com/en/assets/1/event/115/Moose%20is%20Perl_%20A%20Guide%20to%20the%20New%20Revolution%20Presentation%202.pdf "Moose is Perl".

I find it instructive to write Perl 5 programs showing each example found in presentations, blogs, and tutorials.

Here is one:

Perl 5 program: mip-087-001.pl

#!/usr/bin/perl

# Program name: mip-087-001.pl  Run it, thusly: See C++ comment below.
+  Moose is Perl (i.e., MIP or mip) presentation.

# From https://cdn.oreillystatic.com/en/assets/1/event/115/Moose%20is%
+20Perl_%20A%20Guide%20to%20the%20New%20Revolution%20Presentation%202.
+pdf.

# The code below is from https://www.perl.com/pub/2002/08/13/comment.h
+tml/

use Acme::Comment type => 'C++', one_line => 1, own_line => 0;

/*
cd C:\2020-Raku-001\
perl C:\2020-Raku-001\Perl-scripts\mip-087-001.pl
*/

use strict;
use warnings;
use feature 'say';
local $| = 1;    # Forces a flush.

package Employee {

use strict;
use warnings;

use Moose;
use namespace::autoclean;  # Code this instead of coding "no Moose;" a
+t the end of the package block.

has name  => (
  is       => 'ro',
  isa      => 'Str',
  required => 1,
);
has title => (
  is       => 'rw',
  isa      => 'Str',
  required => 1,
);

sub name_and_title {
  my ($self) = @_;

  my $name  = $self->name;
  my $title = $self->title;

  return "$name, $title";
}
1;  # Magic true value.
}

package Employee::Former {

use strict;
use warnings;

use Moose;
use namespace::autoclean;  # Code this instead of coding "no Moose;" a
+t the end of the package block.

extends 'Employee';

override name_and_title => sub {
  my ($self) = @_;
  my $old = super;
  return "$old (Former)";
};

# Slide #85.
# The + says "we're overriding the definition in our superclass.  Ever
+ything stays the same 
# except for the provided changes."

# Here, we give a default.  If no value is given, the default is used,
+ which lets us satisfy the 
# "required" even when no value was given in the call to the construct
+or.

has '+title' => (
  default => 'Team Member',
);
1;  # Magic true value.
}

# Commented out.  use Employee;
my $peon = Employee->new({
  name  => 'William Toady',
  title => 'Associate Assistant',
});

say $peon->name_and_title;  # Get name_and_title.

# Commented out.  use Employee::Former;
my $ex_peon = Employee::Former->new({
  name  => 'William Toady',
  title => 'Associate Assistant',
});

say $ex_peon->name_and_title;  # Get name_and_title.

# Where is the test code?  Answer: Slide #87.
# Name this script mip-087-001.pl

my $ex_peon2 = Employee::Former->new({
  name  => 'William Toady',
});

say $ex_peon2->name_and_title;  # ===> William Toady, Team Member (for
+mer)

__END__

Results:

William Toady, Associate Assistant
William Toady, Associate Assistant (Former)
William Toady, Team Member (Former)

I recently looked at Raku after the name change. (The Windows version is actually easy to install now.)

So, after reading much of the "Think Raku" book, I tried to write counterparts to my collection of Perl 5 RJBS presentation "Moose is Perl" programs.

Well,I am stuck. (I thought Moose was 1:1 Raku OOP. Boy, was I wrong.)

Raku program attempt: mip-087-001.raku

class Employee {
  has Str $.name  is readonly is required;  # "readonly" is the defaul
+t.
  has Str $.title is rw       is required;
  
  method name_and_title {

  my $name  = self.name;
  my $title = self.title;

  return "$name, $title";
  }
}


class Employee::Former is Employee {
  
  method name_and_title {
  
    my $old = callsame;
  
    return "$old (Former)";
  }

# Slide #85.
# The + says "we're overriding the definition in our superclass.  Ever
+ything stays the same 
# except for the provided changes."

# Here, we give a default.  If no value is given, the default is used,
+ which lets us satisfy the 
# "required" even when no value was given in the call to the construct
+or.

# XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
# Is there not a Raku counterpart for this?
# has '+title' => (
#   default => 'Team Member',
# );
# My poor attempt:
  has $.title is default('Team Member');
# XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
}

# Commented out.  use Employee;
my $peon = Employee.new(
  name  => 'William Toady',
  title => 'Associate Assistant',
);

say $peon.name_and_title;  # Get name_and_title.

# Commented out.  use Employee::Former;
my $ex_peon = Employee::Former.new(
  name  => 'William Toady',
  title => 'Associate Assistant',
);

say $ex_peon.name_and_title;  # Get name_and_title.

# Where is the test code?  Answer: Slide #87.
# Name this script mip-087-001.raku

my $ex_peon2 = Employee::Former.new(
  name  => 'William Toady',
);

say $ex_peon2.name_and_title;  # ===> William Toady, Team Member (form
+er)

Results:

William Toady, Associate Assistant
William Toady, Associate Assistant (Former)
The attribute '$!title' is required, but you did not provide a value f
+or it.
  in block <unit> at C:\2020-Raku-001\Raku-scripts\mip-087-001.raku li
+ne 60

[tobyink]

Here's an example using my latest plaything, MooX::Pression. Raku-like syntax, but it's Perl 5.

use v5.14;
use strict;
use warnings;

package Local {
    use MooX::Pression;
    
    class Employee {
        has name  (type => Str, required => true);
        has title (type => Str, required => true);
        
        method name_and_title () {
            my $name  = $self->name;
            my $title = $self->title;
            return "$name, $title";
        }
    }
    
    class Employee::Former {
        extends Employee;
        factory former_employee;
        has +title = "Team Member";
        
        around name_and_title () {
            my $old = $self->$next(@_);
            return "$old (Former)";
        }
    }
}

my $peon = Local->new_employee(
    name  => 'William Toady',
    title => 'Associate Assistant',
);

say $peon->name_and_title;

my $ex_peon = Local->former_employee(
    name  => 'William Toady',
    title => 'Associate Assistant',
);

say $ex_peon->name_and_title;

my $ex_peon2 = Local->former_employee(
    name  => 'William Toady',
);

say $ex_peon2->name_and_title;

__END__
William Toady, Associate Assistant
William Toady, Associate Assistant (Former)
William Toady, Team Member (Former)

Of course, this uses Moo instead of Moose. If you prefer Moose...

use MooX::Pression toolkit => "Moose";

[holli]

The submethod BUILD is (indirectly) called by .bless. It is meant to set private and public attributes of a class and receives all names attributes passed into .bless. The default constructor .new defined in Mu is the method that invokes it. Given that public accessor methods are not available in BUILD, you must use private attribute notation instead.

C&P, straight from the docs

class C {
    has $.attr;
    submethod BUILD (:$attr = 42) {
        $!attr = $attr
    };
};
 
C.new.say; C.new( attr => 666 );
# 42
# 666

Edit:
The difference between BUILD and TWEAK (as mentioned by anonymous monk below) is, that the former happens while the object is constructed, whereas TWEAK happens after that, when the object has been fully constructed.

[Anonymous Monk]

class A {
     has $.a;
     method TWEAK(:$!a = "foo") { }
}
dd A.new;               # A.new(a => "foo")
dd A.new(a => "bar");   # A.new(a => "bar")

[duelafn]

There may be a better way, but you can do this by overriding new (and removing the title attribute from Employee::Former):

class Employee::Former is Employee {
    method new(*%params) {
        %params<title> //= 'Team Member';
        return callwith(|%params);
    }

    method name_and_title {
        my $old = callsame;
        return "$old (Former)";
    }
}

[Anonymous Monk]

has $.title = 'Team Member';