Given that trigger is used (at least by me) to verify/limit the value of what we used to call a class variable (do we call them attributes now?) I mostly wanted to avoid the case where the silly but otherwise innocent user of my code calls a function thinking it might be just the thing only to have it; a. not be 'the thing' and b. screw things up. Here is the small class where this came up:

#!/usr/bin/perl
# Piece_m.pl -- 'Moose' version of Piece.pm
use strict;
use feature ":5.10";
use Data::Dumper::Simple;

our $VERSION = '0.02';

package Piece;
use Moose;
use Moose::Util::TypeConstraints;

my %pieces;

enum 'Side' => qw(white black);
has 'side' => ( is => 'ro', isa => 'Side', required => 1 );

enum 'Name' => qw(K Q R B N P);
has 'name' => ( is => 'ro', isa => 'Name', required => 1 );

enum 'ID' => qw (
  WQR WQN WQB WQ  WK  WKB WKN WKR
  WQRP WQNP WQBP WQP WKP WKBP WKNP WKRP
  BQR BQN BQB BQ BK BKB BKN BKR
  BQRP BQNP BQBP BQP BKP BKBP BKNP BKRP
);
has 'id' => ( is => 'ro', isa => 'ID', required => 1, trigger => \&che
+ck_id );

sub check_id {
    my $self = shift;
    if ( exists( $pieces{ $self->{id} } ) ) {
        confess "", $self->{id}, " already declared";
    }
    else {
        $pieces{ $self->{id} }++;
    }
}

package Main;
use Test::More 'no_plan';
use Test::Fatal;

my $wk = Piece->new( side => 'white', name => 'K', id => 'WK' );
isa_ok( $wk, 'Piece' );
isa_ok( $wk, 'Moose::Object' );
is( $wk->side, 'white', '... got the right side for wk' );
is( $wk->name, 'K', '... got the right name for wk' );
is( $wk->id, 'WK', '... got the right id for wk' );
my @methods = Piece->meta->get_method_list();
print Dumper(@methods);
[download]

As you can see an invocation of Piece->check_id() would not be a good thing— although having just said that I suppose I could examine the passed parameters and be able to tell if the call was correct or not. If not complain else do what it was designed to do. At the moment this is a toy so that I can begin to understand Moose with an eye towards converting my CPAN stuff accordingly (21st century and all that…)

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