Hello dear esteemed monks,

I have a class that is read-only. It does have parameters, but for the most cases a default instance would fit. So I'd like to avoid unneeded initialization and just return $ONE_AND_TRUE_INSTANCE if such exists and no parameters are given to new().

Here's how I would've done this outside Moo/Moose:

my $ONE_TRUE_INSTANCE = __PACKAGE__->new;
sub new {
    my ($class, %args) = @_;

    return $ONE_TRUE_INSTANCE
        if ($ONE_TRUE_INSTANCE and $class eq __PACKAGE__ and !%args);

    # normal constructor here
    ...
};

[download]

Can the same be achieved with Moo? How can my example code be improved? Maybe there's a proper architectural solution and/or pattern for this?

Thank you,

UPDATE: actually I can do this with Moo, so the question remains - what can be fixed in architecture to avoid the need for "defaulton": 

package Defaulton;

use Moo;
has foo => is => ro => default => sub { 42 };
my $INST = __PACKAGE__->new;

around new => sub {
    my ($orig, $class, @args) = @_;

    return $INST if $INST and $class eq __PACKAGE__ and !@args;

    return $class->$orig(@args);
};

1;

[download]

In reply to Caching a default instance to avoid initializing it every time (with Moo) by Dallaylaen

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