adt.pl

use v5.10;
use Data::Dumper;
use Carp;

sub aref { \@_ }

package Opt {
    use overload
        '""' => \&stringinfy;

    sub new {
        my ($cls, $tag, $payload) = @_;
        bless {
            tag => $tag,
            payload => $payload,
        }, $cls;
    }

    sub value {
        shift->{payload};
    }

    sub some {
        Opt->new("some", shift);
    }

    sub none {
        Opt->new("none");
    }

    sub is_some {
        shift->{tag} eq "some";
    }

    sub map(&$) {
        my ($block, $self) = @_;
        if ($self->is_some) {
            local $_ = $self->value;
            return some($block->());
        } else {
            return $self;
        }
    }

    sub default(&$) { 
        my ($block, $self) = @_;
        return $block->() unless $self->is_some;
        $self->value;
    }
    
    sub stringinfy {
        my ($self) = @_;
        if ($self->is_some) {
            return "Some(" . $self->value . ")";
        } else {
            return "None"
        }
    }

    sub bind(&$) {
        my ($block, $self) = @_;
        if ($self->is_some) {
            local $_ = $self->value;
            return $block->();
        } else {
            return $self;
        }
    }
}

package Opt::Match {
    sub some(&$) {
        my ($block, $self) = @_;
        return $self unless ref($self) eq "Opt";
        if ($self->is_some) {
            local $_ = $self->value;
            return $block->();
        } else {
            return $self;
        }
    }

    sub none(&$) {
        my ($block, $self) = @_;
        return $self unless ref($self) eq "Opt";
        if (! $self->is_some()) {
            local $_ = undef;
            return $block->();
        } else {
            return $self;
        }
    }
}




my $some = Opt::some(1);
my $none = Opt::none();
say 
Opt::default { "oops" }
Opt::map { $_ + 2 } 
Opt::map { $_ + 1 } 
$some;

say 
Opt::Match::some { 1 }
Opt::Match::none { 0 }
Opt::bind { $_ > 0 ? Opt::some($_ - 1) : Opt::none() }
Opt::bind { $_ > 0 ? Opt::some($_ - 1) : Opt::none() }
Opt::none(10);