#!/usr/bin/perl

package Monad;
use Data::Dumper;
use base 'Exporter';

our @EXPORT = qw/Identity Monad Fail Bind Return L /;

use overload '>>'   => 'Bind',
             q("") => sub { Dumper(shift) }
            ;

sub L (&) { shift } # instead of "sub", looks a bit prettier

sub Identity {
    my ($value) = @_;
    return bless \$value, 'Monad';
}

sub Monad {
    my $monad = caller(1);
}
sub Lift {
    my $monad = caller(1);
    $monad->lift(@_);
}
sub Fail {
    my $monad = caller(1);
    $monad->fail(@_);
}
sub Bind {
    my $monad = shift;
    $monad->bind(@_);
}
sub Return {
    my $monad = caller(1);
    $monad->return(@_);
}

sub return {
    my ($class, $data) = @_;
    bless $data, $class;
}
sub lift {
    my ($class, $f) = @_;
    return sub {
        $class->return( $f->(@_) );
        };
}
sub bind {
    my ($self, $f) = @_;
    return $f->($self->mjoin); # default Identity monad
}
sub mjoin {
    my $self = shift;
    return $$self;
}
sub fail {
    my ($class, $exception) = @_;
    die $exception;
}

1;