The Past Monad

past.php

<?php

namespace igorw;

class Past {
    private $value;
    private $pastValues;

    function __construct($value, array $pastValues = []) {
        $this->value = $value;
        $this->pastValues = $pastValues;
    }

    function map(callable $fn) {
        return $this->update($fn($this->value));
    }

    function update($value) {
        return new Past($value, array_merge($this->pastValues, [$this->value]));
    }

    function get() {
        return array_rand($this->pastValues);
    }
}

$answer = new Past(42);
$answer = $answer->map(function ($number) { return $number + 1; });
$answer = $answer->update(-3);
$answer = $answer->map(function ($number) { return $number * -1; });
$answer = $answer->map(function ($number) { return $number * 3; });
$answer = $answer->update('completely new value, hehehe');

var_dump($answer->get());

Temporal paradoxes be dammed!