gistfile1.txt

<?php

abstract class SnakStorage
{
    public function canStore( Snak $snak )
    {
        throw new \RuntimeException("Not implemented, yet.");
    }

    public function storeSnak( $snak, $snakRole /* ... */ )
    {
        throw new \RuntimeException("Not implemented, yet.");
    }
}

class PropertyValueSnakStorage
{
    public function canStore( Snak $snak )
    {
        return ( $snak instanceof PropertyValueSnak );
    }

    public function storeSnak( $snak, $snakRole /* ... */ )
    {
        // Perform storage
    }
}

class Updater
{
    private $snakStorages = array();

    public function __construct( array $snakStorages )
    {
        // .. add snak storages
    }

    public function insertSnak( Snak $snak, $snakRole )
    {
        foreach ( $this->snakStorages as $snakStorage )
        {
            if ( $snakStorage->canStore( $snak ) )
            {
                return $snakStorage->storeSnak( $snak /* ... */ );
            }
        }
        throw new \RuntimeException( 'Unknown snak ...' );
    }
}