Contravariance for multiple interface

contra

interface Document {}

interface JSONDocument extends Document {}

interface XMLDocument extends Document {}

// If PHP had it we could use covariance on the params
// interface Printer {
//     function printDocument(Document $document);
// }

interface JSONPrinter {
    function printDocument(JSONDocument $jsonDocument);
}

interface XMLPrinter {
    function printDocument(XMLDocument $xmlDocument);
}

class MultiplePrinter implements JSONPrinter, XMLPrinter {
	// Param of this function has to be contravariant to 
        // support multiple interfaces.
	function printDocument(Document $document) {
		if ($document instanceof JSONDocument) {
			//...
		}
		else if ($document instanceof XMLDocument) {
			//...
		}
		else {
		    //Unsupported type
		}
	}
}