I've mixed feelings about this.
An external link (book, article, code) could solve completely the OP's problem. But considering it a valid way to answering may lead quickly to ridiculous situations (I spare here some gedanken examples born out of my particular sense of humor), not to speak of the problem with future dead links, etc.
On the other hand, some topics are too cumbersome and/or have canonical and good answers already published and re-writing/condensing/analyzing them could be really time demanding.
So: when is appropriate flagging such answers as "not an answer" so they reincarnate as comments and when should we let them be in this site?
Please Note: There is a clear exemption not to be considered here when the OP explicitly asks for references.