The meaning of "mainstream" changes throughout the years. Remember that Common Lisp started as quite literally the XKCD joke of "14 competing standards", except it actually worked -- in the sense that this 15th competing standard killed all the other ones and gained widespread adoption in the Lisp community. Tons of vendors threw money and effort at the situation, and it all started as an ARPA manager's idea. Of course, we live in different times now... But Zetalisp is one fairly popular Lisp I can think of that had funcallable objects. Whether the idea was appreciated or preferred over alternatives is a separate thing entirely, of course. I would assume most people would use a simple let-over-lambda to achieve the same effect as funcallable objects.